Landscapes and Landforms of Nigeria 3031179714, 9783031179716

Table of contents :
Series Editor’s Preface
Introduction
References
Contents
Editors and Contributors
Physiography
1 Geology of Nigeria
Abstract
1.1 Introduction
1.2 The Geological and Geotectonic Framework of Nigeria
1.3 Geological Setting of Nigeria
1.3.1 The Precambrian Basement Complex
1.3.1.1 The Migmatite–Gneiss Complex (MGC)
1.3.1.2 The Schist Belts
1.3.1.3 Older Granites (Pan-African Granitoids)
1.3.1.4 Undeformed Acid and Basic Dykes
1.3.1.5 The Younger Granites
1.3.2 The Sedimentary Rocks
1.3.2.1 The Benue Trough
1.3.2.2 The Calabar Flank
1.3.2.3 Anambra Basin
1.3.2.4 Chad (Bornu) Basin
1.3.2.5 Sokoto Basin
1.3.2.6 Mid-Niger Basin
1.3.2.7 The Dahomey Basin
1.3.2.8 Niger Delta Basin
1.4 Geological Setting and Geomorphological Implications
1.5 Geology: Economic Considerations, Geoheritage and Way Forward
References
2 The Climate of Nigeria and Its Role in Landscape Modification
Abstract
2.1 Introduction
2.2 General Overview of the Climate of Nigeria
2.3 Spatio-Temporal Distribution of Rainfall in Nigeria
2.4 Temperature Profile
2.5 Conclusion
References
3 Vegetation and Human Impact
Abstract
3.1 Vegetation and Its Varieties in Nigeria
3.2 Mangroves and Mangrove Landscapes
3.2.1 Distribution of the Mangroves
3.2.2 Mangrove Landscapes
3.2.3 Human Impacts on the Mangroves
3.3 Rainforest and Rainforest Landscape
3.3.1 Distribution
3.3.2 Rainforest Landscape
3.3.3 Human Impact
3.4 Savanna and Savanna Landscapes
3.4.1 Factors Controlling the Occurrence of the Savanna
3.4.2 Savanna Landscape
3.4.3 The Human Impact on the Savanna
3.5 Conclusion
References
4 Lower Plains of Northern Nigeria
Abstract
4.1 Introduction
4.2 Lower Plains of the Sokoto Basin
4.3 Lower Plains in North-Central Northern Nigeria
4.4 Lower Plains of Sand Dune Belt of North-eastern Nigeria
4.5 Lower Plain of the Chad Basin
4.6 Conclusion
Acknowledgements
References
5 Hills and Ridges in Southwestern Nigeria
Abstract
5.1 Introduction
5.2 Inselberg Description and Classification
5.3 Types of Hills
5.3.1 Inselbergs
5.3.2 Tors
5.4 Inselberg Occurrence
5.5 Some Aspects of Inselberg Morphometry
5.6 Ridges
5.6.1 Ridges Around Ibadan
5.6.2 Ridges East of Ilesa
5.6.3 The Low Ridges South of Ipetu-Ijesa
5.7 The Development of the Hills and Ridges
5.8 Tourism Potentials of the Hills and Ridges
5.9 Conclusion
References
6 South-East Hills and Ridges
Abstract
6.1 Introduction
6.2 Geology
6.3 The Eastern Borderland Highlands
6.4 The Nsukka Escarpment and Plains
6.5 Conclusion
References
7 The Niger Delta Region
Abstract
7.1 Introduction
7.2 Physiography
7.3 Geology and Geomorphological Units
7.4 Land-Use and Vegetation Changes in the Niger Delta
7.5 Oil and Gas Pollution in the Niger Delta
7.6 Conclusions
References
Internet Sources
Specific Landforms
8 Landforms of the Chad Basin
Abstract
8.1 Introduction
8.2 Location and Extent of Lake Chad Basin
8.3 Geology and Soils of the Mega-Chad Basin
8.4 The Bama Beach Ridge
8.5 Soils
8.6 Landforms in the Lake Chad Basin
8.6.1 The Bodele Depression
8.6.2 Paleo-Dunes
8.6.3 Sand Dunes
8.6.4 Inselbergs
8.7 Economic Implications of the Desiccating Chad Basin
8.8 Conclusions
References
9 Dune Fields on the Plains of Northern Nigeria
Abstract
9.1 The Formation of Sand Dunes
9.2 The Application of Remote Sensing Techniques in Dunes Studies
9.3 Dunes of Northern Nigeria
9.3.1 Origin
9.3.2 Studies of Dunes in Northern Nigeria
9.3.3 Age of the Dunes of Northern Nigeria
9.3.4 The Spatial Extent of the Dune Fields of Northern Nigeria
9.4 Morphology of Dune Landscapes
9.5 Paleoclimatology and Paleobotany Information in Dunes
9.6 The Impact of Dune Re-Mobilization on Settlement
9.7 Dune Fields as Tourist Attractions
9.8 Conclusions
References
10 Landscapes and Landforms of the Jos Plateau
Abstract
10.1 Introduction
10.2 Landforms Units of the Jos Plateau
10.3 Landscape Features of the Basement Complex
10.4 Ring Complexes in the Younger Granite Areas
10.5 Landscape Features Supported by Volcanic Rocks
10.6 Fluvial Morphology
10.6.1 Rivers
10.6.2 Waterfalls
10.6.3 Springs
10.7 Economic Activities
10.8 Conclusions
Acknowledgements
References
11 Kainji Dam and Lake
Abstract
11.1 Introduction
11.2 Kainji Dam and Lake
11.2.1 Kainji Dam
11.2.2 Lake Kainji
11.2.3 Lake Islands
11.3 Lake Kainji Area
11.3.1 Climate
11.3.2 Geology and Geomorphology
11.3.3 Resettlement of Displaced People
11.4 Lake Kainji National Park
11.5 Conclusion
References
12 Riparian Vegetation Along Nigeria Rivers: The River Ogun Example
Abstract
12.1 Introduction
12.2 Riparian Vegetation Along Major Nigerian Rivers
12.2.1 Rivers in Nigeria
12.2.2 Riparian Vegetation Along Nigeria’s Rivers
12.3 The River Ogun Case Study
12.3.1 Setting
12.3.2 Geomorphological Features Along the Alluvial Channel
12.3.3 Riparian Species Diversity Along River Ogun
12.3.4 Above-Ground and Below-Ground Diversity of the Riparian Vegetation of River Ogun
12.4 Human Activities Along the River Ogun
12.5 Conclusion
References
13 Quartzite Ridges in Southwestern Nigeria
Abstract
13.1 Introduction
13.2 The Distribution of Quartzite Ridges in Southwestern Nigeria
13.3 Physiography and Geology of Quartzite Ridges
13.3.1 Drainage
13.3.2 Geomorphic Activities on Quartzite Ridges
13.4 Notable Quartzite Ridges—Examples from Ibadan
13.4.1 Geomorphology of Ibadan Quartzite Ridges
13.4.2 Gully Erosion on Ibadan Quartzite Ridges
13.4.3 Human Activities on Ibadan Quartzite Ridges
13.5 Conclusions
References
14 Geology and Landscapes of the Southwestern Nigeria
Abstract
14.1 Introduction
14.2 Overview of Geology of Southwestern Nigeria
14.2.1 Gneiss–Migmatite–Quartzite Complex
14.2.2 Schist Belt
14.2.2.1 Iseyin–Oyan River Schist Belt
14.2.2.2 Ilesa Schist Belt
14.2.2.3 Igarra Schist Belt
14.2.2.4 The Older Granites (Pan-African Granitoids)
14.2.2.5 Minor Felsic and Mafic Intrusives
14.2.3 Sedimentary Rocks
14.3 Landscapes and Landforms
14.3.1 Inselbergs—Origin and Examples
14.3.1.1 Evolution of Inselbergs
14.3.1.2 Aseke Hill on Banded Gneiss
14.3.1.3 Inselbergs in Some Parts of Southwestern Nigeria
14.3.2 Pediments
14.4 Corestones
14.5 Conclusions
References
15 Geology, Geomorphology and Evolution of the Landscapes of Cross River Region, South-Eastern Nigeria
Abstract
15.1 Introduction
15.2 Setting and Location
15.3 Geology
15.3.1 Basement Igneous and Metamorphic Rocks
15.3.2 Cretaceous Sedimentary Rocks
15.4 Major Relief Features
15.4.1 The Obudu Plateau
15.4.2 The Oban Massif
15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation15.4.3 Mfamosing Karst Formation
15.5 Human Impacts on the Cross River State Landscape
15.6 Conclusion
References
16 Gully Erosion Sites in Southeast Nigeria: Prospects for Geotourism
Abstract
16.1 Introduction
16.2 Gullies and Gully Erosion
16.3 Gully Formation Processes
16.3.1 Knick Point Formation and Slumping
16.3.2 Gullies as a Result of Swelling and Shrinkage of Soil
16.3.3 Lateral Bank Failure and Undercutting
16.3.4 Culvert Failure and Undercutting
16.3.5 Factors of Gully Development
16.4 Identifying Major Gully Hot Spots Across the South East
16.4.1 Anambra State
16.4.2 Imo State
16.4.3 Abia State
16.4.4 Enugu State
16.5 Summary and Conclusions
References
17 Erosion and Accretion Along the Coastal Zone of Nigeria
Abstract
17.1 Introduction
17.2 Coastal Zone of Nigeria
17.3 Coastal and River Bank Erosion
17.4 Erosion of the Transgressive Mud Coast, Ondo State
17.5 Depletion of Victoria Deach, Lagos
17.6 Beach Erosion at Forcados South Point, Youbebe, Delta State
17.7 Sand Bars and Recurved Sand Spits
17.8 Conclusion
References
Geoheritage, Conservation of Geomorphological Sites and Geotourism
18 The Islands of Lagos
Abstract
18.1 Introduction
18.2 Geographic Location
18.3 Geology, Landforms and Terrain
18.4 Beaches
18.5 Mangroves
18.6 Sand
18.7 Human Activities
18.8 Geoheritage and Shoreline Change
18.8.1 The Lagos Harbour/Commodore Channel and the East and West Moles
18.8.2 Victoria Island and Lekki Erosion
18.8.3 The Great Lagos Sea Wall, Shoreline Protection and Eko Atlantic City Reclamation Project
18.9 Conclusions
References
19 Urban Geoheritage Site: The Example of Olumo Rock in Abeokuta City, Ogun State, Nigeria
Abstract
19.1 Introduction
19.2 Location
19.3 The Geological Landscape of Olumo Rock
19.4 Geomythology of the Olumo Rock
19.5 Olumo Rock as Geomorphological Heritage
19.6 Biogeomorphology of the Olumo Rock
19.7 The Anthroscape of the Olumo Rock
19.7.1 Transformation of Natural Landscape
19.7.2 The Cultural and Aesthetic Values of Olumo Rock
19.8 Conclusion
References
Author Index
Subject Index

Citation preview

World Geomorphological Landscapes

Adetoye Faniran L. K. Jeje Olutoyin A. Fashae Adeyemi O. Olusola   Editors

Landscapes and Landforms of Nigeria

World Geomorphological Landscapes Series Editor Piotr Migoń, Institute of Geography and Regional Development, University of Wrocław, Wrocław, Poland

Geomorphology – ‘the Science of Scenery’ – is a part of Earth Sciences that focuses on the scientific study of landforms, their assemblages, and surface and subsurface processes that moulded them in the past and that change them today. Shapes of landforms and regularities of their spatial distribution, their origin, evolution, and ages are the subject of geomorphology. Geomorphology is also a science of considerable practical importance since many geomorphic processes occur so suddenly and unexpectedly, and with such a force, that they pose significant hazards to human populations. Landforms and landscapes vary enormously across the Earth, from high mountains to endless plains. At a smaller scale, Nature often surprises us creating shapes which look improbable. Many geomorphological landscapes are so immensely beautiful that they received the highest possible recognition – they hold the status of World Heritage properties. Apart from often being immensely scenic, landscapes tell stories which not uncommonly can be traced back in time for millions of years and include unique events. This international book series will be a scientific library of monographs that present and explain physical landscapes across the globe, focusing on both representative and uniquely spectacular examples. Each book contains details on geomorphology of a particular country (i.e. The Geomorphological Landscapes of France, The Geomorphological Landscapes of Italy, The Geomorphological Landscapes of India) or a geographically coherent region. The content is divided into two parts. Part one contains the necessary background about geology and tectonic framework, past and present climate, geographical regions, and long-term geomorphological history. The core of each book is however succinct presentation of key geomorphological localities (landscapes) and it is envisaged that the number of such studies will generally vary from 20 to 30. There is additional scope for discussing issues of geomorphological heritage and suggesting itineraries to visit the most important sites. The series provides a unique reference source not only for geomorphologists, but all Earth scientists, geographers, and conservationists. It complements the existing reference books in geomorphology which focus on specific themes rather than regions or localities and fills a growing gap between poorly accessible regional studies, often in national languages, and papers in international journals which put major emphasis on understanding processes rather than particular landscapes. The World Geomorphological Landscapes series is a peer-reviewed series which contains single and multi-authored books as well as edited volumes. World Geomorphological Landscapes – now indexed in Scopus® !

Adetoye Faniran • Lawrence Kosoko Jeje • Olutoyin A. Fashae • Adeyemi O. Olusola Editors

Landscapes and Landforms of Nigeria

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Editors Adetoye Faniran Department of Geography, Faculty of the Social Sciences University of Ibadan Ibadan, Nigeria Olutoyin A. Fashae Department of Geography, Faculty of the Social Sciences University of Ibadan Ibadan, Nigeria

Lawrence Kosoko Jeje Department of Geography, Faculty of Social Sciences Obafemi Awolowo University Ilé-Ifè, Nigeria Adeyemi O. Olusola Faculty of Environmental and Urban Change York University Toronto, ON, Canada

ISSN 2213-2090 ISSN 2213-2104 (electronic) World Geomorphological Landscapes ISBN 978-3-031-17971-6 ISBN 978-3-031-17972-3 (eBook) https://doi.org/10.1007/978-3-031-17972-3 © Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Series Editor’s Preface

Landforms and landscapes vary enormously across the Earth, from high mountains to endless plains. On a smaller scale, nature often surprises us creating shapes that look improbable. Many physical landscapes are so immensely beautiful that they received the highest possible recognition—they hold the status of World Heritage properties. Apart from often being immensely scenic, landscapes tell stories that not uncommonly can be traced back in time for tens of million years and include unique events. In addition, many landscapes owe their appearance and harmony not solely to the natural forces. For centuries, and even millennia, they have been shaped by humans who have modified hillslopes, river courses, and coastlines, and erected structures that often blend with the natural landforms to form inseparable entities. These landscapes are studied by geomorphology—‘the science of scenery’—a part of Earth Sciences that focuses on landforms, their assemblages, surface, and subsurface processes that moulded them in the past and that change them today. To show the importance of geomorphology in understanding the landscape and to present the beauty and diversity of the geomorphological sceneries across the world, we have launched a book series World Geomorphological Landscapes. It aims to be a scientific library of monographs that present and explain physical landscapes, focusing on both representative and uniquely spectacular examples. Each book will contain details on the geomorphology of a particular country or a geographically coherent region. This volume presents the geomorphology of Nigeria—an equatorial African country of immense geomorphological diversity. The variety of Nigerian landscapes and landforms reflects both their geological foundations and the presence of rocks of different origins, ages, and structural arrangement, as well as a strong climatic gradient, from humid tropics in the south to the Saharan desert in the north. Thus, within the geographical limits of Nigeria, we find mountains and plateaus, inselbergs and residual ridges, plains and dune fields, landslides and gullies, large rivers and extensive wetlands, and a diverse coastline. The history of geomorphological research in Nigeria is more than 100 years long, and several important concepts in geomorphology matured here, to name the role of deep weathering in the origin of denudational landscapes or the model of the two-phase evolution of granite inselbergs. The advent of satellite imagery, digital elevation models, and GIS tools helped to advance our understanding of Nigerian landscapes, and these recent efforts by Nigerian geomorphologists are the foundations of this volume. The World Geomorphological Landscapes series is produced under the scientific patronage of the International Association of Geomorphologists (IAG)—a society that brings together geomorphologists from all around the world. The IAG was established in 1989 and is an independent scientific association affiliated with the International Geographical Union (IGU) and the International Union of Geological Sciences (IUGS). Its main aims are to promote geomorphology and to foster the dissemination of geomorphological knowledge. I believe that this lavishly illustrated series, which sticks to the scientific rigour, is the most appropriate means to fulfil these aims and to serve the geoscientific community. To this end, my great thanks go to the editors of the volume, particularly to Dr. Adeyemi Olusola, who committed themselves to this challenging project and coordinated the work of a large group of authors, from different institutions and backgrounds. I am also grateful to all individual contributors who agreed to take part in this endeavour and shared their local expertise with the v

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global geomorphological community. I believe that the final product will serve as a much-needed and appreciated reference source about the geomorphology of Nigeria, which certainly has many more landform gems to reveal to the global world than we know about. Wrocław, Poland

Piotr Migoń

Introduction

Nigeria is a country with wide and varied landscapes and landforms from the North to the South and from the East to the West. The beauty of the country is seen as one travels from the coastal south through the highlands towards the hinterlands and the savannahs in the northern part of the country (Fig. 1). These varied landforms tell us so much about the climate and the history of this part of West Africa and Africa at large. The broad pattern of landform development in any given area is normally determined by the structural and lithological variations. As rich and diverse as the country is, not much is known about the landforms within the country. Some authors have attempted to provide examples of the hidden beauty across each region such as but are not limited to Udo (1970), Faniran (1970), Faniran and High (1971), Faniran and Areola (1974), Jeje (1970, 1980), Olumide and Olusola (2017), and Olusola (2019). These studies, apart from Udo (1970), were examples of landforms and their processes in selected locations or regions across the country. Udo (1970) was a blend of physical and human dimensions across the entire country.

Fig. 1 vii

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Most of the geomorphic evidence as presented in the literature confirms that between ca. 31,000 and 3000 B.P. two periods of desert conditions alternated with two wet phases in the present rain-forest areas (Jeje 1980). These two periods, the first arid period, witnessed pedimentation, while the second was characterized by chemical weathering, etc. The two wet phases are based on evidence provided by Grove and Warren (1968), Shaw (1976) and Burke et al. (1971). This evidence was based on lake-level fluctuations, lake deposits, and longitudinal and traverse dunes within and around Lake Chad in the northeastern part of the country. Over the years, the study of geomorphology, especially interest in landforms, has gradually given way to more process-oriented studies, especially focused on fluvial and erosional processes in Nigeria. However, some structural geomorphologists/geologists are still very much interested in landform studies, processes, and materials. This is evidenced in some of the chapters presented in this book. This book showcases the rich diversity of landscapes and landforms of Nigeria and demonstrates the opportunity for geotourism. In common with other volumes in the series, the book is structured into three main parts. Part I (Chaps. 1–7), written by various authors and editors of this book, gives an overview of the geomorphic diversity of Nigeria while setting the tone for what is to be expected in subsequent parts. The book begins with the Geology of Nigeria (Chap. 1, Moshood Tijani): from the coast to the savannahs. The chapter provides a basis for understanding landforms and landscapes within the country. Reference was also made to the rich mineral resources associated with various geological deposits across the country that have led to the development and economic growth of Nigeria. Chapter 1 is followed by the climate of Nigeria (Chap. 2, Olumide Onafeso), discussed in relation to the varying geological landscapes. The next chapter (Chap. 3, Gbadegesin and others) gives a broad view of the vegetation of the country, from the coastal mangroves to the Sudan savannah. The discussion focuses on the biogeographical characteristics of the country as influenced by landscapes and landforms, whereas the influence of man intensifies changes in vegetal distribution across the landscape. Chapter 5 (Lower Plains, Tasi’u Rilwanu) presents the geomorphology of the Lower Plains of Northern Nigeria, their distribution and evolution. The distinctiveness of these landscapes against the plains in Southern Nigeria is discussed, chiefly considered as a result of the underlying geology. The next two chapters (Hills and Ridges, Southwest (Jeje and others) and Southeast (Olayinka Ogunkoya), present interesting landform features within these regions in Nigeria. The evolution of these landforms is portrayed across these regions: also providing locations, names, and ancient histories that could serve as interesting geotourism spaces. The physical environment part ends with Chap. 7 (Oyegun and others, The Niger Delta). The Niger Delta is one of the largest deltas in the world, with the largest freshwater swamp in Africa and is a biodiversity hot spot because of its rich variety of plant and animal species. This chapter describes geomorphological/geological units, with an emphasis on the physical environment, land cover characteristics, and hydrocarbon pollution. The plains described here as against those of the Lower Plains in the North are homoclinal geomorphic structures that trend westwards and south-westwards. Part II of the book, focused on specific landscapes and landforms, present ten studies that represent the enormous geodiversity of the Nigerian landscapes. The specific landscapes and landforms are identified in Fig. 1. These landscapes are distinct and portray the geomorphological richness of the humid tropical geomorphology. Part II begins in the northern territory of the country and then moves south, toward the coast. The first chapter in this part (Chap. 8, Daura and others) describes Lake Chad Mega Lake: a vast inland drainage system with geomorphic, hydrologic, and climatological significance across the West African Region and beyond. The geology of the Chad Basin shows that Quaternary sands of various origins dominate the basin. The Bodele Depression in northern Chad, one of the major landforms here, is a typical example of a large deflation hollow. Historically, the Bodele Depression used to be part of the paleo-lake, being the deepest part of the lake basin, at 155 m above sea level. Dust arising from this depression travels across the Atlantic. The next chapter (Chap. 9, Aliyu Nabegu) on the dunes of Northern Nigeria describes the unique and spectacular patterns of the

Introduction

Introduction

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dunes on the plains of Northern Nigeria. These dunes are dominantly linear, and they are relicts of the Quaternary period when the climate of northern Nigeria was drier. Today, the dunes are found in areas that receive as much as 1000 mm of rainfall per year. Moving southward, the focus shifts to the Jos Plateau (Chap. 10, Tasi’u and others). The Jos Plateau is situated in the north-central part of Nigeria and is surrounded by plains on all sides. The landforms and landscapes of the plateau can be grouped into three units, namely hills and mountains, dissected terrains, and undulating terrain. The Jos Plateau is drained by a radial river network, with numerous waterfalls at topographic escarpments. Regional geology includes two major structural units: an Older Basement Complex and the Younger Granites. The Jos Plateau is an important site in the country with interesting ring complexes. The next chapter addresses the Kainji Dam and Lakes (Chap. 11, Olayinka Ogunkoya). The area covered by the Kainji dam, Lake and National Park constitutes a distinct landscape within the Nigerian scenery. The dam and lake were created along the River Niger approximately 105 km upstream from Jebba. Though the primary focus of the dam and associated impoundment is hydroelectricity generation, other ancillary purposes are flood control, navigation enhancement, irrigation, and fishery development. The lake’s annual hydrograph shows there are two peak inflows: the ‘White’ and ‘Black’ floods, occurring in September and February, respectively. Having completed a discussion about the landscapes and landforms within the northern regions of Nigeria, attention is shifted to southwestern Nigeria, with a discussion of the riparian vegetation (Chap. 12, Fashae and others), especially along the Ogun River Basin. Across rivers draining Nigeria, the riparian composition varies significantly. The southern part of the country presents more luxuriant vegetation along river courses than the northern part. However, biodiversity along river courses in Nigeria presents a scenery that is attractive to view and serves as economic gains for the local people. River Ogun, an alluvial river that is reported in this study, largely epitomizes the situation of riparian ecosystems in the humid tropics. Typically found along this alluvial stretch are woody plants and light forests which influence riverbank stability. Still, within the southwestern part of Nigeria, the next chapter (Chap. 13, Fashae and others) describes the quartzite elevations. These landforms occur mainly as ridges and lines of hills, although bedrock outcrops are rare because the ridges are often covered by debris due to rock weathering. The next chapter (Chap. 14, Eludoyin and others) presents various landforms from southwestern Nigeria: their geology, evolution, and location. The landforms discussed include inselbergs, pediments and corestones. The next three chapters focus on the southern part of the country, starting with Chapter 15 (Azubuike Ekwere) about the Obudu Plateau and the Oban Massif in the Cross River State. These Precambrian geologic domains bear relics of ancient geodynamics and evolutionary land forming processes, showcasing scenic mountain ranges, deeply incised valleys and cascading waterfalls. Also within the Cross River state are landforms associated with sedimentary rocks, typically exemplified by the karst geomorphology of the Mfamosing Formation. The following is Chap. 16 on gully erosion in the eastern parts of the country (Gordon Amangabara). Gully erosion occurs in many parts of Nigeria under different geologic, climatic and soil conditions, with varying degrees of severity. However, the southeastern part of Nigeria is the major gully erosion precinct of the country. This chapter describes geomorphological processes leading to gully formation in the study area and highlights various gully erosion hotspots. Concluding the part on specific landscapes and landforms is Chap. 17 (Oyegun and others), which shows the coastal region of Nigeria. The coastal zone stretches from the 20 km barrier island ridges, which abut directly with the Atlantic Ocean, through the tidal basins and creeks, to the ferruginous sands of the Benin Formation. The 800 km long coastline of Nigeria is witnessing accelerated erosion at some sections, while active accretion is dominant at other stretches. Finally, the focus shifts from specific landscapes and landforms to geoheritage and geotourism. We have two chapters within this part. Chapter 18 (Fasona and others) introduces the Islands of Lagos, which are part of the 200 km barrier–lagoon coast of southwestern Nigeria and show considerable geodiversity, including beaches, mangroves, and sand. The Islands are

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Introduction

occupied by the Lagos Megacity—the largest human agglomeration in the sub-Saharan Africa. The book concludes with Chap. 19 (Hezekiah Olaniran). In this chapter, the Olumo rock in the city of Abeokuta in Ogun state—a granite inselberg with a variety of minor granite landforms due to weathering and mass movement—is considered an important geosite. The Olumo rock attracts tourists from all parts of Nigeria and other countries of the world, generating up to about $18,000 (approximately ₦6.3 million) annually for the government. On a final note, the editors are grateful to the International Association of Geomorphology for providing the space to connect with Springer team (during the 9th IAG held in India). Also, the editors would like to appreciate Prof. Piotr Migoń for his relentless work: reading and re-reading the drafts. In addition, we would also like to acknowledge the contributions of Late Emeritus Adetoye Faniran and Late Prof. Charles Oyegun who passed away during the compilation of this book. Adeyemi Olusola Faculty of Environmental and Urban Change York University, Toronto, ON, Canada

References Burke K, Durotoye AB, Whiteman AJ (1971) A dry phase south of the Saharan 20,000 years ago. West Afr J Archaeol 1: 1–8 Faniran A (1970) Landform examples from Nigeria. No. 2. The deep weathering (duricrust) profile. Niger Geogr J 13:87–88 Faniran A, Areola O (1974) Landform example from Nigeria. Niger Geogr J 17(1):57–60 Faniran A, High C (197) Landform examples from Nigeria No. 3: Differential Weathering. Niger Geogr J 14 (1):105–106 Grove AT, Warren A (1968) Quaternary landforms and climate on the south side of the Sahara. Geogr J 134 (2):194–208 Jeje LK (1970) Some aspects of the geomorphology of South Western Nigeria. Unpublished Ph.D. Thesis submitted to the University of Edinburgh Jeje LK (1980) A review of geomorphic evidence for climatic change since the late pleistocene in the rain-forest areas of southern Nigeria. Palaeogeogr Palaeoclimatol Palaeoecol 31:63–86 Olusola AO (2019). Process-form dynamics of upper Ogun river basin, Southwestern Nigeria. Unpublished Ph. D. Thesis, University of Ibadan, Ibadan, Nigeria Onafeso O, Olusola A (2018) Urban stone decay and sustainable built environment in the Niger River Basin. In: Urban geomorphology. Elsevier, pp 261–276 Shaw BD (1976) Climate, environment and prehistory in the Sahara. World Archaeol 8(2):133–149 Udo RK (1970) Geographical regions of Nigeria. University of California Press

Contents

Part I

Physiography

1

Geology of Nigeria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Moshood N. Tijani

3

2

The Climate of Nigeria and Its Role in Landscape Modification . . . . . . . . . . Olumide David Onafeso

33

3

Vegetation and Human Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adeniyi Gbadegesin, Francis Adesina, Oluwagbenga Orimoogunje, and Folasade Oderinde

39

4

Lower Plains of Northern Nigeria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tasi’u Yalwa Rilwanu

53

5

Hills and Ridges in Southwestern Nigeria . . . . . . . . . . . . . . . . . . . . . . . . . . . Lawrence Kosoko Jeje, Oluwagbenga Orimoogunje, and Adeyemi Olusola

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6

South-East Hills and Ridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Olayinka O. Ogunkoya

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7

The Niger Delta Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Charles Uwadiae Oyegun, Olanrewaju Lawal, and Mark Ogoro

Part II

Specific Landforms

8

Landforms of the Chad Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 Mala M. Daura, Emmanuel D. Dawha, and John O. Odihi

9

Dune Fields on the Plains of Northern Nigeria . . . . . . . . . . . . . . . . . . . . . . . . 135 Aliyu Baba Nabegu

10 Landscapes and Landforms of the Jos Plateau . . . . . . . . . . . . . . . . . . . . . . . . 145 Tasi’u Yalwa Rilwanu and Yakubu Samuel 11 Kainji Dam and Lake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 Olayinka O. Ogunkoya 12 Riparian Vegetation Along Nigeria Rivers: The River Ogun Example . . . . . . 175 Olutoyin Adeola Fashae and Rotimi Obateru 13 Quartzite Ridges in Southwestern Nigeria . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Olutoyin Adeola Fashae, Adeyemi Olusola, Rotimi Obateru, and Adetoye Faniran 14 Geology and Landscapes of the Southwestern Nigeria . . . . . . . . . . . . . . . . . . 201 Adebayo Oluwole Eludoyin, Adeyemi Olusola, Olutoyin Adeola Fashae, Lawrence Kosoko Jeje, and Adetoye Faniran

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Contents

15 Geology, Geomorphology and Evolution of the Landscapes of Cross River Region, South-Eastern Nigeria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 Azubuike Solomon Ekwere 16 Gully Erosion Sites in Southeast Nigeria: Prospects for Geotourism . . . . . . . 225 Gordon Tami Amangabara 17 Erosion and Accretion Along the Coastal Zone of Nigeria . . . . . . . . . . . . . . . 243 Charles Uwadiae Oyegun, Lawal Olanrewaju, and Ogoro Mark Part III

Geoheritage, Conservation of Geomorphological Sites and Geotourism

18 The Islands of Lagos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 Mayowa Fasona, Tamarabrakemi Akoso, and Akinlabi Akintuyi 19 Urban Geoheritage Site: The Example of Olumo Rock in Abeokuta City, Ogun State, Nigeria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 H. D. Olaniran Author Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281

Editors and Contributors

About the Editors Lawrence Kosoko Jeje is a professor of physical geography at the Obafemi Awolowo University, Ile-Ife, Nigeria. His research interests include Land Evolution in Humid Tropics, Geoconservation, Erosion Processes and Problems. He has published many academic papers and articles in these fields. In addition, he has co-authored/co-edited books, including Humid Tropical Geomorphology (1983, 2002), Essentials of Geomorphology (2006, 2022), Mans Physical Environment (1995). He is a Fellow of the Nigerian Geographical Association, a member of the Nigerian Geomorphological Working Group, the International Association of Geomorphologists, the International Advisory Board, Royal Scottish Geographical Magazine, and a corresponding member of the International Geographical Union Commission on Geomorphological Processes. The late Adetoye Faniran was a professor emeritus of physical geography at the University of Ibadan, Ibadan, Nigeria, a fellow of the Nigerian Geographical Association, the International Association of Geomorphology, the International Water Resources Association, and the British Geomorphological Research Team, among many other professional organizations. By the last count, he had written or collaborated on over 30 books or monographs, 150 papers for national and international journals, and many book chapters. Also, he has made significant contributions to deep weathering, particularly in Syndey, Australia, where he earned his PhD in 1968, and to the environments of West Africa. Olutoyin Adeola Fashae is a Senior Lecturer at the University of Ibadan, Ibadan, Nigeria. She is a member of the Nigerian Geomorphological Working Group, the International Association of Geomorphologists, the Association of Nigerian Geographers, African Association of Remote Sensing and Environments. Her specialization areas are Fluvial and Biogeomorphology, GIS and Remote Sensing, Disaster Risk Assessments and Environmental Dynamics. Specifically, Dr. Fashae focuses on the study of geomorphic forms, processes, and their contribution to the environmental sustainability of water and land resources, emphasizing rivers, lakes, coasts, river health and restoration, river ecology, and river ecology river pollution, erosion, and climate change. She has published in both national and international journals. Her most recent publications were printed in Natural Hazards, Hydrological Sciences, Acta Geophysica and Catena. Adeyemi Oludapo Olusola is an Assistant Professor at the Faculty of Environmental and Urban Change at York University. He is a member of the Nigerian Geomorphological Working Group, International Association of Geomorphologists, Association of Nigerian Geographers, Canadian Geophysical Union, and American Association of Geographers. He is a river catchment scientist who strongly focuses on rivers, their dynamics, human impacts, and extreme events at the reach-scale and basin scales. In addition, he has co-authored/co-edited xiii

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books, including Essentials of Geomorphology (2006, 2022) and Remote Sensing of African Mountains (2022). He has published in several national and international journals. His latest publications appeared in Catena, Hydrological Sciences, International Journal of Climatology and Acta Geophysica.

Contributors Francis Adesina Department of Geography, Obafemi Awolowo University, Ile-Ife, Nigeria Akinlabi Akintuyi Department of Geography, University of Lagos, Lagos, Nigeria Tamarabrakemi Akoso Department of Geography, University of Lagos, Lagos, Nigeria Gordon Tami Amangabara Department of Environmental Management, School of Environmental Science, Federal University of Technology, Owerri, Nigeria Mala M. Daura Department of Geography, University of Maiduguri, Maiduguri, Borno State, Nigeria Emmanuel D. Dawha Department of Geography, University of Maiduguri, Maiduguri, Borno State, Nigeria Azubuike Solomon Ekwere Department of Geology, University of Calabar, Calabar, Nigeria Adebayo Oluwole Eludoyin Department of Geography, Obafemi Awolowo University, Ile-Ife, Nigeria Adetoye Faniran Department of Geography, University of Ibadan, Ibadan, Nigeria Olutoyin Adeola Fashae Department of Geography, University of Ibadan, Ibadan, Nigeria Mayowa Fasona Department of Geography, University of Lagos, Lagos, Nigeria Adeniyi Gbadegesin Department of Geography, University of Ibadan, Ibadan, Nigeria Lawrence Kosoko Jeje Department of Geography, Obafemi Awolowo University, Ile-Ife, Nigeria Olanrewaju Lawal Department of Geography and Environmental Management, University of Port Harcourt, Port Harcourt, PH, Nigeria Ogoro Mark Department of Geography and Environmental Management, University of Port Harcourt, PH, Nigeria Aliyu Baba Nabegu Department of Geography, Kano University of Science and Technology, Wudil, Kano State, Nigeria Rotimi Obateru Department of Geography and Planning Sciences, Adekunle Ajasin University, Ondo State, Nigeria Folasade Oderinde Department of Geography, Tai Solarin University of Education, Ijebu-Ode, Nigeria John O. Odihi Department of Geography, University of Maiduguri, Maiduguri, Borno State, Nigeria Mark Ogoro Department of Geography and Environmental Management, University of Port Harcourt, Port Harcourt, PH, Nigeria

Editors and Contributors

Editors and Contributors

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Olayinka O. Ogunkoya Department of Geography, Obafemi Awolowo University, Ile Ife, Nigeria H. D. Olaniran Department of Geography, Faculty of the Social Sciences, University of Ibadan, Ibadan, Nigeria Lawal Olanrewaju Department of Geography and Environmental Management, University of Port Harcourt, PH, Nigeria Adeyemi Olusola Faculty of Environmental and Urban Change, York University, Toronto, ON, Canada Olumide David Onafeso Department of Geography, Olabisi Onabanjo University, Ago-Iwoye, Nigeria Oluwagbenga Orimoogunje Department of Geography, Obafemi Awolowo University, Ile-Ife, Nigeria Tasi’u Yalwa Rilwanu Department of Geography, Bayero University Kano P.M.B, Kano, Nigeria Yakubu Samuel Department of Geography, Osun State University, Osogbo, Nigeria Moshood N. Tijani Department of Geology, University of Ibadan, Ibadan, Nigeria Charles Uwadiae Oyegun Department of Geography and Environmental Management, University of Port Harcourt, Port Harcourt, PH, Nigeria

Part I Physiography

1

Geology of Nigeria Moshood N. Tijani

Abstract

1.1

Three major rock types are present in Nigeria: igneous, metamorphic and sedimentary. Igneous and metamorphic rocks constitute the Precambrian Basement Complex which is the oldest, crystalline, solid physical foundation of the country. Sedimentary rocks of the Cretaceous and Cenozoic age fill up the basins, which are depressions within the basement landmass. The Basement Complex and the sedimentary basins are equally dispersed in Nigeria. Quaternary to Recent age alluvial deposits occur along the main river valleys as thin and discontinuous sandy beds to thick sedimentary units up to 15 km wide and 15–30 m thick along the channels of rivers Niger and Benue, as well as along the courses of major ephemeral streams and fadamas, especially in the northern parts of Nigeria. The diverse geology of Nigeria offers opportunities for the exploitation of varied mineral deposits contained in different rock units. Abundant mineral deposits occur in all components of Nigerian geology with mineral deposits of economic significance that include gold, iron ore, cassiterite, columbite, wolframite, pyrochlore, monazite, marble, coal, limestone, clays, barites, lead–zinc, etc. The geological, structural and tectonic settings of different rock units have significant implications on the geomorphological evolution of the landscapes in Nigeria. In addition, a number of the associated geological–geomorphic features and scenery have considerable aesthetic and touristic potential, which in some cases combines with cultural importance. Keywords











Precambrian Basement complex Sedimentary rocks Cretaceous Quaternary Alluvial deposit

M. N. Tijani (&) Department of Geology, University of Ibadan, Ibadan, Nigeria e-mail: [email protected]




Introduction

Geology, which is the study of rocks, minerals and the physical make-up of the solid earth, determines the environment and natural resources and in essence the industrial potential and wealth of a nation (Petters 2004). Geomorphology is the study of the physical features of the Earth’s crust as related to the geological features and geomorphic processes (Whittow 1984), involving physical and chemical interactions of natural environmental forces acting upon the Earth's surface and the resulting morphological characteristics and associated landforms. However, among the natural environment variables, geological factors exert, perhaps, the greatest impact on economic activities, since soils, water supply and vegetation are, to a large extent, influenced by the nature of the underlying bedrock. By and large, the evolution of landforms and landscapes is greatly influenced by geology, climate and vegetation. Arising from the above concept, there is no doubt as to the fact that the vast ocean margin that borders Nigeria to the south; the intricate drainage systems that sculpture the physical landscape; the mountainous eastern frontiers; and the wind-swept northern hinterland, with the great plains on the Saharan fringe, are all part of the surface geological features that bestow upon Nigeria its unique physical and geomorphological attributes (Petters 2004). Nigeria lies south of the Sahara within West Africa, with the Atlantic Ocean bordering the southern coastal region. The total land surface area is about 923,768 km2, with an average distance of 1120 km from south to north. Nigeria lies within longitudes 2° 50ʹ to 14° 20ʹ E and latitudes 4° 10ʹ to 13° 48ʹ N. It is bordered to the north by the Republics of Niger and Chad and to the west by the Republic of Benin. It shares the eastern borders with the Republic of Cameroon right down to the shores of the Atlantic Ocean forming the southern limits of the Nigerian territory (Fig. 1.1). The country is the most populous in Africa, with an estimated population of about 196 million,

© Springer Nature Switzerland AG 2023 A. Faniran et al. (eds.), Landscapes and Landforms of Nigeria, World Geomorphological Landscapes, https://doi.org/10.1007/978-3-031-17972-3_1

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Fig. 1.1 The location of Nigeria and adjoining West African countries showing the topography of Nigeria. Source http://en.wikipedia.org/

while about half (52%) of the population is rural (WHO/UNICEF 2007), with an annual demographic growth rate of about 3%. By and large, the diverse landscapes and landforms of Nigeria are the function of the interplay of geotectonic and geomorphic processes vis-à-vis the geological framework, which has remained stable in the last 1.5 Ga, and the associated mobile belts that have been affected by Late Proterozoic Pan-African Orogeny. The relief and physical features of a given area depend on three major factors. These are the type of erosion at work which is responsible for the wear and tear of the landscape, the nature of rocks (geology) and the time required for the change to take place. Nigeria displays physical environment regions of varying characters in relief, nature and spatial distribution (Fig. 1.2). The general relief of Nigeria can be categorized into high plateaus or uplands and the lowlands. The eastern part of the country is locally over 1500 m asl, whereas the North has a general average relief of 600 m asl and the West has an average of 300 m asl in height. The areas below 300 m are found in the South and the centre of the country (Iloeje 1982). Significantly, the highlands correspond with the areas of volcanic rocks and uplifted areas of basement complex rocks. The lowland is characterized by plains and valleys filled with sedimentary formations. The three trunk rivers of the Niger-Benue River System cut the highlands into three blocks, namely the central

plateau in the North, the western uplands in the West and the eastern and north-eastern highlands in the East (Jimoh 2012). Consequently, the major geographical units are: (a) The Jos Plateau in the north-central upland region, with an elevation of about 1500–1800 m asl. (b) The north-eastern highland characterized by Mandara and Mambila mountains, with a range of 1800– 2400 m asl, separating Cameroon and Nigeria. (c) The Udi Hills in the south-eastern part, characterized by flat-topped lateritic iron-capped hills. The rest of the country can be divided into lowland areas which are mainly made up of sedimentary rock formations and include (a) Sokoto plains in the North-West; (b) interior coastal lowlands of western Nigeria; (c) the lowlands and scarplands of south-eastern Nigeria between (the blue extent) the Udi Hills and the Oban Massif; (d) the Niger Delta plains; (e) the Bida Basin, (f) Chad Basin and (g) the Niger-Benue Trough. The corresponding underlying geological architecture of Nigeria (Burke and Dewey 1972; Rahaman 1976; Kogbe 1979; Black et al. 1979; Caby et al. 1981; Petters 2004; Obiora 2005; Dada 2006; Obaje 2009) is highlighted in Fig. 1.3. The geology of Nigeria is made up of three major litho-petrological components, namely the Basement

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Fig. 1.2 Physical environment and relief map of Nigeria (gisystematix.com 2015)

Fig. 1.3 Geological map of Nigeria highlighting the different geological units. Modified after Tijani (2004)

Complex, Younger Granites and Sedimentary Basins. The Basement Complex, which is Precambrian in age, is made up of the Migmatite–Gneiss Complex, the Schist Belts and the Older Granites. The Sedimentary Basins, containing sediment fill of Cretaceous to Cenozoic ages, comprise the

Niger Delta, the Anambra Basin, the Lower, Middle and Upper Benue Trough, the Chad Basin, the Sokoto Basin, the Mid-Niger (Bida-Nupe) Basin and the Dahomey Basin. The Basement Complex and the Sedimentary Basins are equally dispersed in Nigeria. In addition, the Quaternary to Recent

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age alluvial deposits (including gravel, coarse and fine sand, silt and clay) occur along the main river valleys as thin discontinuous sands to thick sedimentary packages. The Precambrian Basement Complex consists of crystalline igneous and metamorphic rocks, which constitute the oldest, solid physical foundation of the country. Major petro-lithological units of the Precambrian Basement Complex are: (a) Pan-African Crystalline Igneous–Metamorphic Rocks (> 600 Ma). (b) Jurassic Younger Granites and Cenozoic to Recent volcanics (200 to < 65 Ma). The Cretaceous–Cenozoic–Recent sedimentary series were deposited unconformably on the dissected Precambrian crystalline rocks and define the X-shaped nature of the depressions. Main sedimentary successions in seven different basins are: (a) Cretaceous to Cenozoic Sedimentary Units (< 125– 2.6 Ma). (b) Quaternary to Recent Alluvial Deposits (2.6– < 0.12 Ma). Usually, recent alluvial deposits (including gravel, coarse and fine sand, silt and clay) occur along the main river valleys as thin discontinuous sand sheets to thick alluvial packages up to 15–30 m thick along the channels of rivers Niger and Benue (Adelana et al. 2008). Thin deposits of unconsolidated and mixed sands and gravels occur also along the courses of major ephemeral streams and on their floodplains, especially in the northern parts of Nigeria (Fig. 1.4).

Fig. 1.4 Deposits of unconsolidated mixed sands and gravels along a major. Ephemeral river along Gusau–Sokoto Road, in Northern Nigeria. Photo April 2010

M. N. Tijani

In the following sections, the main structural geotectonic events and associated igneous, metamorphic, sedimentary and volcanic activities that characterized the geological framework of Nigeria will be highlighted. In general, these activities are related to Pan-African orogenic cycles that followed the separation of Africa and South America continents in the wake of continental fragmentation of Gondwanaland. Consequently, the principal aim of this chapter is to provide a review of the geological setting of Nigeria with a geological history spanning the Precambrian through the Mesozoic–Cenozoic to the Quaternary. Also, the extents to which this geological architecture influences or determines the evolution of the geomorphological in the different geological regions of the country are briefly highlighted.

1.2

The Geological and Geotectonic Framework of Nigeria

The regional structural geotectonic setting of Nigeria is an integral part of the Precambrian geology of Africa and West Africa in particular. The geological framework consists of a few cratons, which have remained stable in the last 1.5 Ga, and mobile belts which have been affected by Late Proterozoic Pan-African Orogeny around 500 Ma ago (Wright 1985). Regionally, Nigeria lies within the Pan-African mobile belt between the West African and Congo Cratons and south of the Tuareg shield (Black et al. 1979; Black 1980). Nigeria occupies the reactivated region (Fig. 1.5), i.e. the so-called mobile belt, which resulted from plate collision between the passive continental margin of the West African Craton and the active Pharusian belt (Tuareg shield), about 600 Ma (Burke and Dewey 1972; Caby et. al. 1981; Dada 2006). This Pan-African mobile belt is separated from the West African Craton (Proterozoic) by Infra-Cambrian to lower Palaeozoic sediments of the Volta Basin and the intensely thrust-faulted rocks of the Togo belt (Wright 1985). The geological setting is characterized by elements of the “basin and swell” pattern of mainly Pan-African rocks of the Togo– Benin–Nigeria swell (Fig. 1.6), also known as the Benin-Nigeria shield (Wright 1985). In general, the geological setting of Nigeria is predominantly made up of Precambrian basement rocks, which were intruded by the Mesozoic calc-alkaline ring complexes (Younger Granites) around the Jos Plateau area. However, the basement rocks are believed to be the results of at least four major orogenic cycles of deformation, metamorphism and remobilization, corresponding to the following episodes; (a) Liberian (c. 2500 Ma) (b) Eburnian (c. 1850 ± 250 Ma)

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Fig. 1.5 Location of Nigerian within the mobile belt zone between the cratons. Modified after Gubanov and Mooney (2009)

Fig. 1.6 The generalized geotectonic setting of Nigeria within the framework of the geology of West Africa. Modified from Wright (1985)

(c) Kibarian (c. 1100 ± 100 Ma) (d) Pan-African (c. 550 ± Ma) The first three cycles were characterized by intense deformation and isoclinal folding accompanied by regional metamorphism and followed by extensive migmatization. However, the Pan-African orogenic deformation (c. 550 ± Ma) was also accompanied by regional

metamorphism, migmatization and extensive granitization and gneissification which produced syntectonic granites and homogenous gneisses (Abaa 1983). The structural grain of the Precambrian rocks of the Basement Complex of Nigeria generally lies between N-S, NNE-SSW and NNW-SSE. This is believed to be related to the Pan-African Orogeny, the latest orogenic activity affecting the Precambrian rocks of the region.

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Late-tectonic emplacement of granites and granodiorites and associated contact metamorphism accompanied the final stages of this last deformation phase. The end of the orogeny was also said to be marked by faulting and fracturing (Gandu et al. 1986; Olayinka 1992). The Precambrian Basement Complex of Nigeria is unconformably overlain by Mesozoic to Recent formations (Obaje 2009), forming sedimentary covers. Hence, the Basement Complex is crossed by the narrow Cretaceous Bida Basin, Benue Trough and Anambra Basin, which link up with the Cenozoic to the Recent sediments of the Niger Delta (Fig. 1.6). Further to the north-east, the Upper Benue Trough links up with the Bornu Basin (Fig. 1.6) which is mainly occupied by Quaternary sediments, while other basins such as the Cretaceous-Tertiary Dahomey Basin and the Calabar Flank, along the southern coastline, also link up with the Niger Delta at the western and eastern edge, respectively (Wright 1985; Petters 2004). In addition, there is an important Palaeozoic to Jurassic and Cenozoic province of granitic intrusions in the Jos Plateau area as well as isolated Cenozoic basaltic volcanism in the eastern edge of Nigeria bordering Cameroun (Fig. 1.6). In summary, the greater parts of Nigeria are within the Pan-African mobile belt and underlain by Precambrian basement rocks, whereas the basins constitute a sedimentary cover on the dissected depressions of the older basement rocks, forming the “swell-basin” framework, characteristic of the African continent. The swells represent areas underlain by older igneous and metamorphic rocks, while the basins are mostly filled with younger sediments.

1.3

Geological Setting of Nigeria

The surface of Nigeria is underlain, in nearly equal proportions, by Precambrian crystalline rocks and sedimentary rocks, with crystalline basement rocks most extensive in northern Nigeria; less so in the south-western part of the country; and least along the eastern margin (Fig. 1.7). Details of the geological architecture of Nigeria are well documented (see Burke and Dewey 1972; Rahaman 1976; Kogbe 1979; Black et al. 1979; Caby et al. 1981; Petters 2004; Obiora 2005; Dada 2006; Obaje 2009).

1.3.1 The Precambrian Basement Complex The rocks of the Precambrian Basement Complex are the most abundant and widespread of the group of crystalline rocks. They underlie nearly half of the surface of the country (Fig. 1.8). A number of petro-lithological groups of the Basement Complex are commonly distinguished:

(a) The Migmatite–Gneiss Complex (MGC). This is dominated by amphibolite-grade gneisses, migmatites and granite-gneisses. There are also lenses of other rock types such as amphibolites, quartzites and older metasediments. (b) The Schist Belts (metasedimentary and metavolcanic rocks). These are NNE-SSW elongated supracrustal belts composed mainly of greenschist- to amphibolitefacies metasediments, which include biotite schists, phyllites, quartzites, marbles and amphibolites. (c) The Older Granites represent a syntectonic to late-tectonic Pan-African Granitoids of granitic and granodioritic composition, which intruded both the Migmatite–Gneisses and the metasediments. They also comprise syenites, monzonites, gabbro and charnockites. (d) Undeformed Acid and Basic Dykes comprising muscovite-, tourmaline- and beryl-bearing pegmatites, aplites and syenite dykes; basaltic, doleritic and lampropyric dykes. (e) The Younger Granites and Volcanic rocks represent a distinctive group of intrusive and volcanic rocks that are bounded by ring dykes or ring faults.

1.3.1.1 The Migmatite–Gneiss Complex (MGC) The Migmatite–Gneiss Complex (Fig. 1.8) is generally considered the Basement Complex sensu stricto (Rahaman 1988; Dada 2006), and it is the most widespread, constituting about 60% of the Nigerian Basement Complex (Rahaman and Ocan 1978). The Migmatite–Gneiss complex is the oldest basement rock and is believed to be of sedimentary origin but profoundly altered later into metamorphic and granite rocks. The Migmatite–Gneiss Complex has ages ranging from Pan-African (900–450 Ma) to Eburnian (2000 ± 200 Ma). Three main petro-lithological units within the Migmatite–Gneiss Complex are: (a) Biotite gneiss, which is widespread and normally fine-grained, with strong foliation caused by the parallel arrangement of alternating dark and light minerals. (b) Banded gneiss, which shows alternating light-coloured and dark bands and exhibits intricate folding of their bands. (c) Older metasediments, initially of sedimentary origin, with a more extensive distribution. The older metasediments underwent prolonged, repeated metamorphism and now occur as quartzites (ancient sandstones), marble (ancient limestones) and other calcareous rocks, including relics of highly altered clayey sediments and igneous rocks. In general, the migmatites and gneissic metasediments are often intruded by pegmatite veins and dykes (Oluyide et al.

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Fig. 1.7 Generalized geological map of Nigeria showing major geological units. Modified after MacDonald et al. (2005a, b)

1998). Along the border with Cameroon, the crystalline basement rocks include biotite-hornblende gneiss, kyanite gneiss, Migmatite–Gneiss and granites, all well-fractured (Ekwueme 1987). Mineral deposits in the Basement Complex include gold, molybdenite and non-metallic minerals such as feldspars and talc among others. Many areas in the northern (Abuja, Keffi, Akwanga, Bauchi, Kaduna, Kano, Funtua, Okenne, Egbe, Ajaokuta), western (Ibadan, Ile-Ife, Akure, Ikerre) and eastern (Obudu and the Oban Massif) blocks are covered by rocks of the Migmatite–Gneiss Complex (Figs. 1.9, 1.10 and 1.11).

1.3.1.2 The Schist Belts Most parts of the Basement Complex are characterized by belts of roughly north-south trending, slightly metamorphosed ancient Pre-Cambrian sedimentary and volcanic rocks known as the younger metasediments forming an NNE-SSW elongated supracrustal belts. The schist belts are best developed in the south-western and north-central parts of Nigeria, west of 8° E longitude and are confined to an NNE-trending zone of about 300 km wide (Fig. 1.12). The schist belts are considered to be Upper Proterozoic supracrustal rocks that have been in-folded into the Migmatite–Gneiss–Quartzite Complex (Oyawoye 1972; McCurry; 1976; Rahaman 1976; Grant 1978; Obaje 2009).

The lithological diversity of the schist belts includes coarse to fine-grained clastic, biotite schists, phyllites, banded iron formations, carbonate rocks (marbles/dolomitic marbles) and mafic metavolcanics (amphibolites). The major rock types are ancient shales, which are now referred to as quartz–biotite–muscovite schist. These change laterally into coarse-grained feldspar-bearing micaceous schists. In the north-western block of the Basement Complex, the metasediments form narrow, tightly folded north-south trending belts with igneous rocks, biotite schists, phyllites and banded ironstones, in addition to ferruginous quartzites and talc schists in places. The evolution, lithological associations, deformational histories as well as structural relationships between the schist belts and the basement are described in detail by Truswell and Cope (1963), Ogezi (1977), Grant (1978), Ajibade et al. (1979), Olade and Elueze (1979), Ajibade (1980), Ajayi (1980), Rahaman (1981), Egbuniwe (1982), Holt (1982), Turner (1983). Generally, these younger metasediments are mineralized and contain most of the gold deposits in Nigeria. These are located in the northwest around Maru and Anka, at Kusheriki-Kushaka and Zungeru-Birnin Gwari, near Kaduna in the north-central block, and also at Efon-Alaye, Ijero, llesha and Iseyin-Oyan in the south-western block (Fig. 1.13).

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Fig. 1.8 Basement geology of Nigeria showing the different petro-lithologic units: The Migmatite–Gneiss Complex, the Schist Belts, the Older Granites and Younger Granites. Modified from Wright (1985)

Fig. 1.9 Low-lying batholithic outcrop of migmatitic rock (a), near Ikare, SW-Nigeria and outcrop of banded gneiss (b) at Aba-Eku in Ibadan, SW-Nigeria

1.3.1.3 Older Granites (Pan-African Granitoids) Older granites (Pan-African Granitoids) are widespread throughout the Basement Complex and represented by syntectonic to late-tectonic granites, granodiorites and charnockites (Oyawoye 1972). Compositionally, the granites plot in the field of calc-alkaline rocks on the AFM diagram, although they contain a significant amount of

alkalis (Obaje 2009). They vary widely in age and represent a varied and long-lasting magmatic cycle (750–450 Ma) associated with the Pan-African orogeny (Rahaman 1988). The Older Granites are believed to be pre-, syn- and post-tectonic rocks, which intruded both the Migmatite– Gneiss–Quartzite Complex and the schist belts. The Older Granites suite occurs intricately associated with the

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Geology of Nigeria

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Fig. 1.10 Typical micro-fold structures in migmatite and banded gneiss as imprints of metamorphism at Arigidi Area, South-western Nigeria

Fig. 1.11 Typical examples of assimilated or reactivated older rock (xenolith) in a banded gneiss and b migmatite

Fig. 1.12 Schist Belt localities in Nigeria within the context of the regional geology of parts of West Africa. Modified after Wright (1985), Obaje (2009)

Migmatite–Gneiss Complex and the Schist Belts and occurs as large circular masses. Pan-African Older Granites appear to be particularly noteworthy in and around Wusasa (Zaria), Abuja, Bauchi, Akwanga, Ado-Ekiti and Obudu areas),

where these occur as isolated intrusions (McCurry 1973) supporting inselbergs (Figs. 1.14, 1.15 and 1.16). However, outcrops are notable for the general lack of associated mineralization.

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Fig. 1.13 Outcrops of metasedimentary rocks at a Itawure and b Ijero area, SW Nigeria

Fig. 1.14 Inselbergs in the Older Granites. a Iconic Zuma rock inselberg at Suleja, near Abuja, b an inselberg in the Oban Hills (Google images) (b) in the southern section of the Cross River National Park, SE-Nigeria. Sourced from pinterest.com

Fig. 1.15 Inselbergs in the Older Granites cont’d a aerial view of Ado-Awaye township and the surrounding hills and inselbergs b Inselberg at Ibarapa area, SW-Nigeria. Inset Ado-Awaye suspended Lake at the summit of one of the inselbergs. Source Google images

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Fig. 1.16 Outcrops of Older Granites as domed inselbergs in Oke-Ogun region, SW-Nigeria: a near Irawo-Ile, b near Ago-Are and c near Saki Township

In the Bauchi area and some parts of south-western Nigeria, most of the Older Granite rocks occur as dark, greenish-grey granites with significant quantities of olivine (fayalite) and pyroxene, along with quartz, feldspars and micas. For this unusual composition, the Older Granites in these areas are termed Bauchite (in Bauchi area) and Oyawoyite (after Professor M. O. Oyawoye who first mapped them) in south-western Nigeria. For uniformity of terminology, both the Bauchites and Oyawoyites constitute the charnockitic rocks (Charnockites) of the Basement Complex (Obaje 2009). Charnockites form an important rock group emplaced during this period. They are generally high-level intrusions and anataxis has played an important role (Rahaman 1981). Apart from Toro, other localities of charnockites include Bauchi, Ado-Ekiti, Ikere (Ekiti), Akure, Idanre and the Obudu Plateau (Wright 1970; Cooray 1975; Van Breemen et al. 1977; Dada 1989; Olarewaju 2006). Fig. 1.17 Outcrops of dykes cross-cutting older Migmatite– Gneiss within the Basement setting at Akobo area, Ibadan, metropolis, SW-Nigeria

In the central region, extensive Older Granites of the Precambrian age were intruded by Younger Granites of the Jurassic age, which formed characteristic ring complex structures (Oyawoye 1972; Oluyide et al. 1998; Adelana et al. 2008).

1.3.1.4 Undeformed Acid and Basic Dykes Generally, the undeformed acid and basic dykes are late to post-tectonic Pan-African, comprising muscovite-, tourmaline- and beryl-bearing pegmatites, applites and syenite dykes; basaltic, doleritic and lampropyric dykes. These cross-cut the Migmatite–Gneiss Complex, the Schist Belts and the Older Granites (Obaje 2009). The undeformed acid and basic dykes include (Fig. 1.17): (a) Felsic dykes, associated with Pan-African granitoids such as the muscovite, tourmaline and beryl-bearing

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Fig. 1.18 Geological map of Younger Granite Province of Nigeria. Modified after Jacobson et al. (1958), Kinnaird (1981)

pegmatites, microgranites, aplites and syenite dykes (Dada 2006). (b) Basic dykes are generally regarded as the youngest units in the Nigerian basement such as dolerites and less common basaltic, felsites and lamprophyric dykes. The age of felsic dykes has been put at between 580 and 535 Ma from Rb–Sr studies on whole rocks (Matheis and Caen-Vachette 1983; Dada 2006), while the basic dykes have a much younger suggested age of ca. 500 Ma (Grant 1970).

1.3.1.5 The Younger Granites The ring tectonic complexes of the Jurassic age, i.e. Younger Granites, intruded into late Precambrian Basement rocks in an N-S trending zone. The Younger Granites suite is 160– 170 million years old. Its emplacement was associated with epirogenetic uplift (Kogbe 1976). The Younger Granites consist of non-orogenic (anorogenic) magmatic rocks that are essentially high-level granite intrusions and their volcanic and hyperbassal equivalents, mainly rhyolites and granite porphyries. The Younger Granites and associated rock types occur in the form of separated bodies or ring complexes, which cut the Basement Complex in the north-central part of Nigeria (Fig. 1.18).

Mineralogically, the Younger Granites are characterized by high amounts of sodium and potassium, with albitic, perthitic feldspar as well as sodic amphiboles and alkaline pyroxenes. In Nigeria, they are found mainly on the Jos Plateau area, with fewer members scattered to the north and south of the Jos Plateau (such as Bauchi, Kaduna, Kano and Nasarawa areas), forming a distinctive group of intrusive and volcanic rocks that are bounded by ring dykes or ring faults. Structurally, Younger Granites plutons show a north-south trend (Fig. 1.18) and were emplaced in Mesozoic time (about 160 Ma) along ring fractures caused by cauldron subsidence, forming the most impressive ring dyke province in the world. Other occurrences approximate a north-south belt towards the middle Benue in the south, where the ages are younger and towards the Niger Republic in the north where the Younger Granites are older. However, in Nigeria the ages of emplacement decrease southward, confirming a definite southward migration of centres of igneous activity. Several workers showed that the ages of these intrusive centres vary from 173 to 156 Ma, north to south (Bowden et al. 1976; Rahaman et al. 1984; Bowden and Kinnaird 1984; Bowden 1985). Three major types of granites have been recognized in the Younger Granite Province based on dominant mafic mineral

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Geology of Nigeria

contents: the biotite-granite, hornblende-fayalite-granite and riebeckite-granite. Biotite-granites are the most abundant and widespread in the Younger Granite Province and are host to tin, columbite and wolframite mineralization. Other minerals of economic value in association with the Younger Granites are pyrochlore, monazite, thorite, zircon, molybdenite and beryl. Major characteristics of the Nigerian Younger Granite rocks in comparison with the Older Granite suites as presented by Obaje (2009) are highlighted as follows: (a) The Younger Granites are anorogenic, while the Older Granites are orogenic. (b) The Younger Granites intrude the basement discordantly to form highly stepped hills, while the Older Granites intrusions are generally flat/low lying. (c) The Younger Granites occur generally as ring dykes and cone sheets, sometimes with outer and inner rings, while the Older Granites occur as massive batholiths. (d) The Younger Granites are of Jurassic age, while the Older Granites are Precambrian (Pan-African). (e) The Younger Granites are generally peralkaline and are the source of tin mineralization in the Jos Plateau region, while Older Granites are generally calk-alkaline and peraluminous.

1.3.2 The Sedimentary Rocks The sedimentary rocks range in age from Lower Cretaceous (about 120 Ma) to Quaternary (< 2 Ma) and include even recent sedimentation. Generally, they overlie the Basement Complex within the depressions between the dissected basement blocks. Two main sedimentary successions are (a) Cretaceous to Cenozoic Sedimentary Units and (b) Quaternary to Recent Alluvial Deposits. A number of major depositional episodes responsible for the sedimentation and sedimentary successions can be distinguished: (a) Early to Late Cretaceous, Pre-Santonian Transgressive and Regressive Complexes when the rift-like Benue depression and Abakaliki trough were formed as a result of the separation of South America and Africa. Igneous activities and mineralizations accompanied this geological episode. (b) The Bima complexes, which developed during the Albian–Cenomanian at the end of the Benue depression in north-eastern Nigeria, marking the beginning of sedimentation in the Nigerian sector of the Chad Basin. (c) Late Cretaceous–Palaeocene (post-Santonian) Anambra Basin situated on the northwest flank of the Abakalilki fold belt, which began to develop as a regressive off-lap sequence during Campanian times.

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(d) Maastrichtian (latest Cretaceous) transgression when the sea moved northwards from the Gulf of Guinea through the Mid-Niger Basin and southwards from the Mediterranean through the Sahara. (e) The Cenozoic regressive off-lap sequence, which led to the development of the Niger Delta complex. The culmination of these depositional events resulted in the formation of about seven sedimentary basins in Nigeria (Fig. 1.19). These are broadly divisible into coastal basins (Calabar Flank, Niger Delta and Dahomey Basin) and interior basins (Benue Trough, Anambra Basin, Chad Basin, Bida/Nupe Basin and Sokoto Basin). Sedimentary successions in these basins are of middle Mesozoic to Recent age. Older sedimentary rocks were not preserved, probably because during the Paleozoic–early Mesozoic the area of contemporary Nigeria was a broad regional basement uplift, with no major basin subsidence allowing for sediment accumulation. Nonetheless, the seven basins altogether occupy about half of the surface area of Nigeria. Based on lithology and biostratigraphy, each of the seven basins is subdivided into a number of stratigraphic successions.

1.3.2.1 The Benue Trough The Cretaceous sediments of the down-faulted and failed rift, which has become the Benue Trough, occur in a series of basins that extend north-east of the confluence of the Niger and Benue Rivers, bounded by the Basement Complex to the north and south of the Benue River (Reyment 1980). The Benue Trough is an elongate-rifted depression with sediment thickness well over 5000 m in places, and it is in many ways the most interesting of the sedimentary basins in West Africa. It probably provided the major link between the Mediterranean (Tethys Ocean) and the Gulf of Guinea via the Ilullmedden and Chad Basins during Late Cretaceous times. The Benue Trough bifurcates near its north-eastern end (Fig. 1.20). The northern branch (the Gongola arm) continues beneath the Chad Formation as an elongate depression that extends well beyond the Lake Chad, while the southern branch (the Yola arm) is aligned with another deep depression beneath the southern boundary of the Chad Basin. North of the Zambuk ridge, the sediments are part of the Chad Basin succession and are also related to the evolution of the Benue Trough. At its south-western end, the Benue Trough merges with the important petroleum-bearing Tertiary Niger Delta Basin. The Bida Basin, now occupied by the Niger River, is a shallow Late Cretaceous branch of the Benue Trough, but there is no evidence of folding or faulting of sediments within it. Regionally, the Benue Trough is part of an Early Cretaceous rift complex known as the West and Central

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Fig. 1.19 Location and geological settings of the sedimentary basins in Nigeria. Modified after Tijani (2004) Fig. 1.20 Location and geological setting of the Benue Trough, Nigeria vis-à-vis otter sedimentary basins. Modified after Tijani (2004)

African Rift System. It forms a regional structure which is exposed from the northern margin of the Niger Delta and runs northeastwards for about l000 km to underneath the Lake Chad, where it terminates. The evolution of the Benue Trough, as well as that of the associated basins, is intimately linked with the Ridge-Ridge-Ridge (RRR) triple junction fault system responsible for the separation of Africa and South America during the break-up of the Gondwana (King 1950) and eventual opening of the South Atlantic ocean. The Benue Trough is said to be a “failed

arm” of a triple fault system, i.e. an aulacogen that formed when Africa and South America separated in the Cretaceous (Fig. 1.21). Tectonic phases and basin development within the Benue Trough are divisible into: (a) An Early Cretaceous rift phase, with fluviatile and lacustrine deposits; (b) Late Early to Middle Cretaceous phase of rapid basin subsidence and initiation of marine transgression in all

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Fig. 1.21 RRR triple junction fault system associated with the separation of Africa and South America continents. Modified after Gubanov and Mooney (2009)

rift segments, characterized by submarine gravity flows with megaslumps and turbidites; (c) Prolonged shelf and deep basin deposition, especially in the southern part (Abakiliki Rift section), under predominantly oxygen-deficient bottom conditions, (d) Tectonism, involving deformation and magmatism and the formation of lead–zinc deposits from circulating hot brines, (e) Late Cretaceous post-deformation subsidence with westward displacement of depocentres, especially in the Anambra Basin, where extensive coal-forming swamps developed. Although several sub-basins or depocentres have been identified in the Benue Trough on geophysical (gravity) evidence, structurally and in terms of sediments, three segments have been recognized, namely the Lower, the Middle and the Upper Benue Trough. The observed irregular basement floor in the Benue Trough is probably the result of extensive block faulting, initiated when the trough began to develop as the “failed arm” of the triple junction. The bifurcation at the north-eastern end has been identified as another triple junction, though without any crustal spreading. Nonetheless, apart from the above Rift-faulting model evolution, there are a number of theories or hypotheses regarding the tectonic evolution of the Benue Trough, namely (a) Geosynclinal model: This suggests the evolution in form a normal synclinal structure or trough development involving subsidence and sedimentation (Farrington 1952). This idea is supported by Lee (1952) based on the absence of marginal fault-lines and the

observed thick sedimentation as well as the relatively greater width of the trough. (b) Rifting model: The plausibility of the rift-hypothesis model is greatly supported by a number of geophysical investigations (in particular, gravity measurements), and these have greatly assisted in determining the structure of the Benue Trough (Cratchley and Jones 1965; Ajakaiye and Burke 1973; Adighije 1979). (c) Mantle Plume-faulting (Aulacogen) model: Following a review of the above existing model, Olade (1975) proposed a pulsating mantle plume-faulting model whereby the up-doming and faulting during the Aptian to the Albian led to rift formation (Aulacogen). This model, which incorporates most of the other models, highlights the overall tectonic evolution of the Benue trough as presented in Fig. 1.22. In line with the above tectonic evolution of the Benue Trough, the stratigraphic nomenclature changes from one end of the trough to the other, while the stratigraphic profile indicating correlations is presented in Fig. 1.23. The main features of stratigraphy and sedimentation history, as highlighted in Fig. 1.24, revealed that marine sediments of Albian–Cenomanian age abound in the lower segment of the Benue Trough. Especially on its southern side, the Albian Asu River Group is dominated by blue-black shales, often carbonaceous and pyritic, with subordinate limestones and sandstones. In the middle and upper segments of the Benue Trough, Albian to Cenomanian sediments are dominated by fluvio-deltaic sandstones, variously called the Bima and its lateral equivalents (Keana and Makurdi Sandstones). They are massively cross-bedded and often ripplemarked arkosic sandstones and pebbly grits, with clay and

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Fig. 1.22 Tectonic evolution model of the Benue Trough. After Olade (1975). a During Aptian to early Albian, there is mantle upwelling, uplifting and thinning of the crust leading to the formation of fractures/faults, possibly associated with initial basal volcanics around Abakaliki. b In the mid-late Albian, the continuing mantle upwelling leads to further uplift and eventually rifting, followed by subsidence and sedimentation of the Asu-River Group. c Cessation of mantle upwelling and a short regressive folding phase in the Cenomanian leading to the suspected possible deformation (folding) of Asu-River Group and thereby limiting the Cenomanian sedimentation to the

Calabar flank of the lower Benue-Trough as Odukpani Formation. d Reactivation of mantle upwelling and associated subsidence during middle Turonian, leading to subsequent deposition of the Ezeaku Group, i.e. Eze-Aku and Awgu Shales. This was followed by Santonian folding episode consequent to stoppage of mantle upwelling and the development of mantle contraction leading to the deformation of Eze-Aku Group. e Campanian to Maastrichtian periods are characterized by deposition of post-Santonian sediments unconformably on the older sediments and apparently mostly within the Anambra basin to the north-west of the lower segment of the Benue Trough

shale lenses and a well-developed basal conglomerate. The thickness is very variable, ranging up to 3000 m in places. In the Turonian to early Coniacian (Senonian) interval, sedimentation in the lower (south-west) segment of the trough was characterized by a major marine transgression and represented by the Eze Aku and Awgu Shales. These are mainly shallow-water shales and siltstones with interbedded sandstones and limestones (in places pure enough to be quarried) and a variety of fossils including ammonites. However, in the upper (north-east) segment of the Benue Trough, south of the Zambuk Ridge and in the Yola arm, the continental sandstones of the Albian–Cenomanian are overlain conformably by beds of the Yolde Formation, representing a transition to marine conditions in the Turonian–Coniacian period. The Yolde Formation is characterized by thin alternations of shallow-water shales and sandstones with occasional nodular limestones that contain ammonites and other fossils.

The Santonian was a period of folding and regression in the lower segment of the Benue Trough, while the axis of sedimentation shifted to the Anambra Basin. Hence, the succeeding Campano-Maastrichtian Nkporo/Enugu Shales and Coal Measures as well as their lateral equivalents unconformably overlie folded older beds, especially in the Anambra Basin. In the upper (north-east) segment, the Yolde Formation passes conformably up into a sequence of shales with thin nodular limestones, locally gypsiferous and subordinate sandstones represented by the Pindiga Formation (on the Zambuk Ridge) and its lateral equivalents (south of the ridge) that were subdivided into five units (Fig. 1.24).

1.3.2.2 The Calabar Flank The Calabar Flank sedimentary basin is an extension of the Lower Benue Trough, and it extends from the southern margin of the Oban Basement Complex to the boundary with the Niger Delta. The northwest-southeast trending basement

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During the Cenomanian, mantle upwelling ceased and a short regressive folding phase within the main lower Benue Trough led to the suspected deformation of the Asu-River Group. Consequently, Cenomanian sedimentation was limited to the deposition of Odukpani Formation in the Calabar Flank as an extension of Lower Benue Trough (Olade 1975). Thus, the sedimentary succession on the Calabar Flank is mostly of Cretaceous age, comprising an ancient river-borne sandstone, the Awi Formation and the overlying marine Odukpani Group of Albian to Late Cretaceous age. The Odukpani Group is represented only by a very narrow strip of sandstones, shales and limestones and comprises the Mfamosing Limestone, the Ekenkpon Shale and the New Netim Marl, which are all exposed near the Odukpani area. This is unconformably covered by the Nkporo Shale (Tertiary marine shales) and regressive sandstones overlie the Cretaceous succession. The total sediment thickness in the Calabar Flank is over 3500 m.

Fig. 1.23 Generalized Stratigraphy of the Benue Trough spanning Aptian to Maastrichtian periods. After Tijani (2004)

structures underlie the Calabar Flank and define the ltu High and the lkang Trough, thus relating the Calabar Flank to the South Atlantic Cretaceous marginal basins with similar horst-and-graben structures in Angola and Gabon (Wright 1985).

Fig. 1.24 Composite longitudinal cross section of Cretaceous sediments in the Benue Trough, Nigeria. Modified after Obaje (2009)

1.3.2.3 Anambra Basin After Santonian folding and uplift, the main axis of subsidence and sedimentation was displaced to the north-west (i.e. the then Anambra platform). The evolution of the Anambra Basin to the north-west of the lower segment of the Benue Trough is linked with the development of the Benue Trough (Ojoh 1992). During the development of the Benue Trough the former Anambra platform transformed into Anambra depression as the consequence of the Santonian folding and uplift of the Lower Benue Trough (Abakaliki zone) (Tijani and Nton 2008; Tijani et al. 2009). Hence, the Anambra Basin is characterized by post-Santonian deposition of Campanian to Maastrichtian sediments (Figs. 1.25 and 1.26).

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Fig. 1.25 Stratigraphic profiles and depositional environment of sedimentary units within the Anambra Basin (Tijani and Nton 2008)

Sedimentation started with the accumulation of thick Nkporo Shales and Coal Measures which thin rapidly across the Abakaliki anticlinorium and remain relatively thin on the other side of it. The shales (Nkporo) form low ground and rarely crop out, and information about them comes mainly from boreholes. They are of shallow water origin, with thin beds of sandstone, shelly limestone and non-continuous coal seams, as well as being locally gypsiferous. Fossils include the ammonite Libycoceras afikpoensis, which is diagnostic for the lower part of the Maastrichtian, and these sediments are presumed to span the Campanian–Maastrichtian interval. The Coal Measures in Nigeria are of Maastrichtian age and represent a period of non-marine sedimentation at the end of the Cretaceous. They have a total thickness of around 900 m. The lowermost 100 m or so forms the base of the Enugu-Udi escarpment and consists of typical coal measures lithologies (alternations of sandstones, siltstones, mudstones and shales with concretionary siderite and marcasite) and contains at least five workable coal seams (i.e. Mamu Formation). During the Maastrichtian, the Anambra Basin became silted up and extensive, thickly vegetated swamps developed near sea level, on top of a broad delta fan built up by rivers bringing sediment down from the hinterland. Hence, the coal-forming environment in the Anambra Basin was disturbed by a period of rapid fluviatile sedimentation, presumably resulting from rejuvenation in adjoining upland areas. Large volumes of over 400 m of barren massive cross-bedded sandstones (Fig. 1.27) (i.e. Ajali Sanstone) were deposited rapidly, and vegetation had no chance to become established (Ladipo 1988). However, subsequent

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renewed marine sedimentation heralded the return to coal-forming conditions forming about 300 m thick Nuskka Formation (Upper Coal Measures). This became more extensive southwards (in the succeeding Palaeocene), thus marking the beginning of the development of the Niger Delta. Sedimentological studies of sandstones (Fig. 1.28) show a major contrast between Albian to Turonian and Coniacian sediments of the Abakaliki Basin and the post-Santonian sediments of the Anambra Basin. Sandstones in the Abalkaliki sub-basin are commonly feldspathic and relatively poorly sorted, with a significant proportion of angular grains; i.e. they are both texturally and mineralogically immature. By contrast, sandstones in the younger sediments in the Anambra Basin are virtually free of feldspar, though quartz grains are still rather angular. The inference is that there was some recycling of sediment following the Santonian folding episode; i.e. sediments deposited before folding were eroded to contribute to the sediments deposited in the Anambra Basin after the folding and uplift. Hence, sediments supplied to the Anambra Basin were probably also partly derived from basement areas that had undergone more prolonged weathering, resulting in the breakdown of feldspars, but not in any significant rounding of quartz grains.

1.3.2.4 Chad (Bornu) Basin The Chad Basin is more than 1000 km in diameter and most of it is occupied by Quaternary alluvial sands of the Chad Formation that obscure older Cretaceous sediments, which in parts result from sedimentation cycles in the Benue Trough (Petters 1982; Obaje et al. 2004). A subsurface basement high, the Zambuk Ridge, separates the Southern Chad Basin, locally referred to as Bornu Basin, from the Upper Benue (Wright 1985) (Fig. 1.29). The Cretaceous sedimentation in the Nigerian sector of the Chad Basin includes the basal Albian–Cenomanian Bima Sandstone Formation, which is characterized by thickly bedded, poorly sorted feldspathic sandstones and conglomerates of continental fluviatile and deltaic origin. This is followed by the Pindiga Formation/Group, which passes laterally into a Middle Cretaceous limestone, sandstone and shale succession, subdivided into the Gongila Formation at the base and the Fika Shale at the top, both of which constitute marine and transitional deposits that extend from the Upper Benue Trough into the Southern Chad Basin (Avbovbo et al. 1986; Okosun 1995). The Gongila Formation, early Turonian in age, is about 500 m thick and consists of intercalation of marine limestones, sandstones and shales. The Fika Shale Formation of Senonian-Maastrichtian age, 100–500 m thick, consists of gypsiferous shales and limestone of marine and continental origin. This is overlain by the Gombe Sandstone Formation (Maastrichtian), which consists of estuarine and deltaic

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Fig. 1.26 Geological map of Lower Benue Trough and the Anambra Basin, SE-Nigeria. Modified after Olade (1975)

Fig. 1.27 Outcrops of whitish friable cross-bedded Ajali Sandstone

Fig. 1.28 Yellowish friable cross-bedded Ajali Sandstones near Uturu in the Anambra Basin

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Fig. 1.29 Stratigraphic successions in the Nigerian sector (Burno) of the Chad Basin. Modified after Obaje (2009)

sediments. The lower section consists of siltstone, mudstone and ironstone (of about 2 m in thickness), overlain by well-bedded sandstones and siltstones, while the upper section contains impure coals and flaggy, sometimes cross-bedded sandstones (Wright 1985; Avbovbo et al. 1986; Okosun 1995). With the end of Cretaceous characterized by uplift and erosion, the Kerri-Kerri Formation (Palaeocene) was deposited unconformably on the Gombe Sandstone Formation. It thickens towards the basin centre where it is overlain by Chad Formation (Pleistocene age). The Kerri-Kerri Formation, which oversteps on to basement in the west, consists of a lacustrine or fluvio-lacustrine sequence of loosely cemented cross-bedded coarse- to fine-grained reddish sandstones, with locally occurring claystones, siltstones, ironstones, lignites and conglomerates (Wright 1985; Avbovbo et al. 1986). The Kerri-Kerri Formation has not been folded or even tilted, and it is capped by prominent laterite, which is itself overlain unconformably by Chad Formation sediments of Pleistocene age. The Chad Formation, up to 840 m thick, consists of poorly sorted, fine- to coarse-grained sand, with sandy clay, clay and diatomite. Chad Formation sediments bury a varied topography and consist of fluviatile and lacustrine clays and sands, with lenses of diatomite up to a few metres thick (Obaje et al. 1999). Diatoms indicate a Lower Pleistocene age in the deeper parts of the basin where the Chad Formation the stratigraphic succession probably extends down into the Pliocene.

1.3.2.5 Sokoto Basin Marine Late Cretaceous–Palaeocene beds in the SE lullemmeden Basin, i.e. Sokoto Basin, are well exposed in the Sokoto region, in Niger and extending into Mali. The

Mesozoic and Tertiary sedimentation in the Sokoto Basin comprise intercalations of sandstones, clays and limestones. The sedimentary sequence started in the Early Cretaceous, with Illo and Gundumi Formations directly overlying unconformably the Precambrian Basement rocks. This is followed by three sedimentation cycles that constitute the Late Cretaceous–Palaeocene marine sequence, namely (Figs. 1.30 and 1.31): (a) Late Cretaceous Rima Group (Maastrichtian) that comprises the Taloka Formation (50 m of brown, laminated, parallel-bedded, carbonaceous, fine-grained sandstone, siltstones and mudstones), overlain by the Dukamaje Formation (10 m of basal bone bed, gypsiferous, fissile, grey lower and upper shales and middle marls). (b) The Palaeocene Sokoto cycle that comprises the Wurno Formation (20 m of soft, tabular mudstone, muddy siltstones and fine-grained sandstones at the base), which grades upward into the Dange shale (10 m thick) and the Kalambaina Formation (12 m of nodular, marly limestone which becomes shaly at the top). (c) The Eocene-Miocene Gwandu Formation, characterized by a series of overlapping marine transgressions with over 1250 m of sediments in the downwarped Sokoto Basin. Generally, most of the outcropping formations are capped by laterites, forming ferruginous deposits over thousands of square kilometres. These deposits can be subdivided into three major groups. Ferruginous oolites are the primary deposits of the Paleocene age. Crusty-concretionary laterites of the Post-Gwandu Formation constitute secondary deposits, whereas ferruginous sandstones are mostly primary deposits capping continental deposits.

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23

Fig. 1.30 Stratigraphic successions in the Nigerian sector of the Sokoto Basin. After Obaje (2009)

Fig. 1.31 Geological cross section through the Sokoto Basin. Modified after Anderson and Ogilbee (1973), Adelana et al. (2009)

The abundance of ferruginous oolites, laterites and ferruginized sandstones in the Iullemmeden Basin has been known for a long time (Falconer 1911; Kogbe 1976). The iron-rich oolites are primary deposits, and the deposition of ferruginous materials occurred during the late Paleocene. The crusty laterites and ferruginous sandstones were formed during the late Neogene or early Quaternary. They attain the thickness of approximately 4 m or more, forming the duricrust capping flat-topped hills or mesas (Fig. 1.32).

1.3.2.6 Mid-Niger Basin The Mid-Niger Basin runs SE–NW, from the confluence of Niger and Benue rivers in the Anambra Basin towards the Sokoto Basin. The Mid-Niger Basin (Bida Basin) is assumed

to be a northwesterly extension of the Anambra Basin (Akande et al. 2005). The basin fill comprises a north-west trending belt of Upper Cretaceous sedimentary rocks that were deposited as a result of block faulting, basement fragmentation, subsidence, rifting and drifting consequent to the Cretaceous opening of the South Atlantic Ocean. The basin contains mainly continental sandstones, siltstones, claystones and conglomerates (Ladipo 1988). The stratigraphic succession of the Mid-Niger Basin, collectively referred to as the Nupe Group (Adeleye 1973), comprises the Northern Bida Sub-Basin and the Southern Lokoja Sub-Basin (Fig. 1.33). The evolution of the Mid-Niger Basin led to the sedimentation of the Upper Cretaceous depositional cycle,

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and concretionary ironstones deposited within a continental to shallow marine setting.

Fig. 1.32 Example of flat-topped hills or mesas with ferruginized crust capping in the Sokoto Basin. Insets Remnants of crusty laterites and ferruginous sandstones

commencing with the main units of Lokoja and Bida Formation unconformably overlying the basement bedrocks. These are lateral equivalents of the fully marine shales of the Campanian Nkporo/Mamu Formation. The main units in the Mid-Niger Basin are characterized by conglomerates, alluvial to deltaic, coarse- to medium-grained cross-bedded and poorly sorted sandstones and claystones of the Lokoja Formation (Lokoja sub-Basin) and Bida Sandstone (Bida sub-basin), with subordinate siltstones, kaolinitic claystones and shales (Fig. 1.34). The Lokoja Sandstone Formation is succeeded by sandstones of the Lower Maastrichtian Patti Formation, with lateral equivalents of the Sakpe and Enagi Formations in the Bida sub-basin. These are well-sorted quartz arenites that are commonly interbedded with siltstones and claystones. The Patti and Enagi Formations are overlain by the Agbaja and Batati Formations (lateral equivalents) of Upper Maastrichtian age (Fig. 1.35). These consist of oolitic, pisolitic Fig. 1.33 Stratigraphic successions in the Mid-Niger or Nupe Basin (also known as Bida Basin). Adopted from Obaje (2009)

1.3.2.7 The Dahomey Basin The Dahomey Basin is an arcuate coastal basin, the onshore parts of which underlie the coastal plains of southwest Nigeria, Benin and Togo. The eastern sector of the Dahomey Basin is referred to locally as the Benin Basin and is one of the sedimentary basins of southern Nigeria. It was separated from the Benue Trough and the Niger Delta Basin by the Okitipupa ridge (the so-called Benin Hinge Line) until the Late Cretaceous subsidence and marine transgression. As summarized in Fig. 1.36, the sedimentary rock sequences in the eastern Dahomey Basin range from Cretaceous to Recent in age (Jones and Hockey 1964). These successions comprise four main sedimentary formations: Abeokuta (Late Cretaceous), Ewekoro (Paleocene), Ilaro (Eocene) and Coastal Plains Sands and Alluvial deposits (Pleistocene-Oligocene-Recent). The early Cenozoic in the Benin (eastern Dahomey) Basin is represented by the basal Abeokuta Group (Fig. 1.37) consisting of the Ise, Afowo and Araromi Formations (Adegoke and Omatsola 1981). The Ise Formation unconformably overlies the Basement Complex and consists of basal conglomerates and sandstones, overlain by coarse- to medium-grained sands with interbedded kaolins. Overlying the Ise Formation is the Afowo Formation, which is composed of coarse- to medium-grained sandstones with variable but thick interbedded shales, siltstones and claystones. The Araromi Formation overlies the Afowo Formation and is the youngest Cretaceous sediment in the eastern Dahomey Basin (Adegoke and Omatsola 1981). It is composed of fine- to medium-grained sandstone and bitumen-bearing sandy black shales with thin sandstones and limestones in places (Okosun 1998). The Araromi Formation is overlain by Ewekoro Formation (Palaeocene) consisting of shales, siltstones, with interbedded extensive limestone unit, marl and lignite

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Geology of Nigeria

Fig. 1.34 Angular unconformity showing a the Lokoja Sandstone overlying basement rocks at the margin of Lokoja sub-basin along Lokoja-Okene Road and b Patti and Enagi Formation overlying the

Fig. 1.35 Oolitic ironstone concretion of Agbaja and Batati Formations within the Lokoja sub-basin of the Mid-Niger or Nupe Basin

Fig. 1.36 Regional geology and stratigraphy of Benin (eastern Dahomey) Basin (Jones and Hockey 1964)

25

basal Lokoja Formation of Bida Basin along Lokoja-Abuja Road. Modified after Ojo et al. (2021)

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M. N. Tijani

sandstones, while the Ilaro Formation consists of massive, yellowish poorly, consolidated, cross-bedded sandstones. In summary, a north to south geological section showing the relationship of these sedimentary formations vis-à-vis the underlying basement rock is presented in Fig. 1.39.

Fig. 1.37 Road-cut section in the Abeokuta Formation, comprising mainly sand with sandstone, siltstone, clay-/mudstone interbeds, between Obada-Oko and Lala village along the Abeokuta–Sango Ota highway

Fig. 1.38 Section through the Ewekoro Formation consisting of extensive limestone units with interbedded shales, siltstones and marls at Shagamu Quarry

(Fig. 1.38). The Ewekoro Formation, which is being actively mined in at least three locations (Shagamu, Ewekoro and Ibese) for cement production, is overlain by the Akinbo Formation, which is made up of shaly and clayey sequences (Ogbe 1972). Overlying the Akinbo Formation are the Oshosun and Ilaro Formations. The former consists of Eocene phosphatic greenish-grey clay and shale with interbedded Fig. 1.39 North-south geological section of the Dahomey Basin showing sub-surface stratigraphic units. Modified after Jones and Hockey (1964)

1.3.2.8 Niger Delta Basin The Cenozoic Niger Delta Basin was developed as a regressive off-lap sequence. The delta complex, described as having an arcuate-lobate shape, was built across the Anambra Basin and the Cross River margins and eventually extended onto the Late Cretaceous continental margin. The geological age of the Niger Delta sediments ranges from Paleocene to present. Its sedimentary succession comprises a lower marine unit, the Akata Group; a middle coastal unit, the Agbada Group; and an upper continental sequence, the Benin Group. Each of these units represents an enormous time, because of the advancement of the Niger Delta ocean-ward. The three lithostratigraphical units have been found the main petroliferous units in the Niger Delta in both onshore and continental shelf terrains. The Akata formation is composed of marine shales which are the main source rocks for petroleum and are overlain by the paralic Agbada formation which consists of main reservoir units. The Agbada Formation is made up of intercalations of sand and shale sequences. The sands are mainly unconsolidated reservoir sands, while the shales also function as source and cap rocks. The Agbada formation is overlain by the continental Benin formation. This formation is mainly composed of sand units and has few petroleum-bearing strata (Fig. 1.40). Most of oil and gas so far discovered and produced in the Niger Delta Basin have been supplied by the Agbada formation. Unlike the onshore, the Niger Delta continental shelf is characterized in places by diapirs and growth faults. The presence of growth faults and associated rollover anticlines in nearly all terrains of the Niger Delta Basin points to the fact that structural trapping mechanism for petroleum is common in the delta. Three categories of structural styles are

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27

Fig. 1.40 Schematic cross-section of the Niger Delta Basin illustrating the stratigraphy and diachronous nature of the sediments. Redrawn from Wright et al. (1985)

Fig. 1.41 Diagrammatic cross-section of showing the basic pattern of deformation of the unconsolidated deltaic sediments in the Niger Delta Basin. Redrawn from Wright et al. (1985)

common in the Niger Delta onshore, continental shelf and deep-water terrains. Diapirs typify the translational zone, growth faults occur in the extensional zone, whereas toe thrusts are associated with the compressional zone (Fig. 1.41).

1.4

Geological Setting and Geomorphological Implications

In Nigeria, diverse geological settings created geomorphological and structural features that are of interest in terms of their geotourism potentials as geoheritage and geosites. A comparison of the geological and geomorphological maps of Nigeria (Fig. 1.42) reveals that landforms and physical environment features are more or less reflections of the

underlying rock types, the tectonic processes, as well as associated uplift and deformation of the rock units. Like in most parts of West Africa, the landscape in Nigeria is characterized by the occurrence of large, almost level plains (peneplains), formed by intensive erosion over long periods. Most of the plains and plateaus in northern Nigeria are the result of Pliocene–Pleistocene erosion, the so-called African and Post-African planation (Szentes 2009). These plains and plateaus have an undulating relief 400– 600 m above the sea level and are dissected by streams and rivers, usually in a dendritic drainage pattern. Falconer (1911) and Wilson (1922) believed that active chemical subsurface decomposition resulted in an uneven weathered surface which is reflected by inselberg topography when the surface is exposed to erosion. However, Twidale (1984) emphasized that erosion of the lateritic regolith and tectonic

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Fig. 1.42 Comparison of the outline of the geology of Nigeria vis-à-vis the distribution of physical environment features

movements brought the granite boulders to the surface, thus stripped away the overlying material as weathering processes have shaped the granite boulders into the present-day forms. In Nigeria, the basement rocks (gneiss, migmatites and metasediments of Precambrian age) were intruded by plutonic rocks of late Precambrian to lower Palaeozoic ages, known as the Older Granites. Granite masses, and in some cases gneisses and migmatites as well, form rugged topography with inselbergs rising abruptly to hundreds of metres above the surrounding plains. The inselbergs are bare domes, whalebacks or less regular hills and display onion-skin (sheet) jointing or tor-like capping of boulders in some cases. The granite bodies are widespread, especially in the north and southwest, and range in size from smaller elliptical plutons to masses of batholithic dimensions of over 100 km in length (Szentes 2009). In the south-western part of Nigeria, these granitic outcrops form spectacular inselbergs within the vast rolling landscape and can be as high as 100–300 m; prominent examples are found at Iseyin-Okeho-Saki-Igbeti axis and

Akure-Idanre-Ikare-Akoko axis (Fig. 1.43a). The crystalline bedrock areas of northern-central part of Nigeria are characterized by disintegrated granite hills with balanced rocks and tors in Bauchi, Kano and Brinin-Kudu areas (Szentes 2009). In addition, there are spectacular inselbergs of Older Granites with peaks of over 700 m in some cases, while in the Jos area, the hills rise up from the plateau, consisting of rhyolite that is part of the Younger Granites assemblage (Fig. 1.43b). The Jos Plateau is characterized by ring complexes on the granitic plateau, which occur in a broad N-S zone, 400  150 km. About fifty granite massifs of various size are known, and they are characterized by several inselberg-shaped hills built of the Younger Granites, while the outcrops of volcanic rocks are weathered to form a succession of laterite crust of 5–10 m thick on the Jos Plateau (Farnbauer and Tietz 2000). A geological section cutting across both the Precambrian Basement Complex and the Younger Granite settings of the northern part of Nigeria as described by Szentes (2009) is presented in Fig. 1.44 highlighting the characteristic landforms.

Fig. 1.43 Granite topography. a Aerial view of Idanre Town SW-Nigeria showing the surrounding granite domes (Ige et al. 2011), b stunning view of Shere Hills on the Jos Plateau as one of the highest peaks in Jos area. Retrieved from google images

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29

Fig. 1.44 Sketch geological section across the Precambrian Basement Complex and Younger Granite settings of the northern part of Nigeria. After Szentes (2009)

Unlike the hummocky terrain underlain by crystalline rocks, the sedimentary terrains of southern Nigeria are characterized by thickly forested, undulating hills and swampy lowlands. The low-lying coastal strip and extensive alluvial plains of the major rivers are expressions of topographic depressions. In general, the south-eastern uplands typically consist of low-rounded hills with secondary forest cover, which are capped with ferruginized lateritic crust that can rise to about 100 m above the ground level in some cases. However, due to the unconsolidated nature of sedimentary units in some locations and partly due to deforestation, gully erosion and landslides are common, especially in areas underlain by the Nnaka and Ajali sandstone formations (Fig. 1.45). There is no doubt that the geological framework of Nigeria is reflected in the diverse nature of geological settings. The associated geomorphological processes have resulted in the evolution of geomorphic features and scenery that have aesthetic and touristic potentials, apart from the cultural importance in some cases. The Basement Complex settings of Nigeria are characterized by a high degree of geodiversity of granitic landforms, mostly the plains and inselbergs, while the sedimentary terrains are also characterized by low hills and mesas capped with ferruginized lateritic crusts. Fig. 1.45 Incipient landslides in parts of south-eastern Nigeria in areas underlain by unconsolidated a Ajali Sandstone and b Nnaka Sandstone (Tijani and Nton 2008)

A number of geomorphological features as highlighted earlier are clear evidence of wide geodiversity in Nigeria. These features are usually abound in form of lakes, caves, impressive hills/ridges, beaches, spring and waterfalls among others (Figs. 1.46 and 1.47). Therefore, there is a need to develop a detailed inventory and characterization of these landforms and geosites vis-àvis their scientific/educational values and geotouristic potentials on one hand and also to ensure their protection and preservations on the other hand. Apart from the cultural/traditional values, these potentials if properly harnessed can be developed for geotourism with attendant economic benefits to Nigeria.

1.5

Geology: Economic Considerations, Geoheritage and Way Forward

Geologically, Nigeria comprises crystalline basement rocks and sedimentary basins, almost in equal proportions. The ages of the basement rocks span Precambrian to early Palaeozoic, having been consolidated during the Pan-African Orogeny. The sedimentary basins are of Cretaceous to Recent age. Abundant mineral deposits occur in all components of

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Fig. 1.46 Waterfalls of Nigeria. Gurara waterfalls in Niger state (a) (https://guardian.ng/saturday-magazine/destination-gurarawaterfalls/) and Owu Water Falls with a height of about 120 m in

Kwara state (https://www.nigeriagalleria.com/Nigeria/States_Nigeria/ Kwara/Owu-Waterfall.html) (b) both in the North-central region of the country

Fig. 1.47 Granite inselbergs as tourist attractions. a The famous Olumo Rock in Abeokuta (Google images), b Ikyogen Hill located about 34 km from Adikpo, near Gboko in Benue State, with evergreen

vegetation that supports holiday tours and ranching. Source https:// www.visitnigerianow.com/tours/ikyogen-hills/

Nigerian geology (Basement, Younger Granites and Sedimentary Basins). Jurassic igneous and volcanic rocks occur on the Jos Plateau, where they are sources of mineral deposits such as tin, columbite, tantalite, wolframite, monazite and gemstones. Gold, molybdenite and non-metallic minerals such as feldspars and talc are among the mineral deposits in the Basement Complex. Apart from oil and natural gas, groundwater and a host of industrial minerals, the older sedimentary rocks are mineralized with lead and zinc. Coal and lignite occur extensively in the southern and middle Benue Trough. Bitumen and tar sands occur in Cretaceous sediments extending across the Benin (eastern Dahomey) Basin in the south-western part of

the country. Lateritic superficial deposits as construction soil aggregates also abound in Nigeria. In summary, the solid mineral sector offers viable prospects for mining, mineral processing and manufacturing of a host of intermediate raw materials for local industries as well as for foreign exchange earnings, which can provide a unique source of mineral wealth and unparalleled opportunities for rapid economic development and opportunity for diversifying Nigeria's hitherto petroleum-dominated economy. Furthermore, the geological framework of Nigeria reveals diverse geological processes and associated geomorphological processes. There is a marked contrast between the hummocky terrain underlain by crystalline rocks, and the

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Geology of Nigeria

thickly forested undulating hills and swampy lowlands of the sediments to the south. The overall evolution of geological–geomorphic process resulted in scenery and features that have aesthetic and touristic potential, apart from the cultural importance in some cases. Hence, there is the need to develop a detailed inventory and characterization of these landforms and geomorphological heritage (geosites) to ensure their protection and preservations. This will ultimately offer many great opportunities and potentials in geotourism, an aspect of economic development that is largely untapped in Nigeria.

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31 Cooray PG (1975) The charnockitic rocks of Nigeria, Pitchamutu volume. Banglore University, India, pp 50–73 Cratchley CR, Jones GP (1965) An interpretation of the geology and gravity anomalies of the Benue Valley, Nigeria. Overseas Geolog Surv Geophys Pap 1:1–25 Dada SS (1989) Evolution de la croute continental au Nord Nigeria: apport de la geochimie, dela geochronologie U-Pb et des traceurs isotopiques Sr, Nd et Pb. PhD thesis, University of Science and Technology Languedoc, Montpellier, France Dada SS (2006) Proterozoic evolution of Nigeria. In: Oshi O (ed) The basement complex of Nigeria and its mineral resources (a tribute to Prof. M. A. O. Rahaman). Akin Jinad & Co. Ibadan, pp 29–44 Egbuniwe IG (1982) Geotectonic evolution of the Maru Belt, NW Nigeria. Unpublished PhD thesis, University of Wales, Aberystwyth Ekwueme BN (1987) Structural orientation and Precambrian deformational episodes of Uwet area, Oban Massif, SE Nigeria. Precambr Res 31:269–289 Falconer JD (1911) The geology and geography of northern Nigeria. Macmillan, London Farnbauer B, Tietz G (2000) The individuality of laterites developed on the Jos-Plateau/Central-Nigeria (in Deutsch). Zbl. Geol. Palaeont. Tiel I (5/6): 509–525 Farrington JL (1952) A preliminary description of the Nigerian lead-zinc field. Econ Geol 47(6): 583–608 Gandu AH, Ojo SB, Ajakaiye DE (1986) A gravity study of the Precambrian rocks in the Malumfashi area of Kaduna State, Nigeria. Tectonophysics 126(2–4): 181–194 Grant NK (1970) Geochronology of Precambrian basement rocks from Ibadan, south-western Nigeria. Earth Planet Sci Lett 10:19–38 Grant NK (1978) Structural distinction between a metasedimentary cover and an underlying basement Gubanov AP, Mooney WD (2009) New global geological maps of crustal basement age. In: AGU fall meeting abstracts, vol 2009, pp T53B–1583 Holt RW (1982) The geotectonic evolution of the Anka Belt in the Precambrian basement complex of N.W. Nigeria. Unpublished PhD thesis, The Open University Ige O, Adeyemi C, Ogunfolakan A, Ayansola A, Olayemi A, Taiwo Y, Olayiwola M, Oyelade J (2011) An inventory of the geological, biological and cultural resources on Ufe-Oke Hill, Idanre, Southwestern Nigeria. Nat Res 2(03):180 Iloeje OC (1982) Effects of parallel channel interactions, steam flow, liquid subcool and channel heat addition on nuclear reactor reflood transients. Niger J Technol 6(1) Jacobson RR, MacLeod WN, Black R (1958) Ring complexes in the younger granites of northern Nigeria. Geol Soc Lond Mem 1:5–9 Jimoh HI (2012) Leading Issues in Geomorphology. Ilorin. Haytees Press and Publishing Company, Nigeria Jones HA, Hockey RD (1964) The geology of part of south-western Nigeria. Geol Surv Niger Bull 31:101pp Kinnaird JA (1981) Geology of the Nigerian anorogenic ring complexes 1:500,000 geological map. John Bartholomew and Sons King L (1950) Speculations upon the outline and the mode of disruption of Gondwanaland. Geol Mag 87(5): 353–359 Kogbe CA (1976) Outline of the geology of the Lullemmeden basin in north-western Nigeria. In: Kogbe CA (ed) Geology of Nigeria. Elizahethan Publishing Co., Lagos Kogbe CA (1979) Geology of the south-eastern (Sokoto) sector of the Lullemmeden basin. Dept Geol Ahmadu Bello Univ Zaria Bull 32:142pp Ladipo KO (1988) Paleogeography, sedimentation and tectonics of the upper Cretaceous Anambra basin, south-eastern Nigeria. J Afr Earth Sci 7:865–871 Lees GM (1952) Foreland folding. Quart J Geol Soc 108(1–4): 1–34

32 MacDonald AM, Kemp SJ, Davies J (2005a) Transmissivity variations in mudstones. Ground Water 43:259–269 MacDonald AM, Cobbing J, Davies J (2005b) Developing groundwater for rural water supply in Nigeria. British geological survey commissioned report CR/05/219N Matheis G, Caen-Vachette M (1983) Rb–Sr isotopic study of rare-metal bearing and barren pegmatites in the Pan-African reactivation zone of Nigeria. J Afr Earth Sci 1(1): 35–40 McCurry P (1973) Geology of degree sheet 21, Zaria, Nigeria. Overseas Geol Miner Res 45:1–30 McCurry P (1976) The geology of the Precambrian to lower Palaeozoic rocks of northern Nigeria—a review. In: Kogbe CA (ed) Geology of Nigeria. Elizabethan Publishers, Lagos, pp 15–39 Obaje NG (1999) Biostratigraphic and geochemical controls of hydrocarbon prospects in the Benue Trough and Anambra Basin, Nigeria. Nigerian Association of Petroleum Explorationists (NAPE) Bulletin 14: 18–54 Obaje NG (2009) Geology and mineral resources of Nigeria. Springer, Dordrecht, p 221 Obaje NG, Wehner H, Scheeder G, Abubakar MB, Jauro A (2004) Hydrocarbon prospectivity of Nigeria’s inland basins: from the viewpoint of organic geochemistry and organic petrology. AAPG Bull 87:325–353 Obiora SC (2005) Field descriptions of hard rocks, with examples from the Nigerian basement complex. SNAAP Press (Nig.) Ltd., Enugu, p 44 Ogbe FGA (1972) Stratigraphy of strata exposed in the Ewekoro quarry, Western Nigeria. Afr Geol 1: 205–322 Ogezi AEO (1977) Geochemistry and geochronology of basement rocks from northwestern Nigeria. Unpublished PhD thesis, University of Leeds Ojoh KA (1992) The southern part of the Benue Trough (Nigeria) Cretaceous stratigraphy, basin analysis, paleo-oceanography and geodynamic evolution in the equatorial domain of the south Atlantic. NAPE Bull 7:131–152 Okosun EA (1995) A review of the geology of the Bornu basin. J Min Geol 31(2):113–212 Okosun EA (1998) Review of the early tertiary stratigraphy of southwestern Nigeria. J Min Geol 34:27–35 Olade MA (1975) Evolution of Nigeria's Benue Trough (Aulacogen): a tectonic model. Geol Mag 112(6): 575–583 Olade MA, Elueze AA (1979) Petrochemistry of the Ilesha amphibolite and Precambrian crustal evolution in the Pan-African domain of SW Nigeria. Precambr Res 8:303–318 Olarewaju VO (2006) The charnockitic intrusives of Nigeria. In: Oshi O (ed) The basement complex of Nigeria and its mineral resources (a tribute to Prof Rahaman MAO). Akin Jinad & Co. Ibadan, pp 45–70 Olayinka AI (1992) Geophysical siting of boreholes in crystalline basement areas of Africa. J Afr Earth Sci 14:197–207 Oluyide PO, Nwajide CS, Oni AO (1998) The geology of Ilorin area with explanations on the 1:250,000 series, sheet 50 (Ilorin). Geol Surv Niger Bull 42:1–84 Oyawoye MO (1972) The basement complex of Nigeria. In: Dessauvagie TFJ, Whiteman AJ (eds) African geology. Ibadan University Press, pp 66–102 Petters SW (2004) Nigeria—geological background. Retrieved on Feb 2019, from https://www.onlinenigeria.com/geology/?blurb=499. Posted on 20 Feb 2004, 1:58:44 PM

M. N. Tijani Petters SW (1982) Central west African Cretaceous-Tertiary benthic foraminifera and stratigraphy Rahaman MA (1976) Review of the basement geology of south-western Nigeria. In: Kogbe CA (ed) Geology of Nigeria, 2nd edn. Elizabethan Publishers, Lagos, pp 41–58 Rahaman MA (1981) Recent advances in the study of the basement complex of Nigeria. In: Abstract, 1st Symposium on the Precambrian geology of Nigeria Rahaman MA (1988) Recent advances in the study of the basement complex of Nigeria. In: Geological Survey of Nigeria (ed) Precambrian geology of Nigeria, pp 11–43 Rahaman MA, Ocan O (1978) On relationships in the Precambrian migmatite-gneisses of Nigeria. Niger J Min Geol 15:23–32 Rahaman MA, Van Breeman O, Bowden P, Bennett JN (1984) Age migration of anorogenic ring complexes in northern Nigeria. J Geol 92:173–184 Reyment RA (1980) Biogeography of the Saharan Cretaceous and Palaeocene epicontinental transgressions. Cretac Res 1:299–327 Szentes G (2009) Granite formations and granite cavities in northern Nigeria. Coruna 34:13–26 Tijani MN (2004) Evolution of saline waters and brines in the Benue-Trough, Nigeria. Appl Geochem 19(9): 1355–1365 Tijani MN, Nton ME (2008) Hydraulic, textural and geochemical characteristics of the Ajali Formation, Anambra basin, Nigeria: implication for Groundwater quality. Environ Geol 56(5): 935–951 Tijani MN, Nton ME, Kitagawa R (2009) Textural and geochemical characteristics of the Ajali Sandstone, Anambra basin, SE Nigeria: implication for its provenance. CR Geosci 342:136–150 Truswell JF, Cope RN (1963) The Geology of Parts of Niger and Zaria Provinces, Northern Nigeria: Explanation of 1: 250,000 Sheet No. 31 Turner DC (1983) Upper Proterozoic schist belts in the Nigerian sector of the Pan-African province of West Africa. Precambr Res 21:55– 79 Twidale CR (1984) So-called Pseudokarst in granite. Bol Soc Venez Espeleol 21:3–12 Van Breemen O, Pidgeon RT, Bowden P (1977) Age and isotopic studies of some Pan-African granites from north central Nigeria. Precambr Res 4:317–319 Whittow J (1984) Dictionary of physical geography. Penguin Books, London, p 1984 Wilson LE (1922) Elements of Engineering Geology. By Ries and Watson. Published by Chapman & Hall. Price 22s. Geol Mag 59(4): 181–182 World Health Organization (2007) WHO-UNICEF policy statement on the implementation of vaccine vial monitors: the role of vaccine vial monitors in improving access to immunization. In WHO-UNICEF policy statement on the implementation of vaccine vial monitors: the role of vaccine vial monitors in improving access to immunization Wright JB (1970) Controls of mineralization in the older and younger tin fields of Nigeria. Econ Geol 65:945–951 Wright JB (1985) Geology and mineral resources of West Africa. George Allen & Unwin, London, p 187 Wright LD, Short AD, Green MO (1985) Short-term changes in the morphodynamic states of beaches and surf zones: an empirical predictive model. Mar Geol 62(3–4): 339–364

2

The Climate of Nigeria and Its Role in Landscape Modification Olumide David Onafeso

Abstract

Nigeria generally has a warm climate all year round, with marked differences between dry and wet seasons. The wet season is normally from April to October, while the dry season is from November to March, although slight alterations have recently been observed in the onset of rains and the interludes between seasons. There are three main climatic regions and types of climate according to Köppen classification. These include the tropical monsoon occurring mostly in the south with rain almost all year round, the dry semi-arid Sahelian climate in the northern parts of the country usually with higher temperatures throughout the year and the savannah climate in the central parts. Keywords

Rainfall

2.1




Tropical




Modification




Monsoon




Climate

Introduction

Climate exerts major controls on landscapes and landforms among other physical features of the earth’s surface. Evidence of climatic imprints on landform processes in the River Niger basin of Nigeria has recently been explored by Onafeso and Olusola (2018). Besides, Watkins (1967) exemplified the relationship between climate and the development of landforms, with particular reference to the impact of the decline in the annual rainfall amount; the spell of rainfall events and intensification in the interludes between rainfall events. All these factors, including fluctuations in temperatures, have been shown to have significant effects on the availability of water, biomass rates as well as O. D. Onafeso (&) Department of Geography, Olabisi Onabanjo University, Ago-Iwoye, Nigeria e-mail: [email protected]

the characteristics of soil organic matter content which, in turn, affect the soil aggregate size and stability. Variations in soil structures and vegetal cover similarly influence the rates of overland flow as well as those of the associated erosion processes (Lavee et al. 1998). Other studies (cf. Mutz et al. 2018; Palmtag et al. 2018) have also shown that climate plays an important role in originating geomorphic processes, particularly in the adjustments to surficial forms. Likewise, the climate is responsible for most if not all of the agents of denudation, degradation and erosion. The shaping of different landscapes has been known to take place under the influence of one or more climatic parameters, particularly precipitation, temperature, solar radiation and wind. As such, landforms in any part of the planet have been discussed in relation to or alongside climatic elements and climatic processes over time (Zhu et al. 2014). Specifically, the sub-discipline of climatic geomorphology has been involved in investigating the roles of both past and the current climate in moderating morphogenetic processes and landform evolution, together with their spatio-temporal distributions and the mechanisms of these climatic predictors of landform change. It is on this basis that regions have been categorized into morpho-climatic zones (Williams 2015; Onafeso and Olusola 2015): indeed, climatic conditions have proved to be the utmost substantial determinants in the advancement of typical landform assemblages. Furthermore, there is a recent growing interest in the roles of climatic variables in the geomorphological analysis due largely to global warming and demand for prognostication on the impacts of the projected climate change scenarios on geomorphic systems (Beckinsale and Chorley 1991). In short, landforms and landform regions in Nigeria are shaped by distinctive climatically driven processes, indicated by the corresponding geomorphologic responses to the intensity, frequency and duration of rainfall, for example. However, while landforms may have been shaped by mechanisms and developments largely climatically generated, they vary genetically. Physical features are

© Springer Nature Switzerland AG 2023 A. Faniran et al. (eds.), Landscapes and Landforms of Nigeria, World Geomorphological Landscapes, https://doi.org/10.1007/978-3-031-17972-3_2

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dynamic, occurring in a wide range of predictably delimited climatic regions, thus transgressing random climatic boundaries sometimes. As such, Twidale and Lageat (1994) have suggested uncertainties relating to the climatic influence on landform evolution and thus not of overt significance across half the global land surface. Notwithstanding, this chapter attempts an appraisal of the climate of Nigeria and the plausible connections to the predominant landforms and landscapes.

2.2

General Overview of the Climate of Nigeria

Nigeria, with geographic coordinates between latitudes 4° −14° N and longitudes 2°−15° E, has a total area of 923,768 sq. km (comprising a land area of 910,768 km2 and a water surface of 13,000 km2). Except for the mountainous eastern borderlands, the country generally rises northward from the sea coast in the south to about 600 m in the north. This northward rise in altitude is only interrupted by the valleys of the Niger and Benue rivers and their tributaries, as well as by the drop in the trough of the Chad Basin. In parts of the eastern border, extensions of the Cameroon Mountain range penetrate Nigeria. In the Obudu and Mambilla Plateau areas, elevations reach above 1500 m where Afro-tropical highland conditions occur. Another upland area is the Jos Plateau in the center of the country, which peaks at about 1500 m and so reflects elements of montane influence; it also serves as an important watershed for many of the country’s large rivers, including the Kaduna, Gongola, Jama’ are and Hadejia rivers. The climate of Nigeria generally falls within the humid tropical type due to its proximity to the equator. With a seasonal tropical climate of distinct wet and dry periods, annual rainfall and length of the wet season generally decrease northward from the coast. Rainfall varies from close to 3000 mm per annum in some coastal locations to about 600 mm in the far north. Variations in mean daily, monthly and annual temperatures are small in coastal and southern areas but overtly greater further inland, ranging from about 20 ºC to over 40 ºC. Like many West African countries, Nigeria’s climate is characterized by strong latitudinal zonation, turning progressively drier northward (Gbadegesin and Onafeso 2010). Rainfall is the main climatic variable with a clear alternation of wet and dry seasons in most areas. Two air masses have been observed to control the rainfall pattern in the region, particularly the moist maritime air mass coming from the Atlantic Ocean and the dry continental air mass originating from the Sahara Desert (Gbadegesin and Onafeso 2010). Topographic relief also has a substantial impact on the local climate around the Jos Plateau and along the eastern border highlands.

Around the coast, the rainy season usually begins in February or March, when moisture-laden southwesterly monsoon wind reaches the region. The commencement of the rains is typically marked by the prevalence of strong winds, while heavy dispersed squalls occur in the north; in other places, rain may be profuse in certain regions, while other adjoining locations are utterly dry. The rains increase all over southward of the Niger-Benue trough around April to early May; northward, however, the rain gets intense around June, with the peak occurring around August, when a break occurs in the southern parts of the country. This period is notably referred to as the August dip in precipitation. Even though it is seldom entirely dry, this break in rainfall, which is especially significant in the southwestern parts of the country, is of immense advantage for agriculture since it permits a transitory dry passé for grain harvesting. Between September and November, when the northeast trade winds produce clear skies, modest temperatures and a decline in humidity for most of the country, another significantly distinct season occurs. Finally, between December and February, the northeast trade winds blow strappingly, often bringing fine dust from the Sahara and a season of “Harmattan”, with dense fog cover. These four distinct climatic seasons, which have also been described by Onafeso (2012), are further explained below.

2.3

Spatio-Temporal Distribution of Rainfall in Nigeria

There is a substantial variation in total rainfall across Nigeria (Fig. 2.1), with the heaviest occurrence in the coastal south. Port Harcourt (Fig. 2.2a) for example experiences very high annual rainfall, averaging about 2477 mm, typical of the equatorial rainforest regions in the southern parts of Nigeria. Many places along the coastal fringes also experience high rainfall. This is the region where all the rivers empty into the Atlantic Ocean. Worthy of note, however, is the significant decline in the annual rainfall trend in Port Harcourt since the early 1980s, indicating that yearly the total rainfall is dropping at about the rate of 10%. The impact of such declining rainfall, which is largely due to climate change, on geomorphic processes and landscape evolution has however not yet been explored. Similarly Lagos (Fig. 2.2b), with an average annual total rainfall of about 1406 mm, seats on the barrier coastline in the southwestern part of the country. A very significant feature here, aside from the Atlantic Ocean, is the Lagos Lagoon which is more than 50 km long and about 3−13 km wide. It is separated from the Atlantic Ocean by a long sand spit, 2−5 km wide sand surface area of approximately 6354.7 km2, with swampy margins. The lagoon is fairly shallow and is not plied by ocean-going ships, but by smaller

2

The Climate of Nigeria and Its Role in Landscape Modification

35

Fig. 2.1 Rainfall total (mm) across Nigeria

barges and boats essentially for transportation and fishing; it also receives the discharge of the Ogun and Osun rivers and their tributaries and is connected by a channel passing south of the town of Epe to the Lekki Lagoon. Narrow winding channels connect the system through broadband of coastal swamps and rivers, as far away as Sapele in the Delta State, some 250 km to the east. The Lagos Lagoon is the largest of the three lagoon systems occurring in the Lagos area, receiving over 80% of the land-derived run-offs laden with various types of waste. It lies within longitudes 6°25″N and 6°43″N and latitudes 3° 22″E and 3°40″E, with depths ranging from 3 m in most parts to between 6 and 10 m in the deeper portions. The Lagos Lagoon maintains a fairly constant volume of water throughout the year. During the rainy season, the lagoon is fed by the numerous coastal rivers draining into it, whereas during the dry season, the loss of water due to evaporation and the reduced amount of water from the rivers and creeks are compensated for by the underground seepage under the active sandy barrier formation and inflow of the tidal waters from the sea through the Lagos harbor and other lagoon outlets. Fashae and Onafeso (2011) have however suggested

that sea-level rise and ocean surges are likely impacts of climate change in Lagos, with possible coastal inundation and increased flooding as well as the intrusion of seawater into freshwater sources, but the effect of all these on the landscape has not been explored. The rest of the southwestern parts of the country receives rainfall similar to that of Lagos, including Ibadan (1335 mm), Abeokuta (1626 mm), Osogbo (1304 mm), Oyo (1312 mm), Ogbomoso (1300 mm), Ado-Ekiti (1329 mm) and Akure (1324 mm). In the southeast and south–south region, the total annual rainfall ranges around 2000 mm for example Enugu (1716 mm), Onitsha (1860 mm), Owerri (2248 mm), Okigwe (1986 mm), Aba (2390 mm), Abakaliki (1779 mm), Umuaihia (2122 mm), Arochuckwu (2462 mm) and Awka (1880 mm). The incidence of accelerated erosion in the southeastern part of the country has been linked to a high incidence of rainfall and the highly erodible soils of the area (Okorafor et al. 2018). Precipitation in the central parts of Nigeria, usually called the middle belt, is similar to its southern counterpart, especially the southeastern segment. Abuja has an average annual total rainfall of 1555 mm (Fig. 2.2c) and Gembu experiences

36

O. D. Onafeso

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 2.2 a Rainfall trend in Port Harcourt b Rainfall trend in Lagos c Rainfall trend in Abuja d Rainfall trend in Gembu e Rainfall trend in Sokoto f Rainfall trend in Maiduguri

1981 mm (Fig. 2.2d), reflecting orographic influence, similar to the Jos Plateau with 1429 mm. Around Kaduna and most of the Northern Guinea savanna as well as the Sudan savanna zones, total annual rainfall and the duration of the rainy season decline progressively, with Kano having an average of 120– 130 rainy days, and Katsina and Sokoto having about 10 −20 days less. Sokoto (Fig. 2.2e) experiences about 626 mm average annual rainfall, Kaduna (1201 mm), Kano (751 mm),

Minna (1124 mm), Ilorin (1268 mm), Gusau (933 mm), Birmin-Kebbi (757 mm), Gombe (504 mm), Bauchi (1029 mm), Yobe (807 mm), Damaturu (718 mm), Jalingo (1172 mm) and Yola (1078 mm). Maiduguri (Fig. 2.2f) with an average annual total rainfall of about 636 mm has been experiencing a significant increasing trend in rainfall of about 4–5 percent as indicated by Onafeso (2012) from GCM rainfall predictions under the A1, A2 and B1 SRES emission scenarios.

2

The Climate of Nigeria and Its Role in Landscape Modification

2.4

Temperature Profile

Odjugo (2010) observed the temporal and spatial variations in air temperature across Nigeria suggesting the trend showing persisting amplification since 1901. Although a slight decline was experienced from the late 1940s to the early 1950s, there has been a sharply marked intensification from the early 1970s in keeping with the global trends. The long-term mean air temperature in Nigeria is 26.7 °C, although this has shown intermittent differences with time (viz., 26.0 °C for 1901–1935 climatic periods; 26.5 °C for 1936–1970; 27.8 °C for 1971–2005; and 28 °C for 2006– 2018). According to Odjugo (2010), the decadal fluctuations in temperature suggested only 0.5 °C difference between 1901–1935 and 1936–1970, whereas about 1.3 °C between 1936–1970 and 1971–2005. He further suggested that from 1901 to 2005 (105 years), the temperature rose in Nigeria by 1.7 °C, whereas global temperatures have increased by 0.4– 0.8 °C with a mean of 0.74 °C since the record started in 1860. Also, Odjugo (2010) concluded that the temperature anomalies attest to global warming (IPCC 2007) and the climate change signal increased from the 1970s in Nigeria. Spatial distribution of temperature in Nigeria varies minimally, ranging from 25.5 °C in Port Harcourt to 28.2 °C in Nguru between 1901 and 1935, but between 1936 and 1970, observations showed Port Harcourt to have recorded 25.8 °C, while Nguru had 29.1 °C. This trend has been noted to have risen to 30.2 °C in Nguru and 26.7 °C in Port Harcourt from 1971 to around 2005 implying Port Harcourt and Nguru got hotter at about 1.2 and 2 °C, respectively, in the 105 years of consideration (Odjugo 2010). The temperature all over Nigeria is mostly high, with diurnal variations more evident than seasonal although relatively higher temperatures ensue in the dry season because precipitation eases wet season heats. Average highs and lows for Lagos are 31 °C and 23 °C in January and 28 °C and 23 °C in June. Although average temperatures vary little from coastal to inland areas, the latter, especially in the northeast, have greater extremes. There, temperatures reach as high as 44 °C before the onset of the rains or drop to as low as 6° C during an intrusion of cool air from the north from December to February.

2.5

Conclusion

The climate of Nigeria varies through three different regions such that the southern parts are largely equatorial, whereas the central region is a tropical savannah, while the northern regions of the country are almost entirely arid. The Köppen Climate Classification System suggests the southern equatorial region, which can also be taken as having a tropical

37

rainforest climate or the equatorial monsoon climate, is that of the “Am” in the southernmost coastal areas (e.g., Port Harcourt, Calabar, Brass and Bonny) and “Af” in the southern hinterlands (Lagos, Ibadan, Benin-City, Enugu and Onitsha). This type of tropical rainforest climate produces heavy rainfall, with a constant temperature range with little differentiation. Correspondingly, the central region of Nigeria considered to be tropical or rather tropical savanna climate is a tropical wet and dry climate. This region is considered in the Köppen classification as “Aw” and “As” in the south (e.g., Abuja, Ilorin, Lafia, Makurdi and Kaduna) and the north (e.g., Gombe, Bauchi, Kano, Jalingo), respectively. Heavy rainfall is experienced during the wet season, whereas the dry season comes with high recurrent temperatures due largely to the African trade winds blustering from the Sahara Desert in the north. Finally, in the north of Nigeria, the arid climate otherwise referred to as the Sahel climate or tropical dry climate is experienced, and according to the Köppen Climate Classification System, it is classified as “BSh”, which is a hot semi-arid climate, and “BSn”, which is semi-arid climate with frequent fog. This type of climate is typified by low rainfall, with Northern Nigeria experiencing less rainfall than any other area of the country. The rainy season is short, lasting only a few months, while the rest of the year is dry and hot. The geomorphological features and landscapes further described in this book all get their origins and modifications from the type of climatic condition prevailing in the particular region where they occur.

References Beckinsale R, Chorley R (1991) In: The history of the study of landforms. vol 3. (Routledge Revivals). London, Routledge Fashae OA, Onafeso OD (2011) Impact of climate change on sea level rise in Lagos, Nigeria. Int J Remote Sensing 32(24):9811−9819. https://doi.org/10.1080/01431161.2011.581709 Gbadegesin AS, Onafeso OD (2010) The physical basis of spatial organization in West Africa. In: Maitrise de I’espace et development en Afrique Igue OJ, Fodouop K, Aloko-N’Guessan J (eds) Collection Maitrise de I’ espace et development. Paris, Karthala, cop. 2010. vol 1. Chapter 1, 340 p, pp 19–42. ISBN 978-2-8111-02647 http://www. karthala.com/2203-maitrise-de-lespace-et-development-en-afriqueetat-des-lieux-9782811102647.html Intergovernmental Panel on Climate (IPCC) (2007) Climate change 2007: synthesis report. Summary for policy makers, available at http://www.ipcc-wg1-ucar.edu/wg1/wg1-report.htm. Accessed 10 August 2018 Lavee H, Imeson AC, Sarah P (1998) The impact of climate change on geomorphology and desertification along a mediterranean-arid transect. Land Degrad Dev 9(5):407–422 Mutz SG, Ehlers TA, Werner M, Lohmann G, Stepanek C, Li J (2018) Estimates of late Cenozoic climate change relevant to earth surface processes in tectonically active orogens. Earth Surf Dynam 6:271– 301. https://doi.org/10.5194/esurf-6-271-2018

38 Odjugo PAO (2010) Regional evidence of climate change in Nigeria. J Geography Regional Plann 3(6):142–150. http://www. academicjournals.org/JGRP Okorafor OO, Akinbile CO, Adeyemo AJ (2018) Determination of soils erodibility factor (K) for selected sites in Imo State Nigeria. Resour Environ 8(1):6–13. https://doi.org/10.5923/j.re.20180801.02 Onafeso OD (2012) Analysis of changes in rainfall pattern and runoff predictions for the lower River Niger, Nigeria. Unpublished PhD Thesis submitted to the Department of Geography, University of Ibadan, Ibadan Onafeso OD, Olusola AO (2015) Evapotranspiration modelling and ecogeomorphological classification of landscapes in Nigeria. In: Gbadegesin AS, Eze EB, Orimoogunje OOI, Fashae OA (eds) Frontiers in environmental research and sustainable development in the 21st Century. Ibadan University Press. 630 p. pp 265–278. ISBN 978-978-8456-92-6 Onafeso OD, Olusola AO (2018) Urban stone decay and sustainable built environment in the Niger River Basin. In: Thornbush M, Allen C (eds) Urban geomorphology: landform and processes in cities. Elsevier Science Publishing Co Inc., USA.—354 p, 1st edn.

O. D. Onafeso Chapter 13, pp 261–276. ISBN: 9780128119518. https://www. elsevier.com/books/urban-geomorphology/thornbush/978-0-12811951-8 Palmtag J, Cable S, Christiansen HH, Hugelius G, Kuhry P (2018) Landform partitioning and estimates of deep storage of soil organic matter in Zackenberg, Greenland. Cryosphere 12:1735–1744. https://doi.org/10.5194/tc-12-1735-2018 Twidale CR, Lageat Y (1994) Climatic geomorphology: a critique. Progress in Phys Geography: Earth and Environ 18(3):319–334. https://doi.org/10.1177/030913339401800302 Watkins JR (1967) The relationship between climate and the development of landforms in the cainozoic rocks of Queensland. J Geol Soc Aust 14(1):153–168. https://doi.org/10.1080/00167616708728651 Williams M (2015) Interaction between fluvial and eolian geomorphic systems and processes: examples from the Sahara and Australia. CATENA 134:4–13. https://doi.org/10.1016/j.catena.2014.09.015 Zhu B, Yu J, Rioual P, Gao Y, Zhang Y, Min L (2014) Geomorphoclimatic characteristics and landform information in the Ejina Basin, Northwest China. Environ Earth Sci. https://doi.org/10.1007/ s12665-014-3927-9

3

Vegetation and Human Impact Adeniyi Gbadegesin, Francis Adesina, Oluwagbenga Orimoogunje, and Folasade Oderinde

Abstract

3.1

Natural vegetation in Nigeria, as elsewhere, is the plant cover that develops with little or no influence or modification by humans and is dominated by native species. Some introduced species, like Gmelina arborea and Tectona grandis, have naturalized and become important components of the local vegetation varieties in the different parts of the country. The wide variation of environmental factors in the country has produced a range of vegetation types, with forests (mangrove forest and rainforest) in the south and savanna vegetation (Guinea, Sudan and Sahel savanna) in the middle and northern parts. Vegetation communities together with the country’s landforms influence considerably the nature, pattern and biophysical processes taking place in the country. Human interference and climate change are posing a serious challenge to the biophysical processes, and there is a need to continue green growth to mitigate human impact on the country’s landscape. Keywords








Vegetation Species Environmental factors Conservation Green economy




A. Gbadegesin Department of Geography, University of Ibadan, Ibadan, Nigeria F. Adesina
O. Orimoogunje (&) Department of Geography, Obafemi Awolowo University, Ile-Ife, Nigeria e-mail: [email protected] F. Adesina e-mail: [email protected] F. Oderinde Department of Geography, Tai Solarin University of Education, Ijebu-Ode, Nigeria

Vegetation and Its Varieties in Nigeria

Vegetation is an important ecological parameter of the physical environment. The term describes an assemblage of plants of one or several species growing in an area. Vegetation is often described based on its dominant plant growth forms or physiognomy, e.g. as woodland or grassland vegetation, or in terms of its location, e.g. desert vegetation, cliff vegetation, montane vegetation or by some generic descriptions of plant communities such as wetland or dryland vegetation types. In reality, vegetation is a product of the environment in which it is located. This is defined by factors such as climate, relief, soils and geology, human interferences as well as relationships among these parameters. Two main vegetation types derive from the effects of these factors in Nigeria: the tropical forest and savanna (Fig. 3.1). The tropical forest is broadly divided into the mangrove and freshwater swamp forests in the coastal areas. These forests transit into the lowland and dry forests in areas farther away from the coast. Generally, with increasing latitude northwards, increasing length of the dry season and declining amount of rainfall received, savanna vegetation types take over the landscape in fairly sequential order as Southern Guinea, Northern Guinea, Sudan and Sahel savannas (Keay 1953; White 1983). These varieties of plant communities play important roles in determining the nature and pattern of biophysical processes over Nigeria’s physical space. Vegetation resources are an important factor of livelihood, especially in rural areas.

3.2

Mangroves and Mangrove Landscapes

3.2.1 Distribution of the Mangroves The mangrove swamp forests occur in coastal wetlands where diurnal flooding by brackish water takes place. They are consequently dominated by halophytic, i.e. salt-loving

© Springer Nature Switzerland AG 2023 A. Faniran et al. (eds.), Landscapes and Landforms of Nigeria, World Geomorphological Landscapes, https://doi.org/10.1007/978-3-031-17972-3_3

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Fig. 3.1 Nigerian vegetation zonation

plants. Brackish water is more saline than freshwater, but not as much as seawater. Mangroves are common in estuaries, where fresh and saltwater meet and are characterized by their maze of woody growth up to the ground level. This vegetation type is found virtually throughout the country’s coastline but is most widely distributed in the Niger Delta, particularly between the Benin River in the west and part of the Rio del Rey estuary in the east. Here, the mangrove communities extend over 30 and 40 km from the coast inland. The second major delta sub-system along the coast is associated with the Cross River, which also has an extensive distribution of mangroves covering an area of 7–8 km wide on the two sides of the estuary and up to 26 km at the head of the estuary. Also, two large lagoons in Lagos and Lekki, which dominate the coastal systems in the western part of the country, used to be fringed by mangroves, followed in sequence by freshwater swamp forests inland.

3.2.2 Mangrove Landscapes Nigeria’s Niger Delta hosts the third largest mangrove ecosystem in the world, extending over an area of about 36,000 km2 (Nwilo 2004) and representing about one-tenth of Nigeria’s forest cover. The mangrove communities are widest in their distribution on the sides of the Niger Delta and narrow towards the centre to a width of about 15 km. Also, the channel of the Brass River, which is a part of the Delta System, has an extensive distribution of mangroves far upstream. This ecosystem houses a wide variety of flora and fauna (NDES 1997). It is characterized by acidic sulphate, silty clay, clay loam and peat, and the locally called “chikoko” soils predominate. The soils are saline and have an almost neutral pH when wet. When exposed, they dry up quickly and become acidic with a pH of 3. When bare areas are dredged in the mangrove landscapes of the Niger delta,

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as occurs commonly, their soils become more acidified. At a range of pH from 3.0 to 3.4, which characterizes the acidified dredged areas, the soils are generally too toxic for plant establishment (Abere and Ekeke 2011). As a result, the soils of the mangrove belt vary markedly within short distances. Also, at different depths, the soils vary significantly with respect to their clay mineralogy, total organic carbon content and carbon stock (Ferreira et al. 2010). Thus, overall, the soils of the mangrove belt are very complex (Hossain and Nuruddin 2016). In terms of flora and physiognomy, which are both factors and a reflection of the biological productivity of the mangroves, these plant communities are highly variable (UNESCO 1998). They also reflect temporal and spatial variations in the edaphic conditions in the mangroves, especially in terms of salinity and nutrient availability (Koch 1997; Fromard et al. 1998) (Fig. 3.2). As a result of the low salinity gradient and increasing salt tolerance up-river, freshwater plant species characterize the mangrove landscape in these locations (Ukpong 1991). The mangrove vegetation communities are dominated by the families of

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Rhizophoraceae (Rhizophora racemosa, R. harrisonii and R. mangle), Avicenniaceae (Avicennia africana) and Combretaceae (Laguncularia racemosa and Conocarpus erectus) (Fig. 3.3) (Tomlinson 2016). Of all the mangrove plant species, R. racemosa is the most abundant in the lagoons, and the delta. R. racemosa occurs as the pioneer at the edge of the swamps, while R. harrisonii dominates the middle of the zone and R. mangle is commonly found in the inner edge. Avicennia germinans occurs very sparingly. Species composition in the estuaries is often different and Nypa fruticans, an invasive species native to the coastlines of the Indian and Pacific Oceans, becomes bountiful. The mangroves seldom grow more than 10 and 12 m tall, but can occasionally exceed 40 m. The strand vegetation at the top of the beach, with C. erectus and other woody species that grow along swamp edges, is associated with the main mangrove formation, especially on the seaside. Nigeria’s mangrove landscape supports a large human population that virtually lives on natural resources offered by the mangrove ecosystem (NDES 1997; Polidoro et al. 2010). Several valuable species of animals are found in the

Fig. 3.2 Sandy-silt soil of the Mangrove near the Atlantic Ocean, Ikuru Town in Andoni LGA. Authors’ fieldwork 2018

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Fig. 3.3 Mangrove plants a Nypa fruticans (Nypa palm), b Laguncularia racemose (black mangroves), c Conocarpus erectus (buttonwood), d herb e Rhizophora racemose (red mangrove), f mangrove

associated fern, g Avicennia germinans (white mangrove), h Heritiera littoralis and i Acrostichum aureum (mangrove fern) (Numbere 2018)

mangroves and have been documented (e.g. Howell 1968; Child 1974; Milligan 1979). The major species associated with fringing forests are the red-flanked duiker, hippopotamus and manatee, which live in pools and rivers (Abere and Ekeke 2011).

mangroves into aquaculture and growing dominance of invasive species almost as soon as the mangrove trees are removed (Fig. 3.4). Hitherto, Nigeria’s mangrove forests were dubbed the least disturbed of the forests in the country because of the relative difficulty of accessing the terrain. Today, this is no longer the case. Extensive land development, particularly for oil and gas exploitation, has led to transformation and indeed, degradation of the mangrove vegetation. Several hundreds of square kilometers of mangrove forests have been removed as a result of oil and gas exploration (e.g. James et al. 2007; Abere and Ekeke 2011). The subtle role of the invasive N. fruticans (Nypa palm) in the degradation of mangrove formation in Nigeria deserves further elaboration. N. fruticans was introduced into the Niger Delta as a measure to curtail coastal erosion in 1906. It, however, turned invasive and is now a threat to the mangrove ecosystem (CEDA 1997; Keay et al. 1964).

3.2.3 Human Impacts on the Mangroves A large proportion of the natural mangrove cover of the world is already lost (Erwin 2009). The lost proportion may be as high as 50% in countries such as India, the Philippines and Vietnam, while in the Americas, they are being cleared at a rate, which is faster than in tropical rainforests (Gibbs et al. 2010). In Nigeria, the major factors responsible for the rapid disappearance of mangrove forests are the extraction of fuelwood and charcoal, expansion of human settlements, oil exploration, conversion of

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Fig. 3.4 Fuelwood extraction in the Mangrove, Ikuru, Andoni Local Government Area, Rivers State. Authors’ fieldwork 2019

The growing expansion of human settlements in the mangrove belt, both in the Lagos axis in the west and in the Niger Delta region, is another significant factor in the decimation of the mangroves (Okoye et al. 1991; Hayden and Granek 2015). The vegetation naturally gives way to population expansion. Waste deposition in the mangroves, which has been growing with increasing population, is also making the environment conducive to the growth of invasive N. fruticans, while the shallow root systems of the palms impact biodiversity, sediment distribution and navigation along the waterways (Udofia and Udo 2005). Mangrove landscapes are also prone to other natural and anthropogenic disturbances. Once disturbed, mangroves take a long time to rejuvenate (Burns et al. 1994). The most important ecological role of the mangroves along the coast is that they provide the first barrier against currents and strong waves. Consequently, when they are destroyed, waves and currents reaching the coast can undermine the fine sediment in which they grow (Alongi 2008). This prevents seedlings from taking root and results in washing away nutrients essential for mangrove

ecosystems and accelerating ocean erosion along the coastline. All these further impede the recovery of the mangroves. Furthermore, fertilizers, pesticides and other toxic man-made chemicals carried by river systems from sources upstream negatively affect animals living in mangrove forests, while oil pollution smoothers mangrove roots and suffocates the trees.

3.3

Rainforest and Rainforest Landscape

3.3.1 Distribution Tropical forests are the world’s richest and most complex terrestrial ecosystems (Richards 1996). The forests form an integral part of life on Earth, providing a wide range of benefits on a local, national and global scale. They cover approximately 31% of the world’s total landmass (FAO 2010). The forest in Nigeria covers a total land area of 360,000 km2. More than a quarter of this (27%) makes up the forest reserve of the country.

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3.3.2 Rainforest Landscape The Cross-Niger transitional forest and the montane forest ecoregion in mountain ranges dominate Nigeria’s forest landscape. This landscape extends from the Gulf of Guinea to the border between Cameroon and Nigeria (Blackburn et al. 2010). In Nigeria, the zone covers an area of about 39,000 km2 and stretches into western Cameroon (Moreau 1963). The area is comprised of a chain of extinct volcanoes and is located above 823 m asl (Mani 2013). The vegetation distribution varies with altitude: the lower elevations between 914.4 and 1829 m are covered by montane forests, while at higher elevations vegetation comprises grassland patches, shrublands and bamboo forests. The region is populated by some endemic fauna. Some of the endangered primate species in the area include Piliocolobus badius (red Columbus), Gorilla gorilla diehli (the Cross-River Gorilla), and Cercopithecus preussi (the Preuss’s monkey). Several species of reptiles and amphibians are native to the area. Gillespie et al. (2012) described a variety of dry forests as the “Cross-Niger Transitional Forest”, together with tropical moist forests whose distribution extends to Abia, Akwa Ibom, Anambra, Ebonyi and Imo states in eastern Nigeria. These formations cover 53,613 km2 in total. The climate is wet but turns out to be drier as one goes farther inland from the south. The dry season is brief and lasts from December to February. The area’s flora comprises transitional plant species from the Upper Guinean forests to the lower forests of the Guinea–Congo border. Noteworthy trees in the area are Afzelia and Borassus aethiopum. The region is devoid of large mammals. The dominant species are bats and frogs (Gillespie et al. 2012). Generally, the forest landscape has a special vegetative structure that is made up of many vertical layers including understory canopy, shrubs and ground layers. The dense ceiling of leaves and tree branches formed by closely spaced forest trees give a distinctive canopy. The upper canopy is about 30–50 m above the forest floor and is infiltrated by developing trees that make up the understory level. Multiple leaves and branch levels are located below the canopy ceiling and are jointly known as the understory. The lowest part of the understory is the shrub layer, 1.5–6 m above the forest floor. This is made up of shrubby plants and tree saplings. The upper canopy of the forest, which may be over 30 m above the ground, is comprised of converging leaves and branches of emergent trees. Many of the rainforest flora and fauna are associated with this canopy. The regular shade from the leaves of canopy trees implies that the rainforest floor is almost always humid and dark. In addition to blocking direct sunlight, the canopy also restricts shrub growth and reduces wind and rain. As a result, it promotes

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complex relationships and interchanges between organisms. The forest floors are important for the decomposition of litter from the forest, a process that is crucial to the continuity of the whole forest. The rainforest is also home to a wide range of animals including G. gorilla diehli (gorillas), Panthera tigris (tigers), Tapirus indicus (tapirs) and Loxodonta africana (elephants) and some others. Some important tree species found within this region include Irvingia gabonensis, Treculia africana, Pentaclethra macrophylla, Triplochiton scleroxylon, Pterygotama crocarpa, Milicia excelsa, Lophira alata, Entandrophragma spp., among others.

3.3.3 Human Impact The widely held belief that Nigeria’s forest landscape is a huge resource is largely true regarding the original vegetation cover. Today, the original plant communities have either been destroyed or severely degraded as a result of human activities within and outside the forest landscape (Fig. 3.5). Probably, 70–80% or more of Nigeria’s original forest has disappeared. The present coverage may just be about 12% (Alongi 2008). For example, between 1990 and 2005, Nigeria lost probably 35.7% of its forest cover or about 6,145,000 ha (FAO 2005). These losses imply that many plants and animals are on the verge of extinction. The USAID Report on Biodiversity and Tropical Forestry Assessment (2002) remarked long ago that there are too many environmental threats affecting biodiversity in Nigeria. This situation could only have worsened in the country. A National Assessment (NCF 2012) also reiterated the reality of the high-rise and fast-tracked increase in biodiversity loss in Nigeria. As earlier shown, the underlying factors affecting Nigeria’s forest landscape are legion and interconnected. One of these is high population growth. Nigeria is the most populous country in Africa, and its population is characterized by a high percentage of illiteracy, unemployment and poverty. All of these are powerful drivers of deforestation. The forest landscape resources had hitherto supported large rural and urban populations. However, the current pressure may already be approaching a tipping point (Orimoogunje 1999) as the consequences of the impact are growing in spread and severity. Natural regeneration is unable to match the high rate of exploitation, leading to a vicious decline in the quality of forest-based resources. Connected with this is the growing significance of exotic species such as Gmelina arborea and Tectona grandis which have naturalized in many areas. Associated with population growth is urbanization and the expansion of rural settlements. The development of new

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a. Sand Digging within the Forest Landscape

b. Human Settlement in the heart of the Forest

c. Clearing of a Portion of Oluwa Forest reserve

d. lumbering activities within the forest

Fig. 3.5 Key human activities within the forest landscape

roads and other physical infrastructure has continued to cut deep into existing forest lands, further reducing the resources extractable from the forests. The growing human population can thus potentially create irreversible damage to the environmental resources of the country. Estimates of species diversity and the number of individual trees in the forest estates are variable but, in general, raise concerns about the sustainability of the forests for wood supply. In the Ondo State, for example, an estimate of 111,377 timber stems belonging to 62 different indigenous hardwood species of the tropical rainforest ecosystem and are distributed among 16 families was made in 2005. In the Onigambari Forest Reserve, there were estimates of 308 and 776 trees per hectare (Oguntala 1981; Gbadegesin 1996; Akinyemi et al. 2002). The figures are expectedly different, but the general conclusion from each study points to a declining stock of woody plants in the forests. There is a need to pay greater attention to activities that can enhance the biodiversity and physiognomy of forests in the country. In this context, for instance, communities that can support forest regeneration and protection need to be enabled to do

this, and Nigeria’s REDD program offers an excellent platform to achieve this. Other factors influencing the transformation and degradation of the forest landscape include declining human resources for managing forest estates, especially in the Forestry Department, illegal activities of loggers, and stoppage of the payment of annual royalties from the proceeds of logging to rural communities, outdated forestry legislation and regulations. Quite clearly, the status of human resources available for forestry exerts influence on other factors highlighted here. Poor implementation of forestry laws and regulations is related to poor human resources in the forestry sector.

3.4

Savanna and Savanna Landscapes

3.4.1 Factors Controlling the Occurrence of the Savanna Savanna is a tropical vegetation type most distinguishable by the presence of a dominant grassy ground layer in all of its

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varieties. Thus, a typical savanna community may or may not contain woody elements but must have a layer of grasses. This attribute can be explored further, highlighting that deforestation in the forest environment, even where all the trees are removed, does not necessarily produce savanna units. Many forest units devoid of their trees are often dominated by forb species such as Chromolaena odorata (formerly Eupatorium odoratum) and Aspilia africana rather than grasses. In areas bordering dry rainforests and along with watercourses, tree elements in the savanna are usually conspicuous. This is observable, particularly where human disturbances have remained minimal. In such locations, the crowns of the trees usually form one distinct stratum, spreading widely, almost touching each other and giving a visual resemblance to the closed tropical forest communities in wetter locations (Hopkins 1965; Morgan and Moss 1965). Farther from the coast and into the drier environments, the significance of trees declines and grasses becomes more important in the composition of plant communities. Indeed, grasses often overwhelmingly dominate the landscape in places that have been heavily impacted by human activities. Savanna vegetation is divided into four types. These are the derived, Guinea, Sudan and Sahel savanna. The dominant grass species belong to the Andropogoneae, especially the genera Andropogon and Hyperrhenia, which are native to Africa. Some of the most frequent trees are Danielia oliveri, Parkia biglobosa, Ficus capensis, Hymenocardia acida and Vitellaria paradoxa (formerly Butrospermum parkii). These trees are some of the most widely distributed woody plants in Nigeria. Climate, particularly rainfall, is a key factor influencing the floristic and physiognomic distinctions as well as the variability of the savannas in Nigeria, as elsewhere in Africa (e.g. Knapp 1973; Cowling et al. 1994). The amount of rainfall and its annual distribution influence water availability, and these reflect markedly in the various stages of the development of vegetation communities (D’Onofrio et al. 2014). In areas, where savanna occurs as a belt, rainfall is normally concentrated within about three to six months of the year, followed by a rainless period, typified by high temperatures that on average could be well above 35 °C in the northern parts. The typical seasonal pattern of rainfall and temperature supports the development and establishment of the varieties of savanna vegetation. For example, if the amount of rainfall received in many of these areas is well spread throughout the year, the savanna would not develop properly and the vegetation could become some variety of the rainforest. Furthermore, dry season fires which are common in the savanna belt are in many ways a significant factor responsible for the variable character of the savanna (Adejuwon and Adesina 1992; Williams et al. 2003). Fire refreshes the grassy

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layer and is frequently used by herdsmen to generate fresh fodder for their animals in the region. The remnant tussocky roots of the grass which are not destroyed by fire make rapid sprouting of grasses possible after the burning. Also, the regularity of fires reduces the huge herbaceous fuel that accumulates during the dry season, thus making subsequent fires relatively less severe. Furthermore, fire inhibits the establishment of many tree species in the belt, while it encourages the development of others. The typical savanna trees have thick, fissured and sometimes flaking barks (Fig. 3.6) (e.g. Adejuwon and Adesina 1992; Lawes et al. 2011), which insulate the inner tissues of their stems and thus protect them from being damaged by the usually lethal fires. The soils also affect the distribution of savanna vegetation. Compared with those of the forests, soils of the savanna tend to be relatively nutrient-poor in many areas because of the low organic matter cover on the surface (Gbadegesin and Akinbola 1995). This is partly due to the annual fires which regularly remove litter that had accumulated on the soil surface before the onset of the wet season. Thus, during the rains, in which more decomposition of organic materials takes place, very little accumulated organic material is often available. The relatively poor nature of the soils of the

Fig. 3.6 A typical Savanna tree with fissured thick bark, Zuma 11, Bwari Area Council, Abuja. Authors’ fieldwork 2018

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savanna is a key explanation for the occurrence of many patches of savanna in the forest zones in western Nigeria. Such patches of savanna are found in areas around Ikire, Ile-Ife, Odeomu and Ede in the Osun State, South-Western Nigeria (Adejuwon 1970). The soils are poorly drained and contain low concentrations of nutrients. Nutrient concentrations are generally insufficient to support the development of forest communities. Following Clements’ theory of climaxes (Clements 1916), the zonally occurring savanna is a form of savanna climatic climaxes, while those occurring azonally as patches in areas where the substrate on which they develop is a major determinant are edaphic climaxes. The coastal savanna in western Nigeria (e.g. Adejuwon 1970), which Sowunmi (2004) described as a “gap” in the regional distribution of the rainforest along the coastal belt, is connected with the presence of soils with low nutrient concentrations. Rainfall intensity and distribution are thus insufficient for explaining the occurrence of savanna vegetation. The edaphic factors are also critical. Landforms influence the distribution of savanna vegetation. The larger regional expression of the distribution of savanna vegetation is found in the plains of central Nigeria. However, high ranges, which are found in many areas in the central part of the country, also moderate the continuity of the distribution. Apart from punctuating the distribution, areas on the windward sides of the high ranges tend to support more luxurious vegetation that sometimes approximates the forest community. This in part explains why the southern part of the Jos plateau extending towards the River Benue flood plain is covered by riparian woodland. Landforms form an important part of the savanna landscape in Nigeria. In many places, the landscape is dotted by granitic outcrops, inselbergs, tors and woolsacks (boulders), which have an important influence on the pattern of occurrence of the savanna. This can happen in two ways: landforms can influence the amount of rainfall received, but they can also affect the characteristics of the soil which, as shown, influence the occurrence of the savanna.

3.4.2 Savanna Landscape As already indicated, the distribution of savanna vegetation is defined broadly by climate at the regional scale and largely by the substrate on which it grows (edaphology) at the local level. The derived savanna occupies the transitional zone between the tropical rainforest and the Guinea savanna. It is characterized by an admixture of fire-tolerant and fire-tender trees. As its name implies, it is directly a derivative of human impacts at the forest/savanna boundary. Its persistence is a function of the strength of anthropic factors, especially slash

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and burn agriculture that engendered it (Adejuwon and Adesina 1992). If these factors intensify, the savanna is sustained and can expand into the forest zone. On the other hand, if these factors are weakened, the forest could re-capture more of the nearby savanna areas. The area of these dynamics is relatively narrow, fizzling out towards the core savanna belt where rainfall is dominantly the limiting factor. The Guinea savanna covers the larger portion of the country and can be divided into the southern and northern Guinea savanna. The southern Guinea savanna is a close neighbour of the dry forest in terms of physiognomy. Some of the tree species found in it, especially in wetter locations along river courses, such as M. excelsa, are typically dry forest trees. Such trees are also more commonly found where human interferences are minimal. In general, however, fire-tolerant trees are more prevalent. Some of the frequent tree species are Mangifera indica, P. biglobosa, V. paradoxa, Afzelia Africana and D. oliveri. Grass species such as Panicum kerstingii and Setaria pallide-fusca are also important, particularly in drier locations (Fig. 3.7). The Northern Guinea Savanna covers much of central Nigeria and is characterized by open woodland, usually made up of a blend of shorter trees and tall grasses. The total annual rainfall ranges between 1000 and 1250 mm, with 5 and 6 months of dry season, and the mean annual temperature is 27.3 °C. The trees characteristically have long tap-roots, with which they can reach deep-seated groundwater during the long dry season when surface water is usually not available. Among the prevalent tree species are Isoberlina spp., Terminalia spp. and Acacia spp. The grass cover is composed of genera such as Aristida, Pennisetum and Andropogon. The grasses are also able to survive dry season fires, having durable roots kept below the ground surface. As a result of human impact, vegetation is no longer in its natural state (Fig. 3.8). The Sudan savanna covers more than twenty-five percent of the country’s total land area. The vegetation zone is comprised of scattered trees in an open grassland. Intense cultivation, grazing by livestock and bush burning characteristic of the area produce lower vegetation cover, which may be completely devoid of trees in many locations. The dry season is longer, extending between 6 and 9 months. Some of the trees found here, usually smaller in stature, are V. paradoxa, Parkia filicoidea, Tamarindus indica, M. indica, Acacia albida, Adansonia digitta and Vitex doniana. The belt supports a wide range of annual crops and particularly grains such as Zea mays and Sorghum bicolor. It also supports livestock such as cattle, goats and sheep. A large part of the zone is free of tsetse flies, which makes it a suitable environment for the rearing and breeding of ruminant livestock (Fig. 3.9).

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Fig. 3.7 A typical Southern Guinea Savanna unit during the wet season, Ikere, Oyo State. Authors’ fieldwork 2018

Fig. 3.8 Savanna at the northern part of the Guinea Savanna during the dry season, Bwari, Abuja. Authors’ fieldwork 2018

The Sahel savanna is found largely in the areas near Lake Chad. It occupies about 18,130 km2 and represents a dry transitional stage between the savanna and the Sahara to the

north (Sinclair and Fryxell 1985; Nwaneke and Chude 2015). The dry season lasts for about nine months, and rainfall is sparse. Annual rainfall is commonly less than

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Fig. 3.9 A part of the Sudan Savanna belt during the rains, Rabah, Sokoto State. Author’s fieldwork 2018

700 mm. Vegetation is scattered, with very short grasses less than 1 m tall. The more luxuriant plant cover occurs in the proximity of Lake Chad. Crop failures are common because of the erratic nature of rains. Inhabitants are largely nomadic herdsmen. The common tree species include Acacia senegal, Grewia spp. and Adansonia digitata, together with short grasses such as Aristida spp. and Chloris spp.

3.4.3 The Human Impact on the Savanna Human impact on the vegetation is a key factor of savanna derivation and its stability, although it also supports the incidences of fires, which moderate the development of plant cover in the savanna (Adejuwon and Adesina 1992). The most important elements of human impact apart from the fires are tree felling for timber and fuelwood, farming and grazing. These present a contest for common resources— vegetation (wood and fodder) as well as land. A single most important factor putting pressure on wood resources is the production of charcoal. Many households still depend on fuelwood for domestic and other purposes, even in major urban areas and hence the pressure on the natural supply of wood remains high. There is also a high demand for Nigeria’s charcoal in the international markets, which is making

more production attractive to those involved in it despite its huge negative impacts (Worldatlas 2018). Nigeria exports hundreds of thousands of tonnes of charcoal to Europe and other parts of the world annually (Charcoal TFT Research 2015). The impact of fuelwood exploitation in the savanna is thus enormous and is growing as local and international demand increases. For charcoal production, tree harvesting on a given parcel of land can be total, since virtually any woody plant can go through the process of carbonization to give charcoal (Adesina et al. 2015). The impact of fuelwood exploitation on the vegetal cover is consequently probably far more severe than had been discussed in the literature. It is, for example, more devastating than the much talked about the expansion of cultivated areas due to the growing population. While farmers often retain some trees on their farms for various purposes, wood harvesters for charcoal usually remove all. Grazing, which is a major feature of the savanna, also affects the savanna biomass (Charles-Dominique et al. 2016). The presence of large numbers of grazing animals in Nigeria’s savanna has continued to generate conflicts between herdsmen and farmers. Herdsmen normally graze their animals on unfarmed or abandoned farmlands. The farms become an “alternative” when the usual grazing lands do not provide enough fodder for the animals. Human

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impact is thus not only the key in the determination of the physiognomy of savanna vegetation, especially in the drier margins of the forests, but also in maintaining it once established. The variable floristic and physiognomic characteristics of the savanna are influenced by the current climate, edaphic conditions and human activities. This is why the savanna landscape remains highly dynamic in every respect. Climate change is adding some dimensions to the state of the vegetation. Given projections of possible wetter conditions in Nigeria’s Guinea savanna, in particular, some positive modifications to the current landscape may be expected. However, a drier savanna will exacerbate the dynamics and make the contest for resources more severe (Figs. 3.10 and 3.11).

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Fig. 3.10 Animals grazing in Rarah, Rabah Local Government Area, Sokoto State. Authors’ fieldwork 2018

Conclusion

The vegetation landscape in Nigeria is naturally controlled by climate, soil and topography. The influence of these three parameters is obvious right from the mangrove environment of the brackish water-flooded terrain of the coast to the dry Sahel savanna environment of the far northern part of the country. The amount and distribution of rainfall are usually the most important climatic determinants since temperature, which provides the crucial energy for plant growth and development, is generally high throughout the year. The

Fig. 3.11 Fuelwood collection in Rabah, Rabah Local Government area, Sokoto State. Authors’ fieldwork 2018

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Vegetation and Human Impact

latitudinal distribution of rainfall is thus responsible for the broad regional expression of the occurrence of vegetation types, while edaphology and topography create local variability. Growing human populations and the associated demands on the environment such as livestock rearing, development of transport infrastructure and expansion of settlements, are making anthropic factors of overwhelming significance in the observable spatial patterns of various vegetation types. This has to be addressed to optimize the benefits, which the natural vegetation landscapes offer. This is important to meet the needs of the growing population in the country. Restoration of damaged sites can be strengthened through greater attention to vegetation enrichment practices in the form of afforestation and reforestation. The development of alternative resources to those derived from the forests, e.g. introduction of cooking gas to replace fuel wood, will help to reduce the contemporary pressure on vegetation communities which will help their restoration.

References Abere SA, Ekeke BA (2011) The Nigerian mangrove and wildlife development. In: 1st International technology, education and environment conference, p 320 Adejuwon JO (1970) The ecological status of coastal savannas in Nigeria. J Trop Geogr 30:1–10 Adejuwon JO, Adesina FA (1992) The nature and dynamics of forest-savanna boundary in Southwestern Nigeria. In: Furley PA, Procter J, Ratter JA (eds) Nature and dynamics of forest-savanna boundaries. Chapman and Hall, London, pp 331–352 Adesina FA, Odekunle TO, Ali H (2015) Charcoal production and threat to savanna in Western Nigeria. An unpublished report Akinyemi OD, Ugbogu OA, Adedokun D, Sefiu H, Odewo TK (2002) A floristic study of Onigambari lowland rainforest reserve. In: FAN Conference proceeding, pp 346–357 Alongi DM (2008) Mangrove forests: resilience, protection from tsunamis, and responses to global climate change. Estuar Coast Shelf Sci 76(1):1–13 Blackburn DC, Gvoždík V, Leaché AD (2010) A new squeaker frog (Arthroleptidae: Arthroleptis) from the mountains of Cameroon and Nigeria. Herpetologica 66(3):335–348 Burns KA, Garrity SD, Jorrisen D, MacPherson J, Stoelting M et al (1994) The Galeta oil spill; II, unexpected persistence of oil trapped in mangrove sediments. Estuar Coast Shell Sci 38:349–364. https:// doi.org/10.1006/ecss.1994.1025 CEDA (1997) Coastal profile of Nigeria. Federal Environmental Protection Agency, Press, Abuja, Nigeria Charles-Dominique T, Davies TJ, Hempson GP, Bezeng BS, Daru BH, Kabongo RM, Bond WJ (2016) Spiny plants, mammal browsers and the origin of African savannas. Proc Natl Acad Sci 113(38): E5572–E5579 Child GS (1974) An ecological survey of Borgu game reserve: technical report no. 4. FI: SF/NIR 24 FAO/UN, Rome Clements FE (1916) Plant succession: an analysis of the development of vegetation (No. 242). Carnegie Institution of Washington Cowling RM, Esler KJ, Midgley GF, Honig MA (1994) Plant functional diversity, species diversity and climate in arid and semi-arid southern Africa. J Arid Environ 27:141–158

51 D’Onofrio D, Baudena M, D’Andrea F, Rietkerk M, Provenzale A (2014) Tree-grass competition for soil water in arid and semiarid savannas: the role of rainfall intermittency. Water Resour Res 51:169–184 Erwin KL (2009) Wetlands and global climate change: the role of wetland restoration in a changing world. Wetlands Ecol Manage 17 (1):71 FAO (2005) State of the world’s forests. FAO, Rome FAO (2010) Timber harvesting and the problem of deforestation. For Harvesting Bull 4(1):1–3 Ferreira TO, Otero XL, de Souza VS Jr, Vi dal-Torrado P, Macias F, Firme LP (2010) Spatial patterns of soil attributes and components in a mangrove system in Southeast Brazil (Sao Paulo). J Soils Sedim 10:995–1006 Fromard F, Puig H, Mougin E, Marty G, Betoulle JL, Cadamuro C (1998) Structure, above-ground biomass and dynamics of mangrove ecosystems: new data from French Guiana. Oecologia 115:39–53 Gbadegesin A (1996) Management of forest resources by women: a case study from the Olokemeji forest reserve area, southwestern Nigeria. Environ Conserv 23(2):115–119 Gbadegesin A, Akinbola GE (1995) Nigeria: reference soil of the southern Guinea Savanna of South Western Nigeria. Soil brief Nigeria 7. University of Ibadan and International Soil Reference and Information Centre, Wagenigen, p 13 Gibbs HK, Ruesch AS, Achard F, Clayton MK, Holmgren P, Ramankutty N, Foley JA (2010) Tropical forests were the primary sources of new agricultural land in the 1980s and 1990s. Proc Natl Acad Sci 107(38):16732–16737 Gillespie TW, Lipkin B, Sullivan L, Benowitz DR, Pau S, Keppel G (2012) The rarest and least protected forests in biodiversity hotspots. Biodivers Conserv 21(14):3597–3611 Hayden HL, Granek EF (2015) Coastal sediment elevation change following anthropogenic mangrove clearing. Estuar Coast Shelf Sci 165:70–74. https://doi.org/10.1016/j.ecss.2015.09.004 Hopkins B (1965) Vegetation of the Olokemeji forest reserve, Nigeria: 111. The microclimates with special reference to their seasonal changes. J Ecol 53(1):125–138 Hossain MD, Nuruddin AA (2016) Soil and mangrove: a review. J Environ Sci Technol 9(2):198 Howell C (1968) Omo research expedition. Nature 219(5154):567 James GK, Adegoke JO, Saba E, Nwilo P, Akinyede J (2007) Satellite-based assessment of the extent and changes in the mangrove ecosystem of the Niger delta. Mar Geodesy 30:249– 267. https://doi.org/10.1080/01490410701438224 Keay RWJ (1953) An outline of Nigerian vegetation, 3rd edn. Government Printer, Lagos Keay RWJ, Onochie CFA, Standfield DP (1964) Nigerian trees. Federal Department of Forestry Research, National Press Limited, Ibadan, Nigeria Knapp R (1973) Die vegetation von Afrika. Gustav Fisher Verlag, Stuttgart Koch MS (1997) Rhizophora mangle L. seedling development into the sapling stage across resource and stress gradients in subtropical Florida. Biotropica 29:427–439 Lawes MJ, Adie H, Russell-Smith J, Murphy B, Midgley JJ (2011) How do small savanna trees avoid stem mortality by fire? The roles of stem diameter, height and bark thickness. Ecosphere 2(4) Mani MS (2013) Ecology and biogeography of high-altitude insects, vol 4. Springer Science & Business Media Milligan K (1979) Counting animals from the air. Field Notes 3 Moreau RE (1963) Vicissitudes of the African biomes in the late Pleistocene. In: Proceedings of the Zoological Society of London, vol 141, no 2. Blackwell Publishing Ltd., Oxford, UK, pp 395– 421

52 Morgan WB, Moss RP (1965) Savanna and forest in western Nigeria. Afr J Int Afr Inst 35(3):286–293 NCF (2012) A national curriculum framework for all. https:// curriculum.gov.mt/en/n Niger Delta Environmental Survey (NDES) (1997) Final report phase. 1. Environmental and socio-economic characteristics. Environmental Resource Managers Limited Numbere AO (2018) Mangrove species distribution and composition, adaptive strategies and ecosystem services in the Niger river, Delta, Nigeria. Mangrove Ecosyst Ecol Funct 17 Nwaneke PK, Chude VO (2015) Socioeconomic impact of climate change in Northern Nigeria. In: Proceedings of the 39th annual conference of the soil science society of Nigeria Nwilo PC (2004) GIS applications in costal management: a view from the developing world. In: Bartlett et Smith (ed) GIS for coastal zone management. CRC Press, Londres, pp 181–194 Oguntala (1981) The dynamics of tree population in Gambari forest reserve. Niger J For II(I):59 Okoye BCO, Afolabi AO, Ajao AE (1991) Heavy metals in the Lagos lagoon sediments. Int J Environ Stud 37:35–41 Orimoogunje OOI (1999) An assessment of management strategy of forest reserves in Osun state, Nigeria. Unpublished MSc thesis, Department of Geography, Obafemi Awolowo University, Ile-Ife Polidoro BA, Carpenter KE, Collins L, Duke NC, Ellison AM et al (2010) The loss of species: mangrove extinction risk and geographic areas of global concern. PLoS ONE 5:e10095–e10095. https://doi. org/10.1371/journal.pone.0010095 Richards PW (1996) The tropical rain forest: an ecological study, 2nd edn. Cambridge University Press, Cambridge, UK, 575pp

A. Gbadegesin et al. Sinclair ARE, Fryxell JM (1985) The Sahel of Africa: ecology of a disaster. Can J Zool 63:987–994 Sowunmi MA (2004) Aspects of Nigerian coastal vegetation in the Holocene: some recent insights. In: Battarbee RW, Gasse F, Stickeley CE (eds) Past climate variability through Europe and Africa. Developments in paleo-environmental research, vol 6. Springer, Dordrecht TFT (2015) C. T. research [online] Tomlinson PB (2016) The botany of mangroves. Cambridge University Press Udofia SI, Udo ES (2005) Local knowledge of utilization of Nipa palm (Nypa fruticans, Wurmb), in the coastal area of Akwa Ibom, Nigeria. Global J Agric Sci 4:33–40. https://doi.org/10.4314/gjass. v4il.2252 Ukpong IE (1991) The performance and distribution of species along soil salinity gradients of mangrove swamps in southeastern Nigeria. Plant Ecol 95:63–70 UNESCO (1998) CARICOMP-Caribbean coral reef, seagrass and mangrove sites. Coastal USAID (2002) Report on biodiversity and tropical forestry assessment White F (1983) The vegetation of Africa. A descriptive memoir to accompany the UNESCO/AETFATmSO vegetation map of Africa. Natural resources report XX, UNESCO, Paris Williams RJ, Woinarski JCZ, Andersen A (2003) Fire experiments in Northern Australia contributions to ecological understanding and biodiversity conservation in tropical savannas. Int J Wildland Fire 12:391–402 Worldatlas (2018) https://www.worldatlas.com/articles/best-of-2018. html

4

Lower Plains of Northern Nigeria Tasi’u Yalwa Rilwanu

Abstract

4.1

Northern Nigerian lower plains are extensive areas of lower topography situated within the relief region that comprises plains, basins and isolated hills. The region is composed of Sokoto Plain which acted as a watershed for most of the rivers in the sub-region. Among the lower plains of Northern Nigeria, areas around Kano, Kaduna and Zaria are higher than the rest of the region in terms of relief and landscape features, Maisaje hill is the highest at 1593 m above mean sea level. Riruwai Ring Complex with its old tin mining ponds serves as a source to the major rivers in the north-western part of the plain. To the north-east, the lower plain formed features such as sand dunes and isolated hills around Jahun, Katangare, Gudumbali, Bulatura and Lantewa Dunefields. Some of the dunefields are blessed with potash deposits. The major landscape features of the region include inselbergs, lake, dams, rivers, metasediments, sand dunes and isolated hills among others. In the north-east, Gwoza Hills an extension of the Cameroun Mountains appears to be the highest point of the sub-region surrounded by plains, basins, wetlands and the shrinking Lake Chad. The physical nature of lower plains and available water bodies provide a good environment for effective agriculture, fishing and mining among others. For that, it is important to identify, study and describe lower plain regions of Northern Nigeria for understanding the display and interaction between drainage sources, landscape, landform units and development processes. Keywords

Lower plains




Inselbergs




Dunes




Northern Nigeria

T. Y. Rilwanu (&) Department of Geography, Bayero University Kano P.M.B, Kano, 3011, Nigeria e-mail: [email protected]

Introduction

Plains are extensive, nearly level stretches of land that have no pronounced changes in topography. They are mostly lower than the surrounding lands; they can be situated along the coast or inland. Lower plains are associated with depressions or basins. The lower plain regions in Northern Nigeria include the Sokoto Plain, the plain of Central Northern Nigeria, the plain of the Sand Dunes Belt, the Chad Basin and some other pockets of the lowlands and depressions all over the region. The lower plains of Northern Nigeria are surrounded by higher areas such as the Jos Plateau, the Biu Plateau, the Mandara Mountains and the Adamawa Bamenda Highlands among others (Fig. 4.1). The plains which are the dominant features in this part of Northern Nigeria have resulted from alternating denudational and aggradational processes. The African denudational land surface of the early Cenozoic Age is the main landform occurring in the Plains of Hausaland. These wide plains are dissected by mature valleys, an example of which is the Kaduna river valley. They belong to the African denudational surface at a height of 600–730 m above sea level, although there are fragments of post-African and Cenozoic surfaces as well. Above the plains rise inselbergs and castle kopjes which may attain a height of over 300 m. The aggradational land surfaces in the country are usually found in the areas bordering the denudational land surfaces, and they comprise most of the remaining plains. The Sokoto Plain belongs to the Northern Nigerian lower plains and attracts a considerable population due to easy access to water, resources and other socio-economic activities of post-African aggradational surfaces of late Cenozoic Age and is composed of sedimentary rocks of Cretaceous to Tertiary Age, mainly sandstones, shales and sands which lie at about 240–300 m above sea level. East of the Jos Plateau lies the hills and plains of Kerri–Kerri and Gombe, which are underlain by sedimentary rocks of Cenozoic Age in the west and of Cretaceous Age in the east, mainly sandstones and

© Springer Nature Switzerland AG 2023 A. Faniran et al. (eds.), Landscapes and Landforms of Nigeria, World Geomorphological Landscapes, https://doi.org/10.1007/978-3-031-17972-3_4

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Fig. 4.1 Lower plains of Northern Nigeria. Source Cartography Lab Geography Department, Bayero University, Kano, 2019

shales. The higher elevations reach over 750 m above sea level, but these volcanic plugs composed of phonolite and basalt result from Cenozoic volcanism. The plains merge northwards with the Chad Basin, which is underlain by young sedimentary rocks of the Quaternary Age (See Fig. 4.2). The wide featureless plains are interrupted in their north-eastern sector by inactive dunes that have been colonised by vegetation. These plains are dissected by a centripetal pattern of river valleys. This chapter specifically describes the landscape of lower plain regions of Northern Nigeria and associated landscape features.

4.2

Lower Plains of the Sokoto Basin

The lower plains of the Sokoto region serve as a watershed for the major drainage systems in the area. River Rima and its tributaries originate from the dissected plateau of the western Rima Basin that drains this plain through TalatarMafara, Gwandu, Sokoto and Kebbi towns up to River Niger. The region is characterised by discontinuous plains from which rise steep-sided granite, gneiss and quartzite hills (Udo 1970). Around parts of Anka town and Ka river; the

landscape has developed upon an extensive belt of phyllites. There are also extensive plains of sand or wind-blown materials which were believed to have been derived locally. The area also extends to the southern and south-eastern parts of Birnin-Kebbi City (Fig. 4.3). Around the Rima river, up to where the Niger State shares a border with the Kebbi State, there are good examples of such sandplains (Fig. 4.4). The average altitude in the area is around 400 m, with a few exceptions with up to 700 m in some of the hilly areas (Silviconsult 1992). Zamfara areas around the towns of Maru, Talatar-Mafara and Moriki and the north-western parts of the state around Gusau are characterised by granitic highlands of low heights rising above the plain. The relief of such landscapes is usually between 244 and 366 m (Thomas 1995). The landscape of the Sokoto Plain depends on the type of the underlying rock. It displays a monotonous spread of minor irregularities commonly found in areas of thick weathering mantle. The lower plains of Sokoto are categorised into two types: the Gundimi-Rima-Illo Plains, characterised by deposits of softer sedimentary formations which can easily be worn by agents of denudation, and the Gwandu Plains believed to have developed from younger rocks of the Eocene to post-Eocene periods and dominated

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Fig. 4.2 Geomorphological and geological regions of Nigeria. Source Adapted from Dada et al. (2006)

by sandstone deposits, hence referred to as a plateau of sandstones (Fig. 4.5). In the lower plains of the Sokoto Basin, the predominant land use includes limestone quarrying and gold mining, farming and cattle rearing, among others. Remarkable examples of sandstone plateaus are commonly found around Sokoto-Rima, stretching through ridges, escarpments and spurs. Other features include cliffs, the examples of which are the common rugged surfaces around Goronyo (Udo 1970). Other typical landscape features of these areas include hills of metasediments formed out of sedimentary rocks which form a continuous chain of hills that starts near Gusau and extends up to Dogon-Dawa (an extensive forest along Gusau−Sokoto road) where metasediments are also common. A typical example can be seen at a turn along Gusau−Sokoto road called KwanarDogonKarfe (Dogonkarfe Junction) (Fig. 4.6). Another remarkable landscape feature in this area is the confluence point of the Rima and Sokoto rivers at Asare in Sokoto (Fig. 4.7). The River Rima originates from the Adar Plateau

around Maudou district and Birnin Kornni City in the Niger Republic, while the Sokoto river drains from the hilly areas around Talatar-Mafara and KotarKoshi towns in the Zamfara State. The surroundings of KotarKoshi town are another typical plain area surrounded by a well-known gigantic granitic hill range called KotarKoshi Hills (Fig. 4.8).

4.3

Lower Plains in North-Central Northern Nigeria

These are plain areas around most of Kaduna State, parts of Kano State and a small portion of Jigawa State. These are parts of the lower plains of Northern Nigeria that are higher than the rest of the plains in terms of elevation and landform features. The major landscape features in the region include residual hills, made up of lateritic materials like Dala, Gwauron-Dutse, Fanisau and Magwan Hills in Kano City (Fig. 4.9) and Ruwares in different localities within the region. The plain region is also characterised by ring

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Fig. 4.3 Typical wind-blown plain in South-eastern Birnin-Kebbi City. Source Retrieved from Google Earth, January, 2022

Fig. 4.4 Landscape of the south and south-eastern parts of the Rima river

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Fig. 4.5 Typical example of an extensive sandstone plateau around Gwandu town in Sokoto. Source Retrieved from Google Earth, January, 2022

Fig. 4.6 Typical sandstone hill commonly found at KwanarDogonKarfe along Gusau—Sokoto road. Source Retrieved from Google Earth, January, 2022

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Fig. 4.7 Confluence point of Sokoto and Rima rivers at Asare along Sokoto—Wammako road. Source Retrieved from Google Earth, January, 2022

Fig. 4.8 Plain landscape with granitic hills around KotarKoshi town, Zamfara State. Source Retrieved from Google Earth, January, 2022

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Fig. 4.9 Dala and Gorondutse Lateritic Hills in Kano City. Source Retrieved from Google Earth, January, 2022

complex hills of younger granite around Riruwai town in the southern part of Kano State, and the area is characterised by old Tin mining pits like green water and Kwalwa ponds (Fig. 4.10), which extend up to the Jos Plateau through Ningi-Burra Complexes in Bauchi State (Fig. 4.11). The highest point in the entire region is Maisaje Hill with 1593 m above mean sea level (Fig. 4.12). The plain region is also characterised by numerous woolsacks, tors and balanced rocks around Birnin Kudu town (Fig. 4.13). The major rivers that drain this plain are River Challwa, River Kano and their tributaries. The River Kano rises from the top of Jos Plateau foothill around Riruwai in Doguwa Local Government Area, Kano State. Parts of Kazaure town (Fig. 4.14), Amaryawa, Roni and Yankwashi towns in Jigawa State are also characterised by the presence of a plain with numerous hills built of metamorphic schist that rise to 620 m above the surrounding plains in some places, particularly in Kazaure. The hills and undulations commonly found in the Basement Complex areas of the Sahel in Nigeria are probably the results of the intrusion of older granites into the Basement Complex, which have undergone a long period of denudation (Buchanan 1955). Most of the extensive land within this plain is dominated by agricultural, commercial and industrial activities. The areas around Kaduna represent a plain landscape with a morphology that is characterised by inselbergs, gently

undulating plains and small river valleys associated with a bedrock of Proterozoic Age that belongs to the older granitic rocks (Bennett et al. 1977). These plains consist of gently undulating land broken by scattered inselbergs and shallow valleys draining into River Kaduna (Fig. 4.15). In another study conducted around the Kaduna Plain, Hartmann, et al. (2014) identified seven topographical units as inselbergs, upper pediments, lower pediments, river valleys, floodplains, lower plains and upper plains with elevations of 800, 670, 640, 600, 550, 640 and 670 m above mean sea level, respectively. In the southern parts of the Kaduna State, the area is dominated by plain land and the major landform features include granitic hills, rivers and streams. In Southern Kaduna, the area is surrounded by hills, valleys and plains which beautify the area and attract visitors (Omon 2014). There is also the Zaria Plain (Fig. 4.16), which comprises areas surrounding the Zaria City from the west, around Shika and Giwa towns, which includes plain land with granitic outcrops and inselbergs. In the north, it comprises parts of Makarfi and Hunkuyi towns among others, with the vast plain lands with undulations of granitic outcrops that extend up to parts of Rogo town in the Kano State. In the south-eastern parts of the Kaduna and Zaria Plains lie the towns of Soba, Kudan, Kauru, Ikara and Maigana, situated on plain topography characterised by rock outcrops, inselbergs, castle kopjes, streams and river valleys. The plain

60

Fig. 4.10 Riruwai ring complex. Source Retrieved from Google Earth, January, 2022

Fig. 4.11 Ningi-Burra complexes part of Bauchi State. Source Retrieved from Google Earth, January, 2022

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Fig. 4.12 Riruwai ring complex showing the highest point and waterfall. Source Retrieved from Google Earth, February, 2022

Fig. 4.13 Landscape features around Birnin Kudu town, Jigawa Stat. Source Retrieved from Google Earth, January, 2022

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Fig. 4.14 Plain around Kazaure town surrounded by hills of metamorphic schist. Source Retrieved from Google Earth, January, 2022

Fig. 4.15 Plains and rivers around Kaduna City, indicating River Kaduna and vast plain used for agriculture, both rain-fed and based on irrigation. Source Retrieved from Google Earth, January, 2022

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Fig. 4.16 Section of Zaria Plain with rivers, streams and hills. Source Retrieved from Google Earth, January, 2022

area is under intensive agriculture; it is the largest producer of cereals in Kaduna State. The Jos Plateau rises steeply above the surrounding areas of this vast plain to an average height of about 1300 m. In the north-east, the plain starts from where the Sokoto Basin rises and it ends where it grades into the Lake Chad Basin; the area is characterised by remarkably lower elevations, level terrain and sandy soils. To the north-west, the plain descends into the Sokoto lowlands.

4.4

Lower Plains of Sand Dune Belt of North-eastern Nigeria

The major landscape features of the lower plains around these areas comprise dunes and residual hills around Jahun town in Jigawa State (Fig. 4.17). The Jahun dunes were believed to have originated from sands deposited by marine regressions and transgressions during the Tertiary period (Eocene to Paleocene Ages). It is also believed that the dunes were shaped and reshaped by north-east trade winds (tropical continental air mass or Westerlies). The sand dunes are running in NE–SW and are currently static. They are between 12 to 20 metres in height. The name Jigawa was derived from Hausa language meaning “sand dunes”. As one is coming from Dutse town to Jahun town, the road passes

across the dunes through Shuwarin, Andaza, Jigawar-Kurma and Kiyawa towns and Katika village where the dunes ended a few kilometres after Jahun town the Headquarter of Jahun Local Government Area. The dunes are separated by depressions that are rich in shallow groundwater. Likewise in part of North-eastern Northern Nigeria, there is the Manga Plain, which is an extension of sand dune plains that started from Jahun town in the Jigawa State. This plain is composed of alluvial and Aeolian sand deposits overlying the Basement Complex and younger granite pediments up to the hill range that extends north into part of the Niger Republic. To the south-east of these plains, there is also the Yobe river complex which starts from Kano and extends up to Lake Chad in the eastern part of Northern Nigeria. The riverine area is composed of deltas and alluvial plains that are an imprint of the fast fluvial processes believed to have been replaced by wind drift and Aeolian activities The area also displays a kind of flooded remnants of longitudinal sand dunes bordering the Yobe river floodplain that are called the Lantewa Dunefield (Fig. 4.18). It can be seen further that in the region, there is a lower plain area called Bulatura, situated in the Yusufari Local Government Area of the Yobe State (Fig. 4.19), which has a series of swampy valleys separated by the beautiful scenery of sand dunes. The valleys contain rich deposits of potash which serves as an important mineral resource to the people

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Fig. 4.17 Longitudinal sand dunes along Dutse-Jahun road separated by depressions or valleys. Source Retrieved from Google Earth, January, 2022

Fig. 4.18 Lantewa Dunefield along Damaturo-Gashua road, Yobe State. Source Retrieved from Google Earth, January, 2022

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Fig. 4.19 Bulatura sand dune area situated in the Yusufari Local Government Area of the Yobe State. Source Retrieved from Google Earth, January, 2022

of the region and beyond (Michele 2005). The plain extends up to the Gwoudmoni region in the Niger Republic which is characterised by Oasis and potash deposits and mines as well.

4.5

Lower Plain of the Chad Basin

In some parts of the Borno State around Gwoza town, characteristic landscape features include boulders, bedrock hills and inselbergs developed upon the Basement Complex. The Gwoza Hills along the Cameroon border in the Borno State (Figs. 4.20, 4.21 and 4.22) are part of the larger granite chains. The terrain of the Gwoza area is made up of rocky and hilly surfaces. The Gwoza Hills are a continuation of the Mandara Mountains, starting from Pulka town with an average altitude of 1300 m above sea level (Szentes 2009). The area is suitable for agriculture, hunting and fishing, among other economic activities. Part of the sedimentary lower plains of the Chad Basin is an area with wind-blown materials, especially in areas like Jigawa. Here Aeolian deposits overlie younger sediments of the Chad Formation. Desert sands are transported by fierce harmattan winds from the Sahara Desert and deposited to form sand dunes of different shapes in the Borno State and

other northern districts (Iloeje 1982). Landforms of the extreme eastern parts of Kano and some parts of Bauchi State are built from the sediments of the Chad Formation and belong to the sedimentary lower plain of the Chad Basin. Other landscape features in this area include fossil dunes and ridge dunes that are separated by shallow depressions, usually filled up with water in the rainy period of the year. In addition to these landform features, there are mesas and buttes built of residual lateritic materials around Limawa, Katangare, Dutse, Galadimawa, Zai and Iyaka towns. In North-eastern Nigeria, i.e. around the Lake Chad Basin, the landscape is made up of longitudinal dunes at Lantewa Dunefield, which are replaced by transverse north-western dunes of the Gudumbali Dunefield. In terms of height, the dunes rise as much as 15 m above the surrounding plains. Around the northern parts of Munguno town, there are a series of dunes with flat clay surfaces, and between the Lantewa and Gudumbali Dunefields, there are sand ridges called the Bama ridges, up to about 12 m above the surrounding areas. These ridges and sand dunes extend to Geidam through Maiduguri and Bama towns up to the Cameroon border. East of the sand plains and dunefields of the Chad Basin are the Yedseram and El Beid rivers, which flow in northerly direction from the Mandara Mountains to the Lake Chad (Bawden 1972). The Lake Chad area displays

66 Fig. 4.20 Section of Gwoza Hills in Borno State is overlain by shrubs on mountain tops and tall grasses in the valleys between hills. Source Jeremiah and Falaju (2018)

Fig. 4.21 Boulders made of older granites in part of Gwoza town in Borno State. Source Jeremiah and Falaju (2018)

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Fig. 4.22 Section of Gwoza town in the Borno State showing valleys, plains and hills. Source Retrieved from Google Earth, January, 2022

an attractively looking landscape with monotonous silty sand surfaces and tinted grasses (Fig. 4.23). The lake is in a constant state of shrinkage due to a lot of factors including climate change and anthropogenic activities among others. It

has been reported by many researchers that the lake is reducing in size due to siltation and other factors like climate change (Jeremiah and Falaju 2018). Vegetation species commonly found around the area include acacia species,

Fig. 4.23 Part of Lake Chad and its surroundings showing fine sand deposit and wetland vegetation. Source Jeremiah and Falaju (2018)

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Fig. 4.24 Shrinking Lake Chad and its feeding rivers. Source Retrieved from Google Earth, January, 2022

African myrrh, desert date, palms, reeds, Papyrus, ambatch and baobab, among others. The major economic activities in the area are fishing, farming and rearing of animals. The major river that contributes to Lake Chad is River Hadejia with its tributaries like Gana, Yedserem and Yobe rivers (Fig. 4.24).

4.6

Conclusion

Plains in Nigeria are of two types and are described as higher plains and lower plains. The Northern Nigerian lower plains fall under different categories, including those on the Basement Complex and sedimentary formations. Although the chapter focused on the lower plains, it is noteworthy to say that the plains extend into parts of the Kaduna and Zaria cities, north-eastward across the Kano Plains through Riruwai, Ningi-Burra Complexes to Jigawa, Borno and Yobe States. To the north-west, it passes through Katsina up to Zamfara, Sokoto and Kebbi States. The major land uses commonly found in the region include agriculture, farming, fishing, animal rearing and mining. The lower plain regions of Northern Nigeria reflect the geology of the area and are composed of a series of spectacular landform features of

denudational (inselbergs, residual hills, laterite mesas and buttes) and depositional (dunes) origin. Acknowledgements I wish to acknowledge the contributions of the authors consulted for this study as well as others, especially, the leaders of the Nigerian Geomorphological Working Group (NGWG) who encouraged me to participate in the project. I am highly indebted to my research assistants, namely Bara’uYakubu Usman, Olorunnipa Paul Kayode, Alogo Philip Aondoser and Mustapha Sa’idu who assisted with most of the illustrations.

References Adeleye DR (1973) Origin of ironstones: an example from the Middle Niger Valley Nigeria. J Sedimentol Petrol 43:709–727 Ajaegwu NE, Odoh BI, Akpunonu EO, Obiadi II, Anakwuba EK (2012) Late Miocene to early Pliocene Palynostratigraphy and Palaeoenvironments of ANE-1 Well, Eastern Niger Delta Nigeria. J Mining Geol 48(1):31–43 Bawden MG (1972) Geomorphology. The land resources of the North East Nigeria Vol 1–5. In: Tuley P (eds) Land resources division Tolworth tower surbiton Surrey, England Bennett JG, Hutcheon AA, Kerr WB, Mansfield JE, Rackham LJ (1977) Environmental aspects of the Kaduna plains, land forms and soils, land resources report 19, Land Resources Division, Surrey, UK, 1977

4

Lower Plains of Northern Nigeria

Buchanan KM (1955) Land and people in Nigeria. Lincoln Ltd United Kingdom Chukwu GA (1991) The Niger delta complex Basin: stratigraphy, structure and hydrocarbon potential. J Pet Geol 14(51): 114–232 Dada FAO, Jibril MG, Ijeoma A (2006) Macmillan Nigeria. Macmillan Publishers Ltd., Nigeria, Secondary Atlas Dukiya JJ (2012) Remote sensing and Gis assessment of flood vulnerability of Nigeria’s confluence town. Trans-Campus J (JORIND) 10(3):1596–8308 Elueze AA, Abimbola AF (1993) Appraisal of rocks in Oke-Iho district southwestern Nigeria, for decorative and ornamental stones. Mineral Wealth 86(87):11–14 Hartmann L, Gabriel M, Zhou Y, Sponholz B, Thiemeyer H (2014) Soil Assessment along Toposequences in Rural Northern Nigeria: a geomedical approach. Appl Environm Soil Sci 1–9 Article ID 628024 Iloeje NP (1982) A new geography of Nigeria. London, Longman Jeremiah K, Falaju (2018) Why lake chad requires urgent attention. The Guardian News Paper, 26th February, 2018 3:45am. Retrieved on 30th June, 2020 Michele LT (2005). Lake Chad catchment. Freshwater Ecoregions of Africa and Madagascar: a conservation assessment. Island Press. pp 194. ISBN 1-55963-365-4

69 Obaje NG (2009) In: Geology and mineral resources of Nigeria, lecture notes in earth sciences. Springer, Berlin, Heidelberg, pp 120. https://doi.org/10.1007/978-3-540-92685-63,C Omon EJ (2014) Landscape features and tourism development in Nigeria. Develop Country Stud 4(10):163–167 Shettima B, Suleiman A, Abdulkarim, AH (2017) Description of soft sediment deformational structure of the campanoMaastrichtianGombe formation of the Northern benue trough, N.E. Nigeria, Nigerian J Technol Developm 14(2):46–51 Short KC, Stauble AJ (1967) Outline of geology of Niger delta. AAPG Bull 51(5):761–779 Silviconsult (1992) Festivals in Nigeria: useful information for Foreing visitors…backswash and terraces on the International vegetation. KantaMesiumArgungu. Silviconsult Engenharin Ltd. Szentes G (2009) Granite formation and granite cavities in Northern Nigeria: Cad.Lab. Laxe34 P13–26 Thomas MF (1995) Models for landscape development on passive margines. Some implications for relief development in glaciated areas. Sci Direct Geomorphol J 12(1):3–15 Udo RK (1970) Geographical regions of Nigeria. London Heinemann, Nairobi Ibadan. Morrison and Gobb Ltd London and Edinburgh Zaccheus Onumba Dibiaezue Memerial Libraries (ZODML) (2018) Gwoza Hills/Zodml. https://www.gettyimages.com›photos›gwoza. Retrieved on 24 Dec 2018

5

Hills and Ridges in Southwestern Nigeria Lawrence Kosoko Jeje, Oluwagbenga Orimoogunje, and Adeyemi Olusola

Abstract

The Basement Complex rocks of southwestern Nigeria host a variety of residual hills. A distinction can be made among hills based on their parent materials and mode of formation. The hills include mesas, inselbergs, tors, regolith hills (i.e., erosional residuals in regolith), and regolith/forest-covered hills developed in amphibolite and diorite. Most common are inselbergs, which based on size have been classified into whalebacks, turtlebacks, bornhardts, and castle kopjes. They are either symmetrical or perfectly domical—bornhardts, elongated and symmetrical, or elongated and asymmetrical. They occur in clusters and with higher density in granitic plutons, but are scattered in various gneisses. These physical features have become a veritable touristic resource in the area. Good examples include the Idanre hills and the location of Abeokuta around the famous Olumo rock (which is actually a tor) in 1830. Keywords




Basement complex hills Olumo rock

5.1









Granitic hills Ado-awaye

Inselberg Residuals




Idanre

Introduction

Hills are elevations or groups of elevations rising above the level of the surrounding country culminating in well-marked summits. Local relief is generally less than 300 m. Steep-sided types are characterized by slopes above 12°, L. K. Jeje (&)
O. Orimoogunje Department of Geography, Obafemi Awolowo University, Ile-Ife, Nigeria e-mail: [email protected] A. Olusola Faculty of Environmental and Urban Change, York University, 4700 Keele Street, Toronto, ON, Canada

while gently sloping types are characterized by slopes less than 12°. Ridges are elevations whose length much exceeds the width. Hills and ridges occur all over the Basement Complex rocks of Nigeria, whereas outliers of sedimentary rocks form hills over the Basement Complex rocks in Sokoto, Lokoja, and Abeokuta areas. However, given the extent of the country, attention in this chapter will be focused on the Basement Complex rocks of southwestern Nigeria. The Basement Complex rocks of southwestern Nigeria are bounded to the south by the Abeokuta sedimentary formation and to the east by the Niger River. The border with the Republic of Benin marks the boundary to the west, while the appearance of the Nupe Sandstone formation constitutes the northern boundary. The area is thus bounded approximately by latitudes 6°50′N and 9°00′N and longitudes 2° 40′E and 6° 40′E (Fig. 5.1). The whole area lies between 90 m a.s.l at the contact with the sedimentary formation and 550 m a.s.l at the primary watershed between the rivers draining into the Atlantic Ocean and the Niger River, with several hills and ridges rising higher than these levels. The area which slopes from the north to the south is dissected by rivers Ogun, Ona, Osun, Owena, Osse, and Siluko and their tributaries (Fig. 5.1). The emphasis here is on the area draining into the Atlantic Ocean.

5.2

Inselberg Description and Classification

Inselbergs can be classified according to their size, form, and appearance. Large and elongated inselbergs have been designated as whalebacks (Jeje 1973; Olusola 2019), while smaller elongated types have been described as turtlebacks by Oyawoye (1965). Where they are relatively high, rounded, and topped by domes, they are known as bornhardts. Castle kopjes are sub-rounded in ground plan but conical and castellated in appearance. Twidale (1982, 1998) recognized three principal types of inselbergs, viz. castle kopjes, bornhardts, and nubbins, with the first and the last derived

© Springer Nature Switzerland AG 2023 A. Faniran et al. (eds.), Landscapes and Landforms of Nigeria, World Geomorphological Landscapes, https://doi.org/10.1007/978-3-031-17972-3_5

71

72

L. K. Jeje et al.

Fig. 5.1 Map of Southwestern Nigeria showing major lithology, waterbodies, major rivers, and rock outcrops

from the second. Migon (2006) classified bornhardts into domed inselbergs and shield inselbergs. Some of the characteristics of bornhardts outlined by Migon (2006) include steep, bare and upward-convex slopes, a sharp piedmont angle, and a mantle of talus derived from sheet structure at the foot slopes. Shield inselbergs on the other hand are characterized by slope angles as low as 10–15°, flat summit surfaces and in most cases, a less abrupt piedmont junction. However, the major distinguishing factor between these various categories of inselbergs appears to be the ratio between the long and short axes which simplifies the issue of whether an inselberg is a bornhardt or shield/turtle back. This is illustrated in Table 5.1. The Elongation Index, which is the ratio between the short and the long axes of inselbergs, measured for landforms present on the different rock types in the Basement Complex rocks in the extreme southwestern part of SW Nigeria, covering about 5000 km2, is shown in Table 5.1, while Table 5.2 illustrates the occurrence of inselberg as related to the parent rocks. About 83% of inselbergs developed in the migmatized gneisses in the study area are less than 32 ha in area. This Table 5.1 Elongation index of the Inselbergs

Rock types

situation is slightly different regarding inselbergs developed in granites. Seventy-five percent of those developed in the coarse porphyritic biotite and biotite-muscovite granite are less than 32 ha. However, the largest inselberg covering more than 121 ha occurs in this rock. In the undifferentiated older granite, 86% are less than 32 ha, which is quite different from the situation of those developed in coarse porphyritic hornblende granite-syenite, with 67% less than 32 ha. This is illustrated in Table 5.3, while Table 5.4 illustrates inselberg heights in different rocks.

5.3

Types of Hills

Distinctions can be made among hills based on their parent materials and mode of formation. The hills in the study area include mesas, inselbergs, tors, regolith hills (i.e., erosional residuals in regolith) developed in amphibolite and diorite. There are also convex summits with soil cover all over. The emphasis here will be on inselbergs and related forms.

Whalebacks

Turtlebacks

Bornhardts

Castle kopjes

Granites

2.6

1.6

1.1

1.3

Migmatised Gneiss

3.5

2.1

1.2

–

Source Jeje (1973)

5

Hills and Ridges in Southwestern Nigeria

Table 5.2 Percent occurrence of inselberg forms as related to parent rocks

73

Rock type

No. all forms

Total number Coarse porphyritic, biotite, biotite, muscovite granite

157

Whaleback

Turtleback

Bornhardt

Castle kopje

Complex

82

188

67

11

37

31.8

42.4

13.2

5.10

7.6

Migmatite gneiss

153

13.7

62.8

13.7

–

9.8

Undifferentiated older granite

25

12.0

60.0

16.0

–

12.0

Coarse, porphyritic, hornblende, granite-syenite

15

26.7

13.3

6.7

13.3

40.0

Porphyroblastic gneiss

10

30.0

60.0

–

–

10.0

Granite-gneiss

3

–

66.7

–

33.3

–

Augen-gneiss

1

100.0

–

–

–

–

Muscovite, tourmaline, granite-gneiss

1

–

100.0

–

–

–

Source Jeje (1973)

Table 5.3 Inselbergs’ sizes in different rock types

Size hectares

Coarse, porphyritic biotite, biotite muscovite granite

Migmatised gneiss

Number

Number

%

%

Undifferentiated older granite

Coarse porphyritic hornblende granite syenite

Number

Number

%

%

0–8

29

18.6

54

35.5

5

20

2

13.3

8–16

41

26.1

53

34.9

7

28

3

20

16–24

26

16.7

19

12.5

6

24

24–32

22

14

10

6.6

3

12

4

26.7

32–40

7

4.6

2

1.3

–

1

6.7

40–49

5

3.3

4

2.6

1

4

2

13.3

–

–

1

4

–

–

–

–

49–57

5

3.3

57–65

4

2.7

65–73

3

2

1

4

1

6.7

73–81

1

0.7

2

1.3

–

–

2

13.3

81–89

1

0.7

2

1.3

–

–

–

89–97

–

–

2

1.3

–

–

97–105

1

0.7

2

1.3

1

4

105–113

–

–

–

–

–

113–121

3

2

1

0.7

–

–

4.6

–

–

–

–

121

–

–

1

0.7

Source Jeje (1973)

5.3.1 Inselbergs The word “inselberg” is German and stands for “Island Mountain”. It was coined by William Bornhardt (1864– 1946) in 1900. The inselberg is an isolated rock hill, knob,

ridge, or a small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. The term inselberg was initially applied to arid landscape features; however, it has since been used to describe a range of rock features in various geographical contexts, leading to

74 Table 5.4 Inselbergs’ heights in different rocks (in m)

L. K. Jeje et al. Class interval (15.2 m)

Coarse porphyritic biotite, biotite muscovite granite %

Migmatised gneiss %

Undifferentiated older granite %

Coarse, porphyritic, hornblende, granite-gneiss %

15.2–30.4

16.7

30.2

16

7.7

30.4–45.6

21.8

24.3

24

15.4

45.6–60.8

13.5

19

28

7.7

60.8–76.0

14.8

11.8

8

23

76.0–91.2

9.6

4

4

–

91.2–106.4

9

4.6

4

7.7

106.4–121.6

3.2

2.6

–

15.4

121.6–136.8

3.2

0.7

–

7.7

136.8–152.0

2.5

0.7

12

15.4

152.0–167.2

1.3

0.7

–

–

167.2–182.4

0.6

0.7

–

–

Above 182.4

3.8

0.7

4

–

Source Jeje (1973)

Fig. 5.3 A Tor Fig. 5.2 An Inselberg—Olosunta Inselberg near Ikere in Ekiti State

confusion about the precise definition. According to Wilson (1900), “inselberg” has been defined as “steep-sided isolated hill rising relatively abruptly above gently sloping ground”. This definition includes features such as mesas, buttes, conical hills with rectilinear sides typically found in arid regions, regolith covered, concave-convex hills, rock crests over regolith slopes, rock domes with near-vertical sides, and tors formed of large boulders but with solid rock cores. However, the word inselberg is now exclusively applied to

solid rock island hill or mountain protruding abruptly from a plain cut across regolith (Fig. 5.2). Inselbergs are ubiquitous all over the Earth.

5.3.2 Tors Tors are closely related to inselbergs and are also known as boulder inselbergs. These are piles of spheroidally weathered rock blocks rooted in bedrock and exposed as a basal rock

5

Hills and Ridges in Southwestern Nigeria

surface following the removal of the overlying saprolite. When still concealed by the weathering mantle, the blocks are called corestones. Individual blocks may be less than 0.8 m in diameter, while the most common are sizes ranging between 1 and 3 m in diameter (Fig. 5.3). Most are less than 30 m in height and their cross-profiles may be highly irregular. They frequently occur in association with domed inselbergs, particularly in granites, either as clusters of boulders surrounding bornhardts or on the summits of emerging inselbergs. They may, however, also occur separately as described by Linton (1955) from southeastern England. Thomas (1993) attributed the relationship between domes and tors to the spatial variations in the jointing frequencies and the development of different types of hierarchies of jointing in granite. Tors also occur in gneissic rocks as small boulders and tend to disintegrate along the mineral bands (Faniran et al. 2021).

5.4

Inselberg Occurrence

Inselbergs are commonly associated with granitic rocks and various types of gneisses, and occasionally schists. They are partially covered by vegetation in the humid areas, but are virtually bare in the savannas and semi-arid areas. Migmatite and the various gneisses underlie about 75% of the Basement Complex areas, supporting gently undulating and rolling plains. However, where rock structure and Fig. 5.4 Map of Southwestern Nigeria showing groups of inselbergs within the basement complex area

75

lithology are suitable, the plains are dotted with impressive hills/inselbergs all over southwestern Nigeria (Fig. 5.4). Notable ones include the famous Ado-Awaye Hill; many inselbergs are found are around Shaki and between Iseyin and Oyo (see Fig. 5.5). Several such hills dot the plains in Ekiti, Ondo, and Kogi States as evident myriads of inselbergs in Ikare, Akoko, and Kabba area, and specifically in Ife-Ijumu area along Omuo-Kabba-Lokoja road and around Okene. Those in the latter area are often referred to as Kukuruku Hills. Oyinloye (2011) observed that both the pink granite and gneiss give rise to hills in Ile-Ife-Ilesa, Ibadan and Eruwa areas. Granite-gneiss gives rise to series of inselbergs, often less than 100 m high, especially within the campus of Obafemi Awolowo University and along Ife-Ilesa road. The massive amphibolite and hornblende gneiss occurring widely in Ile-Ife-Ilesa area give rise to series of low hills. These forested low hills, less than 100 m high, encircle the southern part of Ilesa. Inselbergs occur mainly in the various types of granites such as the coarse porphyritic, biotite-muscovite granite; coarse porphyritic, hornblende, syenogranite; undifferentiated older granite; medium to coarse grained granite and charnockite among others. These occur as discrete plutonic bodies ranging from about 90 km2 at Eruwa-Lanlate and 310 km2 in Idanre to about 410 km2 in Akure-Ikere-Ado-Ekiti area. Apart from these three areas, these rocks are also found all over the southwestern Basement Complex area, specifically near Odigbo and Ile-Oluji in the Ondo State; in

76

L. K. Jeje et al.

Fig. 5.5 Inselberg pattern NE of Idanre Town

Aiyete-Idere-Tapa-Igboora, Iwere-Ile, Ijio, Oke-Iho-Isemi in the Oyo State; Abeokuta-Odeda in the Ogun State; Imesi-Ile-Igbajo-Iressi and Alakowe (near Ife) in the Osun State among many occurrences. These rocks altogether underlie just about 15% of the Basement Complex area of southwestern Nigeria. Inselbergs in these granites occur in clusters, especially where the parent granite pluton is traversed by several vertical joints as at NE of Idanre town (Fig. 5.5). This section of the Idanre hills is traversed by series of NW–SE and NNE-SSW joints. About forty individual inselbergs occur in a cluster in an area of about 50 km2. Clusters may be circular, with massive inselbergs occurring in a circular form as in Ijio in the Afijio Local Government Area of the Oyo State (Fig. 5.5). However, the occurrence may be also in the form of a massive tableland surrounded/fringed by several inselbergs or half-exposed domes. Good examples are observable in Eruwa, Oke-Iho-Isemi, and Imessi-Ile-Igbajo-Iressi areas (Fig. 5.6). Old Eruwa town is located on a tableland surrounded by about 10 inselbergs of various sizes. The town, about 1.0 km2 in area, is underlain by thick regolith exhibiting few rock outcrops. Afolabi (2017) quoting Thomas (1994) about the granitic terrain in Igbajo area observed that “the Igbajo plateau retains a deeply weathered upper surface, with domes currently emerging at the surface”. Oke-Iho and Isemi are towns located on such a plateau, 16 km2 in the area, standing at about 100 m above the local surfaces. The plateau is surrounded by massive inselbergs, some of which are bornhardts, while several others are whalebacks, all surrounding

an area covered by regolith containing massive corestones, and occasional lateritic ironstone on which the towns of Oke-Iho and Isemi are located. However, granitic inselbergs can also be located in a ribbon pattern of few kilometers apart, such as along the Akure-Iju-Ikere-Ado-Ekiti road. Among these are the massive bornhardts such as Olosunta, Orole, and several others. Both Olosunta and Orole are now locally deified with annual festivals associated with them. Generally, inselbergs occur as singular features and far apart in all the varieties of gneisses and schist as exemplified by the Oyo Plains. In a study carried out in an area of 5000 km2 on the Basement Complex in a part of southwestern Nigeria, Jeje (1973) commented on the frequency of occurrence of inselbergs in different rocks as follows: “in the granitic terrain at Eruwa-Lanlate and Idere-Tapa, inselbergs occur at a rate of 1 per 2.5 km2 (40 per 100 km2), in the gneissic area around Ado-Awaye, they occur at a rate of 1 per 6.5 km2 (15 per 100 km2), but in the same rock type around Oyo, inselbergs occur at a rate of 1 per 15.5 km2 (7 per 100 km2). In the granite-syenite and the Undifferentiated Older Granite terrain at Oke-Iho and Iwere-Ile areas, they occur at a rate of 1 per 0.8 km2 (125 per 100 km2)”.

5.5

Some Aspects of Inselberg Morphometry

In the ground plan, inselbergs are mainly elliptical, rounded or complex. In profile, they are either domical or symmetrical—bornhardts, with little or no basal scree slope

5

Hills and Ridges in Southwestern Nigeria

77

Fig. 5.6 Cluster of inselbergs in Ijio-Igbo-Ora area, Oyo State

Fig. 5.7 A Bornhardt in Ado-Ekiti

(Fig. 5.7), or elongated and symmetrical, or elongated and asymmetrical, with lower-inclination slopes carrying regolith/soil and partially vegetated or complex (Fig. 5.7). In several cases, they are devoid of scree slopes. These various forms are related mainly to rock structure and lithology, especially with regard to:

• Rock foliation • Direction of foliation and angle of dip • The presence of bounding joints. Granitic inselbergs are often elongated and symmetrical as most of those in Idanre or domical as those between

78

L. K. Jeje et al.

Fig. 5.8 Ado-Awaye rocks (Inselberg)

Akure and Ikere-Ekiti (Fig. 5.4). Some have pronounced asymmetry as the case of Ilele hill at Ijio (Fig. 5.4). Inselbergs built of migmatized gneisses are elongated but are largely asymmetrical. These characteristics are determined by foliation, direction of strike, and the angles of dip of foliation. Where the dip and strike of foliation coincide, the inselbergs are mainly elongated and symmetrical as the case of Ado Rock in Ado-Awaye (Fig. 5.8), where the strike of foliation is 340° NNW and the dip is vertical. Locally, the long axes of the inselbergs built of coarse porphyritic granite trend in NNE-SSW direction as the parent rocks. Where vertical joints cut across the dip of foliation, asymmetry is pronounced as observed on an inselberg north of Fasola near Oyo. In most cases, inselbergs formed in gneisses have extremely steep bounding slopes on one side and gentle slopes on the other one, e.g., Aseke hill near the old water work in Oyo. Only occasionally are the slopes uniformly steep, as in granites. The steep bounding slopes are often emphasized by subsurface rotting and erosion at the base of the inselbergs, and this leads to the formation of convex slopes or overhangs (Fig. 5.9). Morphometric features of inselbergs developed in the coarse porphyritic granite areas of Igbajo, Otan, Aiyegbaju, and Iresi as determined on 10 sampled inselbergs by Afolabi (2017) are presented in Table 5.5. As the author did not know the names of individual inselbergs, he substituted numerical values for their identification. The table is meant

to illustrate some characteristics of inselbergs concerning their perimeter, areas, maximum and minimum slope length, relative relief, absolute elevation above the sea level, and maximum slope angles. The high values for the coefficient of variation for perimeter, area, maximum slope length, and relative relief indicate a considerable variance in the sizes of the sampled inselbergs, which may apply to inselbergs on different rock types all over the Basement Complex rocks, but the relatively low values of the coefficient of variations for parameters such as elevation above sea level, minimum slope length, and maximum slope angles indicate that inselbergs developed on the same rock may share some common characteristics as also shown in the Idanre hills.

5.6

Ridges

Ridges are developed in quartzites, quartz-schists, and other types of schists. These rocks are found in parts of the Oyo, Osun, Ekiti, and Kwara States. Their occurrence in Ilesa area of the Osun State was mapped by de Swardt (1953) on a scale of 1:125 000 for the Geological Survey of Nigeria as Sheet 31. It was also mapped by the same agency in Ibadan area on the scale of 1:250 000. The most recently published geological map of Nigeria (2004) shows the rocks to extend from Oke-Igbo (Ondo State) through Ilesa area (Osun State)

5

Hills and Ridges in Southwestern Nigeria

79

Fig. 5.9 An Inselberg with an overhanging slope

Table 5.5 Morphometric properties of sampled inselbergs developed in coarse porphyritic granite

Perimeter (km)

Area (km2)

Minimum slope length (m)

Elevation above sea level (m)

Maximum slope angle (°)

47.78

410.89

30.00

38.57

430.72

31.00

Relative relief (m)

S/n

Inselberg

1

Hill 01

1.476

0.163

352.982

262.843

2

Hill 02

1.131

0.094

652.200

269.765

3

Hill 03

2.143

0.258

270.927

221.195

46.72

408.68

40.00

4

Hill 04

2.537

0.429

777.895

512.968

109.74

410.68

33.00

5

Hill 05

5.075

1.177

733.126

424.682

80.09

530.97

40.00

6

Hill 06

3.996

0.981

1057.558

611.456

138.02

517.81

39.00

7

Hill 07

4.239

0.977

1168.926

752.078

96.59

484.62

28.00

8

Hill 08

4.115

0.895

1067.277

646.796

123.33

471.71

36.00

9

Hill 09

4.181

1.159

990.196

541.365

76.96

517.82

28.00

10

Hill 10

3.128

0.458

478.832

226.252

31.70

593.14

23.00

11

Mean

3.202

0.659

755.00

447.934

78.95

477.69

33.80

12

Std. (d)

1.327

0.421

316.14

51.6

37.41

62.72

5.77

13

Co.V

41.44%

63.88%

11.52%

47.38%

13.13%

Maximum slope length (m)

41.87%

10.07

Source Afolabi (2017)

to the Kwara State, thus traceable for a distance of about 180 km. The occurrence in the Ilesa area, which Rahaman (1988) referred to as Effon Psammite Formation, is in the form of elongated bodies trending N-S and NNE-SSW (Fig. 5.10). In Ibadan area, the rocks appear contorted in ground plan, trending NNW-SSE, and reminiscent of old folds. Three types of quartzite have been recognized: massive, fissile, and mica schist/quartz-schist.

The first type comprising fused, highly metamorphosed quartz is the veritable ridge maker. It occurs as bands several meters thick on the ridges east of Ilesa. A road-cut northeast of Itawure (near Effon Alaye) shows the fissile type which displays folds, faults, and fractures of the original sandstone. It is highly permeable and thus permits no runoff or stream development across the formation. The angle of dip is quite steep grading into

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Fig. 5.10 A map showing Efon ridge and southern part of Imesi-Ile-Igbajo-Iresi group of inselbergs

vertical, while most of the joints follow the NE-SW foliation.

5.6.1 Ridges Around Ibadan Ridges formed in quartzite-quartz-schist are common around the city of Ibadan. The ridges occur mostly as elongated hill chains and only a few are continuous over a few kilometers. The one within Ibadan, the Aremo ridge, is in form of seven hills, each of which accommodates important landmarks in the town such as Mapo Hall, the old Roman Catholic Church Seminary, Bower Tower, and Premier Hotel. The ridge at Olokemeji near Eruwa is in form of a tight loop through which the Ogun River cuts a deep canyon. Table 5.6 shows some morphometric properties of the most prominent ridges.

Table 5.6 Some aspects of the morphometric properties of ridges around Ibadan

5.6.2 Ridges East of Ilesa The ridges to the southeast of Ilesa are more impressive than those around Ibadan (see Fig. 5.10). Between the rivers Oni in the south and Epolu, a tributary of the Osun, in the north are about seven narrow broken chains of forested ridges. The first four immediately SE of Ilesa, each generally less than 0.5 km basal width, trend N–S for distances of 15–20 km, attaining an elevation of 380 m a.s.l. on the northern bank of River Oni, rising to 485 m a.s.l. to the NE of Iwaraja. Local relief hardly exceeds 100 m along all the ridges. Immediately east of these are three higher ridges rising from about 350 m a.s.l. east of Iperindo at 70 m above the local surface to 540 m a.s.l. in the extreme north, trending for about 15 km. These merge into a single ridge (in fact as a chain of hills) north of Erinmo, continuing further north for a distance of about 12 km before fading out. Maximum slope angles on

Ridge

Mean slope (°)

Standard deviation (°)

Local relief (m)

Length (km)

Olokemeji

23

4.1

96

5.5

Arowosegbe

19.6

3.8

45

2

Ijaiye

22.9

4.3

106

10.5

Aremo

12.2

2.3

71

4.5

Ayantola

14.9

4.2

103

10

Source Faniran and Jeje (2002)

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Hills and Ridges in Southwestern Nigeria

these ridges, which are covered by flaggy weathered debris, exceed 35°. In between the ridges, gneisses and biotite-schist crop out (de Swardt 1953). East of the low ridges is the most impressive ridges all-over southwestern Nigeria. These two ridges are separated by an undulating low surface less than 300 m a.s.l. in the south, rising to about 400 m a.s.l. in the north and bounded by the upper Owena river in the south and Osun river in the north. These ridges stand above 645 m a.s.l., about 290 m above the local surface east of Esa-Oke-Oke Imessi Road. The western branch originates north of Ikeji, where it merges with a severely dissected granite pluton with peaks above 750 m a.s.l. The ridge trends NNW-SSE. Its basal width exceeds 2 km for a considerable distance before narrowing down to about 1.5 km, becoming wider again around Effon Alaye, after which it bifurcates into a Y shape. It attains elevations above 700 m a.s.l. for over about 25 km distance. The eastern branch originating south of Ikogosi also trends NNW-SSE, both converging north of Oke Imessi after about 54 km from Ikeji. The summit of the eastern branch is at about 690 m a.s.l. for a long distance. So consistently flat is the summit that de Swardt (1953) pronounced it as the third and uppermost erosion surface in Ilesa area. He traced the middle surface to the summits of the lower narrower series of quartz-schist ridges to the west (already described) and the lowest surface to the upper Owena and Osun basins. The wide summits sustain many rivers as evident near Effon Alaye, Ipole, Erin-Odo, and Oke-Ila. On descending the steep, near-vertical slopes of the ridges, these rivers gave rise to falls and rapids among which are Olumirin (Erin-Odo) and Ayinkunnuigba (Oke-Ila). The slopes bounding the ridges are very steep, displaying the classic four standard hillslope elements of King (1967). The flat summit is followed downslope by cliffs of quartzite outcrops, usually less than 15 m high but higher where the scree slope below has been eroded. Below the cliffs are the rectilinear scree slopes composed of a matrix of scree material ranging from cobbles to massive rock blocks, all immersed in soil aggregates varying from coarse to fine sand, silt, and clay. The scree slope is regularly cultivated to various crops, especially upland rice. The footslope below comprises the flat valleys of the adjoining rivers, Owena and Osun together with their tributaries. Veritable canyons have been eroded into the scree slopes by the rivers originating from the upper summit of the ridges. The ridges survived several cycles of weathering and erosion around them because of the relative resistance of massive quartzite, while the quartz-schists are highly permeable and hardly produce any runoff after the rains, but the water seeps out at the ridge base to sustain perennial rivers, especially in Ekiti area (Ogunkoya and Jeje 1987).

81

5.6.3 The Low Ridges South of Ipetu-Ijesa Two low ridges developed in quartz-schists south of Ipetu-Jesa are separated from the high quartzite ridges to the north by the Ipetu-Ikeji road. The two ridges are divided by the tributary of River Oni. The northern ridge, trending NW– SE, is about 15 km long and about 1 km wide in the NW, declining in basal width to about 400 m in the extreme east. Elevation varies from 380 m a.s.l. in the south to 425 m a.s. l. to the north. It rises for 50 m above the local surface in the south to 110 m in the northwest. The asymmetrical ridge is more or less an escarpment with very steep slopes up to 60° to the west and south and very gentle long slopes facing east. The gentle slope is heavily dissected to depths in excess of 50 m by the upper tributaries of the River Akunri, a tributary of River Oni. The southern ridge, trending approximately E-W and about 12 km long, is higher than its northern neighbor. It rises from 450 m a.s.l. in the extreme west to 500 m a.s.l. at the eastern end, standing about 150 m above the local surface. The symmetrical ridge, about 400 m to 1.2 km of basal width, is bounded by steep slopes about 45° for most of its length, except at the extreme eastern side where the slopes are flatter. It is also dissected by the tributaries of the River Oni into three individual but continuous ridges. South of these ridges are several N-S, NW–SE, and NE-SW trending minor ridges developed in quartz-schists. They vary in length from 2 to 3 km and in elevation from 380 to 390 m a. s.l. These are close to Onipanu, Oja Bamikemo, and Mofere villages in the Ondo State.

5.7

The Development of the Hills and Ridges

The development of landscape on the Basement Complex rocks of southwestern Nigeria including the hills and ridges and indeed elsewhere in the humid tropics has been explained by the concept of etch-planation by Thomas (1966, 1974, 1994), Jeje (1970, 1973, 1986), Twidale (1981, 1990, 2007a, b), Summerfield and Thomas (1986), Ollier (1992, 2010) and Modenesi et al. (2011) among several others. The theoretical underpinning of the concept involves deep chemical weathering, the survival of structurally sound parts of the various rocks, and the subsequent removal of the regolith under several weathering and erosion cycles. Thus, as defined by Thomas (1968), an etchplain is “a form of planation surface associated with crystalline shields and ancient massif which do not display tectonic relief and developed under tropical conditions promoting rapid chemical decomposition of susceptible rocks”. Thus, according to Thomas (1974, 1994), the landscape of the clustered hills (inselbergs) in the various granitic terrains such as in Igbajo-Imesi-Ile (Fig. 5.11) represent “dominantly stripped and incised etchsurfaces”, while

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Fig. 5.11 Igbajo-Imesi group of Inselbergs in Osun State

Jeje (1986) assigned the landscape around Ibadan dominated by ridges developed in quartz-schist as “partially stripped etchplain”. The ridge landscape to the southeast of Ilesa may also fall into this category. However, one of the problems of etch-planation vis-a-vis the occurrence of massive inselbergs and ridges standing up to 700 m above their local surfaces is to establish the number of weathering/erosion cycles that occurred before these features evolved, especially where there are no clear-cut erosion surfaces as those established for the Guyana Massif by Schubert et al. (1986) and Clapperton (1993). The germane question is: Were these massive inselbergs and ridges formed by repeated cycles of weathering and erosion around them as suggested by Thomas (1974) or within a single cycle of weathering and erosion over a very long time? Rabassa (2010) appeared to have shed some light on this problem by the application of Gondwana-paleo-landscape development. His idea is summarized in what follows: The paleo-landscapes were generated when the former Gondwana supercontinent was still in place and experienced similar tectonic conditions all over its surface. Gondwana planation surfaces are characteristic of cratonic regions which have survived in the landscape without being covered by marine sediments over extremely long periods having been exposed to long-term subaerial weathering and denudation. The genesis of the present-day paleo-landscapes relates to extremely humid and warm paleo-climates of “hyper tropical” nature with permanent water-saturated soils, or perhaps extremely paleo-monsoonal climates with

seasonal long-term cyclic fluctuations from extremely wet to extremely dry. Deep chemical weathering profiles with well-defined weathering front perhaps down to many hundreds (up to 1 000) of meters deep developed. The weathering products included clays, kaolinite, pure quartz, other silica form sands, silcretes (silica), calcretes (calcium carbonate), and ferricretes, all formed under different environmental conditions. Annual precipitation was up to 10,000 mm, with extremely high temperatures 25–35 °C. Extremely stable tectonic and climatic conditions prevailed from the Jurassic to mid-Cretaceous. Rifting and continental drift started about 130–115 million years ago and environmental conditions started to differ in the adjacent areas of the rifted continents (South America and Africa). By the Late Cretaceous, some 90 million years ago, the gap between the two continents was considerable and landscape evolution changed. However, it was still warm and wet; weathering proceeded to hundreds of meters. Weathered debris remained stable and in place, with denudation being very slow due to tectonic stability. Denudation commenced vigorously from the middle Eocene due to tectonic instability associated with the Alpine-Andean orogeny which involved epeirogenic uplifts of the continents. Rabassa at this stage introduced the concept of Passive Margin Geomorphology. This states that once Gondwana broke up, new margins were formed on which consequent rivers originated as in West Africa. With tectonic activities involving uplift of the new continents, these consequent rivers cut down and created scarps that retreated inland.

5

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Also, these rivers descending to the coast through steep continental margins were highly erosive and able to cut down through the thick regolith to expose the basal weathering surface to different degrees. Thus at the passive margins, etchplains were developed during periods of tropical climate and long-term intervening tectonic stability, which he estimated to have occurred about six specific times since the mid-Tertiary based on the erosion surfaces in the Guyana Massif. Thus, the massive hills and ridges associated with the etchplains started to emerge from the mid-Tertiary. This possibly induced Thornbury (1969) to assert that no current area of the earth surface is older than the Tertiary period. Rabassa concluded by observing as follows: “The deep chemical weathering was the main agent in the formation of these Mesozoic paleo-landscapes, with weathering fronts reaching to depths perhaps up to 1000 m. When the climate changed in the latest Cretaceous and then again later in the Paleogene, the huge thickness of weathered debris was removed by continuous denudation. The weathered materials mostly montmorillonite-beidellite-hydromicas and kaolinite were transported by superficial runoff toward the ocean basins, most of which were opened by the rifting process in the Cretaceous where they were deposited during most of the Tertiary. When the denudation was complete or almost complete, the ancient weathering front became exposed and typical landforms and deposits related to its roots are found in the most significant paleo-landscapes. Corestones, duricrusts of many different types (ferricretes, silcretes, calcretes), inselbergs, bornhardts, tors, domes are the most relevant landforms present in these paleo-landscapes. These landforms are found as landscape elements forming part of planation surfaces of which the most important are the etchplains generated by deep chemical weathering and later by prolonged denudation. Other planation surfaces such as pediplains are found.” Thus, with the explanation by Rabassa et al. (2010), the development of massive inselbergs and ridges can be explained as follows: The landscape got weathered to depths more than 1000 m ever before the supercontinent Gondwana broke up, and with both tectonic and climatic instability since the middle Eocene, the thick regolith got eroded gradually to expose the hills and ridges. These features rose higher and higher as the regolith around them got eroded under several tectonic and climatic instability regimes. Thus, almost all the inselbergs are not older than mid-Tertiary as argued by Thomas (1965).

5.8

83

the Basement Complex rocks, even where very isolated, attract the attention of various religious groups whose members visit the hills regularly for purposes of praying and in some instances these people do live on these hills. Good examples include Oke-Olorunkole on the outskirt of Ibadan on old Oyo road and King Solomon Hill on Ilesa-Ife road in Osun State. Apart from the above, the clustered granitic hills in Igbajo-Imessi Ile served as the war theater for the Ibadan and Ekiti Parapo-Ijesa warriors during the Yoruba Civil War of 1886–1893. The area has become a veritable tourist attraction, more so when a war museum has been established in Igbajo. In fact, most of the hills served as places of refuge or as defensible locations during the internecine wars in Yorubaland during the nineteenth century. Good examples include the Idanre Hills and the location of Abeokuta around the famous Olumo Rock (which is actually a tor). One of the most famous hills in this regard is Ado rock, also known as Oke-Ado Mountain in Ado-Awaye, 20 km southwest of Iseyin. The hill served as home to Awori dwellers who chose the hilltop as refuge from Dahomey armies in the nineteenth century. The hill exhibits several features of tourism interest among which are the staircase leading to the hill top (Fig. 5.12), the hanging lake (Fig. 5.13), the “elephant” tree (Fig. 5.14), the so-called mysterious cluster of foot prints known as foot prints of the elders (Plate 10). There are also features such as Ishage rock (Fig. 5.15) and smaller lakes— Iya Alaro, Iya Oniru, and Agbomofunyake. There is also a system of rills on the western face, especially on the upper dome as the hill is a good example of dome-on-dome inselberg. In addition, eight large and deep gullies are eroded along joints in the rock on its eastern face (Fig. 5.16). A staircase with 369 steps leads to the top of the hill and as one climbs it, the beautiful vista of extremely flat Oyo

Tourism Potentials of the Hills and Ridges

This issue is examined in the chapter on geoheritage, with particular emphasis on the Idanre Hills. However, individual hills are of high potential tourist interest. Most of the hills on

Fig. 5.12 Staircase on Ado-Awaye Inselberg

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Fig. 5.13 Hanging Lake on Ado-Awaye

Fig. 5.15 Grinding pits on Ado-Awaye Inselberg

Fig. 5.14 Elephant tree

Plains dotted with several inselbergs unfold. The major problem with the staircase is that unlike the one on Idanre hills, it has no resting stations along its entire length and there are no railings. The suspended lake is one of the two in the world, the other being located in Colorado, USA—the Hanging Lake in Colorado is located 11 km east of Glenward Springs (Akande 2012—https: even trends wordpress. com/2012/03/16/ado-awaye-hill). The Ado-Awaye hanging

lake is about 4 m in diameter but elongated along joints at one end to about 6 m. The water is green in color at sight (not algae induced) but crystal clear when fetched into containers. It is reported to maintain the same level at all seasons and also reputed to be bottomless. As it is credited with therapeutic powers, religious groups throng the venue for prayers. The lake appears to have developed at the intersection of vertical joints. The intersection was subject to deep chemical weathering and gradual removal of the decayed material. The question is: Was it formed by weathering when the rock was still buried by superjacent regolith and exposed by the gradual removal of the regolith? Smaller lakes on the hill include Iya alaro (“the dying mother”), Iya oniru (“the mother of locust beans”), and Agbomofunyake (“the one who drops the baby for the mother to care for”). Several pits in form of footsteps up to 15 cm deep with the length of the long axes more than double the short axes, referred to as “footprints of the elders”, are found on the

5

Hills and Ridges in Southwestern Nigeria

85

Fig. 5.16 Isagha rock, a delicately balanced rock on the Inselberg

hilltop surface. What has been referred to as mysterious clusters of indentation are grinding pits of which Akande (2012) claimed to have recognized about 100 on the hill uppermost flat slope. However, this author recognized about 14 some distance from the hanging lake. Some of these are larger than the others, indicating communal usage. Akande (2012) also observed that grinding pits of various sizes are still currently in use by women to grind pepper, while bigger ones are used as water reservoirs during the rainy season. The “elephant” tree could be found at some distance from the lake. It is a tree that fell but somehow its trunk got mangled in such a way that it resembles an elephant trunk at first glance (https://titiwonderlust.wordpress.com/2014/09/ 21/. Another interesting feature is the Ishage rock, a huge boulder balanced precariously and delicately and standing upright on one of its smaller edges. Apart from Ado rock, several other inselbergs are noted for their tourism potentials. Good examples include the massive bornhardts near Ikere-Ekiti (Olosunta and Orole) collectively known as Ikere Hills; Agunrege inselberg in Oke Ogun; the Iwere Hill in Iwere-Ile; and Ijio Hill both in the Oyo State near the boundary with Benin Republic. The quartzite-quartz-schist ridges are notable mainly for the waterfalls developed across them in many places. The waterfall near Oke-Ila is actually a series of short falls more or less in form of cataracts, but the one close to Erin-Ijesa is more spectacular. Before the advent of Christianity, this waterfall was deified by the local people who designated it Olumirin, i.e., another god or deity and were worshiping it. The upper part comprises four distinct falls, while the lower part is in the form of rapids which cut deeply into the scree

slope at the lower part of the ridge. Of the upper four falls, the upper three are short, with none exceeding 5 m as perpendicular falls, but the lowest one is the most impressive. At the peak of the raining season, it drops about 100 cumecs of water through a free fall of about 10 m below which there is a shallow pool about 15 m in diameter, accommodating swimmers especially in the raining season (Fig. 5.17). The importance of the waterfall has long been appreciated by the state government who provided accommodation near the waterfall in form of fenced self-contained bungalows and built a staircase and walkway to link the area of accommodation with the area of the waterfall. The waterfall in Ipole and Effon Alaye in form of rapids interspersed with short free falls is yet to attract any developmental attention as much as the one near Erin-Ijesa. Overall, the inselbergs and ridges though endowed with high potential tourism attraction are yet to be comparably developed with the way the Idanre Hills have been developed by successive governments in the Ondo State.

5.9

Conclusion

A variety of hills have developed on the Basement Complex rocks of southwestern Nigeria, but the most common are inselbergs, which based on sizes have been classified into whalebacks, turtlebacks, bornhardts, and castle kopjes. They are either symmetrical or perfectly domical—bornhardts; elongated and symmetrical; or elongated and asymmetrical. They occur in clusters and with higher density on granitic plutons, but are scattered on various gneisses. Those on

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Fig. 5.17 A pool at the base of Olumirin Falls near Erin-Odo

granites appear to be higher and larger than those on gneisses, but their slope angles are equally steep. Quartzite and quartz-schist are the ridge makers. Ridges on the latter are by far lower and narrower than those developed on quartzite as observed around Ibadan, and to the southeast and east of Ilesa. Ridges on quartzite are massive, with elevation generally above 650 m. Variably dissected two major ridges extend for more than 50 km east of Ilesa to the Kwara State and sustain several waterfalls developed by rivers originating from the summits. The concept of etch-planation applied to account for the development of the hills and ridges has been further highlighted by the paleo-landscape model developed by Rabassa (2010), who postulated that these hills and ridges developed on paleo-landscapes from the ancient Gondwana continent, which experienced deep weathering over 1000 m, with a highly irregular weathering front. With the fragmentation of the supercontinent from the middle Cretaceous, the regolith was removed following several cycles of uplift/climate change. Though in need of further refinement, this theory appears to satisfactorily explain the origin of the inselbergs and ridges more than 500 m above their footslopes on the Basement Complex rocks of southwestern Nigeria and indeed in most of the tropical areas.

References Afolabi RA (2017) Morphology and spatial distribution of residual landforms in Igbajo-Imess Ile Osun Drainage Basin, South Western Nigeria. M.Sc. Thesis, Obafemi Awolowo, University, Ife

Clapperton CM (1993) Quartenary geology and geomorphology of South America. Elsivier, Amsterdam, p 774 De Swardt RMJ (1953) The geology of the area around Ilesa. Geological Survey of Nigeria, Bull 23 Faniran A, Jeje LK, Ebisemiju SF, Olusola AO (2021) Essentials of geomorphology, 2nd edn. Penthouse Publishers, Ibadan, p 455p Faniran A, Jeje LK (2002) Humid tropical geomorphology. Heinemanns, Ibadan, 414 p Faniran A, Jeje LK, Ebisemiju FS (2006) Essentials of geomorphology Jeje LK (1970) some aspects of the geomorphology of South Western Nigeria, Ph.d. Thesis, Edinburgh Jeje LK (1973) Inselbergs evolution in a humid tropical environment: the example of south western Nigeria. Zeits Fur Geomorph N.F. 17 (2):194–225 Jeje LK (1986) Landscape evolution in the humid tropics and implications for land resources evaluation. In: Inaugural Lecture, vol 81, Obafemi Awolowo University, Ife King LC (1967) Morphology of The Earth; a study and synthesis of world scenery Linton DL (1955) The problem of tors. Geogr J 121(4): 470–487 Migon P (2006) Geomorphological landscapes of the world. Oxford University Press Modenessi-Gautheri MC, Moltade MC, Toledo SK, Hiruma F, Taioli F, Shimada H (2011) Deep weathering and landscape evolution in a tropical plateau. CATENA 85:221–230 Ogunkoya OO, Jeje LK (1987) Sediment yields from some third order basins on the basement complex in South Western Nigeria. CATENA 14:387–397 Olusola AO (2019). Process-forn dynamics of Upper Ogun River Basin, Southwestern Nigeria. PhD Thesis submitted to the Department of Geography, University of Ibadan, Ibadan, Nigeria Ollier CD (1992) Age of soils and landforms in Uganda. Israel J Earth Sci 41:227–231 Ollier CD (2010) Very deep weathering and related landslides. In Calceterra D, Parise M (eds) Weathering as a predisposing factor to slope movements. Geological Social Engineering Geological Special Publication No 22 Oyawoye M (1965) Review of Nigeria pre-cretaceous. In: Rayment A (ed) Aspects of the geology of Nigeria. University Press, Ibadan

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Oyinloye AO (2011) Geology and geotectonic setting of the basement complex rocks in southwestern Nigeria: implications on provenance and evolution. vol 5 chapter Rabassa JO (2010) Gondwana paleolandscapes: long-term landscape evolution, genesis, distribution and age Rabassa J, Carignano C, Cioccale M (2010) Gondwana paleosurfaces in Argentina: an introduction. Geociências 29(4):439–466 Rahaman MA (1988) Recent advances in the study of the basement complex of Nigeria. In: Symposium on the geology of Nigeria. Obafemi Awolowo Univversity Schubert C, Brieno H, Fritz D (1986) Palaeo-environment aspects of the Caroni-Paraguay river basin (south-eastern Venezuela). Interciencia, V 11:278–289 Summerfield MA, Thomas MF (1986) Long-term landform development, editorial introduction. In: Gardiner V (ed) International geomrphology, 1986, Part II, Wiley, pp 927–933 Thomas MF (1965) Some aspects of the geomorphology of domes and tors in Nigeria Zeit. Fur Geomorph 9:63–81 Thomas MF (1974) ‘Tropical Geomorphology: a study in weathering and landform development in warm climates. Macmillian, London Thomas MF (1993) Geomorphology of the humid tropics: a study of weathering and denudation in low latitudes. John Wiley, London

87 Thornbury WD (1969) Principles of geomorphology, 2nd edn. Wiley, New York Twidale CR (1981) Granitic Inselbergs, domed, block strewn castellated. Geog J 147(1) Twidale CR (1982) Granitic landforms. Elsevier Scientific Publishing Twidale CR (1990) The origin and implications of some erosional landforms. J Geol 98:343–364 Twidale CR (1998) Granitic bornhardts: their morphology, characteristics and origins Twidale CR (2007a) Ancient Australian Landscapes, New South Wales. Rosenberg Publishing Coy, 144 p Twidale CR (2007b) Bornhardts and associated fracture patterns. Revista Assoc Geologica Argentina 62(1):139–153 Thomas MF (1994) Geomorphology in the Tropics. Wiley, Chichester Thomas MF (1966) Some geomorphological implications of deep weathering patterns in crystalline rocks in Nigeria. Trans Inst Br Geogr 173–193 Thomas MF (1968) Some outstanding problems in the interpretation of the geomorphology of tropical shields. British Geomorph Res Group, Publ 5, p 4 Wilson HW (1900) XX. On the velocity of solidification and viscosity of super-cooled liquids. Lond Edinb Dublin Philos Mag J Sci 50 (303): 238–250

6

South-East Hills and Ridges Olayinka O. Ogunkoya

Abstract

South-east Nigeria is endowed with picturesque massifs, escarpments, residual hills and ridges, some of which are hydrothermally mineralized or contain beds of fossil fuels. In addition, dry valleys, springs, and caves occur. The landscape displays a gradual rise from the south to the Obudu Highland in the north-east, but a similar ascent from the west to the east, towards the maximum elevation in the Obudu Highland, is interrupted by the Nsukka Escarpment and associated hills, which form an elongated north-east to the south-west ridge. The escarpment is a 400-km-long seahorse-shaped structurally monoclinal line of hills, extending from beyond the Nsukka area in the north to Afikpo in the south. It is about 60 km wide in the Nsukka area but tapers to 2 km at Afikpo and has a maximum elevation of 590 m a.s.l. at Ukehe. Keywords










South-east Nigeria Massifs Escarpments Springs Dry valleys Falls Rapids

6.1




Caves




The three regions comprise distinctive relief features including highlands, plateaus, uplands, escarpments, plains, the delta, and coastal wetlands. Thus, the north has the Jos Plateau located in its eastern central area, the Mandara Mountains along the eastern border north of the Benue valley, and the Hausa Plains in its western central area. The east, a section of which is the main focus of this chapter, has the Eastern Borderland Highlands (Obudu Highland and Oban Massif), escarpments, plains, and the Niger delta. South-east Nigeria is the part of eastern Nigeria having a western border formed by the main trunk valley of the River Niger, and an eastern border defined by the international boundary between Nigeria and the Republic of Cameroon. The area excludes the Niger delta and the coastal part east of the delta. Specifically, the area could be taken to commence at Idah (7° 06′ N 6° 44′ E) on the main trunk of R. Niger, and terminate at Aboh (5° 33′ N 6° 32′ E), just before the bifurcation of the R. Niger. Along the international border with the Republic of Cameroon, it has a southernmost point at Ikang (4° 48′ N 8° 33′ E), south-east of Calabar, and a northernmost point at Sonkwala (6° 28′ N 9° 28′ E), east of Obudu (Fig. 6.2). This latter demarcation is based on the limits of the Eastern Highlands and the geological history of the area.

Introduction

Nigeria is naturally divided into three regions (the north, west, and east) by the Niger and Benue rivers. The River Niger enters Nigeria through the north-western section of the country, while the River Benue enters approximately midway along the eastern section. The rivers join at Lokoja, from where the Niger flows southwards and forms a large delta extending over 29,000 km2 (NDES 1999) at the Gulf of Guinea (Fig. 6.1). O. O. Ogunkoya (&) Department of Geography, Obafemi Awolowo University, Ile Ife, Nigeria e-mail: [email protected]; [email protected]

6.2

Geology

Geologically, the area is a part of the Southern Benue Trough and constitutes the eastern, and partly the northern borders of the modern Niger delta. It could be considered, except for the Eastern Borderland Highlands, as an extension of the delta, but its sedimentary rocks were deposited between the mid-Cretaceous (Albian) and Paleocene, while the formation of the delta began in the Eocene. However, the sedimentary rocks are typical of most deltaic environments and were laid down under different environments - marine, continental, and mixed (Asseez 1989; Kogbe 1989). This

© Springer Nature Switzerland AG 2023 A. Faniran et al. (eds.), Landscapes and Landforms of Nigeria, World Geomorphological Landscapes, https://doi.org/10.1007/978-3-031-17972-3_6

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Fig. 6.1 Generalized relief map of Nigeria

Fig. 6.2 South-east Nigeria

accounts for some of the economic resources, e.g. coal and limestone associated with the area. South-east Nigeria can be divided into four physical (geographic) zones:

• the Eastern Borderland Highlands, being extensions of the Bamenda Highlands of the Republic of Cameroon; • the Abakaliki—Cross River Plains, located to the immediate west and south of the Borderland Highlands;

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South-East Hills and Ridges

• the Nsukka Escarpment and associated hills which border the Cross River Plains to the west; and • the Anambra Plains, which lie between the Nsukka Escarpment and the main trunk of R. Niger (cf. Floyd 1969; Nwachukwu 1978) (Fig. 6.2). The Eastern Borderland Highlands comprise igneous and metamorphic rocks, mainly of the Precambrian Basement Complex. To the west and south of this zone, the area is geologically known as the Abakaliki Anticlinorium, the Mamfe Embayment, and the Calabar Flank (Murat 1972). The Basement Complex in this area is believed to have undergone a series of igneous and metamorphic deformation

Fig. 6.3 Structural units of SE Nigeria (after Short and Stauble 1967)

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(Ekwueme 1990) and has been overlain by approximately 1000-m-thick Cretaceous to Tertiary sedimentary rocks— shales, sandstones, siltstones, and sandy limestone lenses that have formed the Abakaliki—Cross River Plains (Figs. 6.2 and 6.3). The sedimentary formations, which belong to the ‘Asu River Group’—the oldest (Cretaceous) in southern Nigeria, and comprise the Eze Aku Shale Group and the Awgu Sandstones (Figs. 6.2 and 6.4), are folded in the area south of Abakaliki, with the fold axes aligned NE-SW (Kogbe 1989). The sedimentary rocks of the Anambra Basin, comprising both Tertiary marine shales and continental sediments, occur to the west of the Abakaliki— Cross River Plains. The mainly dark shales and mudstones

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with occasional thin beds of sandy shales, sandstones, limestones and oolitic ironstones, crop out in the western part of the basin. They constitute three formations of the Nkporo Group—Nkporo Shale, Owelli Formation, and Enugu Shale. The eastern section of the Anambra Basin was further subjected to down-warping induced by tectonic activities, and subsequent sedimentation. These latter sediments are the youngest (Campanian—Palaeocene) formations in south-east Nigeria and comprise the Coal Measures Group – Mamu, Ajali and Nsukka Formations, from base to top. The Mamu (Lower Coal Measures) and Nsukka (Upper Coal Measures) Formations range from paralic to marine and are both coal-bearing cyclothymic alternating successions of sandstone, dark shale, and sandy shale with thin coal seams at various horizons. The Ajali Formation (‘False-bedded Sandstones’) is paralic, consisting predominantly of thick, friable, poorly sorted, iron-stained sandstones, mudstones, and clays. Where the Nsukka Formation has been eroded, the Ajali Formation is overlain by a considerable thickness of sandy red earth formed from the weathering and ferruginization of the formation (Nwachukwu 1978; Asseez 1989; Figs. 6.3 and 6.4). The entire area now known as the West African region, and more so the South-east Nigeria part of it was subjected to multiple cycles of tectonism. Initial tectonic movements during the Cenozoic in relation to magmatism fostered uplift, particularly on the eastern frontier of Nigeria (Grove 1968; Grant et al. 1972). Others created the Benue Trough. The Trough subsequently experienced cycles of marine transgression and regression, resulting in various environments of sedimentation and different types of sediments. Nwajide (1990) reported another phase of tectonic activity that involved deformation, folding, faulting, and uplift of the older sediments leading to the formation of the Anambra Basin, which evolved as a depression to the west of the Eastern Borderland (see also Benkhelil 1987; Adeigbe and Salufu 2009). This was succeeded by epochs of sedimentation and tectonic movement. There was differential uplift along a NE-SW axis giving a greater rise in the east than in the west. This westward tilt caused a scarp slope on the eastern side, a gentle slope on the western side, and warping along the NE-SW axis. These are held to have fostered the formation of the Nsukka Escarpment and the present-day landscape of South-east Nigeria (Umeji 1988). Tectonic activities also affected the sediments of the Abakaliki Anticlinorium, Mamfe Embayment, and the Calabar Flank. These were folded, faulted, and intruded by sills and dykes. The sediments are folded particularly to the south of Abakaliki, with the fold axes stretching NE-SW and the beds associated with lead–zinc mineralization (Kogbe 1989, p. 325). Offodile (1989, p. 375) suggested that the mineralization is hydrothermal in origin and associated with Tertiary to recent volcanism in the Eastern Borderlands. The

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Fig. 6.4 Geology of the Nsukka–Okigwe Scarpland (after Pritchard 1979, p. 81)

ores occur in the fractured zones in shales, mudstones, and sandstones. Consequently, the landscape of South-east Nigeria shows a gradual rise from the south to the mountains of the Obudu Highland in the north-east (maximum elevation 1935.5 m a. s.l.). A similar trend of ascent from the west to the east, towards the maximum elevation on Oban Massif (1067 m a. s.l.) and Obudu Highland, is interrupted by the Nsukka– Udi–Okigwe–Afikpo Escarpment and associated hills, which form an elongated NE–SW-trending ridge (Floyd 1969) (Fig. 6.1).

6.3

The Eastern Borderland Highlands

The Eastern Borderland Highlands, comprising the Oban Massif, Obudu Highland, and Sonkwala Mountains, consists of extensive high-level plains on which numerous inselbergs and ridges have developed (Figs. 6.5 and 6.6). They are

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Fig. 6.5 Typical topography of the Obudu Highland. https://hotels.ng/guides/everything-need-know-obudu-mountain-resort/.Clarke

denudation surfaces cut across igneous and metamorphic rocks, mainly of the Precambrian Basement Complex suite (granite, gneiss, quartzite, amphibolite, metagabbro, dolerites, and schist), but also comprising some more recent igneous intrusive rocks (emplaced during the Pan-African thermotectonic events c.550 ± 100 Ma) (Ekwueme 2003; Ephraim et al. 2006). The highlands rise gradually north-eastwards from the south near the valley of River Kwa (at 5° 24′ N, 8° 30′ E), where they have an average elevation of 160 m a.s.l. The initial peak (1067 m a.s.l.) in the rise towards the north-east is on the Oban Massif. From here, the elevation declines towards the valley of River Cross both to the west and north (Fig. 6.2) and then rises north-eastwards to attain the maximum elevation of 1935.5 m a.s.l. on the Obudu Highland. The highlands are divided into two groups (a southern group—the Oban Massif and a northern group—the Obudu Highlands and Sonkwala Mountains) by the north-westward/ westward-trending valley of the River Cross, which has its headwaters in the Bamenda Plateau of the Republic of

Cameroon. The highlands constitute headwaters for some of the tributaries of the principal rivers in the area, namely the River Cross and River Katsina-Ala, a tributary of the Benue (Fig. 6.2). These rivers and their tributaries sculpt the landscape, while their valleys, marked by falls and rapids, constitute the lowest parts of the area. The area is well-drained, with the drainage patterns controlled by fractures and joints. The Oban Massif comprises a series of flat-topped hills located between 5° 00′ and 5° 51′ N, and 8° 00′ and 8° 55′ E. The massif, which has an average elevation of 1006 m a.s.l., has been sculpted into a number of isolated hills, separated from each other by deeply incised valleys of, from west to east, the Rivers Calabar, Kwa, Iyang Itu, Ebe, Eku, and Aning. All these drain southwards into the estuary of River Cross or the Rio Del Rey. The massif, a south-western extension of the Bamenda Highlands of the Republic of Cameroon into Nigeria, has steep northern and southern flanks, but much gentler western slopes. The dominant rock is the pelitic schist. Other rocks are gneisses, amphibolites,

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Fig. 6.6 View from a part of Obudu Highland, looking across the mountain range. Inyang effiong. Creative Commons Attribution-Share Alike 4.0 International licence

Fig. 6.7 Geologic map of Amingeje and its environments (After Momta and Essien 2016)

quartzites, and some igneous intrusive rocks, mainly pegmatites and dolerites, which occur as veins, dikes, and sills. They foster the occurrence of falls and rapids, e.g. the Kwa Falls, where they inhibit the formation of smooth longitudinal profiles along river valleys. Momta and Essien (2016) presented a detailed distribution of rock types around the valley of River Kwa, along the channel to which the rocks are well exposed (Fig. 6.7). The massif is located in a thick equatorial rain forest, and the rocks have been intensely weathered and altered to reddish-brown saprolite (Fig. 6.8).

The Obudu Highland (Latitude 6° 30′–6° 45′ N; Longitude 9° 15′–10° 00′ E), described as a Precambrian Basement horst (Ushie and Anike 2011), is a part of the Sonkwala Mountain range, a westward extension of the Bamenda Plateau of Cameroon into south-eastern Nigeria (Orajaka 1964; Umeji 1988; Ekwueme 1991). The Obudu Highland is a large massif covering an area >100 km2 and with a peak height of 1716 m a.s.l. The rocks are similar to those of the Oban massif (migmatite-gneiss schist complex with the subordinate occurrence of amphibolites, charnockites, and minor intrusions of granites, dolerites, and

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include Guinea Savanna and rain forests. The Guinea Savanna is characterized by very tall grasses interspersed with trees. Tall trees of the rainforest type abound around the river valleys. The common trees of the Guinea Savanna are acacias, baobab, and shea butter.

6.4

Fig. 6.8 Reddish-brown saprolite of the Kwa Falls area

gabbros). The topography is rugged, comprising northeasterly trending ridges separated by low-lying valleys and passes. As is the case with the Oban Massif, the northern and southern flanks of the Obudu Highland are steep, but the western slopes are much gentler and fade out into the Abakaliki—Cross River Plain. The Obudu Highland constitutes the highest part of south-eastern Nigeria. The highland receives 4200 mm of rainfall during the rainy season, which extends from April to November. The Sonkwala Mountains promote an orographic effect on incoming moisture-bearing South-Westerlies, fostering heavy rainfall. The rainfall and tropical location facilitate deep weathering (Ushie and Anike 2011). However, most rivers are seasonal, over-flowing their banks in the rainy season, but completely drying out in the dry season. The drainage is structurally controlled. The vegetation types

The Nsukka Escarpment and Plains

The landscape referred to as the Nsukka Escarpment is part of a seahorse-shaped structurally monoclinal line of hills, which in its full length extends for approximately 400 km from the eastern bank of the main trunk of River Niger in the area immediately south of the Niger–Benue confluence, eastwards beyond Ankpa (7° 22′ N 7° 38′ E), and then southwards to Okigwe (5° 49′ N 7° 20′ E), beyond which it turns eastwards to terminate at Afikpo (5° 54′ N7° 57′ E) (Iloeje 1961) (Figs. 6.2 and 6.9). The landscape is not one continuous unit, but consists of two distinct escarpments that are semi-parallel to each other for a distance of approximately 15 km, with a gap between them ranging from 4.8 to 6.4 km wide (Fig. 6.10; Ofomata 1975). The elevated land narrows from north to south and changes shape in the south to become a hog-back ridge. The dip slope of the northern and southern sections of the escarpment (i.e. the head and tail of the ‘sea horse’) trends southward, while the dip slope of the main body trends westward. The landscape can be resolved into several prominent landforms that include cuestas, residual hills, dry valleys, and caves. In South-east Nigeria, the escarpment is known as the Nsukka–Udi–Okigwe–Afikpo Escarpment. It constitutes a drainage divide separating rivers flowing westwards (River Anambra and its tributaries) and draining into the River Niger, from those flowing eastwards and southwards and draining respectively into Cross, Imo, and Kwa Ibo rivers. Numerous springs and rivulets issue from the base of the east-facing scarp slope and these form the headwaters of tributaries of River Cross, namely Asu, Aboine, Ivo, and the other aforementioned rivers. All these rivers drain the Cross River Plains. Fewer streams (tributaries of River Anambra, namely Adada, Nnom, Ajali, and Oji rivers) rise from the dip slope (Fig. 6.10). The Nkporo Shales form the foot of the escarpment, while resistant mudstones of Mamu Formation form the lower slope. The friable Ajali Formation forms the middle slope, while the Nsukka Formation, which consists of coarse sandstone, ironstone, and ferruginized shale, forms the upper slope and the top of the plateau, though most of this formation has been removed by erosion. In the Nsukka area, the caprock of the scarp face is the Nsukka Formation (Upper Coal Measures) (Fig. 6.11a, b). In the Enugu area, it is the thick, friable, permeable, poorly sorted medium-grained sands of the Ajali Formation (False-bedded Sandstones)

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Fig. 6.9 Nsukka Escarpment and plains (the red lines are section boundaries)

Udi-Awgu Sec on

Okigwe-Afikpo Section

overlain by a considerable thickness of ferruginized red sands. In the Udi area, the red sands are up to 30 m thick, while in the Enugu area, they have a thickness of approximately 45 m. Further south in the Awgu area, the caprock comprises the resistant Awgu Sandstone, while in the Okigwe area, the Mamu Formation (Lower Coal Measures) constitutes the caprock. The diverse lithology of these different formations accounts for the variation in the prominence of the relief of the escarpment and in particular, that of the scarp face (Ofomata 1967). Assuming the 200-m contour as the boundary of the Nsukka Escarpment, the landscape can be divided into four

sections based on the maximum elevation of the scarp face, and the width of elevated land: 1. 2. 3. 4.

Enugu Ezike (Nsukka)–Udi Udi–Awgu Awgu–Okigwe Okigwe–Afikpo.

The escarpment is widest in the Nsukka—Udi section, with a width ranging from *60 km in the Nsukka area to *18 km in the Udi area. It also has the highest elevation rising 250–400 m above the Cross River Plains, with the

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Fig. 6.10 Nsukka–Udi–Awgu Scarpland (after Ofomata 1975, p. 36)

crest elevation increasing northwards from 453 m a.s.l. on a hill (6° 16′ 30″ N 7° 23′ 40″ E) south-west of Udi and north-east of Umuagu through 524 m a.s.l. on Nsude Hill (Figs. 6.4 and 6.10), south of the Ninth Mile Corner, and 542 m a.s.l. at Abor, north-west of Enugu, to 590 m a.s.l. on a hill (6° 38′ 30″ N 7° 24′ E) in Ukehe. Based on measurements from the Nigeria 1:50,000 topographic maps, the maximum scarp slope in the Ukehe area is 55°. Elevation declines northwards from here to 472 m a.s.l. on a hill (6° 58′ N 7° 28′ E) at Enugu Ezike (Fig. 6.10). Elevation also declines southwards from Udi, and as already noted, the escarpment is narrowest and has the lowest elevation of about 240 m a.s.l. in the Okigwe–Afikpo section, where it has a width ranging between 2 and 7 km. In the Udi–Awgu and Awgu–Okigwe sections, the escarpment is supported by massive, resistant Awgu sandstones and the Mamu Formation (Lower Coal Measures). Here, the maximum scarp slope

is 27°, with a maximum elevation of 427 m and a width of 13 km, while southwards of Okigwe, the slope is gentler and the crest lower (244 m). The maximum scarp slope in the Nsukka–Enugu area ranges from 34° near Nsukka, through 55° at Ukehe to 69° at Enugu. The relative prominence of relief of the four sections of the escarpment and the extent of denudation, which are functions of lithology, climate, and the relative importance of fluvial erosion over mass wasting, can be depicted using hypsometric analysis (cf. Hutrez et al. 1999; Singh et al. 2008). The analysis was originally designed to study the erosional status of a drainage basin (Strahler 1952) and thus to express the unconsumed volume of a drainage basin as a percentage of that delimited by the summit plane, base plane, and perimeter (Dowling et al. 1998). It has therefore also been extended to describe the percentage of any landform yet to be consumed by erosion, assuming the landform

98 Fig. 6.11 a, b Ironstone of the Nsukka Formation forming the (approximately 30–40 m thick) caprock in the Nsukka area

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a

b

originated as an uplifted mass. The hypsometric analysis relates elevation to the area and indicates the distribution of mass above the datum. The form of the curve is indexed by the Hypsometric Integral (HI), which expresses, as a percentage, the magnitude of the original landform that remains after the uplifted mass began to be subjected to denudation. Most HI values range from 20 to 80%, with higher values indicating that large areas of the original landform have not yet been altered into slopes. The shape of a hypsometric curve is also an indicator of dominant geomorphic processes

at work, whether mass movement or fluvial erosion. A convex curve suggests the dominance of mass movement, while a concave curve suggests that fluvial processes dominate (cf. Talampas and Cabahug 2015). Figure 6.12a, d present the results of hypsometric analyses of the four sections of the escarpment and show among others that though the Nsukka–Udi section has the steepest scarp face, a function of the degree of resistance and latitudinal extent of the Nsukka Formation caprock, the landscape here has been generally significantly eroded by fluvial

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(a)

(b)

(c)

(d) Fig. 6.12 a Hypsometric analysis of the Nsukka–Udi section of the escarpment. b Hypsometric analysis of the Udi–Awgu section of the escarpment. c Hypsometric analysis of the Awgu–Okigwe section of the escarpment. d Hypsometric analysis of the Okigwe–Afikpo section of the escarpment

action, hence the concave hypsometric curve. The Udi– Awgu section has the highest hypsometric integral, with the hypsometric curve depicting the highest convexity of all four sections. This, as noted earlier, should be due to the degree of resistance to erosion of the Awgu Sandstone and the Mamu Formation components of the escarpment in this section (Fig. 6.4), and probably mass movement is the dominant process of material transport. The Okigwe–Afikpo section is the most subdued of the four sections, probably because of the very limited extent of resistant rocks in the area and the dominance of fluvial processes. It is apparent from the above that the main issue is to account for the development of the escarpment and its associated features. This implies explaining how an escarpment that tapers southwards to a tenth of its width in the north, and with a highly indented scarp face, and dip slopes studded with residual hills, has developed from an initial land surface underlain by a series of sedimentary rock formations comprising different lithologies. In this context, it could first be pointed out that southern Nigeria was subjected to a series of tectonic events commencing with the formation of the Benue Trough and the separation of the South American continental landmass from the West African landmass. There were subsequent tectonic activities after the deposition of a series of sedimentary formations, culminating in the formation of anticlines and synclines west of the present-day Bamenda Hills and its western extension, the Eastern Borderland Highlands of South-east Nigeria (Pritchard 1979, p. 81). Compressional fracturing of the longer anticline (the current Okaba– Nsukka–Okigwe–Afikpo anticline) and erosion by consequent and subsequent rivers created the Nsukka–Udi– Okigwe–Afikpo cuesta, with an east-facing scarp slope and a west-facing dip slope, and the exposure of the sedimentary formations that constitute that part of the Anambra Basin. Subsequent rivers created the low-lying terrain in the northern part of the Cross River Plain. Figure 6.13 shows in more detail the probable development of the longitudinal consequent river along the main axis of the synclines and the other rivers in different parts of the landscape. The consequent rivers, e.g. River Aboine (eastern syncline) and River Anambra (western syncline), developed lateral consequent tributaries. Subsequent rivers may also have developed along compressional lines of weakness at the crest of the anticlines. Tributary obsequent rivers could have drained into a subsequent river. The degree of resistance of the underlying geology, lateral erosion, valley incision, and headward erosion by the combined action of these rivers has probably culminated in the development of the current landscape, its characteristic three zones and associated landforms, particularly in the Nsukka–Udi section, and the contemporary drainage pattern. The three zones are the plateau (the dip slope of the escarpment), the scarp face, and the plain (the eastern syncline of the original folded structure) (Fig. 6.14).

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their occurrence is due to lateritic concretions of the Nsukka Formation, which render them resistant to erosion; hence the flat tops (cf. Nwadialo 1989). The hills are however more numerous and massive in the Okigwe area than in the Nsukka area. The slopes of tabular and conical hills are concave.

Fig. 6.13 Block diagram showing drainage initiation and cuesta formation

The landscape in the Nsukka area is characterized by residual hills separated by wide dry valleys, particularly on the dip slope (Figs. 6.14 and 6.15a, b). These residual hills are remnants of the weathered and eroded Nsukka Formation. Some distinct forms can be identified (cf. Ofomata 1967): 1. Domical and conical hills. These occur where the caprock is not indurated or concretionary, e.g., Nsude Hill and Idi Hill (579 m a.s.l.) near the Opi Junction (Fig. 6.16a, b). 2. Ridges. These are common between Okutu (6° 57′ N 7° 15′ E) and Afa (6° 36′ N 7° 18′ E), and Nsukka and Leje (6° 45′ N 7° 22′ E), e.g. Ochue-Nkpologu Hill (6° 47′ N 7° 16 E). They appear as long, obtusely shaped landforms with a flat top (Fig. 6.16c). 3. Mesas. A typical example is the Elenkeu Hill (6° 34′ N 7° 25′ E) at Okpatu-Ukehe. It has been suggested that

The east-facing scarp slope of the Nsukka–Udi section of the escarpment has been considerably indented by deep river valleys and intense gullying. Most of the rivers flow in deep ravines incised into the False-bedded Sandstones (Ajali Formation) and their surficial sands. The streams that rise near the base of the False-bedded Sandstones are perennial, but valleys at higher levels are dry except for short periods during rains (Ofomata 1967). It is believed that the dry valleys were formed by the incision of consequent/insequent rivers through less permeable formations such as the Nsukka Formation (Upper Coal Measures) to the permeable Ajali Formation (False-bedded Sandstones), within which the rivers disappeared only to reappear at the contact with the Mamu Formation (Lower Coal Measures). In the Enugu area, the scarp face is virtually vertical reflecting basal sapping and collapse. The resulting scree lies on the concave slope formed by the Mamu Formation (Ofomata 1967; Umeji and Chigbu, nd). There are numerous short, shallow caves on the scarp face, particularly where more resistant rock is underlain by sandy lithology (Fig. 6.17). Caves are natural cavities in rock masses and include features such as shallow rock overhangs commonly described as rock shelters. The caves are believed to have developed as part of fluvialdenudational processes shaping scarplands underlain by clastic lithology. Thus, cave formation and their life-cycle of incipience, maturity, decay, and collapse are part of the process of scarp retreat (Umeji and Chigbu, nd https://www.

Fig. 6.14 Block diagram showing the current topography and geological structure of the Nsukka–Udi Escarpment and its development (after Pritchard 1981, p. 81)

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a

b Fig. 6.15 a, b Remnant (after road construction) indurated caprock in the Nsukka area

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a

b Fig. 6.16 a Domical hill near Opi, Nsukka. b Conical hill near Opi, Nsukka. c. Ridge (long, obtusely-shaped landform with flat top) in the Nsukka area. a, b and c Ridge, domical and conical hills near Opi, Nsukka

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c Fig. 6.16 (continued)

Fig. 6.17 Caves in the Ehamufu area, North-east of Nsukka

researchgate.net/profile/Obianuju_Umeji/publication/32076 5205_Quaternary_landforms_in_eastern_Nigeria/links/59f9 e3b5aca27221807e94e3/Quaternary-landforms-in-easternNigeria). These authors noted the association between

sandstone formations, fluvial degradation of escarpments, and cave formation, and that low dips to horizontal attitudes exhibited by the sediments of the escarpment have influenced the development of the caves. They explained that the

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caves, which are invariably found along the scarp face, may have been formed through two sets of processes. According to Nwadialo (1989), weathering and removal of weathered debris by mass wasting can lead to cave formation. These include rock-shelter-type caves, which are held to develop from damp concavities at the foot of cliffs. They are formed through differential chemical weathering of soft bands beneath more resistant overlying beds, and the removal of weathered debris by water through hydration and mass wasting. The more resistant bed becomes the cave roof. Examples such as the Ugwu-Nruru caves at Eha-Alumona (Nsukka area) are common in the thinly bedded layers of the sandstone of the escarpment. Ofomata et al. (1981) explained the formation of the caves as due to weathering and erosional processes of corrasion (wearing down; deepening or down-cutting of river floor or channels) and corrosion (solution and lateral widening of river valleys), e.g. Okpu Chukwu Cave at Isuikwuato. At the senile stage of cave history, the roof rock is worn down by spalling on the inside, which reduces roof thickness. The pervasive action of anastomosing roots and rootlets in the roof shatters the rocks. Leakage eventually causes roof collapse. A deep gully is left at the former site of the cave which might be connected to an existing fluvial drainage channel. Cave decay results in the general wearing down of the scarp face and the generation of new slopes.

6.5

Conclusion

The dominance of the R. Niger delta in the topography of south-eastern Nigeria has fostered a perception that this part of Nigeria comprises a subdued landscape of plains and wetlands. This is however far from the reality. South-east Nigeria is imbued with picturesque plains, hills, ridges, and massifs, some of which are well mineralized or contain beds of fossil fuels. This chapter has described these massifs; escarpments, ridges and hills; and plains in terms of their shape, constituent rocks, and mode of formation. These geomorphological features are respectively, the Oban Massif, Obudu Highland, and Sonkwala Mountains of the Eastern Borderland Highlands; the Nsukka–Udi–Okigwe– Afikpo escarpments, and the plains respectively to the west of these landscapes. The Eastern Borderland Highlands comprise mainly Precambrian Basement igneous and metamorphic rocks, which have been denuded into extensive high-level plains on which numerous inselbergs and ridges have developed. They constitute headwaters for some of the tributaries of the principal rivers in the area, namely the River Cross and River Katsina-Ala, a tributary of the Benue. The valleys of these rivers and their tributaries are marked by falls and rapids. The escarpment comprises two semi-parallel structurally monoclinal lines of hills, which

narrow from north to south and change shape in the south to become a hog-back ridge. They also constitute drainage divides separating rivers flowing westwards (River Anambra and its tributaries) and draining into the River Niger, from those flowing eastwards and southwards and draining respectively into Cross, Imo, and Kwa Ibo. The escarpment can be resolved into several prominent landforms that include cuestas, mesas, domical and conical hills, dry valleys, and caves.

References Adeigbe OC, Salufu AE (2009) Geology and depositional environment of Campano-Maastrichtian sediments in the Anambra basin, southeastern Nigeria: evidence from field relationship and sedimentological study. Earth Sci Res J 13(2):148–166 Asseez LO (1989) Review of the stratigraphy, sedimentation and structure of the Niger Delta. In Kogbe CA (ed) Geology of Nigeria. 2nd revised edition. Published by Rock View (Nigeria) Ltd., Jos, Nigeria, pp 311–324, 538p Benkhelil J (1987) Cretaceous deformation magmatism and metamorphism in the Lower Benue Trough. Nigeria Geol J 22:467–493 Dowling TI, Richardson DP, O’Sullivan A, Summerell GK, Walker J (1998) Application of the hypsometric integral and other terrain-based metrics as indicators of catchment health: a preliminary analysis. CSIRO Land and Water, Canberra. Technical Report 20/98. April 1998, 49p Ephraim BE, Ekwueme BN, Adamu IC (2006) Preliminary report on the geology of northeast Obudu, Bamenda massif, southeastern Nigeria. Int J Nat Appl Sci (IJNAS) 1(1):84–89 Ekwueme BN (1990) Petrology of southern Obudu Plateau, Bamenda Massif, Southeastern Nigeria. In Rocci G, Deschamps M (eds) Recent Data in African Earth Sciences. CIFEG: Paris, France, vol 22, pp 155–158 Ekwueme BN (1991) Geology of the area around Obudu cattle ranch, south eastern Nigeria. J Min Geol 27(1):129–134 Ekwueme BN (2003) The Precambrian geology and evolution of the southeastern Nigerian basement complex. University of Calabar Press, Calabar, 135p Floyd B (1969) Landforms, relief and associated drainage features, Chapter 5. In: Eastern Nigeria. Palgrave Macmillan, London, pp 82–97, 359p Grant NK, Rex D, Freeth SJ (1972) Potassium-Argon ages and Strontium isotope ratio measurements from volcanic rocks in northeastern Nigeria. Contrib Miner Petrol 35:277–292 Grove AT (1968) Quaternary landforms and climate on the south side of the Sahara. Geogr J 134:194–208 Hutrez JE, Lucazean F, Lave J, Avouac JP (1999) Investigation of the relationship between basin morphology, tectonic uplift and denudation from the study of an active fold belt in Siwalik hills (Central Nepal). J Geophys Res 104:779–796 Iloeje NP (1961) The structure and relief of the Nsukka-Okigwi Cuesta. Nigeria Geograph J 4(1):21–39 Kogbe CA (1989) The Cretaceous and Paleogene sediments of southern Nigeria. In Kogbe CA (ed) Geology of Nigeria. 2nd revised edition. Rock View (Nigeria) Ltd., Jos, Nigeria, pp 325–334, 538p Momta PS, Essien NU (2016) The geology and structural evolution of the Aningeje Metasediment in the lower part of the Oban Massif, south-eastern Nigeria. J Geography, Environ Sci Int 4(1):1–16 Murat RC (1972) Stratigraphy and Palaeo-geography of the cretaceous and lower Tertiary in Southern Nigeria: In: Dessauvagie TPJ,

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Whiteman AJ (eds) African Geology. University of Ibadan, Nigeria, pp 251–266, 425p NDES (Niger Delta Environmental Survey) (1999) Niger Delta Environmental Survey Phase 1 Report (Revised Edition), vol 1: eNVIRONMENTAL and Socio-economic characteristics. ERML, Lagos, 270p Nwachukwu SO (1978) The geology of Nsukka area. In: Ofomata GEK (ed) The Nsukka environment. Fourth Dimension Publishers, Enugu, pp 47–58 Nwadialo BE (1989) Soil-landscape relationships in the Udi-Nsukka Plateau. CATENA 16(2):111–120. https://doi.org/10.1016/03418162(89)90035-0 Nwajide CS (1990) Cretaceous sedimentation and paleogeography of the Central Benue Trough. In: Ofoegbu CO (ed) The Benue trough structure and evolution International Monograph Series, Braunschweig, pp 19–38 Offodile ME (1989) A review of the geology of the cretaceous of the Benue valley, Chapter 22. In: Kogbe CA (ed) Geology of Nigeria, 2nd revised edition. Published by rock view (Nigeria) Ltd., Jos, Nigeria, pp 365–376, 538p Ofomata GEK (1967) Landforms on the Nsukka Plateau of eastern Nigeria. Nigerian Geograph J 10:3–9 Ofomata GEK (1975) Nsukka-Okigwe cuesta. In: Ofomata GEK (ed) Nigeria in maps: Eastern States. Ethiope Publishing House, Benin City, Nigeria, pp 35–37, 146p Ofomata GEK, Nwafor JC, Okafor DO (1981) Caves in the Agbogugu area of Anambra State Nigeria. Nigerian Field 46(3):129–145

105 Orajaka S (1964) Geology of the Obudu area, Ogoja Province eastern Nigeria. J Sci 3:73–96 Pritchard JM (1979) Landform and Landscape in Africa. Edward Arnold, London, 136p Short KC, Stauble AJ (1967) Outline of geology of Niger delta. Am Assoc Petroleum Geolog Bullet 51:761–779 Singh O, Sarangi A, Sharm MC (2008) Hypsometric integral estimation methods and its relevance on Erosion status of North-Western Lesser Himalayan Watersheds. Water Resour Manage 22:1545– 1560. https://doi.org/10.1007/s11269-008-9242-z Strahler AN (1952) Hypsometric (area-altitude) altitude analysis of erosional topography. Geol Soc America Bull 63:1117–1142 Talampas WD, Cabahug RR (2015) Catchment characterization to understand flooding in Cagayan De Oro River Basin in Northern Mindanao, Philippines. Mindanao J Sci Technol 13:213–227 Umeji AC (1988) The Precambrian of South Eastern Nigeria, a magmatic and tectonic study. In: Oluyide PO, Mbonu WC, Ogezi AEO (eds) Precambrian Geology of Nigeria. Geological Survey of Nigeria Publication, pp 69–75 Umeji OP, Chigbu E (nd) Quaternary landforms in eastern Nigeria: caves in non-karstic lithology. https://www.researchgate.net/profile/ Obianuju_Umeji/publication/320765205_Quaternary_landforms_ in_eastern_Nigeria/links/59f9e3b5aca27221807e94e3/Quaternarylandforms-in-eastern-Nigeria Ushie FA, Anike OL (2011) Lateritic weathering of granite-gneiss in Obudu Plateau, south eastern Nigeria. Global J Geol Sci 9(1):75–83

7

The Niger Delta Region Charles Uwadiae Oyegun, Olanrewaju Lawal, and Mark Ogoro

Abstract

7.1

The Niger Delta is the largest in Africa and the third-largest in the world, with a total area of 112,106 km2. The region has the largest freshwater swamp in Africa and is a biodiversity hot spot because of its rich variety of plant and animal species. This chapter describes geomorphological/geological units, with an emphasis on physiography, land cover characteristics and hydrocarbon pollution. The plains are homo-clinal geomorphic structures that trend westwards and south-westwards. This trend is broken in many places by small hogback ridges and shallow swamp basins, which abut at the coast against the sandy beach-ridge barriers lying between the tidal basins and the open sea. The gently undulating coastal lowland inland from this beach-ridge barrier zone has height ranges between 2.05 and 45 m above sea level. The geomorphological units of the region include the outer barrier island complexes adjoining the Atlantic Ocean, the lower tidal floodplain which consists of estuaries, mangroves, and creeks, and the upper freshwater riverine floodplain, within which seven ecological units are formed. Keywords





Niger Delta Geomorphologic units Deforestation Sea-level rise




Land-use change

C. Uwadiae Oyegun
O. Lawal
M. Ogoro (&) Department of Geography and Environmental Management, University of Port Harcourt, Port Harcourt, PH, Nigeria e-mail: [email protected] C. Uwadiae Oyegun e-mail: [email protected]




Introduction

The arcuate Niger Delta is situated between latitudes 4° and 6° north of the equator and longitudes 4° and 8° east of the Greenwich Meridian (Fig. 7.1). This sedimentary environment is built by the Niger/Benue river system at its entrance into the Atlantic Ocean in the Gulf of Guinea. The River Niger with its tributaries forms the major drainage system of the West African sub-region, with an approximate length of over 5000 km and a catchment area of 2,117,700 km2. The Niger Delta is the largest in Africa and the third-largest in the world, with a total area of 112,106 km2 (Amangabara and Obenade 2015; Williams 2018). The region also hosts the largest freshwater swamp in Africa, and it is a biodiversity hot spot because of its rich variety of plant and animal species. The delimitation above covers both the geological and political definition of the Niger Delta because the adjoining states which produce oil are locally integrated into the definition of the region. The Niger Delta has a projected population of 42,436,000, with a population density of 200 persons per square kilometre (as of 2018), 185 local government areas, and 40 different ethnic groups with over 250 languages and dialects (Akuodu 2011; Williams 2018). According to UNDP (2006), the states with the highest population include Rivers, Delta, Akwa Ibom and Imo State. The region is also the largest wetland in Africa and extends from Onitsha in the north for about 240 km southwards to the tip of the Nun River on the Atlantic Ocean. The geological Niger Delta is over 480 km wide, stretching from the Benin River in the west to the Imo River estuary in the east. This vast area contains the largest mangrove forest in the world and serves as the spawning ground for 60% of the fishes in the Gulf of Guinea (IUCN 2013). This chapter describes the geomorphological/geological units within the delta, with an emphasis on physiography, land cover characteristics, and hydrocarbon pollution. The adjoining states that produce oil included and Edo to the

O. Lawal e-mail: [email protected] © Springer Nature Switzerland AG 2023 A. Faniran et al. (eds.), Landscapes and Landforms of Nigeria, World Geomorphological Landscapes, https://doi.org/10.1007/978-3-031-17972-3_7

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Fig. 7.1 Niger Delta. Source Oyegun (2012)

west of the geological Niger Delta, while Imo, Abia, and Cross River states are located east of the River Niger.

7.2

Physiography

Figure 7.2 shows the digital elevation model of the region, which is a nearly flat terrain that slopes gently from the north to the sea. This topographic configuration has evolved from the sedimentation patterns of the last 75,000 years (Allen 1970). It is tempting to describe the topography of the Niger Delta as a flat, monotonous landform region. The notion is held that it is a horizontal structure of low relief formed from aggradation materials (Ofomata 1975). A close examination of local relief of the region shows the plains as homoclinic (gently inclined) geomorphic structures that trend westwards and south-westwards. This trend is broken in many places by small hogback ridges and shallow swamp basins, which abut at the coast against the sandy beach-ridge barriers lying between the tidal basins and the open sea. The gently undulating coastal lowland inland from this beach-ridge barrier zone shows height ranges between 2.05 and 45 m above sea level. This local variation in relief has evolved in response to erosional processes, lithological differences in the rock groups and differences in the diagenesis of these groups (Oyegun 1991, 1999). Parts of Ogoni, Etche, Egbema, Owerri and Benin areas have land surfaces over 30 m above

sea level. The Port Harcourt region in the eastern delta has height ranges of between 2.5 and 15 m above sea level. This relative relief is shared with towns that include Omoku, Mbiama, Ibaa and Obigbo. A large central portion of the delta from Forcados and Oporoma communities in the west across the Opobo community in the east, enclosing Degema, Buguma, Abonema, Brass and Bonny communities shows a height ranges of 3– 5 m above sea level. A steep terrace-like descent trending north–south from Obigbo through Nchia to Bonny is prominent, while a broad coastal plain topography is observable between Ndoni, Ahoada/Abua and Warri areas of the delta, trending in NW–SE direction. The deltaic plains of the main drainage systems of the Niger Delta generally lie below 2.5 m above sea level, except where the swamp has been reclaimed. The entire Niger Delta topography is characterized by a maze of creeks and swamps which criss-cross the uniclinal, low-lying plains of varying dimensions. The drainage pattern of the region is changing along with shifts in the position of various distributaries of the Niger River; hence, many parts of the drainage systems, e.g. the Orashi–Sombreiro river systems, occupy mature channels despite their relatively small sizes. Drainage density in the deltaic plains is 6 km/km2 (Oyegun 1993). The arcuate Niger Delta has about 21 outlets to the sea, a majority of which are tidal inlets, which allow the flood and ebb tides into the tidal basins (Fig. 7.3). The materials which make up this

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Fig. 7.2 Digital elevation model of the Niger Delta. Source Oyegun (2012)

Fig. 7.3 Geomorphological units of the Niger Delta. Source Cartography Lab. GEM Dept. Uniport

sedimentary environment consist mainly of coarse unconsolidated sands, silt, clay and peat. Aside from the River Niger, the major drainage systems east of it are those of Cross, Imo, Qua Ibo, and Orashi rivers, while major rivers to the west of the Niger include Benin, Oluwa and Siloko rivers.

The region has a humid, warm equatorial climate characterized by heavy rainfall which ranges from 4000 mm in the coastal towns of Bonny (Rivers State) and Brass (Bayelsa) to 3000 mm in the central portion of Ahoada (Rivers), Yenagoa (Bayelsa) and Warri (Delta). Locations situated at the northern extreme of the region, in Abia and

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the Imo States, record annual rainfall values of 2400 mm, which is further reduced in the more interior areas of Cross River and the Ondo states to 2000 mm closer to the coast and 1500 mm further inland. Temperatures are high throughout the year, with the warmest months of February– April recording temperatures that range from 28 to 33 °C. June to September are the coolest months, with temperatures varying from 21 to 23 °C.

7.3

Geology and Geomorphological Units

The geology of the Niger Delta consists of three diachronous sedimentary rock formations that prograde into the Atlantic Ocean. The uppermost rock formation is the Benin formation, which is sandy in nature and rich in iron oxides. The Benin formation is followed in the middle of the delta by the Agbada formation which is rich in oil and natural gas deposits. The materials of this formation are fine-to-mediumgrained sands with intercalations of shale. The oldest of these diachronous sedimentary rocks is the Akata formation, within which the Imo shales are typical rocks. The geomorphological units of the region include the outer barrier island complexes adjoining the Atlantic Ocean, the lower tidal floodplain which consists of estuaries, mangroves and creeks, and the upper freshwater riverine floodplain, within which are the geomorphological units shown in Fig. 7.3. Table 7.1 summarizes the attributes of some of these units. The upper floodplain is seasonally flooded and covers about 50% of the region, extending for 168 km from the beginning of the delta south of Onitsha downstream to the northern limits of the lower floodplain south of Aboh. Within the upper floodplain, the Niger River occasionally shows braided patterns (Fig. 7.4a) and bifurcates into the Nun and Forcados rivers, which further break into numerous distributaries (Fig. 7.4b). Well-developed meanders are present along some of these distributaries, including the Nun River (Fig. 7.4c). Overall, 92.1% of the land area of Akwa Ibom State is occupied by the seasonally flooded upper floodplain, while the corresponding coverages in other states are 60.2% in

Bayelsa State, 78.8% for Delta and 7.6% for Rivers State. The lower floodplain coverages are 6.5% (Akwa Ibom), 17% (Delta), 32.4% (Rivers) and 32.8% (Bayelsa). The upper floodplain is non-tidal, but it is flooded during the rainy season and is characterized by a mosaic of small lakes of rain-fed dark-coloured water. On the other hand, ‘white’ water systems are fed by the waters of the Niger River which makes them more aerated to support comparatively a higher biodiversity of aquatic organisms. The floods associated with the dam collapse in Cameroon, which occurred in 2012, resulted in the discharge of large volumes of water into the Niger Delta. This resulted in the inundation of large expanses of the lower Niger floodplain (Fig. 7.5a, b). A survey carried out during the event by the author that included interviewing persons, who were seventy years old and above, revealed that flood levels had previously not been as high as the ones recorded during the event under reference. Plate 1C shows the seasonal increase in water level of at least 3 m from dry season to wet season levels along the creeks of the Niger Delta. The Lower Floodplain has 20 large estuaries and 7700 km2 of mangrove forest, the largest in Africa and the third-largest mangrove forest in the world. The tidal range in this zone varies between 1 and 3 m and the tidal regime oscillates every six hours. The mangrove tree zone runs almost parallel to the coast and is spread along the creeks, reaching between 15 and 45 km at the furthest inland limits that are under the influence of the flood and ebb tide. Figure 7.6a shows the Kaa waterfront, at Ogoni, Rivers State, at low tide along with one of the creeks of Imo River. A total of 21 barrier island complexes are separating and protecting the tidal basins from the breaking swell waves of the Atlantic Ocean and extending continuously for over 300 km. On average, these barrier island complexes are 16 km long and 3.3 km wide. These barrier island ridges stand about 1 m above the high-water mark (Stutz and Pilky 2002), are unique to the Niger Delta region and are not found elsewhere in the world. They have shallow freshwater aquifers that enable them to support freshwater flora and fauna. The Niger Delta slopes very gently to the Atlantic Ocean and is characterized by Beach and Barrier Island, at which depositional

Table 7.1 Geomorphologic attributes of Niger Delta Upper flood plain

Geom. units States Akwa Ibom

Size (km2)

State coverage (km2)

Lower 8 % of state

State coverage

Barrier Islands % of state

State coverage

% of state

8412

7747.5

92.1

546.8

6.5

117.8

1.4

Bayelsa

10,773

6485.3

60.2

3533.5

32.8

754.1

7

Delta

16,842

13,271.5

78.8

2863.1

17

Rivers

10,393

789.9

7.6

3367.3

32.4

Total

46,420

28,294.2

Source Modified from IUCN Panel Report (2013)

10310.7

707.4 6235.8 7815.1

4.2 60

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111

Fig. 7.4 a Braided river course south of Onitsha before bifurcation at the south of Aboh (Samabri). b Bifurcation of River Niger south of Aboh at Samabri above is the Forcados and the Nun River. c The meandering reach of the Nun River at Tombia, Bayelsa State. Source Google Earth (2018)

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Fig. 7.4 (continued)

processes predominate (Fig. 7.6b, c). Figure 7.6b shows accumulation forms along the Sangana River estuary, with braided courses and a sand spit growing south of the braided forms.

7.4

Land-Use and Vegetation Changes in the Niger Delta

Population pressure over time has resulted in the degradation of vegetation cover of the Niger Delta region, with its adverse impact on biodiversity conservation. Many of the forest reserves have been encroached upon by adjoining communities for logging, arable and plantation agriculture and exploitation of non-timber forest products (NTFP) (Table 7.2). On average as of 1999, the core states of the Niger Delta had lost 78.5% of their original forest cover. Human activities by way of increased pressure on land for arable and plantation agriculture and the impact of the oil and gas sector have continued to degrade the natural ecosystem of the region. Table 7.3 shows vegetation and land-use changes in the core of the Niger Delta states. While arable agriculture increased in the Delta State and the old Rivers State (now Rivers and Bayelsa), land for arable agriculture and vegetation cover decreased in Akwa Ibom State. This decrease in

land for arable agriculture possibly has arisen from the increase of gully erosion and badland topography, which make more and more land unsuitable for agriculture. In all states, the forest area decreased on average by 53,117 ha between 1976 and 1993 (Table 7.3), with the largest encroachment on natural forest area (118,000 ha) and its adverse impact on biodiversity in the Delta State. Water bodies also increased in coverage by 11,166.7 ha on average, with the Delta State recording the highest area underwater of 20,300 ha. This trend lends support to the speculation of an increasing sea level due to global warming and the extinction of terrestrial flora and fauna, whose habitats have been inundated by seawater. Details of land-use change in the late twentieth century are demonstrated in examples from Kwale (Fig. 7.7, Table 7.4) and Warri (Fig. 7.8, Table 7.5), Delta State. In Kwale, the forest cover and wetland recorded a net loss of 30.337 km2 and 5011.575km2 respectively during the period 1973–1993, urban space increased by 145.567 km2, implying a significant urban sprawl in this period at the expense of forest cover and reclamation of the wetlands. Forest cover decreased by 354.296 km2 in the 1976/78–1993/95 period, while urban land use and wetland acreage increased to 137.559 km2 and 4078.063 km2 respectively. These numbers confirm the inference from the Federal Department of Forestry. The trend is more conspicuous in

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113

Fig. 7.5 Effects of flood on the lower floodplain of the Niger Delta. Source Oyegun, Charles

A – House in Akinima Town, Rivers State, underwater during the 2012 flood in the lower Niger Delta floodplain.

B – Roads became river channels in Akinima, Rivers State, with the use of canoes for transportation during the 2012 flood.

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C – Flood stage at 3m above normal level in the lower Niger Delta at Egbedi creek, Bayelsa State, at the peak of the wet season. Fig. 7.5 (continued)

Fig. 7.6 a Kaa waterfront, Rivers State, at low tide. b Estuary of the Nun River (C) showing Akassa in the background and Sangana River (A). (B) Barrier island complex between Sangana and Nun river systems. Source Google Earth (2018). c Kula beach sloping gently to the Atlantic Ocean– West of St. Bartholomew river estuary. Source Oyegun, Charles

a

A B C b

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115

c Fig. 7.6 (continued)

Table 7.2 Forested area in core Niger Delta states S/N

State

Land area

1

Akwa Ibom

708,000

Total forested area 97,785

% of land area (%)

Land area minus Total forested area (%)

13.8

86.2

2

Delta

1,769,800

380,747

21.5

78.5

3

Rivers/Bayelsa

2,185,000

638,707

29.2

70.8

Source Modified from Forest Resources Study (Federal Department of Forestry, Abuja, 1999)

Table 7.3 Vegetation and land-use changes in the core Niger Delta states (1976/78–1993/95) States

Agricultural land (ha)

Woodlands/Shrublands/Grasslands (ha)

Natural forests (ha)

Built-up area (ha)

Plantation (ha)

Water bodies (ha)

Akwa Ibom

−21,000

N/A

−20,600

N/A

N/A

+10,300

Delta

+57,000

N/A

−118,000

+42,500

+5,100

+20,300

*Rivers/Bayelsa

+4600

N/A

−20,900

N/A

+15,600

+2900

Source Modified from Federal Department of Forestry, 1988 *old Rivers State was not yet split into Rivers and Bayelsa states as at the time of the study + indicates that the class increased between 1976/78 and 1993/95 − indicates that the class decreased in the same period N/A data was not available

Warri, where a total of 137.6 km2 of forest land had been converted to urban land use. The other trend, which is also similar to the previous finding, is the increase in the wetland area of Warri, which lost its forested freshwater swamp to

mangrove forest because of the increase in seawater incursion. The other significant change is that a sizeable area of Olague and Uremure Forest Reserves is now occupied by salt marshes and tidal flats.

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Fig. 7.7 a Land use and vegetation map (1976/78) across Kwale. b Land use and vegetation map (1993/95) for Kwale. Source Oyegun (2012)

Table 7.4 Kwale: land cover changes (1976/78–1993/95)

Land cover

Period 1976/78 (km2)

Period 1993/95 (km2)

Difference

Gain/Loss

Forest

415.902

385.565

−30.337

Loss

Urban

22.807

168.374

+145.567

Gain

Wetland

9299.107

4287.532

−5011.575

Loss

Source Oyegun (2012)

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Fig. 7.8 a Land use and vegetation map (1976/78) for Warri. b Land use and vegetation map (1993/95) for Warri. Source Oyegun (2012)

Agriculture, which was not practised before 1976 in Kwale, had become a major activity in 1993/95. The same can be said of tree crop plantations, which had encroached into the forest reserves. At both locations, road infrastructure had significantly increased in the 17 years between 1976 and 1993. The other credible supposition is that the continuous removal of crude oil from the substratum is inducing subsidence and thus, exacerbating inundation by the rising sea level. Abbas and Fasona (2012) using Landsat and Nigeria sat 1 imagery for 1986 and 2008 established that forest degradation was 10.6% in 1986 and rose to 32.16% in 2008 for a section of Western Niger Delta. Uchegbulam and Ayolabi (2013) also showed that forest land use decreased over time in the region. A more detailed study of land cover changes was done by Onojegho and Blackburn (2011) for the period 1986–2007. They found that deforestation in the region affected 1.38 million ha within 21 years covered by their study. Their other findings were that afforestation involved 1.15 million ha; forest cover was 2.39 million ha while annual deforestation and afforestation rates were 0.95 and 0.75%, respectively. These indicate that

revegetation did not keep pace with deforestation, meaning that forest fragmentation increased considerably within the period. In turn, James et al. (2007) found out that 21,340 ha of mangrove forest was lost due to urban sprawl, dredging activities and the spread of the invasive Nypa Palm in the Niger Delta. Similarly, Kuenzer et al. (2014) did a land-use/land cover study of the region for 1986/87, 2002/2003 and 2013 for the dry season months of November to April. Their findings corroborated the earlier ones. Urbanization increased by 214 km2 over the 26 years with a corresponding increase in agricultural land by 2195 km2, all at the expense of forest and swamp forest land cover over the Niger Delta (Table 7.6, Fig. 7.8). The picture that emerges is that there is a need for all stakeholders in the region to face the challenge of deforestation in the forest and mangrove/swamp forest ecosystem and initiate massive reforestation programmes to replenish the losses. The other theme which demands serious attention is the massive pollution that results from oil spills in the Niger Delta states, which are addressed below.

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Fig. 8 (continued)

Table 7.5 Warri: land cover changes (1976/78–1993/95)

Land cover

Period 1976/78 (km2)

Period 1993/95 (km2)

Difference (km2)

Gain/Loss

Forest

459.584

105.288

−354.296

Loss

Urban

5.934

143.4937

+137.5597

Gain

Wetland

2045.25

6123.313

+4078.063

Gain

Source Oyegun (2012)

The white boxes listed numbers 1–4 refer to the built-up area of Port Harcourt, Warri, Aba and Owerri, respectively, for 1986/87. These were relatively smaller than what they became twenty-six years after in 2013 (Fig. 7.9b).

7.5

Oil and Gas Pollution in the Niger Delta

Baird (2010) affirmed that between 9 and 13 million barrels of oil have been spilled in the Niger Delta since 1958 when commercial oil exploitation began in the region (Fig. 7.9).

The elements at risk of this massive hydrocarbon pollution are agricultural land, the local population whose health is compromised through the consumption of polluted resources, and all plant and animal species that come in contact with the spilled oil. The seasonal flooding of the Niger Delta plain exacerbates the problem by spreading heavy metals of lead, iron, zinc, copper and mercury over farmlands (cf. Oyegun, 1993, 2018; Twumasi and Morem 2006; Achudume 2007; Ugochukwu and Ertel 2008; Emoyan et al 2008; Bayode et al. 2010). In addition, a more disturbing trend is the dredging of canals to create access to oil wellheads in the sensitive mangrove ecosystem (Table 7.7). Approximately

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Table 7.6 Niger Delta land cover changes (1986–2013)

119 Land use

Land cover (1986/87) (km2)

Land cover (2013) (km2)

Direction of change

Urban area

1516

1730

Gain (+ve)

Agricultural area

31,700

33,895

Gain (+ve)

Forest/swamp forest

18,325

15,408

Loss (−ve)

Mangrove forest

10,311

10,072

Loss (−ve)

Source Modified from Kuenzer et al. (2014) Fig. 7.9 a Land use and vegetation cover of the Niger Delta (1986/87). b Land-use and vegetation change in the Niger Delta (2013). Source Kuenzer et al. (2014)

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Table 7.7 Dredged access canal length (oil company activity)—1986– 2013 States

Dredged canal length 1986/87

Dredged canal length 2013

% increase

Bayelsa

91

150

64.8

Delta

112

150

33.9

Rivers

26

40

53.9

Mangrove ecosystem (of all 3 states)

230.4

269.5

17.0

Source Kuenzer et al. (2014)

7.6

Conclusions

The Niger Delta is a vast sedimentary environment with a near-flat topographic sequence that gently slopes from the north southwards, towards the Atlantic Ocean. This gently sloping terrain consists of three diachronous rock formations that prograde into the ocean. These formations are the Benin, Agbada and Akata, with rich deposits of oil and gas in the Agbada formation. Extraction of oil and gas is speculated to induce subsidence of surface and sub-surface rocks. This situation is exacerbated by the rising sea level related to global warming. The net effect of these processes is accelerated erosion at the estuaries of the major drainage systems exposed to breaking swell waves of the Atlantic Ocean. The inundation of low-lying coastal areas facilitates the incursion of seawater into shallow freshwater aquifers of the barrier island complexes. Seawater incursion has resulted in habitat fragmentation and extinction of freshwater species on the barrier island complexes. Massive conversion of wetlands and lowland tropical rainforest areas into urban land uses through reclamation and increased arable agriculture to accommodate the rising population also calls for concern. It is only conscious attempt at revegetation of tree species in the mangrove and low forest belt that can assuage the problem of ongoing deforestation in the region. Also, there is a need to deploy environmentally friendly technologies in the oil and gas business to meet internationally acceptable ‘best practices’ for the sector. This will be a ‘stitch in time that saves nine’, to achieve geomorphologic and environmental sustainability in the region.

References Fig. 7.10 Crude oil seepage in a shallow well, Bidi, Delta State. Source Ogoro, Mark (2015)

65% increase in the canal length in the Bayelsa State was recorded between 1987 and 2013. The corresponding figures for Delta and Rivers states are 34 and 54%. Against the background of the rising sea level, these increases in canal length imply that a lot more freshwater habitats along with their groundwater are now exposed to saltwater intrusion and other habitat changes that directly put the erstwhile freshwater flora and fauna at risk of extinction. The other adverse impact of petroleum development is the effect of gas flares, which Baird (2010) put at a conservative estimate of 167 gas flare sites in contrast to the much lower estimate of Kadafa (2012) (Fig. 7.10).

Abbas II, Fasona MJ (2012) Remote sensing and geographic information techniques: veritable tools for land degradation assessment. Am J Geogr Inform Syst 1(1):1–6 Achudume AC (2007) Assessment of farmland sediments after flooding in Ubeji land in Niger Delta of Nigeria. Environ Monit Assess 135:335–338 Akuodu GE (2011) Oil exploitation and challenges of development in the Niger Delta region. MSc Dissertation submitted to the Department of Political Science, Faculty of Social Sciences, University of Nigeria, Nsukka Allen JRL (1970) Sediments of the modern Niger Delta; a summary and review. In: Morgan JP (ed) Deltaic sedimentation—ancient and modern. Society of Economic Paleontologists and Mineralogists, Oklahoma, pp 138–151 Amangabara GT, Obenade M (2015) Flood vulnerability assessment of Niger Delta states relative to 2012 flood disaster in Nigeria. Am J Environ Prot 3(3):76–86 Baird J (July 25, 2010) Oil’s shame in Africa. Newsweek 27

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The Niger Delta Region

Bayode OJA, Adewunmi EA, Odunwole S (2010) Environmental implications of oil exploration and exploitation in the coastal region of Ondo state, Nigeria: a regional planning appraisal. J Geogr Reg Plann 4(3):110–121 Emoyan OO, Akpoborie IA, Akporhonor EE (2008) The oil and gas industry and the Niger Delta: implications for the environment. J Appl Sci Environ Manag 12(3):29–37 IUCN Niger-Delta Panel (2013) Sustainable remediation and rehabilitation of biodiversity and habitats of oil spill sites in the Niger Delta. Main Report including recommendations for the future. A report by the Independent IUCN-Niger Delta Panel (IUCN-NDP) to the Shell Petroleum Development Company of Nigeria (SPDC), January 2013. IUCN, Gland Switzerland James GK, Adegoke JO, Saba E, Nwilo P, Akinyede J (2007) Satelite-basedassessment of the extent and changes in the mangrove ecosystem of the Niger Delta. Mar Geodesy 30(3): 249–267 Kadafa AA (2012) Oil exploration and spillage in the Niger Delta of Nigeria. Civ Environ Res 2(3):38–51 Kuenzer C, Van Beijma, Gessner U, Dech S (2014) Land surface dynamics and environmental challenges of the Niger Delta, Africa: remote sensing-based analysis spanning three decades (1986–2013). Appl Geogr 53:354–368 Ofomata GEK (1975) Landform regions Chp. 12. In: Ofomata GEK (ed) Nigeria in maps, eastern states. Ethiope Publishing House, Benin City Onojegho AO, Blackburn GA (2011) Forest transition in an ecologically important region: patterns and causes for landscape dynamics in the Niger Delta. Ecol Indic 11:1437–1440 Oyegun CU (1991) Spatial and seasonal aspects of shoreline changes at Forcados beach, Nigeria. Earth Surface Process Landforms 16 (4):293–304 Oyegun CU (1993) Land degradation and the coastal environment of Nigeria. Catena 20:215–225

121 Oyegun CU (1999) Climate, relief and drainage, Chapter 3. In: Alagoa EJ (ed) Land and people of Bayelsa State, Central Niger Delta, pp 31–43 Oyegun CU (2012) Environmental sustainability and growth initiatives in the south-south region. In: 2nd south-south economic summit, Asaba, Delta State, 26–28 April 2012 Oyegun CU (2018) Hydrocarbon pollution and oil spill contingency management in the Niger Delta region, Chapter 10. In: Elenwo EI, Ochege FU (eds) Environment, resources and sustainability in the Niger Delta region, Nigeria. University of Port Harcourt Press, Port Harcourt (Pub.) Stutz M, Pilkey O (2002) Global distribution and morphology of deltaic barrier island systems. J Coast Res Special Issue 36 Uchegbulam O, Ayolabi EA (2013) Satellite image analysis using remote sensing data in parts of Western Niger Delta, Nigeria. J Emerg Trends Eng Appl Sci (JETEAS) 4(4):612–617 Ugochukwu C, Ertel J (2008) Negative impacts of oil exploration on biodiversity management in the Niger Delta of Nigeria. Impact Assess Project Appraisal 26(2):97–125 United Nations Development Programme, Niger Delta Human Development Report (2006) Abuja, Nigeria, pp 185&186 Williams ES (2018) Environment of the Niger Delta region, Chapter 1. In: Elenwo EI, Ochege FU (eds) Environment, resources and sustainability in the Niger Delta region, Nigeria. University of Port Harcourt Press, Port Harcourt (Pub.)

Internet Sources Twumasi Y, Merem E (2006) GIS and remote sensing applications on the assessment of change within a coastal environment in the Niger Delta region of Nigeria. Int J Environ Res Public Health 3(1):98– 106. www.ijeoph.org

Part II Specific Landforms

8

Landforms of the Chad Basin Mala M. Daura, Emmanuel D. Dawha, and John O. Odihi

Abstract

8.1

Lake Chad is the terminus of a vast inland drainage system. It was an inland sea with a basin connected to the sea through the River Mayo Kebbi. With time, fluvial processes cut off the sea by blocking its outlet, thereby turning it into the largest freshwater lake in the world, with a catchment area of 2,537,373 km2 some 5000 years ago, ten times its present size. Since then, the water body area has reduced from 400,000 km2 to between 1500 and 2000 km2. This size reduction has been attributed mainly to anthropogenic activities and climate change. Other factors include a combination of desert winds blowing across portions of the basin that lie in the Sahara Desert and fluvial activities operating in more humid areas that lie within the Savanna belt. Lake Chad Basin has many landforms, including the grotesque-looking rock pedestals of the Ennedi Plateau along its northern boundary in Chad. They also include inselbergs in northern Nigeria, such as the famous Kano Dala Hill, the paleo-dunes that covered northern Jigawa and Yobe states of Nigeria, and the famous Bama Beach Ridge, which essentially forms a depositional ring of sand around the Mega-Chad. Man is a principal player in the Chad scenario; therefore, the study of the basin that is home to over 40 million people must include humans. For this reason, the socioeconomic implications of the dying lake are also briefly reviewed. Keywords

Lake Chad




Mega-Chad




Fluvial




Bama Beach Ridge

M. M. Daura (&)
E. D. Dawha
J. O. Odihi Department of Geography, University of Maiduguri, Maiduguri, Borno State, Nigeria e-mail: [email protected] J. O. Odihi e-mail: [email protected]

Introduction

The name Chad is a local name that means the expanse of water and, indeed, Lake Chad used to be a vast expanse of water. Before 5000 BC, the lake was the largest in the sub-Saharan region as it occupied an estimated area of about 400,000 km2 (Drake and Bristow 2006), with a drainage basin of 2,500,000 km2. Thus, it was the largest endorheic basin in the world (Leblanc et al. 2006). It was larger than today’s Caspian Sea (LCBC 2005). Studies have also shown that the historical Mega-Chad was not only larger than today’s Caspian Sea but was an inland sea formed within a structural depression during the Cretaceous era into which many sub-Saharan rivers flowed (Drake and Bristow 2006; Stewart 2009). During that era, the climate was humid, and Lake Mega-Chad used to fill its basin and spill over into the Benue Valley at Bongor through River Mayo Kebbi and connected the then lake to the Benue and Niger rivers on its way to the Atlantic Ocean (Fig. 8.1) (Burke 1976; Schuster et al. 2005). However, the drying of the climate led to the disappearance of these great rivers, which caused the lake to shrink, losing about 95% of its size from 1963 to 1998, revealing previously submerged dunes, ridges, and beaches.

8.2

Location and Extent of Lake Chad Basin

Historically, Lake Chad has been one of Africa’s major lakes. The size of the lake has been variable seasonally and annually depending on the basin’s receipt of rainfall (Gritzner 2021). The Lake Chad Basin lies between latitudes 6° 00″ to 25° 00′ 0 N and longitudes 7° 0 00″ to 25° 0 00″ E. It drains the area bounded by the mountains of Adamawa, Jos Plateau, Air, Ahaggar, Tassili, Air, Tibesti, and the Ennedi Plateau (Fig. 8.1), as well as the central highland in Nigeria. The basin cuts across territories of eight countries: Cameroon, Central African Republic, Sudan, Libya, Algeria, Niger, Nigeria, and Chad, with an estimated drainage area of

© Springer Nature Switzerland AG 2023 A. Faniran et al. (eds.), Landscapes and Landforms of Nigeria, World Geomorphological Landscapes, https://doi.org/10.1007/978-3-031-17972-3_8

125

126

M. M. Daura et al.

Fig. 8.1 A model of the basin showing the watershed, Mega-Chad, and the present-day Lake Chad

2,537,373 km2. Many rivers drain the basin, including rivers Cheri-Logome, Yadzaram Ngaddabul, Kumadugu-Gana, Kumadugu-Yobe, Ebei Mboli, Botha El-Beed, Yadzaram Ngadabul, Taf-taf, and Serbewel. Drought and the changing climate of the Sahara and the Sub-Saharan areas have rendered the contributions of most of the rivers insignificant. Onuoha (2008) estimated that 90% of today’s Lake Chad water comes from the Chari-Logone River system, while the remaining 10% comes from the Yobe-Kumadugu system (Fig. 8.2).

Fig. 8.2 Lake Mega-Chad, present-day Lake Chad, and open water body superimposed on the catchment map of the basin showing the contributing rivers

The Chad Basin gets its water from the monsoon rains that start in June and end in October. Fortnam and Oguntola (2004) reported that in 1963, the lake occupied about 23,000 km2, after which rainfall started declining from the late 1960s and culminated in the two drought periods of 1972–1974 and 1983–1984 (LCBC 2005). Owing to the shallowness of the lake and the changing balance between intake and evaporation, the lake’s shape and size have been continually changing. By 1975, the northern pool had dried out, turning the erstwhile open water into the marshy ground. Fortnam and Oguntola (2004) Reported that the open water then was between 1500 and 2000 km2, with a large surrounding area of marshy vegetation that covered an area between 2000 and 4000 km2 (Fig. 8.3). This study estimated the area of open water in 2018 to be 1584.44 km2 out of a total lake area of 24,902 km2, the remaining 23,312 km2 existing as marshy ground. Studies by Gao et al. (2011), Lemoalle et al. (2012), and Leblanc et al. (2006) have also shown that the lake size has fluctuated over the years and may have disappeared at a time. The presence of wave-cut paleo-dunes that are appearing as the lake water retreats, revealing previously submerged paleo-dunes features, supports this view. The appearance of paleo-dunes shows that the area was once a desert before the Mega-Chad submerged it, and as the lake continues to retreat, more of such submerged dunes are being exposed (Schuster et al. 2005). Drake (2006), through the analysis of a satellite image, identified up to 41 distinct shorelines marking the process of the retreat of the paleo-lake (Table 8.1). We reconstructed four of these shorelines in Fig. 8.4; Taimanga Ridge in Chad and Bama Beach Ridge (BBR) in Nigeria (Fig. 8.5).

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Landforms of the Chad Basin

127

Fig. 8.3 Satellite image of Lage Chad. a UNEP satellite image of Lake Chad 1972. b UNEP satellite image of Lake Chad 1972. c Satellite image of Lake Chad 1972 Table 8.1 Location of shorelines and shoreline heights of paleo-lake Mega-Chad

1

Shoreline location

Shoreline height (m)

East of Agamma

286

2

West of Agamma

3

Tamanaga Ridge

4

Goz Kerki Spits

5

South of Goz Kerki

305

284

297

305

318

302

316

304

323

333

320

328

320

330

340 332

314

327

6

The Hebil Terrace

329

7

North of the Hebil

329

334

328

335

8

South of the Hebil

9

Moyto Island

10

Chari Delta

11

Mayo Kebi Ridges

12

Bama Ridge

13

Terraces North of Bama

14

Ngelewa Ridge

15

Historical Lake Chad

16

345

304

328 315

328

316

330

335

329

336

372 354

327 290 285

288

Mean (m)

285

289

Kanem

330

Standard Deviation (m)

14

14

Mean Lake Area (1000’s of km2)

114

141

Error (1000’s of km2)

5

7

218

305

316

322

329

335

12

17

21

18

0.8

266

310

333

361

413

5

8

7

13

9

456

497

573

873

Source Drake (2006)

8.3

Geology and Soils of the Mega-Chad Basin

The geology of the Chad Basin shows that Quaternary sands of various origins dominate the basin. Aeolian sand, however, predominates in the northern part, including dune

regions in the Borno and Yobe states in Nigeria. At the same time, the combination of fluviatile, lacustrine, and deltaic depositions, resulted in an alternating sequence of sand and clay, with clayey soils on the surface (Fig. 8.6), dominating the southern part (Vassolo 2009). Vassolo (2009) showed that Quaternary materials make up the topsoil in the Mega-Chad area down to 75 m from the surface. At this

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M. M. Daura et al.

Fig. 8.6 Geological map of the Chad Basin (Adapted From Vassolo 2009)

Fig. 8.4 Selected shorelines of the Mega-Chad paleo-lake based on shoreline analysis by Drake (2006). See Table 8.1 for details

underlie the Quaternary layer to another depth of 30 m below. Sandstones of the Cretaceous containing the deepest aquifer, which goes for another depth of 150 m, underlie the Pliocene. The granitic basement complex forms the foundations of the basin.

8.4

Fig. 8.5 Taimanga Ridge in Chad. Credit Drake and Bristow (2006)

depth, according to him, there is “a thick layer of some 280 m of clay from the Upper Pliocene age” (Vassolo 2009: 34). Pliocene materials consisting of sands and sandstones

The Bama Beach Ridge

The Bama Beach Ridge (BBR) is a long, narrow sand ridge, which is a prominent morphological feature that overlies the Quaternary Chad formation. It is aligned in NNW–SSE direction and is estimated to have been formed during the Late Pleistocene when it was left as an abandoned shoreline of the receding Mega-Chad (Nyanganji 2002). The ridge runs in a discontinuous form from the Cameroon plains through the northern part of the Mandara mountains, through Banki in Nigeria to Bama and Maiduguri in the Borno State up to Gashua in the Yobe State, fanning out beneath the dunes in the Niger Republic (Zarma and Tukur 2015). The BBR has an altitude of about 12 m, making it a conspicuous landform in a generally flat environment. Stratigraphically, the BBR can be regarded as the youngest geological unit within the Chad Basin. Zarma and Tukur (2015) established that the BBR has unique characteristics that separate it from the rest of the Borno sub-basin. Their heavy mineral studies show that the BBR contains a well-diversified suite of both opaque and non-opaque heavy minerals, including hematite, magnetite, luxaxane, ilmenite, zircon, tourmaline, and rutile. The rest of the Borno sub-basin around the BBR, however, shows only four types of minerals, namely zircon, rutile, apatite, and sphene. The stratigraphy of the BBR shows layers of lacustrine and

8

Landforms of the Chad Basin

fluviatile clays and sands of the Pleistocene age. Sediments that formed the BBR have been estimated to have an average thickness of 40 m.

8.5

Soils

The distribution of soils in the drainage basin of Lake Chad and their classification are shown in Figs. 8.7 and 8.8. The predominant soil type is the entisols (in USDA soil

129

taxonomy). The Food and Agricultural Organization (FAO) classifies this type of soil as lithosols. It makes up 20.11% of soils in the basin. They are generally poor in nutrients and can only support scanty vegetation, rarely of savanna type. Lithosols are generally found on steep slopes that render sediment accumulation difficult. Though the Chad Basin is a relatively level environment, the soils are predominantly lithosols being poor and lacking horizon development due to the poor presence of weatherable minerals, especially iron oxide. Because lithosols are poor in nutrients, they support a low level of agricultural activities (Oruonye 2018). The second commonest soil in the Chad Basin is cambic arenosols. It is also poor, horizonless soil that goes to a depth of 50 cm. Cambic arenosols do not support many agricultural activities either. Other soils in the basin include yermosols, gleysols, and fluvisols (Figs. 8.7 and 8.8). These are nutritionally poor soils that cannot support intensive animal rearing and crop cultivation. Thus, with the increasing population, the carrying capacity is being exceeded, forcing herders to move into adjoining regions resulting in clashes between farmers and herders.

8.6

Landforms in the Lake Chad Basin

8.6.1 The Bodele Depression

Fig. 8.7 Classification of soils of the Lake Chad Basin (FAO 2003, version 3.6)

Fig. 8.8 Predominant soil types in the Chad Basin (FAO 2003, version 3.6)

The Bodele Depression in northern Chad is a typical example of a large deflation hollow. Deflation hollows are topographic depressions developed in desert areas through wind erosion. They generally have a saucer shape and may contain oases. They are formed when wind action continues to scoop out loose rocks in a particular area over a considerable length of time. Historically, the Bodele Depression used to be part of the paleo-lake, being the deepest part of the lake basin, at 155 m above sea level. As the climate

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M. M. Daura et al.

Fig. 8.10 Satellite image of paleo-dunes in the northern part of Yobe states of Nigeria

Fig. 8.9 Location of the Bodele Depression and the Lake Chad within the Mega-Chad Basin (adapted from NASA Aqua Satellite Imagery of January 14, 2008)

became more arid, the basin was divided into Lake Fitri and the present Lake Chad (Lake Fitri is located 300 km to the east of N’Djamena, the capital city of Chad, and has an area of about 25,000 km2). According to Simon et al. (2015), the depression is an elongated basin with an estimated length of 500 km, a width of 150 km, and is 160 m deep, with a total area of 133,532 km2 (Fig. 8.9). Simon et al. (2015) also noted that the Mega-Chad, and the Bodele Basin, in particular, dried out in a relatively short time of 1000 years (6000–5000 BP). The dry nature of the basin causes lake bottom muddy sediments to be easily blown away by the wind. The Bodele Depression has been the greatest source of atmospheric dust globally, with Richard (2007) estimating that the basin supplies an average of 700,000 tons of dust per day and a total of 61,000 km3 has been eroded from the basin in the past 1000 years.

8.6.2 Paleo-Dunes Paleo-dunes are natural landform records of past dune activities. They are keys to the reconstruction of past climates (paleo-climates) because they indicate the type of climate that formed them, including predominant wind directions. The paleo-dunes of northern Jigawa and Yobe states of Nigeria show that the environment had experienced desert conditions before the present desertification, indicated

by the active dunes encroaching on them. These paleo-dunes run in the northeast to northwest direction, indicating the direction of the predominant winds of the days they were formed (Fig. 8.10). The paleo-dune fields of Northern Nigeria extend to about 3° of latitude south of the present-day limit of active dunes. They occur in areas that today receive as much as 750 mm annual rainfall. Active dunes, however, can only be observed today in areas with less than 150 mm of rainfall, indicating that the aridity during the period the paleo-dunes were formed was more severe than it is today. Butzer (1983) estimated that such aridity has not been experienced in the region for the last 12,500 years, pegging the time of their formation to more than 12,500 years ago. The paleo-dunes covered an area of up to 500 km2 and persisted until the end of the Heinrich event, some 14,600– 14,500 years ago, which set in the African Humid period that led to the development of many rivers, lakes, and mountain glaciers across Africa. The paleo-dunes of Jigawa and Yobe states of Nigeria are predominantly seif (longitudinal) dunes. They vary in length, with some running for hundreds of meters while the bigger ones run in kilometers generally aligning themselves in NE to NW direction. On average, a seif dune can have a thickness of 100–400 m, with an average altitude of 8 m. The paleo-dunes are generally utilized as settlement sites by the locals, while the furrows in-between the dunes are used for agricultural activities as they tend to retain water, especially in the rainy season. Butzer (1983) used dune sands and windborne dust to estimate the development of the Sahara during the Quaternary era (some 2 million years ago).

8

Landforms of the Chad Basin

8.6.3 Sand Dunes The African humid period ended during the Piora Oscillation, some 6000–5000 years ago, when arid conditions again began to spread in the Sahara region. The aridity expanded to today’s size of the Sahara. The present Sahara, however, Fig. 8.11 Active dune (Barchan) in Tulo-Tulowa region of the Yobe State (Courtesy: Kunle Oladimeji 2011)

Fig. 8.12 Encroaching dunes in Tulo-Tulowa area of Yobe State, Nigeria, overrunning grasses, shrubs, and the Sahelian trees at the fringes of the desert

131

is smaller than the one formed 2 million years ago. Notwithstanding, dunes have also developed in today’s Sahara, especially in Manga areas of Niger Republic and Tulo-Tulowa of the Yobe State in Nigeria. These dunes are relatively young and active, as indicated by Moussa et al. (2016). They analyzed satellite images and found little or no

132

dune fields in 1957. By 1975, dune fields were observable from satellite images and covered around 200 ha. Afterward, they expanded rapidly. By 2007, they covered an area of 900 ha in the Manga area of Niger Republic. There are two types of dunes: longitudinal or seif dunes and crescentic dunes or barchans (Figs. 8.11 and 8.12). The migratory dunes can grow up to 5 m high, burying vegetation and houses in their paths. Their analysis proves that the aridity that brought the new dune fields started as recently as the 1970s.

8.6.4 Inselbergs The Lake Chad Basin is an inland drainage basin, surrounded by mountains, highlands, and plateaus, which serve as watersheds. The basin has developed diverse landforms. One of these landforms, the Ennedi Plateau, is so captivating that UNESCO has designated it a World Heritage site. These landforms were developed by alternating processes of wind and water erosion, as the basin is known to have gone through circles of dry and humid periods over millions of years. Through the processes of weathering as well as wind and water erosions, the surrounding mountain portions in Nigeria have developed unique landforms, among which are inselbergs. Inselbergs are isolated rock outcrops that stand as relics of ancient mountains. They usually have rounded tops and steep sides, being mostly made of hard granite rocks. They can stand as singular elevations or in groups (Fig. 8.13). The Gwoza, Machina, and Jigawa mountains, as well as the north-central highland in Nigeria, have many sites that have been sculptured and reduced to inselbergs. Some inselbergs are only a few meters high while others can be more than 15 m high.

Fig. 8.13 a Inselbergs of Kano (the famous Kano Dala Hill), serving as water shade for river Hadeja (Credit: Teslim O. Omipidan, 2017), b a boulder pile (a tor) in Machina, Yobe State

M. M. Daura et al.

8.7

Economic Implications of the Desiccating Chad Basin

The shrinkage of Lake Chad has been a cause for concern for the countries within its drainage basin. This is because the lake provides a habitat for over 120 species of fish, 372 species of birds and many more animal species, including the hippopotamus. The basin is also home to an estimated population of over 40 million people (projecting the population estimate of 37 million in 2004 by UNEP 2004). The World Conservation Union (IUCN) estimated in 1987 that the Chad Basin contains the world’s largest wetland area in the Sahelian region. According to the estimate, it alone has about 10 million ha while others are found along the Chari– Logone and Yobe Rivers. It also has large floodplains that provide a livelihood for millions of people, especially in the Satequi–Deressia area in Chad; the Yaeres in Cameroon and Chad, and the Hadeja–Nguru in Nigeria. Jolley et al. (2000) also estimated that the basin supplied 130,000–141,000 tons/year of fish in the early 1970s. The shrinking of the lake, attributed to anthropogenic activities and climate change, has already undermined the livelihood and, by implication, peace, and stability of the region. The available estimate by Jolley et al. (2000) indicated the basin’s annual fish catch has dropped to between 60,000 and 85,000 tons by 1977. Drought and resultant poor harvest have also led to migrations and resettlements of farmers and pastoralists moving southward, a situation that has resulted in deadly clashes between farmers and the pastoralists in the basin and the adjoining areas (Fig. 8.14). The fragile nature of the ecosystem has rendered economic activities unsustainable and the inhabitants who depend on the lake and its resources are among the poorest people in the world today (UNDP Human Development Index (HDI) 2007/2008).

8

Landforms of the Chad Basin

133

Fig. 8.14 A typical southward movement of herders in the dry Sahel region of the Chad Basin: Cattle herders venturing into the Sudan vegetation zone in, Nigeria. Credit Luminous Jannamike of Vanguard Newspaper Nigeria

8.8

Conclusions

The Lake Chad Basin has exhibited many landforms over time due to natural factors sometimes building and at other times destroying or covering previous landforms. Climate change which dictates the predominance of the prevailing wind with its attendant dryness or wetness of the basin has been responsible for the observed landforms. More recently, humans have become an important factor in landform formation through their interaction with the natural environment. Together, the nexus of natural and anthropogenic factors have made the Lake Chad Basin exhibit different types of landforms at different times. Water and wind have over the years sculpted diverse types of landforms. Thus, landforms that include pedestal rocks, inselbergs, paleo-dunes, and active and dormant dunes have evolved over the years. The Lake Chad Basin is therefore an interesting geomorphic library assembling diverse landforms of both humid and arid origins, some of which can simply qualify as wonders of natural forces. The tremendous climatic and anthropogenic changes of the Lake Chad Basin over the last 1000 years transformed the environment geomorphically including it being an inland sea to the desert lake it is today. The population in this region is estimated to reach 40 million. Furthermore, over the years, the intensity of competition between different land and water users has continued to increase so much that communal clashes between different tribes as well as farmers and herdsmen, and sometimes disagreements between the constituting governments, have increased. The main cause of these conflicts is the lack of water. These conflicts have today rendered the basin that should be the cradle of development and international cooperation into one of the world’s greatest refugee areas,

with an estimate of 11 million people in serious need of assistance. Among them, 6.9 million have no food security, and 2.5 million have been displaced from their settlements, turning the region into the second country in terms of the number of displaced people (Human Right Watch 2016).

References Burke K (1976) The Chad Basin: an active intra-continental basin. In: Developments in geotectonics, vol 12. Elsevier, pp 197–206. Retrieved on 10/2/2018 from https://www.sciencedirect.com Butzer KW (1983) Paleo-environmental perspectives on the Sahel drought of 1968–73. GeoJournal 7(4):369–374. Retrieved from https://link.springer.com on 7/2/2020 Drake N (2006) The Sahara Mega lake Project retrieved on 10/12/2018 from https://www.kcl.ac.uk/sspp/departments/geography/people/ academic/drake/Research/The-Sahara-Megalakes-Project/LakeMegachad.aspx Drake N, Bristow C (2006) Shorelines in the Sahara: geomorphological evidence for an enhanced monsoon from palaeolake Megachad. The Holocene 16(6):901–911 FAO (1997) Irrigation potential in Africa: a basin approach. Retrieved on 14/12/2018 from http://www.fao.org/docrep/W4347E/w4347e0j. htm FAO of the United Nations (2003) version 3.6 Fortnam MP, Oguntola JA (2004) Lake Chad Basin, Giwa, Regional Assessment 43, University of Kolmor Sweden. Retrieved from https://www.droughtmanagement.info/literature/UNEP_lake_chad_ basin_2004.pdf on 20/7/2019 Gao H, Bohn TJ, Podest E, McDonald KC, Lettenmaier DP (2011) On the causes of the shrinking of Lake Chad. Environ Res Lett 6(3): 034021. Retrieved from google scholar on 3/2/2020 Gritzner JA (2021) “Lake Chad”. Encyclopedia Britannica, 14 February 2021, https://www.britannica.com/place/Lake-Chad. Accessed 5 February 2022 Human Right Watch (2016) Tackling terrorism for socio-economic development in Lake Chad: policy implications for sustainable peace. Retrieved on 14/12/2018 from https://www.ijhssnet.com/ journals/Vol_8_No_3_March_2018/16.pdf

134 Jolley TCB, Neiland E (2000) Lake Chad fisheries: policy formulation mechanisms for sustainable development. Portsmouth University, UK Lake Chad Basin Commission (2005) An information system for water assessment and lake management of the Lake Chad Basin. A sub-regional component of the world hydrological cycle observing system (WHYCOS) A draft project proposal, December 2005 Leblanc MJ, Leduc C, Stagnitti F, Van Oevelen PJ, Jones C, Mofor LA, Razack M, Favreau G (2006) Evidence for Megalake Chad, north-central Africa during the late Quaternary from satellite data. Paleogeogr Paleoclimatol Paleoecol 230(3-4):230–242 Lemoalle J, Bader JC, Leblanc M, Sedick A (2012) Recent changes in Lake Chad: observations, simulations and management options (1973–2011). Glob Planetary Change 80:247–254. Retrieved from google scholar on 2/1/2020 Moussa IA, Somé YSC, Abdourhamane TA, Hassane B, Malam AM, Mamadou I, Abba B, Garba Z, Yacouba H (2016) Spatial dynamic of mobile dunes, soil crusting and Yobe’s bank retreat in the Niger’s Lake Chad basin part: cases of Issari and Bagara. Afr J Environ Sci Technol 10(4):104–110. Retrieved on 72/2020 Nyanganji JK (2002) The morphology and hydrography of the Ngadda catchment and the Bama Beach. Frankfurt am Main Onuoha FC (2008) Environmental degradation, livelihood and conflict: a focus on the implications of the diminishing water resources of Lake Chad for North-Eastern Nigeria. Afr J Conflict Resolut 8(2): 2008 Oruonye ED (2018) A survey of the soils of the Lake Chad resettlement areas of Nigeria. Nigerian J Res Prod 15(1). ISSN 1596 6615. Enugu State University of Science and Technology, Nigeria. Retrieved on 7/2/2020 Richard F (2007) Amazon forest relies on dust from one Saharan valley, 3 January 2007. Retrieved on 8/12/2018 from https://www. newscientist.com/article/dn10880-amazon-forest-relies-on-dustfrom-one-saharan-valley

M. M. Daura et al. Salkida A (2012) Africa’s vanishing Lake: action needed to counter an ecological catastrophe. Retrieved on 13/12/2018 from http://www. un.org/en/africarenewal/vol26no1/lake-chad.html Schuster M, Roquin C, Duringer P, Brunet M, Caugy M, Fontugne M, Mackaye HT, Vignaud P, Ghienne JF (2005) Holocene lake MegaChad paleoshorlines from space. Quarternary Sci Rev 24(16-17): 1821–1827 Simon J, Armitagea C, Bristow S, Drake NA (2015) West African monsoon dynamics inferred from abrupt fluctuations of Lake Mega-Chad. PNAS, June 2015. https://doi.org/10.1073/pnas. 1417655112. Retrieved on 11/12/2018 Stewart R (2009) Dustiest places on Earth—dead and dying seas. In: Environmental science in the 21st century. a new online environmental science book for college students. Retrieved n 10/12/2018 UNDP Human Development Index (HDI) (2007/2008) Retrieved from . undp.org n 20/8/2019 UNEP (1972) Image of the Lake Chad. Retrieved on 13/12/2018 from https://i.unu.edu/media/ourworld.unu.edu-en/article/691/lake_chad_ 72-07.jpg UNEP (2004) Population of people living in the Chad Basin. Retrieved on 13/12/2018 from http://www.worldlakes.org/uploads/ELLB%20 ChadDraftFinal.14Nov2004.pdf www.britannica.com/topic/artesian Vassolo S (2009) Adaptive water management in the Lake Chad Basin, Addressing current challenges and adapting to future needs. Retrieved on 11/12/2018 from http://www.fao.org/fileadmin/user_ upload/faowater/docs/ChadWWW09.pdf World Conservation Union (IUCN) (1987) Lake Chad: experience and lessons learned. Retrieved on 10/12/2018 from http://www. worldlakes.Org/uploads/06_Lake_Chad_27February2006.pdf Zarma AA, Tukur A (2015) Stratigraphic status of the Bama Beach Ridge and the Chad formation. Retrieved on 18/11/2018 from https://www.omicsonline.org/open-access/stratigraphic-status-of-thebama-beach-ridge-and-the-chad-formation-in-the-bornu-subbasinnigeria-2329-6755.1000192.pdf

9

Dune Fields on the Plains of Northern Nigeria Aliyu Baba Nabegu

Abstract

The presence of large areas covered by sand dunes on the plains of northern Nigeria has attracted considerable attention over the years due to the unique and spectacular pattern they form on the landscape. The dominantly linear dunes are relict from the Quaternary period when the climate of northern Nigeria was drier. Today, the dunes are found in areas that receive as much as 1000 mm of rainfall per year. The dunes were first recognized on aerial photographs and later using satellite imagery. The history of the dunes has been further deciphered by the application of luminescence techniques to directly date periods of their formation. In recent years, remote sensing technology has enabled better identification of not only the major units but also the main features. Though degraded, the dunes nonetheless form a distinct and prominent feature on the plains. Besides their significance as a major tourist attraction from within and outside the region, the dunes contain fossil biota that has been used to reconstruct past hydrology and vegetation communities as well as serve as information on the paleoclimatology and paleobotany of the region. Keywords





Paleoclimatology Aeolian Paleobotany Tourism

9.1




Remote sensing




The Formation of Sand Dunes

A principal aeolian depositional landform is the dune (hereafter referred to as dune(s)), which can develop in any environment in which loose particles of sand are exposed to A. B. Nabegu (&) Department of Geography, Kano University of Science and Technology, Wudil, Kano State, Nigeria e-mail: [email protected]

wind action and are free to migrate and accumulate. Dunes are not only associated with desert or coastal environments but can be found in any environment where there is a steady wind regime, a dry environment (less than 250 mm of rainfall annually), and a supply of loose surface material which is small enough to be transported by the wind. In general, dunes are not found in wet environments as wet sand grains adhere (stick) to each other and are not easily moved by wind. Probably the single most important work on dune formation was that by Bagnold (1941) who used wind tunnel experiments to make quantitative predictions about sand movement and accumulation and then successfully corroborated most of those predictions in field tests. Since Bagnold, other important works on aeolian sands have been made by Sharp (1963; 1966; 1978) and McKee (1979) among others. Important work on dune morphology has also been done using aerial photography (Smith 1968) and Landsat imagery (Fryberger and Dean 1979; Breed and Grow 1979). In explaining the process of dune formation, Bagnold (1941) showed that individual sand grains are moved under the force of the wind in two distinct ways: by saltation and surface creep. As wind moves over a sandy deposit, it can pick up grains from the surface and give them a forward momentum, but the weight of the sand grains soon brings the grains back to the surface. If the surface is composed of coarse, immobile particles, such as pebbles, the sand grains will bounce directly off the hard surface and get back into the air, where the wind will once again provide forward momentum. These bouncing grains can move downwind at about half the speed of the wind. If the surface is composed of finer sand grains, however, a saltating sand grain will not bounce off the surface; rather, it will strike the sandy surface and bury itself. The impact will eject a second grain into the air to be blown downwind. This “splashing” form of saltation results in a slower rate of downwind movement than the bouncing motion on hard surfaces. Either process falls under the definition of saltation (Bagnold 1941).

© Springer Nature Switzerland AG 2023 A. Faniran et al. (eds.), Landscapes and Landforms of Nigeria, World Geomorphological Landscapes, https://doi.org/10.1007/978-3-031-17972-3_9

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A. B. Nabegu

Two primary factors are necessary for the accumulation of sand into dunes: (1) an adequate supply of sand and (2) winds strong enough and persistent enough to move the sand (McKee 1979). If these two conditions are met, large quantities of sand can be transported hundreds and even thousands of miles (Fryberger and Ahlbrandt 1979). What makes sand accumulate into piles rather than spread out evenly over an area? In general, sand will tend to accumulate in any place “where a sufficient reduction of wind energy exists along the direction of sand drift in an active extensive system” (Fryberger and Ahlbrandt 1979). Any obstacle, such as a rock outcrop or a stand of vegetation, can force sand accumulation by lowering wind speeds and creating a “sand shadow” in the lee of the obstacle. Any small depression or gentle dip in an otherwise flat surface can fill with sand due to lower wind velocity within the depression (Cooke 1973). Large areas of persistent wind deceleration, such as a basin or the base of a plateau, can spawn the creation of large ergs. Most desert aeolian sand seas do occur in basins (Cooke 1973). Saltation of sand grains along the surface accounts for about 75% of all sand movement by wind. However, since grains average about two thousand times the weight of the atmosphere, not all winds will move sand. Wind speeds must reach what Bagnold (1941) called a “fluid threshold,” defined as the wind speed necessary for sand to start saltating under the direct pressure of the wind. The fluid threshold varies in direct proportion to the predominant grain size of the sand surface, generally ranging from fifteen to thirty thousand meters per hour (Bagnold 1941; Sharp 1963). After sand grains start moving under direct wind pressure, wind speeds lower than the fluid threshold can maintain sand movement. Once saltation has begun, the direct wind pressure is no longer necessary to lift sand grains into the air. The impact of saltating grains provides enough energy to knock new grains into the air (assuming a sandy surface). Thus, the wind needs to provide only enough energy to move the airborne grains downwind. The wind speed necessary to maintain saltation once it has begun is termed the “impact threshold” and defined by Bagnold (1941) as the velocity at which “the energy received by the average saltating grains becomes equal to that lost (by the impact), so that motion is sustained.“ Like the fluid threshold, the impact threshold increases with increasing grain size.

seminal work of McKee (1979) relied on remote sensing for the taxonomy and mapping of dunes. Remote sensing also allowed exploring the influence of controlling parameters such as wind patterns, type of vegetation, and availability of sand on the terrain (Wasson and Hyde 1983). While studies in the 1980s focused on individual dunes, the important progress of computer science that took place in the 1990s induced interest in the understanding of the reflectance of dune surfaces (Blumberg 1998). In the 2000s, more advances were made in quantitative aspects of dunes morphology and dynamics (Vermeesch and Drake 2008; Bishop 2010). The improvements in remote sensing spatial and spectral resolution also paved the way for new applications, such as the high resolution (LiDAR) used to create a digital elevation model for identifying parabolic barchans dunes in Canada (Wolfe and Hugenholtz 2009), or the use of ASTER radiometric data used by Scheidt et al. (2010) to create a mapping for the soil moisture in the White Sands Dune field in New Mexico, USA. Also, the HiRISE camera onboard the Mars Reconnaissance Orbiter allowed for more advanced research on dune morphology (Hansen et al. 2011; Azzaoui et al. 2016). The availability of geospatial datasets, from Corona, Landsat, MODIS, ASTER, HiRISE, MOC, MRO CTX, SRTM, ASTER GDEM, and HiRISE DTM, allowed for more research to be conducted on the dynamics of dune systems including in inaccessible areas, such as other planetary systems (Mars, Venus, Titan). The application of remote sensing in Nigeria started in the early 1960s. Oyelese (1968) used aerial photographs to map land use patterns in the forest zone. Aerial photographs of Sokoto province in northern Nigeria were used in land mapping and soil and vegetation (land unit) reconnaissance survey (see FAO 1969). Adeniyi (1980, 1986) and Adeniyi and Olugbile (1987) used sequential aerial photographs in land use change analysis and urban growth. Fagbami (1980, 1981a and 1981b) applied remote sensing in soil survey and land use inventory in southwestern Nigeria and the Benue valley. In relation to dunes, remote sensing was used in a study of the spatial aspects of dune morphology (Grove and Warren 1968; White 1971). LANDSAT Thematic Mapper images of northern Nigeria were used to identify parallel striations that characterize the dune pattern (Nichol and kabiru 2001).

9.3 9.2

The Application of Remote Sensing Techniques in Dunes Studies

Remote sensing has been widely used to study aeolian dunes. It started in the 1970s (Breed and Grow 1979), when the existence of dunes was established on Mars (Cutts and Smith, 1973) and Venus (Florensky et al. 1977). The

Dunes of Northern Nigeria

9.3.1 Origin Dunes are the prominent and widespread landforms associated with the aeolian process in northern Nigeria. Wherever they are found, they have formed a very impressive and distinctive feature on the landscape (Figs. 9.1, 9.2, and 9.3).

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Dune Fields on the Plains of Northern Nigeria

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Fig. 9.1 Typical sand dune in northern Nigeria. Source Department of Geography, KUST, Wudil

Fig. 9.3 Degraded dunes in northern Nigeria. Source Department of Geography, KUST, Wudil

Fig. 9.2 Trees planted in the interdune to stabilize dunes in northern Nigeria. Source Department of Geography, KUST, Wudil

In the central and western parts of northern Nigeria, around Sokoto, (Fig. 9.4) dunes form a regular pattern of long parallel sand ridges. Likewise, they occur within an ancient erg toward the center of the region, north of Kano. In the Lantewa dune field to the east around Damaturu, they form a complex pattern. The dunes, especially those of linear

form, appear to be composite features, reflecting many generations of dune construction and stability. In explaining the chronology of dunes, Grove and Warren (1968) traced their development to the late Quaternary climate change in North and West Africa, during which the late Pleistocene was characterized by a drier climate. The dry conditions witnessed extensive dune systems, followed by a wetter climate in the early Holocene. They further indicated that vegetated dune fields found in northern Nigeria indicate that the 500 mm isohyets lay about 500 km south of their present position. The explanation was also supported by Sarnthein (1978) who reported that active dunes were most extensive 18,000 years ago and that dunes were generally dormant from 6000 years ago. In northern Nigeria, inactive extensive dunes occur in areas that today receive as much as 1000 mm of rainfall per year.

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Fig. 9.4 1000 m Isohyets (areas where relict dunes are found in northern Nigeria)

In addition to historic climatic conditions, dune development has been influenced by conditions that influenced sand supply, availability, and mobility, as well as the preservation of deposits from prior episodes of aeolian construction. Dune development has also been strongly influenced by variations in vegetation cover and soil moisture. Furthermore, dune development has been impacted by lacustrine transgressions, for instance, the consequence of the transgression of Mega-Chad Palaeolake—in northern Nigeria.

9.3.2 Studies of Dunes in Northern Nigeria The existence of a large expanse of land covered by the stabilized dunes on the plains of northern Nigeria has been of interest to researchers for many years. Studies by among others Grove and Warren (1968); Sarnthein (1978); Buschbeck and Thiemeyer (1994); Thiemeyer (1995); Stokes and Horrocks (1998) and Holmes et al. (1999) extensively reported on the origin, morphology, and the general characteristics of the dune landscape of northern Nigeria. Falconer (1911) offered one of the earliest observations of the dunes of northern Nigeria while investigating the ancient erg of Hausa land. He was followed by other researches, notably Grove (1958) Grove and Warren (1968) Sarnthein (1978) Nabegu (1979) Buschbeck and Thiemeyer (1994) Thiemeyer (1995) Stokes and Horrocks (1998) and Holmes et al. (1999) among others. In recent times, the use of satellite imagery has provided a robust scientific tool not only in the examination of

A. B. Nabegu

the dune landscape pattern but also to identify the age, and origin as well as to interpret conditions under which they were formed. Furthermore, the emergence and utilization of luminescence science made it possible to depicter the date of the periods of dune formation, thus allowing a better appreciation of the history of the dune. Similarly, the utilization of radar technology made it possible to examine in detail the study of the dune systems’ associated features. Studies of dune fields of northern Nigeria have been mainly of two types. First, spatial aspects of dune morphology were used as an indicator of Quaternary environmental change (Grove and Warren 1968), and second, soil surveys of dune fields were carried out for agricultural purposes (Sombroek and Zonneveld 1971). The studies of dunes have provided qualitative environmental information from the paleomagnetic data it contains on the history and changes that occur both in the dry and wet phases of the region (Grove and Warren 1968). Furthermore, dune fields relate directly with the extent of desert conditions, which is of direct relevance to assessments of the magnitude of climatic change in desert marginal areas, as well as of soil surveys for agricultural and land use planning (White 1971).

9.3.3 Age of the Dunes of Northern Nigeria One of the most advocated models used in explaining the age of dunes of northern Nigeria postulates that they are relicts of the southward expansion of the Sahara during the Last Glacial Maximum (LGM), followed by dune stabilization after the LGM and during the early to middle Holocene in the southern margin of the Sahara, which corresponds with the latitudinal extent of the dunes of northern Nigeria. This model showed that older dunes preserved in the south and younger, Late Holocene, reactivation of post-African Humid Period (AHP) dunes to the north (Buschbeck and Thiemeyer 1994; Thiemeyer 1995). Notable studies on the expansion of the Sahara during the LGM such as those by Stokes and Horrocks (1998) and Holmes et al. (1999) indicate that active dunes were most extensive 18,000 years ago, while they were generally dormant 6000 years ago. Stokes and Horrocks (1998) further reported that the ages of linear dunes range between 20.6 ± 3.1 and 7.0 ± 1.1 ka and include a cluster of late-glacial ages (20.6 ± 3.2, 18.2 ± 2.0, 17.8 ± 3.0, 17.6 ± 2.4 ka). They suggested that the linear dunes accumulated over a long period at least 6 ka and that the accumulation was probably episodic. Holmes et al. (1999) report 15 ages from the dunes in the Lantewa dune field, which includes linear and transverse dune forms, although ten of the fifteen samples come from barchans or barchanoid dunes (Holmes et al. 1999). Thus, there appear to be at least two generations of dunes in the Lantewa dune field, an older set of linear dunes that trend

9

Dune Fields on the Plains of Northern Nigeria

ENE-WSW, mapped by Grove (1958), and a younger set of barchanoid dunes which were active from the Middle Holocene. Also in northeastern Nigeria, Preusser (2007) identified aeolian sand with an age of 4.8 ± 0.4 ka, which most likely represents increasingly arid conditions in northeastern Nigeria at that time.

9.3.4 The Spatial Extent of the Dune Fields of Northern Nigeria The extent of dunes has direct relevance to assessments of the magnitude of climatic change in desert marginal areas. Numerous workers have observed air photos and posited that the presence of systems of fixed longitudinal dunes bordering the world’s deserts are indicators of more extensive desert conditions in the past when rainfall totals were much lower than at present (Grove 1958; Grove and Warren 1968; Mainguet 1983, Lancaster 1987). At present, the dunes are found in areas that receive as much as 1000 mm of rainfall. Grove and Warren (1968) noted that limits of the desert have shifted long distances north and south in the late Quaternary period and succeeded in establishing alternating wetter and drier intervals marked by the appearance of different generations of dunes and associated fluvial and lacustrine sediments. The cause of the latitudinal shift of the desert has been linked with the strength of the West African monsoon and the latitude of the intertropical convergence zone (ITCZ). Arbuszewski et al. (2013) estimated that the ITC shifted at least 7° south during the LGM to the latitude of 2°. During the early Holocene, there was a northward shift in the ITCZ and the monsoon rain-belt toward 10° or 12°, from which it has subsequently shifted south to around 5° at the present day (Haug et al. 2001). Dunes formerly occupied a greater area of northern Nigeria than at present. It is estimated that 50% of the dune area has been lost. Pressures arising from human activities such as overgrazing, sand excavation, mining, and climate change are thought likely to affect the extent of some systems. There is currently no legislation on the protection of the dunes.

9.4

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crests are generally between 3 and 9 m above the intervening depressions, and in some places, they are as high as 11 m. Local relief is approximately up to 30 m (Grove and Warren 1968; Nabegu 1979) (Fig. 9.5). The dune lineation is not visible on air photos, (Plate 5) or individual wavebands of MSS imagery (Fig. 9.6). From high-resolution satellite image, the dune pattern appears clear and spectacular due to its superior spectral characteristics (Fig. 9.7).

Fig. 9.5 Air photograph of the dune fields around Kano. Source Nichol (1991)

Morphology of Dune Landscapes

The dune fields run dominantly in an ENE-WSW direction but change to NE-SW near the northern frontier, in alignment with the planetary wind system of the region (Grove and Warren 1968). The dunes range from hundreds of meters to a few kilometers wide and run across the region for tens of kilometers. Over most of the dune fields, there is a regular pattern of parallel ridges between 0.75 km and 1.25 km apart. Some are as much as 16–24 km long, although more often they are broken by small discontinuities. The dune

Fig. 9.6 LANDSAT TM thermal band image of the dune fields around Kano. Source Nichol and Kabiru, 2001

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A. B. Nabegu

Fig. 9.7 Google earth image showing the dune system in Jahun, north of Kano, northern Nigeria

Detailed characterization of the dune system in northern Nigeria began when the lineation was first noticed upon visual inspection of a LANDSAT MSS false color print, as alternating light and dark-toned strips aligned in the prevailing wind direction (Stokes and Horrocks, 1998). These strips are believed to owe their formation to wind funneling between the gaps in the hills, depositing sand in the lee of the gaps and leaving bare those areas in the lee of the hills (Stokes and Horrocks 1998). In areas to the east of Kano, the dune features exhibit considerable topographic variations. In recent years, natural processes, as well as anthropogenic factors such as sand mining (Plate 8), have combined to degrade the dunes in most places. Degradation has resulted in the re-mobilization of dunes, particularly in the extreme northeast and northwest of the region, leading to the abandonment of many settlements and farmlands, and massive relocation of communities (Fig. 9.8).

9.5

Paleoclimatology and Paleobotany Information in Dunes

The quality of paleoclimatic and paleobotanic data provided by dunes has been acknowledged as significant in tracing the environmental history of the areas where they are found. Lancaster et al. (2002) have shown that in periods of

Fig. 9.8 Sand mining on the dunes at Govt. Day Sec. School, Illela, near Sokoto. Source Department of Geography, KUST, Wudil

increased rainfall, the high infiltration capacity and porosity of dune sands favor the growth and persistence of vegetation, which may lead to the partial or complete stabilization of the dunes and formation of soils on dune sands. If the periods of increased rainfall are of sufficient magnitude and duration, then water tables may rise, leading to the formation of pond and marsh deposits in interdune areas. In contrast, periods of aridity will give rise to a very unfavorable biotic environment in which vegetation cover is reduced as a result of low precipitation and active sand movement.

9

Dune Fields on the Plains of Northern Nigeria

Dune systems provide some of the best indicators of past wind regimes and atmospheric circulation patterns (Rognon 1982). Also, the dune systems, particularly those of linear form, appear to be composite features, reflecting many generations of dune construction and stability. Furthermore, dunes may also contain evidence of periods of climates wetter than present, in which dunes were stabilized by vegetation, soil formation occurred, and marshes and shallow lakes formed in interdune areas. Many of these deposits contain fossil organic materials that can be used to reconstruct past hydrology and vegetation communities and provide ages for periods of wetter climates (Teller et al. 1990; Gasse 2002). Stratigraphic relations between such deposits and those of dunes have also been used as a major source of chronologic information on the dune areas (Swezey 2001) and elsewhere. In many areas, the evidence for environmental change obtained from the dunes is corroborated by records of dust flux (indicating arid, windy conditions) from marine sediment cores in the adjacent ocean and shelf areas, which frequently provide a better-dated record (de Menocal et al. 2000; Stuut and Lamy 2004). It has been shown that dunes are an important habitat type providing a suite of both environmental and socioeconomic functions (Arun et al. 1999; Pua Bar, 2009) as they support a broad range of flora and fauna owing to the diversity of ecological niches found within them. Part of this diversity is due to the complex topography and its concomitant vegetation communities, creating a wide range of habitats from dry dune crests to wet interdune areas. In northern Nigeria, the Hadejia-Nguru wetland, which is now a designated Ramsar site, is a good example. The wetlands were formed where the Hadejia and Jama’are rivers entered interdunes in a northeast-southwest alignment and split into numerous channels. The wetlands are important for waterbirds, both for breeding species and for wintering and passage of Palearctic water birds. The estimated waterbird population varies between 200,000 and 325,000. 377 bird species have been seen in the wetlands, including occasional sightings of the near-threatened pallid harrier and great snipe species (BirdLife International, 2019). It has thus attracted bird watchers from across the globe. Further, internal heterogeneity is generated by aspects on steep dune slopes (Jones et al. 2009), the degree of grazing and disturbance by animals (van Dijk 1992; Plassmann et al. 2010) and by succession processes in both dry and wet dune habitats (Jones et al. 2008), with the development from largely bare sand through grassland and eventually scrub or woodland as a natural climax community (Provoost et al. 2011).

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9.6

The Impact of Dune Re-Mobilization on Settlement

The first systematic field study of dune mobility was by Bagnold in 1941. Since then, there is the realization that dunes can move and often they do. Consequently, there have been several studies focused on the movement of dunes, notably in the past (Anderson and Haff 1988; Pua 2009). In recent years, there are also simulation studies of dune mobility (Sørensen 1988; Jones et al. 2008, 2009). These studies are important, because the knowledge of the dynamics of dunes helps in predicting their behavior, in particular when dunes can threaten some human activities. According to a report prepared by the United States Geological Survey, dunes are becoming more mobile as the climate changes. Therefore, monitoring of dune movement is fundamental. This monitoring can be accomplished through several methods, combining surface mapping with aerial and satellite imagery, GPS, and LIDAR measurements (Sørensen 1988). Moreover, historical surveys of archives containing aerial and satellite imagery can be quite useful to compile a database for each dune field. In northern Nigeria, various studies showed that mobile dunes today cover two-thirds of the entire landscape (Nasiru 2007). As the dunes move, they bury villages, roads, oases, crops, irrigation channels, and dams, causing major material and socioeconomic damage. Nasiru (2009) estimated that between 50%-75% of Adamawa, Bauchi, Borno, Gombe, Jigawa, Kano, Katsina, Kebbi, Sokoto, Yobe, and the Zamfara States in northern Nigeria are being affected by dune mobilization. These states, with a population of more than 50 million people, account for about 43% of the country’s total land area. In these areas, entire villages and major access roads have been buried under dunes. These dunes are threatening life-supporting oasis and bury water points. Trees planted by the government as shelter belts to check the advancing dunes are withering due to lack of attention. Similarly, shelters established by the government along the desert fringes of the affected states under the World Bank-assisted afforestation program have not been very effective as the trees have been felled for firewood, while some have withered due to high temperature, inadequate rainfall, and drought. In the same vein, some boreholes dug by the government to provide water have dried up due to acute drought in the affected zone. A report by the Federal Ministry of Environment says that Nigeria plunders its forest with more than 30 million tons of firewood annually due to pressure on the urban poor who resort to the cheapest means of cooking.

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A. B. Nabegu

decisions regarding appropriate developments and associated benefits).” It has been established that ecotourism can “provide local economic benefits while maintaining ecological resource integrity through low-impact non-consumptive resource use while contributing to conservation and community development” (Stem et al. 2003). In the dune fields of northern Nigeria, there is an increasing awareness of the value of ecotourism and the dune fields are now a major tourist and educational center attracting visitors “... and has been tasked by the international community to take responsibility for preparing a world list of national parks in keeping with its role as a network and also to share the world’s knowledge on nature conservation”. Fig. 9.9 Tourists on a visit to the dunes in Jahun, near Kano. Source Department of Geography, KUST, Wudil

9.7

Dune Fields as Tourist Attractions

In recent years, dunes have also become a major tourist attraction (Fig. 9.9), principally due to the unique features they provide to the landscape and also to the history and biodiversity they contain. Also in recent years, the concept of ecotourism as a form of tourism that aims at being ecologically and socially conscious is gaining ground globally. Ecotourism is the fastest-growing sector of the tourism industry (Lippard 1999). In recent years, ecotourism has risen from relative obscurity to become one of the world’s fastest-growing industries. In theory, it can lead to environmental awareness and conservation, local economic benefits, and integration of developing countries into the world market. Salama (2001) has shown that more and more tourists are abandoning traditional vacations for a new type of tourism that gives them a sense of nature. Trekking in mountains, on the dunes, bird watching, archaeological digs, desert and photo safaris, and scuba diving are all new types of vacations that attract tourists to travel to relatively remote and unspoiled areas. This type of travel is referred to as nature-based travel, ecotourism, or environmentally sustainable tourism (Salama 2001). The World Conservation Union’s Commission on National Parks and Protected Areas defined ecotourism as “environmentally responsive travel and visitation to relatively undisturbed natural areas, to enjoy and appreciate nature (and any accompanying cultural features—both past and present) that promotes conservation, has low negative visitor impact, and provides for beneficially active socioeconomic involvement of local populations” (Ceballos-Lascurain, 1996). Ross and Wall (2004) defined ecotourism as a “means of protecting natural areas through the generation of revenues, environmental education, and the involvement of local people (in both

9.8

Conclusions

The dune fields of northern Nigeria are relicts and reflect environmental dynamics, over time frames spanning hundreds of thousands of years. They provide a unique view of the plains as well as information about the past. Although stabilized and degraded, dune fields contain a variety of dune types, often forming complex patterns. Satellite image analysis, combined with studies of dune geomorphology and sedimentology, has been used to analyze dune patterns, identify their genetically distinct generation sand interpret the conditions in which they formed. Anthropogenic factors have resulted in the re-mobilization of the dunes in the northeast, and the mobility has destroyed farmlands and settlements. In recent years, due to their unique and dominant features on the landscape, the dunes have become a major tourist attraction.

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143 of the Manga Grasslands, NE Nigeria: evidence from palaeolimnology and dune chronology. J Geol Soc London 156:357–368 Important Bird Area factsheet (2019) Hadejia-Nguru wetlands Nigeria. BirdLife International Jones MLM, Sowerby A, Williams DL, Jones RE (2008) Factors controlling soil development in sand dunes: evidence from a coastal dune soil chronosequence. Plant Soil 307:219–234 Jones MLM, Norman K, Rhind PM (2009) Topsoil inversion as a restoration measure in sand dunes early results from a UK field-trial. J Conserv Lancaster N (1987) Formation and reactivation of dunes in the southwestern Kalahari: palaeoclimatic implications. Palaeoecology of Africa 18:103–10 Lancaster N et al (2002) Late Pleistocene and Holocene dune activity and wind regimes in the western Sahara of Mauritania. Geology 30 (11):991–994 Lippard L (1999) On the beaten track: tourism, art and place. The New Press, New York, USA Mainguet M (1983) Physics of desertification: The wind and desertification processes in the saharo-sahelian and sahelian region, Mithsonian Institution Archives, Accession 17:262, Farouk El-Baz Papers McKee ED (1979) Introduction to a study of global sand seas. In: McKee ED (ed), A study of global sand seas: United States geological survey, professional paper, 1052, pp 3–19 Nabegu AB (1979) Land use in the Ancient Erg of Hausa land: a case study of Wurma. Un-published BSc. Dissertation submitted to the Department of Geography, ABU, Zaria Nasiru IM (2007) A comprehensive approach to addressing drought and desertification in Nigeria. Unpublished Master’s Thesis, Universiti Teknologi Malaysia Nasiru IM (2009) Combating desertification and drought in Nigeria (II). This Day Newspaper, Sunday, 31 May 2009 23:56 Nichol EA (1991) The extent of desert dunes in northern Nigeria as shown by image enhancement. Geogr J 157(1):13–24 Nichol EA, Kabiru A (2001) A thermal remote sensing of atmospheric boundry layer of homogeneous surfaces. Paper presented at the 22nd Asian conference on remote sensing. Centre for remote imaging, sensing and processing (CRISP), National University of Singapore, Singapore Institute of Surveyors and Valuers (SISV); Asian Association on Remote Sensing (AARS) Oyelese JO (1968) The mapping of landuse patterns from air-photographs in the forest zone of Ibadan Division. Niger Geogr J 11:27–37 Plassmann K, Jones MLM, Edwards-Jones G (2010) Effects of long-term grazing management on sand dune vegetation of high conservation interest. Appl Veg Sci 13:100–112 Preusser F, Degering D (2007) Luminescence dating of the Niederwe ningen mammoth site, Switzerland. Quatern Intl 164–165 Provoost S, Jones MLM, Edmondson SE ( 2011) Changes in landscape and vegetation of coastal dunes in northwest Europe: a review. J Coast Conserv Pua Bar K(2009) Overview of the sand dune situation in Israel and the rationale and goals of the study day on ‘Sand Dunes in Israel: In Dunes in Israel: Policy and Management, Ben-Gurion University of the Negev, Israel Rognon P (1982) Pluvial and arid phases in the Sahara: the role of non climatic factors. Palaeoecol Afr 12:45–62 Ross S, Wall G (2004) Ecotourism: towards congruence between theory and practice. In: Williams S (ed) Tourism: critical concepts in the social sciences. Routledge, pp 240–258 Salama A (2001) Ecolodges: a tool for sustainable tourism development in Egypt. A white paper for the tourism development authority in Egypt, Ministry of Tourism, Cairo, Egypt Sarnthein M (1978) Sand deserts during glacial maximum and climatic optimum. Nature 272:43–46. https://doi.org/10.1038/272043a0

144 Scheidt SP, Lancaster N ( 2010) Sensitivity of automatic determination of sand transport direction and rate to dune morphology in the namib sand sea. Des Res Inst, 2215 Raggio Parkway, Reno, NV 89512 Sombroek WG, Zonneveld IS (1971) Ancient dune fields and fluviatile deposits in the Rima-Sokoto River basin _N.W. Nigeria. Soil Survey Papers No. 5. Netherlands Soils Survey Institute, Wageningen. Colour printed map, 107pp Sørensen M (1988) On the rate of Aeolian sand transport. In: Proceedings of ICAR5/GCTESEN joint conference, International center for arid and semiarid lands studies, Texas Tech University, USA Sharp RP (1963) Wind ripples. J Geol 71(5):617–636 Sharp RP (1966) Kelso dunes Mojave desert California. Geol Soc Am Bull 77(10):1045–1073 Sharp RP, Saunders RS (1978) Aeolian activity in westernmost Coachella Valley and at Garnet Hill. In: Aeolian features of Southern California: A Comparative Planetary Geology Smith HTU (1968) Aeolian geomorphology wind direction and climatic change in North Africa. Bedford MA, US Air Force Geophysic Research Directorate Stem C, James PL, David RL, David J, Deshler M (2003) How ‘Eco’ is ecotourism? A comparative case study of ecotourism in Costa Rica. J Sustain Tourism 11(4):322–347 Stokes S, Horrocks J (1998) A reconnaissance survey of the linear dunes and loess plains of northwestern Nigeria: granulometry and geochronology. In: Alsharan AS, Glennie KW, Whittle GL, and Kendall CGSC (eds.) Quaternary deserts and climatic change. Balkem, Rotterdam, pp 165–174

A. B. Nabegu Stuut JB, Lamy F (2004) Climate variability at the southern boundaries of the Namib (southwestern Africa) and Atacama coastal deserts during the last 120,000 yr. Quatern Res 62(3):301–309 Swezey C (2001) Aeolian sediment responses to late Quaternary climate changes: temporal and spatial patterns in the Sahara. Palaeogeogr Palaeoclimatol Palaeoecol 167:119–155 Teller JT, Rutter NW, Lancaster N (1990) Sedimentology and palaeohydrology of late Quaternary lake deposits in the northern Namib Sand Sea, Namibia. Quatern Sci Rev 9:343–364 Theimeyer H (1995) Dating of palaeo-dunes from NE-Nigeria with thermo luminescence. Z Geomorphol 99:97–106 van Dijk HJ (1992) Grazing domestic livestock in Dutch coastal dunes: experiments, experiences and perspectives. In: Carter RWG, Curtis TGF, Sheehy-Skeffington MJ (eds) Coastal dunes: geomorphology, ecology and management for conservation, Proceedings of the third European Dune Congress, Galway, Ireland, Rotterdam, Balkema, pp 235–250 Vermeesch P, Drake N (2008) Remotely sensed dune celerity and sand flux measurements of the world’s fastest barchans (Bodélé, Chad). Geophys Res Lett 35 Wasson RJ, Hyde R (1983) Factors determining desert dune type. Nature 304:337–339 White LP (1971) Vegetation stripes on sheet wash surfaces. J Ecol 59:615–622 Wolfe SA, Hugenholtz CH (2009) Barchan dunes stabilized under recent climate warming on the northern Great Plains. Geology 37:1039–1042

Landscapes and Landforms of the Jos Plateau

10

Tasi’u Yalwa Rilwanu and Yakubu Samuel

Abstract

10.1

The Jos Plateau is situated in the north-central part of Nigeria, with the highest peak at 1829 m. It is surrounded by plains on all sides. The landforms and landscapes of the plateau can be grouped into three physiographic units, namely hills and mountains, dissected terrains and undulating terrain. The Jos Plateau is drained by a radial river network, with numerous waterfalls at topographic escarpments. Regional geology includes two major structural units: an Older Basement Complex and the Younger Granites. The Basement Complex comprises intrusive igneous rocks and structures long exposed by denudation. For the Younger Granites, ring complexes are characteristic, with escarpments and massive outcrops of Jurassic granites. There are over 22 dormant to extinct volcanoes in the Jos Plateau, which occur as four series of volcanic lines named as Ganawuri, Hoss, Panyam (Sura) and Gu (Jiblik). There are two major waterfalls and three minor ones and springs on the Plateau. One of the waterfalls has hydroelectric power stations for electricity generation. Terracing-based farming, animal husbandry, tin mining, lumbering and tourism are common human activities in the plateau area. Keywords

Jos Plateau Volcanoes





Precambrian Waterfalls




Younger granites




T. Y. Rilwanu (&) Department of Geography, Bayero University Kano P.M.B, Kano, 3011, Nigeria e-mail: [email protected] Y. Samuel Department of Geography, Osun State University, Osogbo, Nigeria

Introduction

The Jos Plateau is an upland with undulations, rivers and streams situated in north-central Nigeria, encircled by plains from all sides. The major rivers that flow from the Plateau are Gongola, Kaduna, Yobe and Hadejia. Other smaller rivers flow from the foothills of the plateau around Riruwai, such as Sansantsa, Magajiya, Tandama and Mace da Ciki. The Jos Plateau is situated in the Plateau State of Nigeria and lies between latitudes 8°24’ and 10°04’ N and longitudes 8°32’ and 10°38’ E (Fig. 10.1). Apart from the southern margin, which is both very steep and rather regular in outline, the plateau is bordered by sinuous margins characterised by gentle slopes. The plateau covers about 8600 km2. The immediately surrounding areas of the plateau are foothills or escarpments situated between 300 to 600 m asl. The highest point on the Jos Plateau is the Shere hill (Figs. 10.1 and 10.2), with 1829 m asl (Morgan 1983), whereas an average altitude is 1280 m asl. Geological framework is the main control of the landscape of Jos area. Bedrock is composed of the Precambrian Basement Complex, with the Younger Granites intruding into the Older Basement Complex during the Triassic to Jurassic periods. The younger granites are predominantly non-orogenic alkaline in nature. Volcanic rocks such as basalts and rhyolites overlie or crosscut both granites as well as other basement rocks (Mallo and Wazoh 2014). The basalt rocks of the Jos plateau are related to volcanic activity and can be grouped into lateritised older basalts and older basalts that suffered from denudation, while the newer basalts consist of preserved volcanic cones. These more recent products of volcanic activity comprise basaltic lava flows, pumice and ash deposits. They are of Cenozoic ages, including the Quaternary (Daniel et al. 2015). The Jos Plateau landscape has been categorised into three different physiographic units based on terrain characteristics and scenery. These are named as are hills and mountains, dissected terrain and undulating terrain (Odunuga and Badru

© Springer Nature Switzerland AG 2023 A. Faniran et al. (eds.), Landscapes and Landforms of Nigeria, World Geomorphological Landscapes, https://doi.org/10.1007/978-3-031-17972-3_10

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Fig. 10.1 The Plateau state showing the Jos Plateau and associated Landscapes and landforms. Source Drawn from Shuttle Radar Topography Mission (SRTM), 2014

2015). The form of relief in the plateau area is closely associated with the underlying rock types, in that the resistant younger and older granites have formed a resistant core and support mountainous topography, with an altitude of over 1500 m. The shape of these hills is largely determined by the joint pattern. The top of the plateau is dominated by unconsolidated materials and an iron hardpan that controls the relief. In terms of landforms, the Jos can be subdivided into the upper plateau, comprising of hills and bare rocky areas, and a lower plateau of depositional nature, with sandy materials and some of the landforms defaced due to tin

mining activities. The latter left mining ponds and waste heaps all over the area. The relief and landscape influence the climate, which is semi-temperate on the plateau, with temperatures ranging from 18 °C to 25 °C, while rainfall is around 2000 mm in the wetter southwest, declining to around 1500 mm in the northeastern parts of the plateau. The western part of the plateau is covered with extinct volcanoes, whereas vegetation is mainly riparian. The western escarpment complex is dominated by volcanoes, ridges and rivers, and shrubs and woodlands all over the area. The eastern escarpment complex is also dominated by

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• landscape features supported by the Younger Granitic formation of Jurassic to Triassic age, • basaltic volcanic cones of Cenozoic age. All these, together with associated landscapes and landform features, rivers and streams, mineral deposits and related human activities, are discussed and described in this chapter.

10.2

Fig. 10.2 Part of Shere hills with a typical granite tor, boulders of different sizes and vegetation around as evidence of biological weathering through root penetration and chemical weathering as a result of humic acid formation by the decay of leaves. Source Field work 2019

shrubs and riparian woodlands. The northeastern Jos is occupied by the Toro rock complex which lies on the Mongu plains (Fig. 10.3), an area covered by shrubs and grasses which provide extensive grazing area (Adekoyejo 2013). Between these three complexes, an alluvial complex occurs, which is swampy and suitable for cultivation of crops such as rice. In terms of geologic history, time scale and mode of formation the landscape of the Jos Plateau can be subdivided into three units: • landscape features developed upon the Older Basement Complex of Precambrian to Paleozoic era,

Fig. 10.3 A section of the Mongu Plain indicating an extensive land area with settlements, metamorphic hills and old mining piles and ponds in the western part of the plain. Source Retrieved from Google Earth on 8th February 2020

Landforms Units of the Jos Plateau

The landforms of the Jos Plateau are grouped into three categories in terms of their physiography. These three physiographic units include hills and mountains, dissected terrain and undulating terrain (Rackham 1973/4, after Bennett et al. 1978) and can be briefly characterised as follows: • Hills and mountains comprise all sort of hills, mountain ranges and steep-sided ridges that occur in groups or in isolation. This category hills like the Shere hills, the Wase hill and all volcanoes. • Dissected terrain includes landforms formed at the edges of the plateau-like low bedrock hill, low height rock outcrops, valleys between hills, deeply incised valleys and pockets of flatlands on the plateau. • Undulating terrain covers most of the surface of the plateau affected by erosion. It is characterised by slight to moderate erosional dissection. Examples of such areas include the Daffo-Pankshin area that is less dissected due to the resistance of newer basalts as compared to areas to the northeast and east.

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Landscape Features of the Basement Complex

The Older Basement Complex areas of the Jos plateau comprise intrusive igneous rock structures exposed by denudation long ago. In many of the areas within the Older Basement Complex, intrusions and extrusions of the Younger Granites were believed to have been embedded within the Older Basement Complex structures. These are the major reasons why it is difficult to separate landscape features of the Older Basement Complex from those developed upon the Younger Granites. The Basement Complex landscapes are associated with chains of highlands of variable height, juxtaposed with areas of almost flat topography at lower elevations. The Wase hill situated in the Wase town east of Langtang North and Langtang South Local Government Areas (Figs. 10.4 and 10.5) serves as an example of a landform with such composition of geologic structures. It is one of the largest hills in the world that displays a massive dome shape, rising to a height of about 350 m above the surrounding areas. The geology around the Wase hill is made up of the Basement Complex rocks such as charnockite, diorite and migmatite-gneiss-quartzite complex of the Precambrian age. The geology and hydrogeology of Langtang and Wase areas are intricately linked to hydrology, and the areas are well drained by seasonal rivers (Turner 1971 after Goyit and Solomon 2018). Towards the western part of Wase town, the River Malmo flows from the top of the plateau and separates Langtang from the Wase town. Around the Malmo River valley, there are riparian

Fig. 10.4 Wase hill. Source Field work 2019

communities and tin mines that are more common at the bank towards Langtang side (Fig. 10.5). The Basement Complex area is composed of two groups of rocks: plutonic and volcanic, both of Precambrian age and formed over 500 million years ago. These groups consist of migmatites, gneisses and intrusive older granites of Pan-African Orogeny (Opara et al. 2015). The typical landscape features associated with this geological formation comprise the southwestern escarpments around the Pankshin town, running from the old Pankshin settlement in the west to the Pankshin waterfalls in the east. Downstream from the escarpment, numerous streams and rivers drain down through the new Pankshin settlement. Landscape features in the Jos North section give it a different scenery that beautifies the area around Dogon Dutse (Fig. 10.6), otherwise known as Usharu Utura rock, a typical older rock that stands uniquely on the Younger Granite at Gwong in central Jos with, an altitude of 1194 m asl (Aga et al. 2011). It has a rounded elevation of a limited extent, rising above the surrounding land with local relief of less than 300 m. In the Jos North area, other Older Basement Complex formations are dissected by rivers and include visually attractive geomorphic features. Among them is the Dilimi River, which is typical bedrock river, with a rocky bed and banks (Fig. 10.7).

10.4

Ring Complexes in the Younger Granite Areas

In northern Nigeria, there are a series of ring complexes that include Fagam, Kilawa-warji, Ningi, Tibchi, Banke, Dutsenwai, Liruie (Riruwai), Kudaru, Zaranda, Mada, Rishua, Amo-Buji, Jos, Bukuri, Shakaleri and Tongolo among others. However, they are particularly concentrated within and around the Jos Plateau and its adjoining areas (Fig. 10.8). The Younger Granites of the Jos Plateau have been intruded into the Basement Complex and form a series of ring complexes, thought to be of the Jurassic age (Jacobson et al. 1958 in Bennett et al, 1978). Actually, the Younger granites of the Jos plateau are part of the numerous high-level, non-orogenic ring complexes that extend from Air region in the republic of Niger to the margin of the northern Benue Valley and parts of Cameroon (Turaki 1983). The Younger Granites of the Jos Plateau form hill masses characterised by a resistant core that make up the landscape. They rise over 1500 m asl, and the morphology of Jos Plateau hills is largely controlled by the joint pattern (Rackham 1973/4 in Bennett et al. 1978). Geomorphological landscapes supported by these rock complexes are characterised by escarpments, hills, mountains and buttes with ferricrete caps that protect the soil profiles against erosion (Thorp 1967; Zeese et al. 1994).

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Fig. 10.5 Geomorphological features around Wase and Langtang towns. Source Retrieved from Google Earth, 3rd February 2020

Fig. 10.6 Dogon Dutse hill in the Jos North behind the Bauchi road. Source Field work 2019

The following ring complexes are distinguished: • The Jos-Bukuru Complex is composed of a mountain chain that extends from the Shere Hills in the northeast to the Forum River in the south, bordering with the Jere-Sanga (Jarawa) Complex to the east. The elevation of Jos-Bukuru Complex ranges from 1158 to 1828 m asl, and the highest point is located near the western and northern margins, around the Shere hills (Opara et al. 2015). The ring complex is elliptical in plan, extending for a distance of 48 km, and covers an area of 430 km2 (et al. 2018b). The complex is believed to have formed during the early Cenozoic era (Mallo and Wazoh 2014). Younger granites of the Jos-Bukuru Complex are not related to any form of orogenic activity or event but are associated with epeirogenic uplift (Edun and Davou 2013). • The Ganawuri Complex situated west of Kigom Complex consists of two prominent hill masses of almost circular

outlines (Fig. 10.8). The larger rock mass occurs at the northern margin of the Complex, whereas the smaller hill mass is in the southern part. The Ganawuri Complex extends for a distance of 16 km with a surface area of 181 km2 (Sabinus et al. 2018b). Inside the complex, there are springs, which are usually thermal ones, and the predominant rock type is mafic riebeckite-aegirine granite and riebeckite-biotite granite (Sabinus et al. 2013). • The Kagoro Complex begins at Jos Terminus market area and extends southwestwards, up to the Kagoro hills in the Kaduna State. Bedrock includes gneiss, diorite, migmatite, biotite granite and granite-gneiss that occur in widely distributed ring dykes. Geomorphologically, the complex represents a continuous plateau that rises between 91 and 305 m above the surrounding plain (Alkali and Hassan 2014) (Fig. 10.8). The age of this complex ranges from Precambrian to lower Paleozoic. The diameter is 21 km, the area covers 232 km2, and the ground plan is elliptical (Sabinus et al. 2018b).

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•

• Fig. 10.7 Bedrock-cut channel of River Dilimi, Jos North area. Source Field work 2019

• The Kwandonkaya Complex is located about 40 km southeast of the central Jos Plateau boadering Jere-Sanga (Jarawa) Complex and consists of a hill mass (Fig. 10.8). It is dominated by a series of outcrops with a high degree of mechanical weathering and fracturing that can be noticed through observing fault lines, boulders, rock fragments and screes. The Kwandonkaya Complex is dominated by amphibole and biotite granites, and it is also rich in minerals containing iron such as fayalite, amphibole and hedenbergite (Imeokparia 1984; Sakoma and Williams-Jones 2000). • The Saiya-Shokobo Complex is a typical anorogenic alkaline Younger Granite complex that is situated 45 km to the north of Jos town (Fig. 10.8). The Saiya-Shokobo Complex was formed through several intrusions and is situated at the boundary between the Plateau, Bauchi, Kaduna and Kano States and is one of the northern ring complexes, extending from Dagga Allah area to the Kwandonkaya Complex. It is made up of three minor units that are named after prominent peaks of Saiya hill (1360 m), Shokobo hill (1315 m) and Tongolo hill (1340 m). It is a distinct hill mass, isolated from the main plateau and characterised by rugged topography and poor

•

•

access, with a maximum altitude of 1680 m, some 650 m above the surrounding plain (Kinnaird 1987). Erosion in a ring pattern is not particularly apparent in the area. The area is also characterised by poorly vegetated rugged hills rising to a maximum height of 1340 m on Tongolo hill, which lies 470 m above the Dilimi Plain to the east. The complex consists of ring dykes built of gabbro, dolerite and other igneous rocks (Aga and Haruna 2019). The diameter of the Saiya-Shokobo Complex is 18 km, with an aerial coverage of 252 km2 and an elliptical outline (Sabinus et al. 2018b). The Jere-Sanga (Jarawa) Complex is among the most extensive Younger Granite complexes of northern Nigeria and is situated in the Jos-East local government area, west of Kwandonkaya Complex (10.8). It consists of over 53 non-orogenic granitic hills (Ajigo and Nyako 2019). The major groups in the complex are the Jarawa and Fusa hills, associated with the Neils Valley granite porphyry. The ring complex of Jarawa is located eastwards from the Jos-Bukuru complex. The rocks in the complex are highly mineralized and contain minerals such as cassiterite, molybdenite, topaz and helvite. The length of the Jarawa Complex is 27 km, the surface area is 66 km2, and the outline is circular (Sabinus et al. 2018b). The Amo/Buji Complexes occur at the northeastern margin of the Jos Younger Granite suite, in the zones of mineral entrapment and tectonic activity east of Kudaru Complex (Fig. 10.8). They are characterised by a succession of granite ring dykes and plutons. They contain a ring complex of superimposed porphyritic granite and are dominated by features like calderas and ring dykes. The Amo Complex extends for a distance of 22.4 km, with an aerial coverage of 357 km2, whereas the Buji Complex is only 9.7 km, with an aerial coverage of 78 km2 (Sabinus et al. 2018b). Both complexes have circular outlines. The Ropp complex is located in the southwestern part of the plateau together with Shakaleri Complex, extending for about 80 km2 (Fig. 10.8). It is characterised by the presence of elongated, rugged hills up to 300 m high and polygonal dykes. The area is characterised by a radial drainage network. The Ropp Complex is 35 km long, and its outline is polygonal (Sabinus et al. 2018b). Shakaleri is the third largest Complex in Nigeria with over 640 km2, and it comprises more than 20 units, of which two of them are characterised by pre-caldera and intra-caldera volcanic material. It is located 40 km east– west of Mangu-Bokkos areas of the Jos plateau (Borley 1963). The Shakaleri Younger Granite Complex, which intrudes into the low-lying Paleozoic-Precambrian Basement, forms the Plateau's southern limit (Fig. 10.8). The Kaleri, Monguna and Tof sub-complexes are the three sub-complexes of the Shakaleri Complex (Daspan et al. 2007).

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Fig. 10.8 The Jos Plateau showing some of the major ring complex groups and heights. Source Drawn from Shuttle Radar Topography Mission (SRTM), 2014

• The Sara-Fier Complex is characterised by a different variety of intrusive and extrusive granitic rocks. It is located south Jos-Bukuru Complexes and west of Ropp and Shakaleri Complexes (Fig. 10.8). Sara-Fier Complex is composed mainly of granites, but also includes minor acid volcanic rocks and a few small intrusions of basic and intermediate composition. Sara-Fier Complex is divided into three type of rock structures as early volcanic rocks made up of rhyolite, ring fractures composed of intrusion of ring dikes and emplacement of successive granites within ring-fault. It is situated at the edge of the plateau to the east characterised by rocky hills of that are

around 1219 m (Turner 1963). The Complex is characterised by unbroken escarpment 4828 m long and a high of 610 m. • The Rishua Complex is situated to the west from the Jos-Bukuru complex, north of Korku, Kigom-Rukuba complexes, south of Kudaru Complex and southeast of Mada Complex in the central Jos plateau (Fig. 10.8). The Rishua complex and the Korku complex are like twins, Rishua being three times bigger and located to the north of Korku. In this complex, there are two important hill masses, among which the one in the north has high-discharge thermal springs with sweet water, while

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the one in the south has highly mineralized water that is not sweet. Rock types in the complex are similar to other complexes of northern Nigeria and include rhyolite, hornblende-fayalite granite, biotite granite, riebeckitebiotite granite, riebeckite and syenite among others (Sabinus et al. 2018a). • Other ring complexes can be found mostly located in Jos plateau adjoining areas or at the foothills of the plateau. These complexes include the Dutsenwai and Banke on the border of Kaduna state, and the Riruwai (Liruei), Tibchi and Ningi-Bura complexes in the northern foothills of the Jos plateau in Kano and Bauchi states, respectively. The Tongolo Complex is located east of the central plateau (Fig. 10.8). Dutsen-Wai Complex is also an extension of Jos plateau that is located 64 km east-southeast of Zaria. The Complex is associated with granitic intrusions and composed of isolated and Ring Complex rocks such as the well-known Dutsen-Wai rock at Dutsenwai town (Fig. 10.8). The Complex occupies an area of 6.3 km2 (Ajakaiye and Sweeney 1974). The Riruwai (Liruei) Ring Complex is an extension of the Jos plateau (Fig. 10.8) with a distinctive topography, it is located l40 km south of Kano City, and it occupies an area of about 129 Km2 (Turner 1963). The area is characterised by hills such as Shetu (1358 m), Ginshi (1345 m), Maisaje (.1700 m), Shuburu, Tandama and Mace-Dashiki. Tibchi Complex is a younger granite characterised by basaltic rock materials. Ningi-Bura Complexes: A series of overlapping ring structures each contain varied, pre-caldera volcanic sequences, overlain by massive intra-caldera rhyolitic ignimbrites and enclosed within ring dykes of fayalite-hedenbergite granite porphyry. Riruwai Ring Complex is characterised by Kaffo valley 10 km to the east of Riruwai town, and it is drained by River Kano and other seasonal rivers. Tongolo complex lies northeast of Saiya-Shokobo in the central Jos plateau, and it occupies 270 km2 (Fig. 10.8). The complex is composed largely of granites but there is little evidence of ring structures. Tongolo Complex is associated with Dagga Allah Complex which formed a polygonal system of dykes to the south.

10.5

In the Jos Plateau area, there are 22 dormant to extinct volcanic cones that are grouped into four series of cones trending from northeast to southwest. The four series are as follows: (a) the Ganawuri volcanic line consisting of Jal, and Kwakwi volcanoes, (b) the Hoss volcanic line consisting of Miango (with Rukuba, Miango north, Miango south, Vom and Kassa volcanoes) and Hoss in the southern Jos, (c) the Panyam(Sura) volcanic line that comprises of Dai (Wushik), Amshel (Kugol), Dutsin, Kerang, Tingyaras, Ampang (Mufil) volcanoes and Pidong Crater Lake, and (d) the Gu (Jiblik) volcanic line consists of Jiblik, Kagu, Katul and Lagdak volcanoes. The four series of volcanic cones belong to the Jos ring complexes. Among them are Jos-Bukuru, Rishua, Saiya-Shakobo, Kwandonkaya, Amo-Buji, KigomGanawuri, Kagoro, Ropp, Sara-Fier and others (Fig. 10.8). An important and specific landscape feature of the volcanic areas of the Jos is the Pidong Crater Lake, one of the most pronounced crater lakes in Africa. It occurs with the Ampang denuded volcano (Patterson 1986). This lake contains water that never disappears irrespective of the season (Fig. 10.9). There are further volcanoes in the plateau area of the Younger Granites such as the Jibki volcano on the way from Kerange to Shenam. The Kerange gigantic volcano is having a crater on its tot called Pidon crater lake around Mangu environments (Figs. 10.9 and 10.10).

10.6

Fluvial Morphology

10.6.1 Rivers The drainage pattern of the Jos Plateau is mostly radial as many rivers flow down from the plateau in all directions. This plateau is the source of numerous rivers, including the Kaduna, Karami and N’gell, which drain into the Niger River; the Mada, Ankwe, Dep, Shamanker, Malmo and Wase, which flow into the Benue; the Lere, Maijuju and Bagei, supplying the Gongola; and the Kano, Dilimi, Bunga, Jamaari and Misau, which ultimately go into Lake Chad (Odunuga and Badru 2015). In some areas, the drainage pattern is trellis due to evident control imposed by fractures in the basement rocks, and typical examples can be seen along the rivers of Mazan, Dilimi and Lamingo-yanshanu (Plate 10.11).

Landscape Features Supported by Volcanic Rocks 10.6.2 Waterfalls

The characteristics of the extinct volcanic activity of the Jos Plateau are that the volcanic rocks occur in layers, reflecting the history of successive eruptions (Wurim 2011). Volcanic rocks are very common in the areas of Bassa, Riyom, Jos North, Jos South, Barkin Ladi, Bokkos, Mangu and Pankshin.

The Jos Plateau is one of Nigeria's foremost areas where waterfalls are common. The plateau serves as a water-divide area, from which rivers originate and plunge pools develop where gradient is particularly high. Some of the waterfalls are used in the generation of Hydroelectric Power (HEP).

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Landscapes and Landforms of the Jos Plateau

153

Fig. 10.9 Pidong crater lake in Mangu local Government area. Source Field work 2020

Fig. 10.10 Kerang Hill, Jos Plateau. Source Google common pictures

The presence of waterfalls and favourable weather conditions attract visitors for recreation and tourism (Yakubu 2018). Two of these waterfalls are particularly well known (Kurra and Assop) and three other waterfalls (Zomgbu falls, Jibam falls and Kwang falls). a. Kurra falls The Kurra falls are located about 77 km to the southeast of Jos metropolis (Fig. 10.12 and Plate 10.13). It is the location of the State’s first hydroelectric power station. It is a beautiful area of rocky hills and lakes, ideal for boating, camping and rock climbing. The nearby Mount Sanderson provides beautiful views of the surrounding area. It is located next to Kent Academy, a non-denominational missionary school. It also features a man-made dam that provides water for

domestic use. The altitude of Kurra fall is 1148 m above mean sea level. The height of the waterfall steps is 229 m (National Electricity Supply Company NESCO, Diary 2008). Beside it, there are granitic hills, lakes and pools (Iyakari and Lar 2012). The Kurra falls are the major source of Hydro Electric Power (HEP) constructed and manage by the NESCO. b. Assop falls The Assop Waterfall is situated about 60 km away from the city of Jos, in an area called Hawa-Kibo Riyom. A gallery forest thrives in closer vicinity of the waterfall. Here, water flows from the top of the highland and runs through the forest as numerous streams (Fig. 10.14). The waterfall is a natural feature, situated at the bottom of the famous Hawan

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T. Y. Rilwanu and Y. Samuel

Plate 10.11 River Dilimi, its tributaries and Lamingo Dam in Jos North. Source Retrieved from Google Earth, 6th February 2020

c. Zomgbu falls Zomgbu fall is among the small waterfalls found on the Jos Plateau. It is situated in Miango village of Bassa local government area in the Jos North Fringe. The waterfall is composed of basalt rock, being part of the volcanic regions of the plateau. The step height of the fall is around 8 m (determined from Google Earth). At the bottom of the falls and uphill, there is riparian vegetation (Fig. 10.15). At the base of the falls, there is a flowing River Miango and its tributaries radiating away. d. Jibam falls Fig. 10.12 Kurra Falls on the Jos Plateau. Source Field work, 2022

Kibo rocky hill at Hawan Kibo village, which serves as a source to Assop River and its tributaries. The altitude of the waterfall is 689 m above mean sea level, and around the waterfall, there are caves, pools of water and minor falls (Iyakari and Lar 2012). The height of the waterfall step is 10.5 m from top to bottom (Determined from Google Earth). The rock type in the waterfall area is medium-grained biotite granite that belongs to the Kigom Complex younger granitic series of the Jos Plateau. There is no HEP attached to this waterfall.

It is in the central Jos plateau area, situated in the Pankshin local government area. It is a river that falls on the River Jibam, part of the Shemankar basin made up of granitic rock and basaltic materials. The Jibam waterfall area is made up of basement complexes and newer basements (Longpia et al. 2013). The bottom of the waterfall is made up of sub-basal, alluvial clay and materials intermixed with lava. It is also characterised by a shallow weathered basement of low groundwater potential, which amounted to bore-hole failures in the area. The step height of the falls is about 12 m (determined from Google Earth) (Fig. 10.16).

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Landscapes and Landforms of the Jos Plateau

155

Plate 10.13 Aerial view of part of Jos plateau showing River Kurra, Kurra falls, surrounding hills, valleys and riparian vegetations and a HEP station at Kurra falls. Source Retrieved from Google Earth, 6th February 2020

Fig. 10.14 The Assop waterfall. Source Wikimedia commons

e. Kwang falls Kwang fall is located in the Du area of Plateau States, Jos South Local Government Area. Kwang means ‘creek’ in Berom tribe; therefore, it ‘is a waterfall that appears like a creek and is the

main source of the Dilimi River (also known as the Dark Enclosure River) (Fig. 10.17). The pond’s pool is distinguished by exposed rocks and pebbles. Riparian vegetation, including grasses, bushes and trees of various sorts, surrounds the waterfalls. Old mining ponds, such as Daru-Du pond, can

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T. Y. Rilwanu and Y. Samuel

Fig. 10.15 Zomgbu waterfalls in Miango District. Source Wikimedia commons

or weathered materials, or where the basement is in contact with volcanic materials (Fig. 10.19). The springs are common in the Kerang Volcanic area (Fig. 10.10). The major springs include (1) Punguk Spring situated 1.5 km from Kerang town, (2) Tsohon Kerang Spring in Kerang (Fig. 10.10), (3) Konji Spring in Kerang, (4) Amshal Springs comprising of Amshal Spring in Kerang and Kogul Spring in Panyan-Mangu area, (5) Ogut Spring in Chwuang-Kogul areas, (6) Kungulang Spring in Pangniyes area and (7) Tungwang Spring located in Panyam about 80 km from the Jos city along Jos-Langtang road (Iyakwari and Lar 2012). These springs serve as a source of water to the local inhabitants, provide water to SWAN bottle Water Company and serve as sources for many rivers and numerous streams radiating from the area.

10.7

Fig. 10.16 Jibam falls. Source Wikimedia commons

be found near the waterfalls (Fig. 10.18). The fall step height is approximately 2.5 metres (determined from Google Earth). Granitic rock, metamorphosed granites, laterites and mining hips are among the rocks that formed the waterfall.

10.6.3 Springs In the Jos Plateau, springs occur at the base of high hills, particularly where a dyke is in contact with permeable rock

Economic Activities

In the Jos plateau, there are abundant deposits of tin and columbite. Other minerals include tin-cassiterite, whereas biotite granite and peralkaline granite contain uranium. Due to abundant mineral deposits, mining activity, both illegal and legal, has become one of the major branches of local economy. Farming activities are also very common, even though in most of the areas terracing has to be implemented due to the rugged nature of the terrain. The plateau edges, valley sides and treeless or grassy areas around granitic hills, as well as old mining piles, provide an extensive area for grazing so animal rearing is one of the major economic activities in the plateau area. In the Kerange mountainous region, some springs provide fresh water for bottling by Swan Water Company. Waterfalls in the Kurra area serve as a major source of HEP, which provides electricity to parts of

10

Landscapes and Landforms of the Jos Plateau

157

Fig. 10.17 Kwang fall. Source Wikipedia Commons

Fig. 10.18 Du old mining pond. Source Wikipedia Commons

Jos and some manufacturing industries. The HEP is controlled by the National Electricity Supply Company (NESCO).

10.8

Fig. 10.19 One of the springs in Kerang volcanic area, Jos Plateau. Source Wikimedia commons

Conclusions

The Jos plateau is the part of Northern Nigeria with the highest peaks, especially in the area around the Shere Hills and Kerange Mountains. Landforms and landscape features of the plateau area are directly related to past geologic history. Regional geology includes two major structural units: an Older Basement Complex and the Younger Granites. The drainage and river systems are structurally controlled, which is why part of the river network shows a dendritic pattern.

158

The dominant drainage pattern in the Younger Granite complexes is radial. The major rivers that drain the plateau include Kaduna, Kano, Gongola, Jama’are, Shamankar, Dillimi, Malmo, Mada and Wase. There are over nine ring complexes of Younger Granites that occur in groups, such as Jos-Bukuru, Rukuba, Kwandonkaya, Rishua, Mangu, Kagoro, Jarawa, Ropp and Sara-Fier among others, with different characteristic features. The geology and geomorphology of the Jos plateau influence the climate, drainage pattern, waterfall formation and mineral deposits, which in turn determine the nature of economic activities in the area. Acknowledgements We wish to acknowledge the contributions of scholars whose materials were consulted for this chapter writing. We would also like to thank the leaders of the Nigerian Geomorphological Working Group (NGWG) whose contributions cannot be mentioned in a very short paragraph of this nature. We would also like to show our appreciation for the free services rendered to us by our research assistants namely Bara’u Yakubu Usman and Olorunnipa Paul Kayode, who assisted with pieces of information and most of the camera pictures used in the chapter and also to Dr. Sulaiman Yunus, Mallam Rabi’u Yarima of the Department of Geography Bayero University, Kano Dr. Ahmad Hamza Abdullhi of the Department of Geography and Environmental Management Ahmadu Bello University, Zaria and Atakpa Adioju of the Zonal Advance Technology Application Laboratory, Kano, Nigeria (ZATLKN) for assisting us in map work and setting of Google Earth images, we remain grateful.

References Adekoyejo DO (2013) Application of amphibole chemistry in determining the petrogenesis of Hornblende-Biotite Granites from Toro complex, Northcentral, Nigeria. Continental J Earth Sci 8(1):1–11 Aga T, Zang JJ, Bala DA (2011) Geotourism potentials in parts of Anaguta Enclaves of Jos, North-Central Nigeria, The Pacific. J Sci Technol 12(1):574–579 Aga T, Haruna AI (2019) The geochemical constraints in the origin of Saiya-Shokobo younger granite complex Central Nigeria. IORS J Appl Geol Geophys 2(7):01–07 Ajakaiye DE, Sweeney JF (1974) Three-dimensional gravity interpretation of the Dutsen-Wai complex, Nigerian Younger Granite Province. Tectonophysics 24(1974):331−341. Elsevier Scientific Publishing Company, Amsterdam Ajigo IO, Nyako AA (2019) Litho-structural analysis of the Jarawa complex North Central Nigeria. Int Adv Res J Sci Eng Technol 6 (1):39–48 Alkali SC, Hassan M (2014) Magnetic studies of Kagoro and Environs, North Central Nigeria. IORS J Appl Geol Geophy 2(1):35–45 Bennett JG, Blair Rains A, Gosden PN, Howard WJ, Hutcheon AA, Kerr WB, Mansfield JE, Rackham LJ, Wood AW (1978) The Jos Plateau, Hill ID (eds) Land resources of central Nigeria Agricultural development possibilities Vol 2B. Land Resources Development Centre, Ministry of Overseas Development, Tolworth Tower, Surbiton, Surrey, England KT6 7DY 1978 Borley GD (1963) Amphiboles from the younger granites of Nigeria. Part 1. chemical classification. Mineralogical Magazine 33:358−76 Bowden P (1966a) Lithium in Younger granites of Northern Nigeria. Geochimica et Cosmochimica Acta 30:555−64 Bowden P (1966b) Zirconium in younger granites of northern Nigeria. Geochimica et Cosmochimica Acta 30:985−93

T. Y. Rilwanu and Y. Samuel Daniel SC, Lekwot VZ, Yakubu AA, Makarau SB (2015) An assessment of landscape segments suitable for agriculture in Kerang Volcanic area of Jos Plateau. Int J Sci Technol Res 4(4):294–298 Daspan RI, Yakubu JA, Lar UA (2007) Geochemical characteristics of gabbroic intrusive bodies in the Sha-Kaleri younger granite complex, Central Nigeria, Continental. J Earth Sci 2:7–13 Edun EO, Davou DD (2013) Inventory of abandoned mine ponds/Dams on Jos-Bukuru North-Central Nigeria using GIS and remote sensing technique. Int J Eng Sci 2(5):1805–2319 Goyit MP, Solomon OA, Kutshik RJ (2018) Distribution of fluoride in surface and groundwater: a case study of Langtang North, Plateau State, Nigeria. Int J Biol Chem Sci 12(2):1057–1067. Available online at http://www.ifgdg.org. Retrieved on 26th January, 2020 Imeokparia EG (1984) Geochemistry of the granitic rocks from the Kwandonkaya complex Northern Nigeria. Sci Direct Elsevier 17 (194):103–115 Iyakwari S, Lar AU (2012) Potential sites for Geotourism on the Jos Plateau Nigeria. Nigerian J Tropical Geography 3(1):190–203 Jacobson RRE, MacLeod WN, Black R (1958) Ring complexes in the Younger Granite Province of Northern Nigeria. Geol Soc Lond Memoirs 1:5–72 Kinnaird JA (1987) Hydrothermal alteration and mineralisation of the Nigerian Anregenic ring complexes: with special reference to the Saiya-Shokobo complex. A published Ph.D Thesis University of St Andrews. Published by St Andrew Research Repository. Available on http:/research-repository.st-andrew.ac.uk. Retirieved on 8th February, 2020 Longpia CB, Dakwo PD, Lar UA (2013) Hydrogeo-electric characteristics of upper river Shemankar Basin, Jos Plateau: a case study of Jibam and environs. IOSR J Appl Geol Geophys (IOSR-JAGG) 5:47–57 Mallo SJ, Wazoh HN (2014) Reclamation of abandoned mined-out areas of Bukuru ray field. IOSR J Environ Sci Toxicol Food Technol 8(2):25–34 Morgan WTW (1983) Nigeria. Longman, London. MSN maps reference: “Encarta on MSN Map”. Encarta on MSNMap. Retrieved 23-11-2020 National Electricity Supply Company, NESCO (2008) A published NESCO diary, Aevertising Publishers, Bukuru Jos, Nigeria Obaje NG (2009) In: Geology and mineral resources of Nigeria, lecture notes in earth sciences. Springer, Berlin, Heidelberg. pp 120. https:// doi.org/10.1007/978-3-540-92685-63,C Odunuga S, Badru G (2015) Landcover change, land surface temperature, surface albedo and topography in the plateau region of North-Central Nigeria. Land 4:300−324. https://doi.org/10.3390/ land4020300 Onyeanuna CC (2017) Interpretation of ring structures in Jos Plateau using NigeriaSat 1 imagery. Int J Mathem Phys Sci Res 4(2):95– 104) ISSN 2348-5736 (Available online). Retrieved on 5th February, 2020 Opara AI, Udoete RL, Emberga TT, Echetama HN, Ugwuegbu IE, Nwokocha KC, IJeoma KC, Chinaka JC, Onyema JC (2015) Structural interpretation of the Jos-Bukuru younger Granite ring complexes inferred from landsat-TM Data. J Geosci Geomatics. 3 (3):56–67. http://pubs.sciepub.com/jgg/3/3/2 Patterson G (1986) Lake Pidong-a preliminary survey of a Volcanic Crater Lake, An Interim Report of the Jos Plateau environmental resources development programme number 10. Department of Geography University of Durham (U.K) and Department of Geography and Planning, University of Jos Sabinus II, Chinedu NO, Kalu IK (2013) Aspects of geology of Ganawuri Area North Central Nigeria: evidence from field, petrographic and geotechtonical studies. Int J Eng Sci Res Technol (IJESRT) 2(12):3568–3577 Sabinus II, Ifeanyi OA, Nisir AN, Mosto OK (2018a) Improved mapping of the structural disposition of some younger granites ring

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complexes of Nigeria using high resolution aeromagnetic data. J Geol Geophys 7(4):1–13 Sabinus I, Oha I, Abdulsalam N, Onuoha MK (2018b) Improved mapping of the structural disposition of some younger granite ring complexes of Nigeria using high resolution aeromagnetic data. Geol Geophys 7(4):1–13 Sakoma E, Williams-Jones AE (2000) The stages of evolution of the Kwandonkaya A-type granite complex Nigeria, as deduced from mafic minerals. J Afr Earth Sc 30(2):329–350 Thorp MB (1967) Closed basins in younger granite massifs, northern Nigeria. Z. Geomorphol. NF 11:459–480 Turaki UM (1983) Bibliography on the younger granite ring complexes and tin mineralisation in West Africa with emphasis on Nigeria. J Afr Earth Sc 1(1):71–81 Turner DC (1963) Ring-structures in the Sara-Fier Younger ), Granite complex, Northern Nigeria, quarterly. J Geol Soc 119:345–366

159 Turner DC (1971) The geology of the Jos Plateau: explanation of 1:100,000 sheets nos. 147, 148, 168, 169, and 189 and 190. Geol Surv Niger Bull 32(1):1–136 Wurim DD (2011) An assessment of Jos Plateau volcanic deposits as pozzolans and its effect on blended ordinary Portland cement concrete, An unpublished Ph.D thesis Department of Building, Ahmadu Bello University, Zaria Yakubu AA (2018). Cost-benefit Analysis of Water Supply Projects in Jos Metropolis, Nigeria, Ph.D Unpublished Thesis, University of Jos, Nigeria Zeese R, Schwertmann U, Gerd FT, Jux U (1994) Mineralogy and stratigraphy of three deep lateritic profiles of the Jos plateau (Central Nigeria). CATENA 21(1994):195–214

11

Kainji Dam and Lake Olayinka O. Ogunkoya

Abstract

11.1

The area covered by the Kainji dam, Kainji Lake and Kainji National Park constitutes a distinct landscape within the Nigerian scenery. The dam and lake were created along the River Niger approximately 105 km upstream from Jebba between 1964 and 1968. Though the primary focus of the dam and associated impoundment is hydroelectricity generation, other ancillary purposes are flood control, navigation enhancement, irrigation and fishery development. The lake’s annual hydrograph shows there are two peak inflows: the ‘White’ and ‘Black’ floods, occurring in September and February, respectively. Draw-down occurs from February to June. A National Park was created from two existing game reserves that were contiguous to the impoundment area. Notable geomorphological features, namely corestones, tors, ruwares, and littoral caves and crevices abound in the area. Corestones, tors and ruwares were etched out from crystalline rocks, while the caves and crevices were created by the impact of waves and seiches on headlands and promontories in the lake’s draw-down zone. Keywords


















Kainji Dam Kainji Lake Kainji Lake National Park Ruwares Tors Corestones Terraces Lake shoreline caves Hydroelectricity Resettlement

Introduction

The massive growth in the demand for electricity in Nigeria by the middle of the twentieth century led to a consideration of hydroelectricity as a supply source option by the government. Consequently, studies of the hydropower potentials of the River Niger and its tributaries were commissioned (e.g. Balfour Beatty & Co. Ltd. and NEDECO 1958; 1961; 1965). Consultancy reports suggested some sites for hydropower development, namely Kainji, Jebba, Lokoja and Onitsha on the River Niger, Shiroro and Zungeru on the River Kaduna, a tributary of the Niger, Makurdi and Yola on the River Benue, Katsina-Ala on River Katsina-Ala, a tributary of the Benue, Beli on the River Taraba, another tributary of the Benue, and Mambilla Plateau on the River Donga, also a tributary of the Benue. Kainji Hydroelectric Power project was the first option pursued, and after its completion, a National Park was created from two existing game reserves that were contiguous to the impoundment area. The whole Kainji area has since become a national heritage comprising the dam, the lake and National Park. Notable geomorphological features including those created by etch-planation on crystalline rocks, and the impact of waves and seiches1 in the lake’s draw-down zone, abound in the area.

11.2

Kainji Dam and Lake

11.2.1 Kainji Dam Kainji Dam (9o51′45″ N 4o36′48″ E) is a multipurpose dam constructed between 1964 and 1968 across the River Niger,

1

O. O. Ogunkoya (&) Department of Geography, Obafemi Awolowo University, Ile Ife, Nigeria e-mail: [email protected]; [email protected]

A seiche is a standing wave oscillating in a body of water and is typically caused when strong winds push water from one end of a body of water to the other. When the wind stops, the water rebounds to the other side of the enclosed area. The water then continues to oscillate back and forth for hours or even days.

© Springer Nature Switzerland AG 2023 A. Faniran et al. (eds.), Landscapes and Landforms of Nigeria, World Geomorphological Landscapes, https://doi.org/10.1007/978-3-031-17972-3_11

161

162

approximately 105 km upstream from Jebba. Though the primary focus of the dam and associated impoundment is to produce hydroelectricity, other ancillary purposes are flood control, navigation enhancement, irrigation and fishery development. The dam, anchored on a rocky island, the Kainji Island, is aligned approximately WSW–ENE across the river. It has a central concrete gravity structure, which houses the penstocks (steel-lined ducts that convey water from the reservoir to the turbines), the hydroelectric power plant and the associated tailrace structure located at its downstream foot. The spillway and its associated tailrace structure are at the eastern extremity of the concrete structure. Kainji Island also hosts the hydroelectric power plant’s Switch Yard (Figs. 11.1, 11.2 and 11.3). Abutting this central area are the western and eastern rock-filled sections, with compacted clay cores (Fig. 11.3). The eastern section is traversed by a 2.4 km long navigation canal with two locks, each capable of lifting four barges over a total elevation of 41.2 m from the river water surface level to the lake water surface level. It takes 15 min for each lock

O. O. Ogunkoya

to fill and two hours for each barge train to pass through the navigation canal across the dam to the reservoir (Figs. 11.4 and 11.5). Beyond the eastern rock-filled dam is a saddle dam built across a tributary that would have bypassed the dam (Fig. 11.3). Other features of the Kainji Dam are presented in Table 11.1. It is one of the longest dams in the world, and it took about 20,000 men of nine different nationalities to construct (Olagunju 1972). The dam was designed to generate 960 MW, but this had to be reduced to 760 MW due to a 30-year drought (1968–1998) that occurred mainly in the Sudan and Sahel ecological zones of West Africa, the preponderant parts of the River Niger drainage basin upstream of the dam. The original design of the Kainji Project was based on rainfall and runoff data between 1914 and 1959, a pluvial period in West Africa. Events that occurred immediately after the commissioning of the dam in 1968 indicated that longer-term climatic conditions in the upstream River Niger drainage basin include much drier epochs during which available water will not meet Kainji’s initial hydroelectric power production target (cf. Sagua 1979).

Bussa Island

Fig. 11.1 Kainji Dam, Kainji Lake and Kainji National Park (after Google Earth), and map of Nigeria showing their situation

11

Kainji Dam and Lake

163

Fig. 11.2 Kainji Dam and the remnant Kainji Island (strip of land between the two Tailraces) now used as Switch Yard (after Google Earth)

Lock 2

Spillway Navigation Canal

Dam

Transformer Yard

Lock 1

Fig. 11.3 The layout of the Kainji Dam (modified after Olagunju 1972)

164

O. O. Ogunkoya

Fig. 11.4 Upper lock (foreground), waiting basin and lower lock of the Kainji Navigation Canal with River Niger in the background (after Olagunju 1972)

Fig. 11.5 (1) Barges and Tug-pusher in the lock, and (2) the lock filling up to enable passage to the higher level (after Olagunju 1972)

11.2.2 Lake Kainji Lake Kainji, the impoundment behind the Kainji Dam, is located in the western extremity of central Nigeria (Ogunjo et al. 2022a, b). It extends over a distance of 136.8 km, well beyond Yelwa (10o50′0″ N 4o44′44″ E), covers an area of 1243 km2 (Fig. 11.1) and has a maximum depth, and maximum width of 55 m and 24 km, respectively. The lake has a high-water perimeter of approximately 720 km and a shore development factor of 5.65. The latter is the ratio of the actual shoreline to the shoreline of a perfectly circular

lake of the same area. High values indicate considerable shoreline extension produced by bays and other indentations of the lake margin and the significance of the lake’s shallow littoral zone to the open deep-water zone. Most natural lakes approach the idealized circular form much more closely than Kainji and have shore development factor values around 2.0. Values for most African reservoirs are much higher than 2.0 (Henderson 1973). The indentations, particularly along the long, narrow northern and southern ends of Lake Kainji, account for its poor non-circularity (Fig. 11.6).

11

Kainji Dam and Lake

Table 11.1 Characteristics of the Kainji Dam

165 Description

Parameters

Type of dam

Concrete gravity and rock-fill

Total length of dam

8.64 km

Maximum height of dam

65.5 m

Maximum dam crest width

14.6 m

Maximum dam base width

91.4 m

Dam crest elevation

158.3 m a.s.l

Reservoir catchment area

Tributary catchments in Nigeria have an area of approximately 100,000 km2

Reservoir length

136.8 km

Maximum water level

155 m a.s.l

Lowest water level

128.9 m

Maximum fluctuation in water depth

26 m

Maximum water depth

55 m

Freeboard

3.3 m

Maximum reservoir capacity

15,000 MCM

Dead storage

3500 MCM

Reservoir area

1243 km2

Annual evaporation

2500 MCM

Spillway discharge

4 radial gates; 7900 m3/s

Penstock

12; 8.53 m diameter

Installed HEP capacity

760 MW

Lock

2 locks (each 198 m long and 12 m wide), each capable of lifting four barges (each 40 m long and 9.1 m wide plus the pusher-tug)

Lock lift

41.2 m from river to reservoir

The lake’s inflow hydrograph shows that discharges increase during the rainy season, from June onward, reaching a peak in October. This inflow, contributed virtually solely by upstream catchments in Nigeria, mainly rivers Sokoto, Dan Zaki, Malendo and Menai, is termed the ‘White Flood’ on account of the high kaolinitic sediment load it carries. The water is consequently highly turbid (Secchi disc transparency approximately 0.3 m) and greyish (Henderson 1973; Ogunjo et al. 2022a, b). The annual draw-down commences with the onset of the dry season in November, but a secondary water level peak is attained between December and February (the ‘Black Flood’—because its waters are desilted and clearer). This flood is caused by rains of the same rainy season at the headwaters of the Niger in the Republics of Guinea and Sierra Leone, but arrives late in Nigeria due to flow retardation in River Niger’s inland delta in the region southwest of Timbuktu, Republic of Mali. The water level falls from February to its lowest levels in June (Sagua 1979). The magnitude of the secondary flood only marginally affects the rapid recession from the White Flood

peak discharges. The Black Flood may however soon become a historical phenomenon given the current and potential growth in the exploitation of water resources of the upstream River Niger. The developments include the Selingue dam (2350 MCM) in Mali and the Fomi dam (5300 MCM) in Guinea built to meet irrigation and hydropower needs. But a most critical one is the Kandadji Dam (1596 MCM), a large multipurpose dam on the River Niger near the small town of Kandadji, 180 km northwest and upstream of Niamey, Republic of Niger. A few hundred kilometres from Nigeria’s border with the Republic of Niger, the project, formally known as the Kandadji Programme for Ecosystem Regeneration and Niger River Development is being constructed to guarantee Niamey’s water supply, generate electricity and provide water for irrigation (Liersch et al. 2019; https://www.iied.org/niger-tough-questionsposed-kandadji-dam-development). Though designed to release a regulated discharge of 120 m3/s, since this amount could be entirely accounted for by channel evaporation, seepage and downstream informal irrigation in the Republic

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O. O. Ogunkoya

Fig. 11.6 Kainji Lake showing lake indentation, tributaries and islands

of Niger, the project’s completion may result in upstream River Niger waters never again reaching Nigeria. The inflow regime imbues a rhythmic fluctuation of the lake level during the year, which determines the extent of inundation of the islands within the lake, the draw-down and the area of the lake floor that is annually exposed, and subjected to wave erosion. Superimposed on this annual rhythm is that caused by variations in the outflow of turbinated water due to power generation needs, daily rhythms of evaporation and the seiches, common in the lake. Seventy (70) percent of the impoundment zone was cleared of all vegetation. An essence was to reduce the effects of vegetation decay on water quality and weed

growth. Another one was to promote fishing, a purpose for the creation of the lake, by removing obstructions to fish gear, particularly gill nets, which will be impossible to use except the lake bed was reasonably free of obstructions that could entangle the nets. Experience of the effects of clearing vegetation at the Kainji has led to the global recommendation that complete clearance should not be undertaken at any new impoundment but rather strip clearance to provide for fish shelter and feeding grounds (Olagunju 1972). A consequence of weed clearance was that most game in the impoundment zone voluntarily moved off the cleared zone. Another was that there was a minimum amount of rotting vegetation that could elevate biochemical oxygen demand

11

Kainji Dam and Lake

leading to deoxygenation of the waters; hence, there was no aquatic weed problem as at Kariba Dam, Zambia, nor mass fish mortality as at Volta Dam, Ghana (Imevbore 1970).

11.2.3 Lake Islands There are three groups of islands in Lake Kainji. One comprises the non-permanently inundated parts of the old Foge Island (04°33'E, 10°13,N), which created the bifurcation of River Niger in the area that now constitutes the main body of the lake. Another is the old Bussa Hill formerly located on the right bank of the river, just downstream of where the two branches around the Foge Island re-join. It is a partly submerged inselberg formed from Basement Complex rocks and now located at the beginning of the narrow southern extension of the lake (Fig. 11.6). The final group comprises isolated bodies of headlands along the coast that have been cut-off by wave erosion (Fig. 11.7). The Foge Island trended N-S and was about 35 km long with a maximum width of 16 km. It was a floodplain island composed of sand and silty alluvium deposited during the last 15,000 years (Halstead 1975). Currently, the western section of the old island is permanently submerged, but two much smaller masses remain in the northern and eastern parts (Fig. 11.6). More parts of the island emerge at low water levels. The current size of the island, therefore, varies depending on the water level of the lake, which varies between the rainy and dry seasons, and reflects the extent of draw-down through power generation and evaporation. The low-lying island topography ensures that erosion is minimal except at the shores where waves and the action of seiches Fig. 11.7 The island that was part of a headland but is now cut off by wave erosion

167

create wave-cut platforms. The permanently exposed sections lie within Kebbi State, are dotted with numerous termite mounds and have a large population of resident and migratory species of birds. They are important breeding grounds for waterbirds, including the White-faced whistling duck (Dendrocygna viduata), Blue-cheeked Bee-eater (Merops persicus), Collared Pratincole (Glareola pratincola) and Spur-winged Lapwing (Vanellus spinosus). Other birds sighted in the area include the Glossy Ibis (Plegadis falcinellus), Spur-winged goose (Plectropterus gambensis), Pintain (Anas acuta) and African Darter (Anhinga rufa). Some people inhabit the remnant island and fish the pools and the lake, and also engage in traditional irrigation agriculture and harvesting of other wild resources, including the birds. The Foge Island forms part of a regional biodiversity hotspot due to its species richness and supports the element of biodiversity that is characteristic of such wetlands in the Guinea Savanna woodland of Nigeria. In 2008, it was declared a RAMSAR Wetland of International Importance by the International Union for the Conservation of Nature (IUCN; https://www.ramsar.org/news/nigerias-new-wetlandsof-international-importance). It is part of the Kainji National Park, though across the waters from the Borgu Sector (western sector) of the park. The Kainji National Park and the Nigerian Institute for Freshwater and Fisheries Research at New Bussa have criteria and stipulations for managing the ecology of the island. The island is now virtually treeless due to vegetation clearance before impoundment. The Kainji Lake has a diverse fish fauna comprising 82 known species belonging to 18 families (Ogunkoya and Dami 2008; https:// www.ramsar.org/news/nigerias-new-wetlands-of-internationalimportance).

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Table 11.2 Rainfall in the Kainji lake area Town

Location

Altitude (m)

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Total

Ilorin

8°29'N 4°35'E

307

10

18

64

102

170

193

142

132

251

165

28

13

1288

Mokwa

9°18'N 5°04'E

152

3

8

25

84

147

196

201

119

269

107

10

5

1174

Kainji Dam

10°10'N 4°38'E

144

0

0

8

53

132

137

163

228

190

59

0

0

970

11.3

Lake Kainji Area

11.3.1 Climate The climate in the Kainji Lake area is the Köppen’s Aw1 (i.e. humid tropical wet and dry climate), with an almost equal length of the rainy season and dry season. The dry season extends from November to April, while the rainy season covers the remainder of the year (May–October). The mean total annual rainfall is approximately 1000 mm, with the peak of the rainy season between July and August (Table 11.2). Mean maximum and minimum temperatures during the rainy season are 35 °C and 24 °C, respectively, while those of the dry season are 38 °C (in late March and April) and 16 °C in December/January, respectively. The dry season is dominated by cool and dusty north-easterly Harmattan winds.

11.3.2 Geology and Geomorphology The Precambrian Basement Complex and sedimentary rocks underlie or are exposed in specific zones of the area in the proximity of and beneath the lake. The northern, northeastern and southern parts of the area are underlain by argillaceous and arenaceous meta-sediments, granites, gneisses, muscovite schists and quartzites of the Basement Complex. The Upper Cretaceous Nupe Formation, comprising conglomerates, grits and sandstones, occurs continuously along the northern half of the western coast of the main body of the lake, and also southeastwards of the lake. It is of both marine and continental origin and is the main source of sediment delivered into the lake. The Late Quaternary Alluvium believed to be derived mainly from the Nupe Formation occurs along the central-eastern coast, and as the bed of the lake (Fig. 11.8). It comprises mainly sand, but is in parts of the draw-down zone characterized by depressions and covered by mud (Adegoke and Kogbe 1975; Halstead 1975). The area has low relief topography, with elevations ranging between 150 and 400 m a.s.l. In general, the underlying geology of the area determines the surface topography. For example, the N-S trend of the valley of the Niger between Yelwa and Jebba, and more so, the gorge-like nature of the area occupied by the southern narrow and deep

Fig. 11.8 Geology of the Lake Kainji area (after Halstead 1975)

extension of the lake, since it is confined by a hard rock on both banks, suggest that the valley follows and occupies a line of faults (cf. NEDECO 1961). This zone was marked by rapids and cataracts, but these features have been blasted off and the area is now submerged. The zone extends beyond the Kainji Dam to Jebba, an area now also inundated and occupied by the Jebba Lake created by the dam at Jebba. Further, areas with low and medium relief are generally restricted to zones underlain by alluvium and argillaceous meta-sediments. Bays and inlets have been formed in these zones, indicating drowned former tributaries of the River Niger (Fig. 11.6). Areas with high relief are on the other

11

Kainji Dam and Lake

hand restricted to zones where the Basement Complex and the Nupe Formation are exposed. In the latter case, the landforms are mainly low-lying buttes (small, flat-topped hills, capped by resistant rock platform), rocky coasts, promontories and headlands. The Basement Complex of the Kainji Lake area is part of the northernmost outliers of a large hilly range, the Yoruba Hills and Ranges and their extension, the Kukuruku Hills, that trend NW–SE extending from Nigeria’s border with the Republic of Benin to the main trunk of the River Niger. The rocks are associated with denudation landforms, mainly inselbergs (isolated rocky hills rising sharply above the general surrounding level). The inselbergs in the Kainji area, however, have smaller dimensions compared to those in the southern parts of the Hills and Ranges. Bedrock outcrops occurring in the Kainji area include corestones, tors and ruwares (Figs. 11.9, 11.10 and 11.11). Ruwares are low-lying, gently sloping rock pavements with smooth convex surfaces, which barely rise above the general ground surface. They are believed (e.g. Thomas 1965; 1974; 1994) to be inselbergs in the early stage of development through exhumation. Tors are piles of exposed, rounded boulders perched atop a solid rock platform, and rising abruptly from the surrounding ground surface. Corestones are piles of rounded or sub-rounded rock boulders lying on a buried underlying rock platform. They and tors are formed from well-jointed rock masses. It is believed (e.g. Thomas 1989; 1994) that these landforms have developed through the process of etch-planation, i.e. deep weathering, multicyclic

Fig. 11.9 Corestones near New Bussa

169

erosion and exhumation. The areas underlain by Quaternary Alluvium are gently undulating, comprising low slopes, irregular small depressions and numerous termite mounds. The depressions are now filled with mud, which on exposure after draw-down are scarred by desiccation cracks. The indentations along the narrow southern extension of the Kainji lake (Fig. 11.6) mark the now-drowned valleys of small tributaries of the River Niger (e.g. rivers Sadoro, Doro, Timo and Menai) that drained the area. Halstead (1975) noted that the indentations are zones where argillaceous meta-sediments occur, while the headlands and promontories are formed by gneisses and arenaceous meta-sediments. Considerable beach erosion is caused by waves and seiches created by the predominant South Westerlies and North Easterlies sweeping across the surface of Lake Kainji in the rainy season and dry season, respectively. With a fetch greater than 24 km, waves break on the shore with some force (Fig. 11.12), creating striking erosional features. Henderson (1973) reported that waves of 50–75 cm in height are common and waves up to 1 m occur during storms. The waves have stripped off the vegetation cover and soils of the draw-down zone and in the area underlain by the Nupe Formation, exposing clean, un-weathered grits and sands (Fig. 11.13). Active undercutting of headland and cliffs by waves at various water levels during the filling up of the lake in the rainy season and its draw-down in the dry season has also created wave-cut platforms featuring terraces. These are common in the areas where the Nupe Formation is exposed along the shoreline or forms a headland

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O. O. Ogunkoya

Fig. 11.10 Ruware near Bussa

Fig. 11.11 Low stature inselberg of the type common in the Kainji area

(Fig. 11.14). Terracing on headlands (Fig. 11.15) represents beaches formed during the retreat of the strand. The terraces are in many areas covered by gravel, evidence of the stripping off of the sand and smaller soil fractions (Fig. 11.16). The wave-cut platforms are submerged during high water but exposed during draw-down. The platform may be bare

(Fig. 11.13) or covered by gravel or sand (Fig. 11.16). The cliffs backing the wave-cut platforms are also subject to wave attacks at high water. The base of the cliffs is the main focus of wave action involving notching by abrasion, hydraulic action and compression, leading to the creation of littoral caves (Fig. 11.17), the collapse of the cliff face,

11

Kainji Dam and Lake

171

Fig. 11.12 Waves breaking on the shore in the dry season near Shagunu

Fig. 11.13 Nupe Formation exposed in draw-down zone swept bare by waves

removal of debris by backwash and the landward extension of the wave-cut platform. Notches are enlarged into littoral caves through compression when waves crash into air pockets in notches causing cracks to spread and pieces of rock to break off (Short 1982). The seiches have, through the oscillations in water level they induce, seriously affected human safety (Abayomi 1979).

there was enough farmland, the possibility of obtaining a good water supply, and within the Native Authority from which they were displaced. 139 new villages and two towns (Yelwa and New Bussa; Fig. 11.1), all comprising 4320 houses, were constructed and given to re-settlers. The resettlement communities and the constituent houses and facilities (schools, markets, health centres and hospitals, mosques and churches) were designed by the renowned tropical architecture experts, ‘Fry, Drew and Atkinson’.

11.3.3 Resettlement of Displaced People 44,000 people from both the present-day Niger State (Borgu and Kontagora Emirates) and Kebbi State (Yauri Emirate) were displaced from the dam, navigation canal and impoundment areas. They were resettled in new housing estates built well above the maximum water level, where

11.4

Lake Kainji National Park

The Kainji National Park, located between latitudes 9°40'N and 10°30'N, and longitudes 3°30'E and 5°50'E, spread across two states: Niger State and Kebbi State. The Park was

172 Fig. 11.14 Terracing on a Nupe Formation headland along the banks of the Lake Kainji (after Halstead 1975)

O. O. Ogunkoya

Terraces

Fig. 11.15 Terrace covered by gravel in Nupe Formation area

formally established in 1979 by the amalgamation of two existing game reserves, Borgu Games Reserve in the Niger State and Zugurma Games Reserve in the Kebbi State. It has a combined area of 5341 km2, made of three distinct sectors: Foge Island Sector; Borgu Sector (3970 km2) to the west of the Kainji Lake and the Zugurma Sector (1371 km2) to the southeast of the lake (Fig. 11.1). The Borgu sector is studded with various types of inselbergs. The sector extends to less than 10 km from the border with the Republic of Benin to the west and is drained mainly by the River Oli, which supports hippopotamus populations. The Zugurma sector is poorly

drained, and the topography is more subdued. The park belongs to the IUCN Management Category II (National Park) and the Biogeographical Province 3.04.04 (West African Woodland/savanna) (https://www.unep-wcmc.org/sites/ pa/0302p.htm Kainji Lake National Park). The park is in the Northern Guinea Savanna, but the humidity of the environment and the soil water regime create the ambience of the Southern Guinea Savanna (https://nigeriaparkservice.org/?p= 148). The flora is typically Guinea Savanna woodland and shrub (Burkea africana, Terminalia avicennioides, Afzelia sp., Acacia sp., Isoberlinia sp., Hyparrhenia sp., Andropogon

11

Kainji Dam and Lake

173

Fig. 11.16 Sandy terraces near Shagunu

Fig. 11.17 Wave cut platform backed by cliff having littoral caves at its base

sp., Parkia clappertoniana, Butyrospermum paradoxum, Uapaca togoensis and Khaya senegalensis). Sixty-five (65) mammal, 350 bird and 30 amphibia and reptile species, all of which are typical of the Guinea Savanna woodland, are recorded in the park. The mammals include lion Panthera leo (VU),2 leopard P. pardus (CR), 2

These are abbreviations used in the IUCN Red List Categories and Criteria as components of an objective framework for the classification of the broadest range of species according to their extinction risk:CR (Critically Endangered)—species that are facing an extremely high risk of extinction in the wild.EN (Endangered)—species that are facing a very high risk of extinction in the wild.NT (Near threatened)—species that do not qualify for Critically Endangered (CR), Endangered (EN) or Vulnerable (VU) now, but are close to qualifying for or are likely to qualify for a threatened category in the near future.VU (Vulnerable)— species that are facing a high risk of extinction in the wild.LC (Least concern)—species that are widespread and abundant.

caracal Felis caracal (LC), elephant Loxodonta africana ( VU), buffalo Syncerus caffer (LC), hartebeest Alcelaphus buselaphus (NT), kob Kobus kob (LC), roan antelope Hippotragus equinus (LC), red-flanked duiker Cephalophus rufilatus (LC), common duiker Sylvicapra grimmia (LC), Bohor reedbuck Redunca redunca (LC), waterbuck Kobus ellipsiprymnus (LC), oribi Ourebia ourebi (LC), bushbuck Tragelaphus scriptus (LC), warthog Phacochoerus aethiopicus (LC), giant pangolin Smutsia gigantea (VU), hippopotamus Hippopotamus amphibious (VU), wildcat Felis silvestris (LC), African Wild dog Lycaon pictus (EN), honey badger Mellivora capensis (LC), clawless otter Aonyx capensis (NT), olive baboon Papio anubis (LC), green monkey Chlorocebus sabaeus (LC), patas monkey Erythrocebus patas (LC), Senegal bush baby Galago senegalensis (LC), aardvark Orycteropus afer (LC), manatee

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O. O. Ogunkoya

Trichechus senegalensis (VU) and Cheetah Acinonyx jubatus (VU). Reptiles include Nile crocodile Crocodylus niloticus (LC), slender snouted crocodile Mecistops cataphractus (CR), rock python Python sebae, royal python P. regius (LC), spitting cobra Naja nigricollis (LC), black cobra N. melanoleuca, puff adder Bitis arietans, gaboon viper B. gabonica, monitor lizard Varanus niloticus and V. exanthematicus, and terrapin Pelomedusa subrufa and Pelusiosa dansoni. Wildlife numbers are much lower in the Zugurma sector due to poor drainage, deteriorating vegetation, heavy poaching and extensive cattle grazing. Ostrich (Struthio camelus (LC)) has been domesticated in the area.

11.5

Conclusion

The Kainji Dam, Kainji Lake and Kainji National Park may indeed have become a national heritage of tremendous benefit to the nation as a whole and the Kainji area in particular, but the heritage is facing significant threats. First, the national quest for food security that has been associated with formal and informal irrigation schemes appears to be impacting inflows into the lake. The impending climate change may reinforce inflow reduction, threatening the capacity of the lake to support an already reduced hydroelectric power generation scope. Climate change may also impact the vegetation of the National Park, and therefore its fauna, and further reinforce the significant threat from poachers. Serious consideration should therefore be given to factors that could promote the sustainability of the National Heritage.

References Abayomi CA (1979) Hydrometeorology and development of large dams, pp 219–222. In: Proceedings, international conference on Kainji Lake and river Basin development in Africa, Ibadan, vol 1. Kainji Lake Research Institute, New Bussa, p 249 Adegoke OS, Kogbe CA (1975) Littoral and suspended sediments of Lake Kainji, pp. 57–66. In: Imevbore AMA, Adegoke OS (eds) The ecology of Lake Kainji—the transition from river to lake. University of Ife Press, Ile Ife, Nigeria, p 209p

Balfour Beatty & Co. Ltd. and Netherlands Engineering Company (NEDECO) (1961) Niger dams project Balfour Beatty & Co. Ltd. and Netherlands Engineering Company (NEDECO) (1958) Jebba project, Nigeria—feasibility report Balfour Beatty & Co. Ltd. and Netherlands Engineering Company (NEDECO) (1965) Kainji hydro-electric development: a brief description. NEDECO Hague Halstead LB (1975) The shoreline of the Lake Kainji, pp. 3–56. In: Imevbore AMA, Adegoke OS (eds) The ecology of Lake Kainji— the transition from river to lake. University of Ife Press, Ile Ife, Nigeria, p 209p Henderson F (1973) Nigeria—Kainji Lake project: a limnological description of Kainji Lake 1969–1971. A report prepared for the Kainji Lake project. FAO, Rome FI: DP/NIR 66/524/10, 1 Feb 1973. http://www.fao.org/3/d8476e/D8476E02.htm Imevbore AMA (1970) Ecology of the newly formed Kainji Lake. Nigeria. Mimeo Liersch S, Fournet S, Koch H, Djibo AG, Reinhardt J, Kortlandt J, Van Weert F, Seidou O, Klop E, Baker C, Hattermann FF (2019) Water resources planning in the upper Niger River basin: are there gaps between water demand and supply? J Hydrol: Reg Stud 21:176–194 NEDECO (Netherlands Engineering Consultants) (1961) Niger Dams project. The Hague Ogunjo S, Olusola A (2022) Signature of teleconnection patterns in river discharge within the Niger Basin. Meteorol Atmos Phys 134 (2):1–15 Ogunjo S, Olusola A, Fuwape I, Durowoju O (2022) Temporal variation in deterministic chaos: the influence of Kainji dam on downstream stations along lower Niger River. Arab J Geosci 15 (3):1–11 Ogunkoya OO, Dami A (2008) Information sheet on Ramsar Wetlands (RIS): Foge Island, Nigeria, p 9. https://www.ramsar.org/news/ nigerias-new-wetlands-of-international-importance Olagunju SO (1972) Kainji Dam: 126 questions answered. Niger Dams Authority, p 124 Sagua VO (1979) Some uses and effects of multipurpose dams in tropical Africa—the Kainji experience, pp 44–54. In: Proceedings, international conference on Kainji Lake and River Basins development in Africa, Ibadan, vol 1. Kainji Lake Research Institute, p 249 Short AD (1982) Cliff erosion. In: Beaches and coastal geology. encyclopedia of earth science. Springer, Boston, MA Thomas MF (1965) Some aspects of the geomorphology of domes and tors in Nigeria. Zeitschrift für Geomorphologie N.F. 9:63–81 Thomas MF (1974) Tropical geomorphology. A study of weathering and landform development in warm climates. The MacMillan Press ltd, London and Basingstoke Thomas MF (1989) The role of etch processes in landform development. I. Etching concepts and their application. Zeitschrift Für Geomorphologie NF 33:129–142 Thomas MF (1994) Geomorphology in the tropics: a study of weathering and denudation in low latitudes. Wiley, Chichester

Riparian Vegetation Along Nigeria Rivers: The River Ogun Example

12

Olutoyin Adeola Fashae and Rotimi Obateru

Abstract

12.1

The presence of vegetation along a stream has been shown to affect a variety of biotic and abiotic processes, resulting in complex effects on stream structure and water flow dynamics. The magnitude of these effects depends on the type of vegetation, its mechanical properties and density, as well as its spatial distribution. Across rivers draining Nigeria, the riparian composition varies significantly. The southern part of the country presents more luxuriant vegetation along river courses than the northern part. However, biodiversity along river courses in Nigeria presents a scenery that is attractive to view and also serves as economic gains for the local people. River Ogun, an alluvial example that is reported in this study, largely epitomizes the situation of riparian ecosystems in the humid tropics. Typically found along this alluvial stretch are woody plants and light forests which influence riverbank stability. Morphological features like sand bars, cut banks, oxbow lakes and braided channels that have been colonized by riparian species are prominent along the reach. The natural functioning of this riparian ecosystem has over time been disturbed by sand mining, plantation agriculture and arable crop farming. Keywords

Riparian




Scenery




Biotic




Abiotic




Biodiversity

O. A. Fashae (&) Department of Geography, University of Ibadan, Ibadan, Nigeria e-mail: [email protected] R. Obateru Department of Geography and Planning Sciences, Adekunle Ajasin University, Akungba-Akoko, Ondo State, Nigeria

Introduction

The pursuit of knowledge of river systems has advanced in leaps and bounds. The river system has been the focus of travel, industry and human culture for millennia. Due to their importance to human activity and quality of life, many disciplines continuously pursue the understanding of their physical properties and behaviour to ensure their judicious management (Fashae 2011). Vegetation is ubiquitous, opportunistically colonizing areas of the river channels that are abandoned or exposed at low flows, as in the case of riparian vegetation which colonizes the adjoining land of most rivers, creeks, streams, floodplains, wetlands, lagoons and lakes. Hence, its interaction with the earth surface processes and landforms is particularly significant for both understanding and interpreting scientific observations (Gran and Paola 2001; Montgomery and Buffington 1993), especially for land management purposes. Riparian vegetation is often the thin line of greens containing trees, shrubs, herbs and grasses along stream banks. This landscape is usually found at the interface between terrestrial and aquatic ecosystems, with plant habitats and communities colonizing river margins and banks, usually hydrophilic. An infinite variety of plant species are known to characterize these riparian landscapes, largely due to the diverse and productive nature of the ecosystem. Riparian vegetation plays a crucial role in river geomorphology by maintaining bank stability and controlling bed erosion, helping to maintain aquatic ecosystems, such as the terrestrial and aquatic habitats, nutrient supply and assisting in protecting the environment by reducing erosion and slowing floodwater (Fashae 2011). The presence of vegetation along the stream has been shown to affect a variety of biotic and abiotic processes, resulting in complex effects on stream structure and water flow dynamics. The magnitude of these effects depends on the type of vegetation, its mechanical properties and density, as well as its spatial distribution. Further, these effects change with channel size (Eaton and Millar 2004) and water flow

© Springer Nature Switzerland AG 2023 A. Faniran et al. (eds.), Landscapes and Landforms of Nigeria, World Geomorphological Landscapes, https://doi.org/10.1007/978-3-031-17972-3_12

175

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O. A. Fashae and R. Obateru

capacity (Gurnell 1997). Gregory and Gurnell (1988) have also suggested that bank stabilization and increased resistance to water flow are two main mechanisms by which vegetated buffers affect channel structure. Vegetation interacts with a range of geomorphological, geotechnical, hydrological and hydraulic factors to affect the type and extent of riverbank erosion (Abernethy and Rutherfurd 1998a, b; 2000). Different types of vegetation affect different processes; hence, it is essential to assess the dominant erosion process correctly so that the stabilizing role of appropriate species can be recognized. While dense herbaceous vegetation has been shown to effectively reinforce stream banks (Micheli and Kirchner 2002), the effect of forest vegetation is more complex because the weight of trees could add significantly to bank stresses, and because large woody debris could deflect river flow in ways that accelerate bank scour. Riparian zones are connected to their landscape, and riparian vegetation is a fundamental component of landscape systems. This chapter thus seeks to report Nigeria's rivers vis-à-vis their riparian ecosystem, with an emphasis on the southwestern river, River Ogun, which has a dense, diverse and lush species representation of the ecosystem.

12.2

Riparian Vegetation Along Major Nigerian Rivers

12.2.1 Rivers in Nigeria Nigeria is a well-drained country, with a dense network of rivers and streams (Ayoade and Oyebande 1978). These rivers are unevenly distributed due to topographic variation (Fig. 12.1). The country is drained by two major rivers, namely River Niger, from where her name is derived, and River Benue, which is the largest tributary of the Niger. These two rivers have their confluence at Lokoja before flowing down south and emptying their content into the Atlantic Ocean. It is worth noting that the flow pattern of Nigerian rivers corresponds to the physiognomy of the landscape, thereby informing the pattern and direction of flow in a southerly manner; this has led to the recognition of some centres of drainage dispersal, for instance, the North-Central Plateau, the Western Uplands, the Eastern Highlands (Oban hill, Adamawa and Obudu Plateau, Shebsi and Atlantika Mountain) and the Eastern Scarpland (Iloeje 1965). Rivers in northern Nigeria are characterized by radial flow in all directions from the North-Central Plateau. Originating from this plateau are the Sokoto River flowing in the northwest direction, the Hadejia flowing north-eastwards, the Gongola running eastwards and the Kaduna going south-westwards, while those in the south, due to the sloping nature of the land to the south, are arranged in a north–south direction. Topography compelled some of the rivers in the

Western Uplands to flow northwards to the Niger, e.g. Rivers Moshi and Awum on the one hand, and others to flow southwards into the Atlantic Ocean, e.g. Rivers Ogun, Oshun, Ouse and Owena. Katsina-Ala and Donga Rivers emanate from the Eastern Highlands, while the Eastern Scarpland forms a minor axis of drainage dispersal between two sets of rivers, one flowing westward to the Niger and the other to the Cross in the east. However, the flow of some of these rivers is impeded and studded with rapids and falls, which limit their use for navigation. The Niger Delta is a vast low-lying region through which the water of the Niger drains into the Gulf of Guinea (Fashae 2014). Meanders, braids, oxbow lakes, levees, point bars and extensive floodplains feature prominently in the region.

12.2.2 Riparian Vegetation Along Nigeria’s Rivers Riparian vegetation decreased from 0.8% of the total area of Nigeria in 1976/78 to 0.6% in 1993/95, amounting to a reduction of about 2148 km2, with considerable parts of this area currently being used for floodplain agriculture (Geomatics International 1998). Generally speaking, low annual rainfall, which is characteristic of Northern Nigeria, makes it difficult for the rivers to support luxuriant riparian vegetation (Fig. 12.2a, b). Mubi (2012) reported a low mean riparian species density and diversity index of 42 and 12, respectively, per 625 m2 quadrant at Moyo Kam along River Gashaka. Species density is a measure of the number of species per unit area while species diversity indicates the variety of species within the delineated quadrant. Specifically, in places where the area is humid enough, these riparian species have been eliminated in favour of extensive farming activities (Fig. 12.3). Common feature of river valleys on the North-Central Plateau, for instance, is the presence of Deleb palms which are readily recognized by the dense growth of woodlands that is supported by soil conditions along the valleys. Generally, in Northern Nigeria, riparian species that feature prominently along undisturbed rivers are Brachystigiaeurycoma, Vitex doniana, Andria enermis, Diospyros spp. Anogeissusleocarpus, Perinariexcelsa, Crossopteryxfebrufuga, Nauclealatifolia. Uapakatogoensis, Terminalia glaucoscens and Grewiamollis. Lying between the Sudan savanna and the drier Sahel to the north is the Hadejia wetland which extends to Nguru (www.nigeria.galleria.com) (Fig. 12.4). River Hadejia is a tributary of River Yobe which is also known as the Komadougou Yobe or the Komadougou-Yobe, that flows into Lake Chad. This wetland lies within the Yobe-Komadougou basin and is drained by River Yobe which flows eastwards into Lake Chad. It is worthy to note that riparian vegetation cannot thrive in the area because some parts are permanently

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Fig. 12.1 Physical map of Nigeria showing the major rivers and hills

a

b

Fig. 12.2 The scenery of a typical river in Northern Nigeria, with its riparian area. a Scanty trees along River Sokoto (Source http://gdb.voanews. com). b Thin-lined grasses colonizing the riparian area of River Kaduna (www.nigeria.galleria.com)

flooded, while other areas are only inundated during the wet season. Scanty grassy vegetation exists in this wet region. In the Middle Belt region, especially around River Niger and Benue confluence area, the valley floors of smaller and

seasonal streams are covered with dense riparian woodlands. However, in more densely settled areas of Kontagora and Borgu riparian forests have been removed for agricultural activities since suitable farmland is mainly restricted to the

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Fig. 12.3 Cultivation along River Kware, Sokoto State (Google Earth Image 2019)

Fig. 12.4 Aerial view of the strip-line Riparian grasses along River Hadejia, Northern Nigeria (Google Earth Image 2019)

valley floors (Udo 1970). A considerable density of trees exists along the rivers that drain the Benue valley, providing an effective screen between the rivers and their floodplains. The southern part of Nigeria, especially south of River Niger and Benue, has a characteristic diverse species of luxuriant riparian vegetation. In some areas, there are thick emergent trees with tall shrubs and grasses which adjoin the rivers. For instance, the secondary (derived) savanna areas of Ibadan, Ede-Osogbo and Ikirun districts in Southwestern Nigeria are characterized by gallery forests along

watercourses, with prominent species like Brachystegia eurycoma with flaming red leaves during January and February (Udo 1970). Also, there are large stretches of mangrove trees along most rivers in the Sapele-Warri and the Port Harcourt-Abonnema axis of the Niger Delta region; these trees are close to the banks of the creeks (Fig. 12.5a, b). Typical of the Cross-River estuary and the lower reaches of Calabar and Kwa Rivers is the mangrove swamp vegetation. Other areas of dense riparian vegetation are Cross-River bend, extending to the foothills of the Obudu

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b

c

Fig. 12.5 Thick Riparian Vegetation in Southern Nigeria Rivers. a Second growth swamp forest and gallery forest habitats at Ethiope

Fig. 12.6 Rice cultivation along River Benue, Nigeria

River, Delta State. b Ijaw River in the Niger Delta Region, Delta State (Adrian Arbib). c Dense riparian vegetation along the River Cross

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12.3

The River Ogun Case Study

12.3.1 Setting

Fig. 12.7 Fish trading along harvesting on River Hadejia, Northeastern Nigeria (www.nigeria.galleria.com)

Plateau, where the majority of the plant species is associated with Central Africa rather than West African species (Fig. 12.5c). A high rate of riparian vegetation depletion has been recorded along the River Shasha, Osun State, with a reduction of 9.31 km2 ha between 1972 and 1986 and 4.58 km2 between 1982 and 2002; and a mean annual depletion rate of 7.6 km2 and 5.2 km2, respectively (Gadiga et al. n.d). Also, in the Osun State, the spatio-temporal assessment of riparian vegetation in Osogbo city as studied by Adesina (2016) revealed a reduction of 49 km2 between 1986 and 2000 and a further 37 km2 between 2000 and 2015. The principal cause of this devastation was farming and fishing along the riverbanks. Some evidence of farming and fishing activities along some rivers in the northern part of Nigeria are depicted in Figs. 12.6, 12.7 and 12.8.

Contemporary studies of riparian vegetation have focussed on southwestern Nigeria. Therefore, this section assesses a typical river characterized by numerous tributaries and perennial flow (Fashae and Faniran 2016), with diverse riparian vegetation. River Ogun, with a catchment area of about 23,700 km2, lies between latitudes 6°33′ N and 8°58′ N and longitudes 2°40′ E and 4°10′ E in southwestern Nigeria (Fig. 12.9). It drains parts of Oyo, Ogun and Lagos States in Nigeria, with the remaining 0.2 per cent of the catchment situated in the Benin Republic to the west. The river, which has two main tributaries: the rivers Ofiki and Opeki, originates from the Iganran Hills and flows southwards, first across the Basement Complex (480 km) cutting down to the hard metamorphic rocks, and then within the sedimentary rock terrain (Olusola 2019). The River Ogun Basin has been subdivided into the upper and lower basin areas. The former, which constitutes the main part of the catchment, is drained largely by the Ogun, Oyan and Ofiki rivers, hence the Oyan-Ofiki River system, whose evolution on the crystalline basement is strongly controlled by the pattern of foliated rocks and by jointing on the more resistant rocks (Olusola 2019). It is important to note that the channel of River Ogun appears to exhibit a considerably uniform gradient of 1 m per 1 km up to the Ogun-Oyan confluence. This confluence marks the beginning of the Lower River Ogun Basin, which is characterized by a narrow strip extending 100 km long from north to south with a drastic gradient decrease to about 0.1 m per 1 km. However, this confluence represents also the boundary between the Basement Complex and

Fig. 12.8 Fish harvesting along River Hadejia, Northeastern Nigeria (www.nigeria.galleria.com)

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Fig. 12.9 Alluvial section of River Ogun (Source Fashae et al. 2018)

sedimentary formations in the basin, with River Ogun flowing sluggishly in an alluvial bed, causing floodplain inundation. The Inter-Tropical Discontinuity (ITD) exerts significant influence on the climate of this area, and as a result, the rainy and dry seasons are well marked. The rainy season begins earlier in the south, where it lasts from March until the end of October or early November, giving at least seven months of rainfall. North of Oyo and Iseyin, the onset of rains is delayed and generally begins late in April or early May and ends in mid-October (River Ogun Basin Development Authority 1981). In late July and early August, dry days are common and sufficiently regular to constitute what has been termed the “little dry season”, with mean monthly figures for rainfall below 100 mm. The mean annual rainfall of the study area ranges from 900 mm to about 2000 mm (River Ogun Basin Development Authority 1981). Temperatures are fairly uniform throughout the year with a mean annual of 26°–27° and an annual range of 5–8 °C, while the relative humidity ranges between 60 and 80%. Annual evaporation rates are also high throughout the year, with monthly amounts varying from about 90 mm in July to over 130 mm in January.

This section examines the alluvial segment of River Ogun (Lower Ogun Basin) comprising a variety of riparian species. This alluvial reach is underlain by sedimentary formations that were deposited in the Cretaceous Basin of the Coastal Plain Sands. Its natural vegetation is mixed with thick woods and light forest typical of the rainforest regions (Fig. 12.10a, b); the most dominant natural species are Cissussp, Alternanthera sessilis, Pistia stretoites, Ficus spp., Monechma ciliatum, Tridax procumbens, Ipomoea acquatica and Setaria bartata.

12.3.2 Geomorphological Features Along the Alluvial Channel Riparian vegetation is widely believed to protect riverbanks from erosion, and these effects on river channel changes are quite quantifiable. The main depositional features found along the stretch of the alluvial channel are sand bars which represent the local storage of sediments (Fig. 12.11a). At the earliest stage of development, the gravel base for the developing bar supports riparian vegetation (Fig. 12.11b).

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Fig. 12.10 a Dense riparian forest upstream of River Ogun near Ibaragun (Source Fashae 2011). b Tufted grasses colonizing the loose sand bars (Source Fashae 2011)

This gives credence to Nanson (1981) who introduced the notion that scroll bars have a vegetative origin and suggested that scroll bars develop around trees and logs. Occurring adjacent to sand bars along this alluvial channel are cut banks (Fig. 12.11c). They are cliff-like shapes formed by the erosion of soil as the river collides with its banks. In areas where undercut banks were found, pools are common within the channel (Fig. 12.11d). At points where pools occur, the depth of the channel is at its maximum (Fashae 2011). Oxbow lakes, a product of meander cut-off, also feature prominently along the River Ogun (Fashae and Faniran 2015; Fashae and Olusola 2017). Satellite imagery taken in 2006 shows an oxbow lake although images taken in 1984 included the cut-off area as part of the channel (Fig. 12.12).

These oxbow lakes comprise swamp vegetation, mainly of the pteridophytes family. Common are fern plants, with leaves that range from small and simple to large broad fronds with branched veins. They have roots and sometimes true stems (Fig. 12.13). A few braided sections exist along the channel, particularly at those sections where maximum deposition due to the inflow of adjoining rivers occurs and at points of intersection with other tributaries where the river flow velocity was reduced significantly. This is evident at Ibaragun where the Ewekoro River, a second-order river, joins the River Ogun (Fig. 12.14). The establishment of braids is essential to create an environment favourable to the establishment of in-stream vegetation.

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a

b

c

d Pool

Fig. 12.11 Riparian landscape (Source Fashae 2011). a Sand bars showing growth of annual grass with shallow roots along the River Ogun at Magbon, b deposition of sands showing the growth of annual

grass along River Ogun bank at Isheri, c cut bank erosion at Mokoloki section of the River Ogun channel, d pool at Magbon section of the River Ogun channel

12.3.3 Riparian Species Diversity Along River Ogun

Pistia stratiotes (water lettuce), Pennisetum purpureum and Sacciolepis Africana, as these species are found almost everywhere, so also are Ficus spp. and Eichhorrioa crassipas encountered on both sides of the riverbank.

Deductions on the probable morphological state of a river channel at a point from the identified riparian vegetation species require adequate knowledge of plant species, species richness and diversity at such a point of the channel (Fashae 2011). A total number of about 80 plant species of different categories was identified in the riparian vegetation zone of the alluvial channel of River Ogun. Simpson’s diversity index on both banks of the river is less than 0.8, suggesting a reduction in the diversity of plant species. However, species composition along bank B (to the east of the channel) is less diverse than those along bank A (to the west of the channel) (Fig. 12.15). Whereas the species diversity index is only important to provide information on the number of different species that are represented in a given ecosystem, an evaluation of the role of vegetation in bank stability requires that specific plant species be identified. The most pervasive of all the plant species in the alluvial section of the River Ogun channel are

12.3.4 Above-Ground and Below-Ground Diversity of the Riparian Vegetation of River Ogun Plant stems, which constitute the above-ground formation of the riparian area of River Ogun, play a significant role in riverbank stability. The importance of stems to channel morphology has not been widely studied even though most of the woody debris that affects river flow is primarily from plant stems (Fashae 2011). Three classes of plant species based on stem diameter can be identified along the alluvial channel of River Ogun: small stems (with diameter