Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershed, Gialias River, Cyprus 9781407302638, 9781407332659

Table of contents :
Cover
Title
Copyright
Epigraph
Acknowledgments
Contents
1. Introduction
2. PHYSIOGRAPHY OF THEGIALIAS WATERSHED
3. MODERN CLIMATE, HYDROLOGY AND EROSION
4. METHODOLOGY: DEFINING AND IDENTIFYING CHRONOSTRATIGRAPHIC FACTS
5. CHRONOSTRATIGRAPHY OF THE MEDIAN SECTOR POTAMIA – AGIOS SOZOMENOS
6. GIALIAS HOLOCENE-DETRITISM HISTORY IN THE REGION OF POTAMIA-AGIOS SOZOMENOS
7. DOWNSTREAM SEDIMENTARY DYNAMICS:FILLING OF THE GIALIAS BASAL PLAIN
8. UPSTREAM-DOWNSTREAM CONTINUUM IN THE GIALIAS SEMI-ARID WATERSHED: DYNAMICS AND MORPHOGENESIS
CONCLUSION
REFERENCES

Citation preview

l na tio ne di nli ad l o ith ria W ate m

BAR S1775 2008  DEVILLERS  HOLOCENE MORPHOGENESIS AND ANTHROPISATION

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershed, Gialias River, Cyprus Benoît Devillers

BAR International Series 1775 9 781407 302638

B A R

2008

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershed, Gialias River, Cyprus

Benoît Devillers

BAR International Series 1775 2008

ISBN 9781407302638 paperback ISBN 9781407332659 e-format DOI https://doi.org/10.30861/9781407302638 A catalogue record for this book is available from the British Library

BAR

PUBLISHING

« The Mesaoria combines every extreme of beauty and ugliness; barren, sand-bedevilled, empty, and under moon-light a haunted waste; then in spring bursting with the shallow splendours of anemone and poppy, and crosshatched with silk-soft vegetation. Only here you realize that things pushed to extremes become their opposites; the ugly barren Mesaoria and the verdant one are so extreme that one wonders whether the beauty or ugliness has not the greater power. » Lawrence Durrell, Bitter Lemons

Acknowledgments

This work could not have been successfully accomplished without the help and the assistance of many people in Cyprus, France, Belgium and Israel. First, my gratitude is due to M. Provansal and C. Morhange (CEREGE) for their support and advice. N. Lecuyer (MMSH) and G. Grivaud (University of Rouen), of the Potamia European research program, helped me with their support and knowledge of the archaeology of Cyprus. I am obliged to George Petrides, Ioannis Panayides and Zomenia Zomeni and the Geological Survey Department of Cyprus. Chemical and magnetic analyses were undertaken in collaboration with A. Elmaleh (University of Paris 7), P.-E. Mathé, F. Vadeboin and F. Rostek (CEREGE); Bettina Schilman and Miriam Bat-Matthews (Geological Survey of Israel) analysed the carbon and oxygen isotopes. H. Bruneton (CEREGE) and Pierre Carbonel (University of Bordeaux) assisted in determining the ostracoda. My thanks to all. I am also grateful to George Stoops and Peter Vanderhaute (University of Gent) for training in soil micromorphology, and Lucy Vallaury (MMSH) and Sandrine Marquié (FNRS) for ceramic identification. The British Sovereign Base Area of Dekhelia gave me permission to drill near the Gialias. This work was supported financially by the French Foreign Ministry, the French School of Athens (EFA) and the European Union (MENRT). I am indebted to many friends for their help in the field, advice in the laboratory and for proof reading this manuscript: Mathilde, Jules, Phillipe, Sébastien, Marie, Isabelle, Thomas, Teiki, Juliana (for the cake), Grégoire, Laurent, Sarah, Bernard, Sylvain and others too numerous to mention. This book was translated from French by the very competent Valère Lounas ([email protected]).

iii

Contents 1 1.1 1.2 1.3 1.4 1.5

GENERAL INTRODUCTION.......................................................................................................................... 1 Overall objective and study area........................................................................................................................ 1 Geopolitical outlines........................................................................................................................................... 1 Preceding works................................................................................................................................................. 1 Problematic and discipline area.......................................................................................................................... 2 Morphogenesis factors........................................................................................................................................ 3

2 2.1 2.1.1 2.1.2

PHYSIOGRAPHY OF THE GIALIAS WATERSHED..................................................................................... 5 Geological and geomorphological context of the study..................................................................................... 5 The watershed and its geological environment.................................................................................................. 5 A brief geomorphological description of the Troodos carbonated surroundings pre-Holocene morphogenesis of the Gialias watershed................................................................................................................................... 9 2.1.3 Structural units and Holocene tectonic mobility ............................................................................................. 11 3 3.1 3.1.1 3.1.2 3.1.3 3.2 3.2.1 3.2.2 3.2.3 3.2.4 3.2.5 3.2.6

MODERN CLIMATE, HYDROLOGY AND EROSION................................................................................ 21 Relating the hydrology of Cypriot water-streams to climate........................................................................... 21 A Mediterranean semi-arid bioclimate............................................................................................................. 21 Hydrographic networks and hydrology............................................................................................................ 23 Hydrological characteristics............................................................................................................................. 23 The Gialias hydrosedimentary context and variability..................................................................................... 27 Hydromorphological specificities of semi-arid fluvial environments.............................................................. 27 Flooding variability at different time scales..................................................................................................... 28 Holocene erosion measured at the watershed scale.......................................................................................... 29 Sedimentary filling of Cypriot retention lakes, a geographical model............................................................. 29 Applying the method to the Gialias wateshed.................................................................................................. 32 Summary........................................................................................................................................................... 33

4 4.1.. 4.1.1 4.1.2 4.2 4.2.1 4.2.2 4.3 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.4 4.5 4.5.1 4.5.2 4.5.3 4.5.4 4.5.5

METHODOLOGY: DEFINING AND IDENTIFYING CHRONOSTRATIGRAPHIC FACTS Methodology..................................................................................................................................................... 39 Definition of stratigraphic units, morphologic elements and sampling strategy.............................................. 40 Sediment granulometry..................................................................................................................................... 53 The fauna markers of littoral and marine paleoenvironments.......................................................................... 61 The benthic macrofauna................................................................................................................................... 61 The ostracofauna............................................................................................................................................... 64 Dating methods................................................................................................................................................. 65 Studied Period.................................................................................................................................................. 65 Strategy and dating methods............................................................................................................................. 65 Radiocarbon dating........................................................................................................................................... 65 Dating based on cultural criterion.................................................................................................................... 67 Relation-based stratigraphic dating.................................................................................................................. 68 Recognition and use of pedological features.................................................................................................... 68 Magnetic sedimentology and fluvial morphogenesis....................................................................................... 69 Objectives......................................................................................................................................................... 69 Methods............................................................................................................................................................ 69 Magnetic measurements applied to finding sediment sources in the Gialias watershed.................................. 72 Magnetic Characterization of exposed rocks in the Gialias watershed............................................................ 73 Summary........................................................................................................................................................... 74

5 5.1 5.2 5.2.1 5.2.2 5.2.3 5.2.4 5.2.5 5.3

CHRONOSTRATIGRAPHY OF THE MEDIAN SECTOR........................................................................... 75 Introducing the Potamia - Agios Sozomenos sector and its surficial............................................................... 75 The Ekhaton Skales sector: 9500 years of detrital activity in the Alykos........................................................ 77 Alluvial Formation A, 11 000 – 8000 B.P.: AY7/3 and AY26 records............................................................. 79 The alluvial formation B’: incision and flow rate reduction between 8370 and 3000 B.P............................... 82 The alluvial formation B................................................................................................................................... 86 Chronostratigraphy of the alluvial formation B in the sector of Ekhaton Skales............................................. 86 Partial Conclusions for the sector of Ekhaton Skales....................................................................................... 93 Holocene alluviation around Agios Sozomenos............................................................................................... 93 v

5.3.1 Presentation...................................................................................................................................................... 93 5.3.2 The alluvial formation B in the sector of Agios Sozomenos: section AY24.................................................... 95 5.3.3 Alluvium exposures upstream Agios Sozomenos: pre-Holocene deposits, alluvial layers B and C (section AY12)............................................................................................................................................................... 96 5.4 Fluvial archives – upstream sector of the Alykos: sectors of Dhiplopotamon and Kakoskalin..................... 103 5.4.1 Record AY29.................................................................................................................................................. 103 5.4.2 Sedimentary characteristics of Section AY32 (alluvial layers B and C)........................................................ 105 5.4.3 Alluvial Terrace B and infilling of a rank 2 affluent of the Alykos: stratigraphic record AY30..................... 107 5.4.4 A new chronostratigraphic and morphologic marker of the alluvial terrace B: Section AY3........................ 108 5.5 The alluvial valley margins: colluvial formations on the slopes of the Agios Sozomenos Mesa ................. 109 5.5.1 Mutas Skales................................................................................................................................................... 109 5.5.2 Kakoskalin...................................................................................................................................................... 109 5.6 The Gialias...................................................................................................................................................... 113 5.6.1 The alluvial terrace C of the Gialias - Section AY31..................................................................................... 115 5.6.2 10m high aggradation in the Gialias plain: Section AY34............................................................................. 116 6 6.1 6.1.1 6.1.2 6.1.3 6.2 6.2.1 6.3

GIALIAS HOLOCENE-DETRITISM HISTORY IN THE REGION OF POTAMIA-AGIOS SOZOMENOS. Chronostratigraphical synthesis...................................................................................................................... 119 Validity and phasing limitations of the Gialias sedimentary archives (sector of Potamia)............................ 119 Alluvial dynamics variability and physicochemical measurements............................................................... 124 Synthesis of hydrosedimentary functioning in the Gialias median sector...................................................... 129 An attempt to evaluate the impact of human societies on the fluvial environments in the Median Sector.... 132 The detection of anthropic impacts................................................................................................................ 135 Summary......................................................................................................................................................... 137

7 7.1 7.2 7.2.1 7.2.2 7.2.3 7.2.4 7.2.5 7.3 7.3.1 7.3.2 7.3.3 7.3.4 7.3.5 7.4 7.4.1 7.4.2 7.5 7.5.1 7.5.2 7.6

DOWNSTREAM SEDIMENTARY DYNAMICS: FILLING OF THE GIALIAS BASAL PLAIN............ 139 Presentation.................................................................................................................................................... 139 Sedimentary records: SBA drillings............................................................................................................... 139 The transgressive Unit A (ca 8200 Cal. yrs B.P.)........................................................................................... 142 Unit B, marine sedimentation between 8200 and 6700 cal. yrs B.P.............................................................. 143 Unit C, shelly-sand deposits (6700-3575 Cal. yrs B.P.)................................................................................. 145 Unit D............................................................................................................................................................. 145 Unit E.............................................................................................................................................................. 147 Paleolandscapes of the basal plain.................................................................................................................. 149 The Holocene transgression paroxysm........................................................................................................... 149 The first signs of progradation: the littoral ridges and ria.............................................................................. 149 Progradation slowdown: formations of littoral ridges on the site SBA.......................................................... 152 Coastline progression and formation of the lagoon of Dhekelia – Enkomi................................................... 152 Final infilling of the Gialias ria...................................................................................................................... 152 Harbour and coastline mobility...................................................................................................................... 156 Harbour sites in their environmental context................................................................................................. 156 Shoreline mobility and factors of harbour mobility....................................................................................... 157 The indicators of detrital activity.................................................................................................................... 157 Volumic sedimentation rates........................................................................................................................... 157 Magnetic mineralogy and detrital contribution during the alluvial plain formation...................................... 159 Summary......................................................................................................................................................... 165

8

UPSTREAM-DOWNSTREAM CONTINUUM IN THE GIALIAS SEMI-ARID WATERSHED: DYNAMICS AND MORPHOGENESIS....................................................................................................... 171 Morphogenesis and compared sedimentary transfer between upstream and downstream: Towards a relative morphogenic synchronicity?........................................................................................................................... 171 Comparison of morphogenesis in the Gialias median and downstream sectors............................................. 171 Different types of hydrosedimentary functioning in the watershed during the Holocene.............................. 174 Rhythms and factors of the Cypriot fluvial landscape formation................................................................... 176 Comparison with regional and global paleoclimatic records......................................................................... 176 Discussion on anthropic and climatic forcings with regard to the Gialias watershed morphogenesis........... 178 From Byzantine to Modern epoch, new data in Eastern Mediterranean and the Cypriot originality............. 179 The limitations of regional comparisons........................................................................................................ 180

8.1 8.1.1 8.1.2 8.2 8.2.1 8.2.2 8.2.3 8.2.4

vi

9 9.1 9.2

CONCLUSIONS............................................................................................................................................ 183 Regional Morphogenesis................................................................................................................................ 183 Future prospects.............................................................................................................................................. 183

ADDITIONAL MATERIAL ONLINE Please note that 130 black and white and colour figures are available to download from www.barpublishing.com/additional-downloads.html

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Figure 1. Map of Cyprus

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

ntroduction

1. I 1.1 Overall objective and study area Interest in studying historical environments expanded quite rapidly parallel to a passion for romantic landscapes in the 19th century and the knowledge of natural resources among the ecologists of the 20th century (Delort & Walter 2001). From the point of view of geomorphology, this interest has generated a wealth of research on coevolution between relief and human population (Philippson 1890). Later, with the study of recent sediment deposits in the Mediterranean Basin, the concept of parallel development between the spread of the Oikumene and erosive phenomena has emerged (Dufaure 1976, Bousquet & Péchoux 1980). Since then, numerous and sometimes contradictory studies have highlighted the diversity of the Holocene sedimentation and morphogenic response to human settlement and population growth according to the geographical variety. With the present work, we intend to contribute to this line of research by studying a semi-arid Mediterranean environment, the island of Cyprus.

where we were not allowed to work. The partitioning is materialized by the Green Line, a buffer zone under the U.N.O surveillance. The buffer zone cuts the Gialias river basin slightly downstream Agios Sozomenos and forbids access to the downstream half-part of the river course. However, downstream the hydrographic basin, near Famagusta, in the military occupied territories under British administration, a remainder of the colonial period, we were permitted access to the Gialias main water channel. 1.3 Preceding Works Cyprus, and in particular the Gialias River basin, are areas for which geomorphological and geological studies are rare, ancient and deficient. Therefore a project of reconstructing the geological context, especially the tectonic elements and the setup of Tertiary and Quaternary topographic ensembles, was absolutely necessary. Works on the mobility of the Holocene fluvial or coastal landscapes are scarce. Deckers (2002, 2005) established a chronology of pebble terraces in the midst of short and steep torrents in the western part of the island near Paphos (Figure 1). The fluvial environments, the morphology, the hydrosedimentary functioning and the bioclimatic surroundings of the watercourses we study are remote from the wide plains of Messaoria. On the northern mountain side of Troodos, a geoarchaeological survey (Given et al. 1999 et 2000) shows that ceramic land fills collected at the surface can be connected with the different alluvial morphologies. This work does not tackle the problem of dating and analyzing the sedimentary environments. Thus, the poorly accurate dating of the terraces and the resulting information do not make possible a comparison with the plain of the Gialias River.

Our study focuses on the Gialias River watershed which extends from the piedmont of the Troodos range to the Eastern Messaoria plains (Figure 1). Its purpose is to reconstruct the history of the Holocene morphogenesis in connection with the forms and structures of fluvial landscapes. This work is inscribed in a both geomorpholigical and geoarchaeological perspective and adds to recent advances on the evolution of sub-arid and semi-arid environments in the oriental part of the Mediterranean Basin. Features common to the morphogenesis of the entire region are brought to the fore in connection with the spatial specificities near arid climate limits (Geyer 1999) where a less abundant and more variable rainfall regime as well as an earlier anthropisation take place. Firstly, we will aim at examining the impact of those characteristics on the area of study at the Holocene period.

In the Messaoria, from Pedehios to Nicosia, Newell et al. (2004) excavated a series of 7 paleosoils in the alluvial deposits of the Pedehios which compare closely to those of the Gialias. Unfortunately, 14C dating carried out on continental gastropods could not be accounted for.

The ancientness and the extent of human settlements in the region have caused the creation of an archaeological programme POTAMIA (Dir. N. Lecuyer), the purpose of which is to reconstruct the history of territory and landscape formation (Lecuyer et al. 2001 & 2002). This programme is supported by the EFA, MAE and MERNT funds and an ACI ‘young researcher’ fellowship.

On the southern coast, in a pioneering work Gifford (1978, 1980) brought to the fore the Holocene filling of the ria of Tremithos, unravelling that the site of Hala Sultan Tekke was connected to the sea at the Bronze Age, at a location which forms today the salt lake of Larnaka. Afterwards, a little further east, Morhange et al. (1999, 2000 & 2001) reconstructed the coastal evolution in the Larnaka sector (Figure 1). The filling steps of this ancient lagoon provide information on the sea level and the alluvia carried down by

1.2 Geopolitical outlines The area of study encompasses three different political administrations. Invasion of the island by the Turkish troops in 1974 resulted in its partitioning into an independent state in the south and a military occupied zone in the north 1

Introduction

the vicinal watercourses. In the same sector, Dalongeville (2000) studied the Holocene sea level variations using beach ridges and uplifted wave-cut notches.

3. The detrital modes and intensity we envisage are related to the context of semi-arid bioclimatic specificity. What is the influence of the bioclimatic context and rhythmicity on erosion and detritism? Are the relations between climate and morphogenesis, which are evidenced in Western Mediterranean for well known episodes such as the Little Ice Age, transferable to a more arid space model? A comparison with regional paleoclimatic data is also carried out.

Our work constitutes as well the first study supported by detailed chronostratigraphy and accurate analysis of the sedimentological and pedological processes which have occurred in the large plains of the Messaoria and its main watercourses: i.e. the upstream-downstream continuum formed by the catchment basin of the Gialias.

4. How ancient and intense are impacts of anthropic origin? How can we identify and measure the influence of human population on morphogenesis?

1.4 Problematic and discipline area In this work, we attempt to measure the rhythmicity, the modes of action and the other factors pertaining to the Holocene morphogenesis of the Gialias River basin.

After presenting the approach we chose to follow and after briefly summarising the state of the art research progresses, the investigation methods we use will be introduced. A special focus will be directed towards the process of identifying detrital dynamics in semi-arid environments. Next, we will deal with the factors of morphogenesis in the median sector of Potamia - Agios Sozomenos, then in the downstream sector with the core drilling campaign in the Gialias littoral plain (Acheritou-Enkomi). The potential discovery of synchronous events in the detrital dynamics recorded on the upstream and downstream sectors and their comparison with other published works constitutes the last part of this study.

This problem implies several questions to be elucidated: 1. The underlying question focuses on the diachronic reconstruction of the erosive dynamics and sedimentary environments of the watershed upstream and downstream areas. It raises the problem of phasing and building a common reference frame for the detrital dynamics in very differing environments such as alluvial terraces and littoral plains. This original approach (Campy & Macaire 2003) is meant to measure longitudinal transfers of solid materials in the watershed. 2. What is the influence of morphosedimentary inheritance during the morphogenic process? Does the inheritance remain static all along the Holocene? This question implies that impacts on the valley topographical evolution must be measured in order to determine areas liable to flooding and, that the mobility of the shoreline at the reference level must be evaluated.

The study of detritism during the Mediterranean Holocene cannot be dissociated from the dialectic: human society and climate, both agents being alternatively held responsible by different authors for alluvial formations during the Holocene (Bintliff 2002). Our approach consists of understanding and measuring

Thème

Discipline concerned

Methodology-tools

Morphology

Geomorphology

Carthography, G.I.S.

Evolutionary Millieu

Stratigraphy Geomorphology Sedimentology Biology

Stratigraphy, Rock magnetism, ostracofauna and marine malacofauna

Pedology Evolution of the hydrosystem (hydrosedimentary responses to environmental forcings)

Geomorphology Sedimentologye Pedology

Stratigraphy, sedimentology, Rock magnetism, soil micromorphology

Societies-morphogenesis dialectic

History

Ancient texts, Archaeological excavation, Geomorphology, palaeoecology etc.

Archaeology Geoarcheology Consequences for societies

Influence and climate forcing

History, archaeology, geoarchaeology

Archaeological excavation

Sedimentology Geomorphology, isotopic Geochemistry, Palynology

Stratigraphy, Rock magnetism, ostracofauna and marine malacofauna

Table 1: Disciplines of the morphogenesis study in the Gialias anthropised landscapes 2

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

et al. 2003, Brückner et al. 2005). It demonstrates the weak chronological resolution-power of these studies as well as the problem of accurate dating in the1970s. The radiocarbon datings published in The Mediterranean Valleys were often carried out on terrestrial molluscs.

landscape evolution in its entirety. The description of these landscapes is inscribed into the dialectic between human settlements and elements which are constitutive of the environment and can be made explicit (Leveau 2000). We follow the involvement of some geomorphologists with the study of strictly urbanized sites (Morhange 2001, Devillers et al. 2007) or ancient cities (Desruelles 2004) in the exploitation of this approach. More commonly and similarly to this work, the choice of studying hybrid geographical objects between natural environments and human societies is to the advantage of the first one. The impact of human societies expresses principally through agriculture and can be sensed in landscape mobility. Diverse studies can be cited on this ‘archaeology of landscapes’ in southern France (Bravard et al. 1986, Provansal 1995, Berger et al. 2000, Van Der Leeuw et al. 2005) or in Greece (Neboit 1980, Van Andel 1990, Lespez 1999, Brückner 2003). It is also in the same trend of ideas that numerous works on erosion in Mediterranean environments must be situated (Dufaure 1985). Our work finds its place resolutely in that problematic and thus constitutes an attempt of geomorphological understanding which integrates human society activities within the studied geographical system. Because geomorphological researches on Holocene sediment deposits in Cyprus are totally new in the study area, our efforts must cover a large field of disciplines (Table 1). Interrelating disciplines turns to be a complex problem but finds its logic in the multidisciplinary programme POTAMIA, and more generally in the study of anthropised landscapes.

Another point of concern is to establish the factors of the Mediterranean morphogenesis. The model of Vita-Finzi is justified by the Holocene paleoclimatic variations. For this author, the relationship climate-morphogenesis is extrapolated from Quaternary climate variations where dry periods favour incision and humid periods of aggradation. With the advent of geoarchaeology, studying the natural sedimentation in and around archaeological sites has lead several authors to propose an anthropogenic model. This is the case in Eastern Mediterranean: Bousquet & Péchoux (1978) in Cyprus, Wilkinson (1988) in the Near East and Middle East countries, Lespez (1999) & Fouache (1999) for continental Greece. Van Andel (1990 & 1995) in his work on the same country explains the Holocene detrital variations by the action of human societies. The argument, sometimes cyclic, ascribes the alluvium deposit phase to deforestation (as soon as the Neolithic), to agriculture activities, but also to the abandonment of soils by the farmers for instance during the late Antiquity. Agricultural activities and land developments can also control erosion by stabilizing slopes. If we follow his argumentation, the key points to understand alluvium deposits reside uniquely in archaeological sciences. Without indefinitely opposing the two respective arguments: actions of the human societies versus climates; the debate on Hellenistic landscapes evolution reveals an important problem on how to define morphologic objects. The correspondence between the agricultural use of soils and the erosion of mountains and valley sides is fully documented for the present periods (Neboit 1983). For the Holocene numerous works show equally that nuanced relationships exist between human occupation and the deposited sediment layers (Jorda & Vaudour 1980, Bichet et al. 1998, Provansal 1995, Berger 1996, Allée et al. 1997, Zielhofer et al. 2002, Bruneton et al. 2003, Devillers & Provansal 2003, Lespez 2003, Lucke et al. 2005).

1.5 Factors of morphogenesis Claudio Vita-Finzi (1969) who established the scheme of alluvium deposits during the Holocene in the Mediterranean basin is a pioneer of Holocene geomorphology in the region. His scheme summarizes alluvial plains evolution in two major phases: the older fill and the younger fill. The older fill represents colluvial and alluvial phases of periglacial origin (stage I). During the Ancient and Median Holocene (stage II), the weakness of erosion allows thalweg incision and sediment transportation favouring littoral progradation. From Late Antiquity to the Middle Age, and locally in post-medieval times, a new phase of aggradation appears in Mediterranean fluvial systems. This younger fill (stage III), often referred to as the ‘historical terrace’, is followed by a general tendency to incision. This latter aggradation is attributed to a humid and cool phase such as the Little Ice Age (16th-19th centuries). Since the end of the 1970s, the synthesis of Vita-Finzi showed its limits. Although morphological statements remain one of the components of the description of Mediterranean landscapes, the chronology and factors of their formation have been questioned vigorously.

However the debate on the anthropic origin of the deposit detritism does not rely on a common definition of what constitutes detrital events. The example of Greece summarized by Bintliff (1992, 2000 et 2002) is symptomatic of this flaw. The opposition of the refined study of sediment series, adopted by example par Van Andel (1990 & 1995), with the study of the alluvial formation around the Mediterranean Basin (Vita-Finzi 1969), inevitably collides more with problems due to differences of spatial and chronological scales than with the debate around anthropisation and paleoclimatology. It seems indeed incongruous to compare the paleosol/deposit pair of accelerated erosion described by Van Andel with the filling/incision pair studied by Vita-Finzi. The time and spatial scales as well as the static and dynamic factors

The absence of aggradation in alluvial plains between stages I and II has been since that time widely contradicted. Numerous works around the Mediterranean Basin bring evidence of alluvial formations during this period (Neboit 1983, Fontugne et al. 1999, Wilkinson 1999, Klinger 3

Introduction

intervening in these dichotomies are differing sensibly. For instance, from mountain sides down to thalweg bottoms and then further down large catchment basins, changes of scales and in morphogenic dynamics take place. Intense precipitations generate slope erosion but also accumulation in thalwegs. In the midst of the 1990s, evidence of the conjugated influence of human activities and climate on morphogenesis is formulated for the Southern France region of Provence (Jorda et Provansal 1996) and for Greece (Bintliff 1992). This position is also adopted for the Middle East by Wilkinson (1999) who established composite indices of climate and human influence on geographical systems with arid margins. These two factors possess spatial and temporal dynamics that are relatively independent. In Provence the observation of Holocene synchronous fluctuations has lead to the identification of still and dynamic morphogenetic periods. The concept of environmental crisis with climatoanthropic causality has occasionally resulted from these observations. Thus, such crisis periods, similarly to the Little Ice Age, the First Iron Age (circa 2800 BP) or the Ancient Bronze Age (circa 4200 BP) have in principle local or regional value.

relationships with landscape evolution at the scale of the whole geosystem. Thus, research on anthropisedlandscape morphogenesis factors reveals a strong dependence with respect to the progresses made in historical sciences. Examples of relationships between archaeological sites and intensity of sedimentation are more and more numerous. As a consequence, explaining the spatial distribution and importance of fluvial detritism throughout the evolution of human settlements at the scale of the whole Mediterranean basin becomes an attracting perspective (Bousquet & Péchoux 1980). Without raising the same problematic as in our approach (climate role, static factors, etc.), at least it sheds light on particular problems related to pedosedimentary events. Because of their size, their distribution, the weight of statistical factors (substrate, relief, inheritances, etc.) and the methods utilised, accumulations of Holocene sediments are not directly observable at the regional scale. Any attempt to regionalisation can only be achieved through the multiplication of isolated observations within a broader ensemble. The systematic observations of relationships between archaeological sites and sedimentation, at the scale of a whole river basin from upstream down to the sea, since it covers an entire region might allow a deeper understanding of the anthropic erosion which takes place in the formation of large fluvial and littoral morphosedimentary systems (Bichet et al. 1998, Van der Leeuw et al. 2003 & 2005). At the scale of the watershed, knowledge of soil occupation during the Holocene requires a programme of archaeological survey and excavations which has not yet been carried out in Cyprus where most of the undertaken researches have directed efforts at addressing problematics linked to the sole site.

Understanding the mechanisms of a morphogenetic process of climato-anthropic origin requires more precise definition and knowledge of the human and natural factors triggering erosion. It is perfectly evident that these processes differ according to the upstream-downstream positioning, the spatial scale and the functioning duration of the observed landscapes forms. Very early, a few authors (Philippson 1890, Athanasiades 1975, Neboit 1977, Bousquet & Péchoux 1980 Dufaure 1985) have set the conditions to analyse the influences of human societies on landscapes. The relationship between soil erosion and human activities turns to be complex. War, economy, demography, agricultural techniques, soil exploitation, habitat spreading, geopolitical evolution, are facts made explicit by historical sciences. However they clash with the geographical delineation and the chronological accuracy necessary to understand their

Firstly, our work will focus on the task of setting up an accurate chronostratigraphic referential system, and on understanding the hydrosedimentary mechanism in the watershed. Secondly, only the relations to external factors, such as anthropisation and climate variations, will be discussed concomitantly to the detection of synchronous events with true causality.

4

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

2. PHYSIOGRAPHY OF THE GIALIAS WATERSHED The Gialias River springs up in the ophiolite range of Machairas (1423m). All the way down to the sector of Agia Varvara, the river follows a straight course with many substratum incisions and without exhibiting a really visible flood plain. The only deposits present are rock blocks and big pebbles at the bottom of the thalwegs. The Gialias valley exhibits there the morphology of a torrent (Figure 2, Figure 3 sector A). Thalwegs are V-shaped and slopes are relatively steep (7-10°): altitude decreases rapidly from the culminating point of the Machairas range. Sides are covered with weakly developed rendzina formed after reforestation undertaken by the British administration since the end of 19th century (Thirgood 1987). Poor soils and steep slopes limit agriculture. The agricultural development of the valleys was made possible due to terraces and retention dams constructed during the Medieval and Modern periods.

monotonous. Slopes are on average less than 1°. The plain is about 20 km wide and strewed with small outlier mounds from the Pleistocene, where some archaeological sites such as Enkomi and Kalopsidha are located. Proximity of the sea level causes problems of soil exploitation since at least the end of the 19th century (Lt. Kenyon Correspondence of 19.5.1881, National Archives of Cyprus & Finkel 1996). These problems are materialized through water logging and soil salinity increase. From upstream to downstream the alluvial plain widens very sensibly. It is non-existent in the Machairas range and it develops a surface area of more than 200 km² as it reaches Eastern Messaoria. This conservative configuration influences the distribution and geometry of the alluvium deposit processes. 2.1 Geological and Geomorphological Context 2.1.1 The watershed in its geological environment

The upstream part of the flood plain in the median sector corresponds to a tectonic bench in the vicinity of Agia Varvara. Up to the village of Tymvou, the Gialias floodplain is bounded by interfluves constituted of marine and fluviatil deposits from the Pleistocene and Pliocene periods which appear under the form of outlier mounds and larger mesas (Figure 3 sector B, Figure 4). From the point of view of topography, the slopes are here less steep (Table 2). The residual hills are bared and therefore witnessing of an ancient erosion which resulted in the disappearance of the soil superficial horizons. The valley sides are not easily tillable because of a reduced water retention capacity and important rain-wash (Christodoulou, 1959). In addition, the presence of nutriments is poor and the strong rock hardness restricts possibilities of tilling. Based on the analysis of correlative deposits, the present study intends to date the main phases of relief scouring. Archaeological sites from the Bronze Age, such as the site of AthienouMallouda (dir. M. Tomazou) which is located upstream smaller watersheds flowing into the Gialias, exhibit soil scouring anterior to their settlements. No evidence of a soil mantle is visible in the stratigraphic levels or in the ditches. It is possible that that a progressive diminution of the agricultural potential in the interfluves had influenced the spreading and economic development of the archaeological sites. Indeed, habitats close to floodplains easily preserve soils whereas interfluve sites loose tillable soils progressively.

The geological history of Cyprus has even influenced the name of the island, which would originate from Copper an important resource on the island since the Bronze Age. More solidly, that history allows us to understand the logics of the geographical distribution of the different rock types constituting the sedimentary supply to the Holocene detritism. It has several interests: firstly, to define contrasts in the erosive potentialities linked to the watershed lithography; secondly, to understand the influence of the substrate granulometry and mineralogy onto the Holocene deposits lithography. Soil geology determines the solifluction modes, superficial or karstic. It enables us to understand and to localize possible tectonic movements. Setting up of large structural ensembles Above all, the geological history of Cyprus is a recount of the Anatolian and African plates colliding with each other (Figure 5). According to the submarine drilling study LEG 16O (Robertson 1998), the Cypriot orogenesis is principally ascribed to the collision of the submarine Eratosthene mounts with the active margin of the Eurasian plate materialized in the north by Southern Cyprus. The distortion of the Eratosthene mounts, subsidence and angular faults, results from the tectonic thrust of South Cyprus and has for consequence the orogeny of the Troodos (Bousquet & Péchoux 1980, Eaton & Robertson 1993). The uplifting of submarine mounts at the Miocene period is more important than the glacial-eustatic variations. There

From Tymvou to the golf of Salamina-Famagusta, the Gialias and Pedehios valleys converge near the base level and form a vast alluvial plain of 280km² remarkably flat and 5

2. Physiography of the Gialias Watershed

Figure 2: Upstream the Gialias River

6

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

Figure 3: Morphology of the Gialias fluvial valley

Surface area (km ) Mean Elevation (m a.s.l) Cef. var. elevation Mean slope (degrees) Minimum slope (degrees) Maximum slope (degrees) 2

Upstream Floodplain

Seasonal flood Broad peak flood Seasonal variation

Surficial

-->

Surficial and subterranean process mixing

Fast surface runoff

-->

Delayed surface runoff

Upstream-downstream Infiltration

-->

Evaporation

Strongly and densely connected drainage pattern

Dominated by wide rivers

--> -->

Numerous morphogenic floods tendency to more pronounced stability

Convective storms Highly localised Highly variable

Output

Hydric transfer

Main water channel

Weakly connected drainage pattern

Convective storms and seasonal masses of Springs, seasonal masses of humid air, humid air glaciers, snowmelt --> Regional --> More regular

Scarce large floods Ephemeral behavior

Table 5: Hydrologic and morphogenic differences between perennial and ephemeral fluvial systems (Knighton & Nanson 2000) 3.2 The Gialias hydrosedimentary context and variability

Nanson (2000, Table 5). Origins of the flows are differing according to the environmental bioclimate. Localized convective storms are the chief source of outflow in arid environments. This spatial discrimination is also the cause of differences in flux coefficients. The mode of fluvial water supply (spring weakness, etc.) and the rainfall intensity lead to surface-water outflow preponderance and velocity. Drainage pattern and mean-water channel evolution are also influenced by the number of morphogenic events always in relation with the bioclimatic surroundings (Table 5).

3.2.1 Hydromorphological specificities of semi-arid fluvial environments The definition of fluvial environment is based on climatic considerations and justified by the precipitation influence on ablation forms and particularly on accumulation forms. The principal variables depending on the perennial nature of liquid fluxes are summarized by Knighton and

27

3. Modern Climate, Hydrology and Erosion

3.2.2 Flooding variability at different time scales At the present time, Cypriot rivers and especially the Gialias watershed exhibit behaviours which are intermediate between transient and perennial at centennial time scale. Flow measurements clearly show a transient or intermittent behaviour (Figure 20). The lack of water discharges, during a large part of the year has noticeable consequences on the Gialias fluvial environment. Hydrology of flooding shows variations at the seasonal time scale. As we can see for the watershed examples (Figure 19 and Figure 20), the hibernal floods are more abundant and last longer with respect to the autumnal and spring events. In spite of large peak floods, they are characterized by a fast return to the null discharge regime (Figure 20). The winter floods do not differ, whereas after a flood event fluvial bed dewatering systematically occurs, but less rapidly than the previous ones. Despite large seasonal variations, there is no sign of noticeable changes over the years.

Traveller recounts must be considered with all the caution necessary to historical studies. These descriptions are made by western Europeans readily describing the Cyprus high dryness (“scorched land”, “deserts”, “furnace”, etc.). Nevertheless they tell with certitude that runoffs much more regular than now occurred. Of course these recounts do not provide any quantitative data on flow rates and flood frequency variations. Travellers, geographers, colonial engineers and countrymen have a similar point of view. Most of the recounts previous to the 17th century, report the damages and number of deaths caused by flood events (Grivaud 1998), what is without direct relation to the hydrologic functioning of water streams. At the multicentennial time scale, hydrologic behaviour variations can be deduced from sedimentary records. Centennial variations of flash-floods and floods pertaining to a more humid Mediterranean climate seem to be confirmed by texts extracted from the literature. As we shall see later on, the stratigraphic approach also confirms the hypothesis of multicentennial variations. The parameters which we can consider to study the nature of flood changes are of several types (Table 6). The first is of climatic type: mean precipitations and pluvial events intensity are, of course, the chief factors determining the nature of the flooding events for rivers controlled by pluvial regimes (Phillips 2002). The second is the vegetal cover variations modifying rainwash coefficients on the entire watershed environment. Its regression leads to an increased proportion of surficial trickling of rainwater and consequently increases the peak flood intensity while reducing its broadening by shortening retention times. At last, domestic, industrial and agricultural water consumptions can largely reduce the phreatic volume (Amorphos et al. 1999). It creates a water buffer, at the level of river channels, which must be filled before water can flow into the channels. The water table disconnection from the fluvial bed increases runoff intermittence and peak flood intensity.

During the 40-year period for which data are available, the Gialias and the Alykos are always intermittent rivers.At the centennial time scale, flow rate data are non-existent. Nevertheless some qualitative information is brought by historical sources. During the 18th century and at the beginning of the 19th century, traveller recounts and first hydraulic engineering studies note the runoff perennial character and insist on the variability of flow rates. So, in June 1909, it is the Gialias low-water level period, but water still flows in its bed. The flow rate is partly supplied by the phreatic groundwater through resurgences in the thalwegs (Reid 1909). Oberhummer (1903) mentions the great drought, accentuated by high summer temperatures, but he also writes that, during these periods, water is abundant in rivers. Hutchinson (1909) observes that the Pedehios and Gialias rivers get during summer abundant water supplied by the everlasting snows of the Troodos. The traveller S. Baker, in August 1879, does not find the Gialias ford making impossible for him to rejoin the Dhali village because of “a few decimetres-deep water flow runs between the two borders separated by about 100 yards” (Baker in Wallace et Orphanides 1990). Complaints lodged with the British colonial administration, in 1883, among others, reveal problems of water-diversion and sharing during the hot season (Colonial Administration notebooks SA1/1883, Cyprus National Archives). Finally, eucalyptus plantation is decided in Nicosia in 1866 (and in the whole island until the beginning of 20th century) in order to fight malaria epidemics which is raging in summer due to the Pedehios stagnant waters (Colonial Administration notebooks SA1/1866). Crue "temperee"

Flash Flood

Deforestation

-

+

Consomation en eau

-

+

Creation de barrage

-

+

Augmentation precipitation moyenne

+

-

Forte intensite des precipitations

-

+

The change of flood type has several geomorphological consequences. First, it influences the width of the mean water channel (Karcz 1972), as well as the morphology of its embankments (Wolman and Gerson 1978, Reid et Frostick 2000). Moreover, the flood type determines the sediment deposit rate, the fluvial lithofacies and pedogenesis. This type of impact will be discussed throughout this work. Different Mediterranean examples illustrate flood type variation during the Holocene. Thus, Zielhofer (2002) brought to the fore flash flood recurrence in Tunisia at the Roman age. In the same way, the hydrological functioning of the wadis was described as being more regular between 8000 and 500 B.P. (Moeyersons 1999). In view of all these elements, it must be noted that identifying the dominant flood type at the multicentennial time scale is important to reconstruct the Gialias hydrosedimentary functioning at the Holocene. Among other paleo-hydrological indicators, the peculiar

Table 6: Principal parameters affecting the Cypriot flood events 28

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

importance of criteria measuring the runoff perennial character must be underlined.

for the Gialias watershed and to identify elements of various upstream and downstream sectors, contributing to the sedimentary supplies in the nowadays morpho-climatic and anthropic context.

3.2.3 Holocene erosion measured at the watershed scale During floods, the amount of suspended material increases with flow intensity. This physical constant is given by the formula C = aQb where C is the concentration in sediment expressed in Mg/liter, and Qb the flow rate in m³/sec. The constant ‘a’ is 6 to 4500 times higher for non-perennial rivers in arid and semi-arid environment than for rivers in temperate and humid environment (Reid et Frostick 2000). This illustrates the morphogenic power of non-perennial rivers, although their flow rate is less important when compared to perennial water streams. The fluvial forms of the Gialias watershed are of particularly large extent and the sediments volumes are considerable. To understand the spatial distribution of the sediment stocks and sources in the watershed, studies carried out on the filling levels of the Cypriot retention dams are used.

This method is not intended to measure erosion during the Holocene, but to provide an overall comparison of the mean sedimentary stocks in the different sectors of the Gialias while allowing for current erosion conditions. The results will then be compared with the probable volumes of Holocene deposits. It allows the present erosional conditions to be roughly situated with respect to the mean conditions from previous periods. In this way, sediment transits and stocking characterizing each spatial unit are not integrated into the overall results. Burdon (1951) provides an estimate of the sedimentary filling volumes for eleven dams using filling measurements of initial retention volume known from engineering documents. In a second phase, the calculated volumes may be related to the watershed of each reservoir (Table 7).

3.2.4 Sedimentary filling of Cypriot retention lakes, a geographical model The objective here is to determine the erosive potential contrast in the different parts of the watershed and possibly to evaluate the volumes involved during the Holocene. To this end, we have at our disposal measurements on eroded sediment volumes in connection with data on the land use and vegetal cover, collected by Burdon (1951). These data are supported by filling-speed direct measurements carried out on the different Cypriot dams. Their reliability is therefore insured. The filling data informs on the erosive activity of the first half of the 20th century, that is to say, between the building dates of the dams and filling measurements (1951). It is difficult to assert that erosion modes have remained unchanged since that time. Although up-to-date documents make possible a more accurate qualification, the soil condition classification can only be the one used in Burdon’s publication (1951). The collected data will allow us to elaborate a transfer model

The reservoirs and their catchment areas are distributed in the main Cypriot geographical systems. Each geographical system (geosystem) is defined by a homogeneous topography and substratum. On the other hand, the soil use is variable. The first geosystem group is the Troodos zone with the reservoirs of Kalokorio, Petra, Lithrodhonda and Galini (Table 8, Table 9 and Figure 21). It is characterized by steep slopes, magmatic rock substratum (essentially basalts) and a comparatively dense forest cover. The second zone is the Troodos and Kyrenean chains together where slopes are quite steep. Here, the substratum is made up of carbonated-derivate rocks. The soils are either cultivated or covered by sparse forests and vineyards. The zones around the villages of Akounda, Syngrassi, Kanli, Kophinou and Lymbia are representative of these types of geographical areas (Table 8, Table 9 and Figure 21). The low plain of Messaoria is of the last type with low gradient slopes. The substratum here is made of Quaternary deposits. Cultivated

Superficie BV

Erosion

Erosion

Year

Dam

Km²

t/Km²/yr

Activity start

Akhyritou Akrounda Galini Kalokhorio Kanli Kophinou Kouklia Lymbia Lythrodhonda Petra Syngrassi

8.0 27.4 23.1 24.3 29.5 10.1 5.3 33.0 7.4 34.8 49.0

m3/Km²/yr c. 1000 1,192.9 3,221.0 5,207.8 c. 2000 5,272.6 c. 0 3,458.6 2,464.7 1,662.5 c. 2000

c. 1600 1,908.7 5,153.7 8,332.5 c. 3200 8,436.1 c. 0 5,533.7 3,943.6 2,659.9 c. 3200

1900 1946 1947 1946 1949 1946 1900 1945 1944 1947 1898

Table 7: Watershed erosion of 11 Cypriot reservoirs (from Burdon 1951) 29

3. Modern Climate, Hydrology and Erosion

Dam Name Akhyritou Akrounda Galini Kalokhorio Kanli Kophinou Kouklia re Lymbia Lythrodhon Petra Syngrassi

Topography Surf Km² 8.0 27.4 23.1 24.3 29.5 10.1 5.3 33.0 7.4 34.8 49.0

Z Min (m) 7.3 136.3 224.1 505.9 168.3 197.2 10.6 200.0 413.0 264.1 27.4

Z Max (m) 34.5 942.4 1,043.5 1,360.7 633.2 462.8 22.9 591.0 711.5 1,357.0 548.9

Average 14.7 470.8 648.6 822.5 255.9 336.1 14.9 296.3 499.5 681.8 108.4

Slope (degrees) Max 6.2 31.6 35.1 33.2 35.4 16.9 2.7 22.8 26.6 34.8 26.5

Average 1.1 13.9 18.2 15.4 4.3 5.4 0.4 4.6 7.3 11.9 3.0

Range 6.2 31.1 34.6 32.7 35.4 16.6 2.7 22.8 26.1 29.7 26.5

Table 8: Topographical characteristics of the Cypriot reservoir watersheds Dam name

Vegetation and soil use (%) Dense Light forest Uncovered forest soil and farmland

Vineyards

Geology Substratum

Akrounda Petra Lythrodhon Galini

57.4 100.0 62.7

99.0 15.5 4.0

24.5 8.1

1.0 2.6 25.2

MN et Q Upl/lpl Upl/lpl Upl/lpl

Lymbia Kalokhorio Kophinou Kouklia Akhyritou Syngrassi Kanli

-

59.1 34.2 18.1 -

39.0 16.7 100.0 100.0 81.9 69.0

2.0 83.3 65.8 -

Upl/lpl Upl/lpl Upl/lpl Holocene Q1 & Q2 MN et Q MN et Q

31.0

Table 9: Use of soil and substratum and bared soils cover the greater part of the areas. Table 9 shows the distribution of soils according to occupation type for each watershed.

(Table 7) are found on vineyard areas in the Troodos (Kophinou and Kalokhorio). That can be explained by the combination of steep slopes (5° and 15°) and the lack of protective vegetal cover. However this type of soil use does not encompass large areas and is, for instance, absent from the Gialias watershed. In spite of considerable rainfall, the lowest erosion rates are found on the same slopes, but under the forest cover (Akounda, Petra and Lythrodonda) (Figure 15). This contrast underlines the dominating role of vegetation on erosion. Similar observations have lead the colonial engineers to abandon plans against erosion in the 60s after that pastoral land use had been stopped in the Troodos and a consecutive biological recovery had followed (Thirgood 1987). The reservoirs for which erosion is considerable have watersheds characterized by naked soils cultivated on the Messaoria and its detrital rock surroundings (Syngrassi et Kanli). The erosion is there important (about 2000m3/Km²/year) – the soil coverage type favours it –, but the relatively weaker slopes (between 3 and 4°), account for the noticeable differences with respect

The supply surface-area description of the structures is divided according to the following nomenclature: • • • •

dense forest sparse forest bared or cultivated soils vineyards

This classification is based on soil coverage and takes into account topographical and geological characteristics. Dense and sparse forests generally cover the magmatic substratum slopes of the Troodos. It is also on these slopes that vineyards generally grow. Detrital rocks can usually sustain bared or cultivated soils, but small areas of sparse forest can also be found, as in the Messaoria. Observing the filling levels in different reservoirs enables us to put forward several facts. The highest erosion rates 30

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

Figure 21: Relief, slope and soil use of the Cypriot reservoir watersheds

31

3. Modern Climate, Hydrology and Erosion

to the Troodos deforested slopes. Naturally, the ablation is negligible or absent from Holocene sedimentation environments (no slopes, deposit areas); data obtained on the Kouklia reservoir corroborate this fact (Table 7).

1993). Those more recent studies cannot be used because the published data provide an insufficient methodological and geographical level of details. 3.2.5 Application to the Gialias watershed These measurements are applied to the watershed, using a soil numerical model (M.N.T.), built from radar data SRTM DTED level 1, produced in 2000 (NASA-NGA 2000). The MNT pixel-unit (3 arcs/sec) covers a surface area of 8100m². The distribution of the arborized vegetation and soil condition and use is measured by the cartography of the forest massif (Department of Forest 1997) and a satellite imagery cover (Image Landsat 7, ETM+ of 02/12/2004 and Landsat 5 - Geocover) with a 712,5m² spatial resolution. The substratum is delimited by the mapping of geological outcrops (GSDC 1995).

Considering that for each type of soil use the erosion is constant, for a given reservoir it is possible to translate the value of the measured erosion into an equation of the type (Burdon 1951): reservoir filling volume = dense forest erosion × area + sparse forest erosion × area + bare soil erosion × area + vineyard erosion × area The erosion is expressed in volume unit (m3) by surface area unit (km²) by year. Each reservoir volume and soil type area is a known variable. Therefore, resolving that equation allows us to know the volume of sediment loss for the different area types (Table 10).

The Gialias watershed can be divided in three homogeneous areas according to the relief, the slopes and the substratum (Figure 22, Table 11 and Table 12). The soil coverage on the upstream zone is divided into dense and sparse forest parts. The bared soils and cultivated lands form the main areas in the median and downstream sectors. Sparse forests can be also observed sporadically.

Excepted in the particular case of the Holocene vineyards and deposits, the erosion is in close relation to the density of the arborized cover and can vary by more than 600%. Considering that only soil-coverage variation can explain erosion, bias in calculations could appear when a same type of soil coverage encompasses very different topographies. It could be the case for sparse forests covering both the Troodos slopes and the Messaoria plain. Nevertheless, the distribution of forests and cultivated areas remains relatively homogeneous and minimize the spatial error made on erosion. More recent studies on soil erosion have adopted a definite division of the geological outcrop distribution bringing to the fore noticeably similar numbers (Tsiourtis

Holocene floodplain Dense forest Light forest Uncovered soil and farmland Vineyards

Erosion m3/Km²/yr ca. 0 75.7 335.4 442.6

t/Km²/yr ca.0 113.1 435.9 552.1

605.2

736.5

As for the different dams and reservoirs, differences in soil coverage characterize different physiographical contexts. The arborized upstream zone belongs to the Troodos massif magmatic environment, with 7° slopes on average and 34° maximum slopes on an area of 8100m² representing one pixel-unit of the MNT SRTM DTED utilized (NASANGA 2000) ; the slopes are evidently the steepest of the watershed. Weak slopes (less of 3° on average), outcrops of detrital carbonated rocks and a very low proportion of arborized cover (less of 1,5%, Table 12) characterize the watershed median and downstream sectors. The maximal slopes, around 15°, are also obviously weaker and show the weakest thalweg entrenchment. The Holocene flooding areas are not taken into account in evaluating the Gialias watershed erosive potential. There are two reasons: - the topic of the study being the product of erosion, Holocene deposits cannot be considered as a material input by themselves. In other words, the Holocene deposit

Table 10: Erosion according to different types of soil surface coverage Sector All BV Upsteam Median Downstream Holocene

Topography Area (km2)

Slope Max. (°)

Min. Z (m)

Max. Z (m)

Average

936.4

0.0

1397.0

196.8

34.6

2.1

116.8

278.0

1397.0

516.9

34.6

7.2

175.5

99.3

509.8

227.7

15.5

2.7

341.6

0.0

246.9

71.4

14.0

1.3

302.2

0.0

360.8

52.8

12.3

0.7

Table 11: Topographical characteristics in different sectors of the Gialias watershed 32

Average (°)

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

Figure 22 : Soil use in the Gialias Watershed (left aside Holocene outcrops) Sector All BV Upstream Median Downstream Holocene

Dense Forest

Sparse Forest

9.8 53.1 -

9.3 45.8 0.8 1.2 2

Bared soils & Cultivated lands 80.9 1.1 99.2 98.8 98

Vineyards

Substrate

-

UPL/LPL Mn/Q Q Holo

Table 12: Soil occupation and substratum in the Gialias watershed reworking pertains to the intrinsic transport mode in the hydrodynamic system and not to the solid material input during the Holocene.

similar to those of watersheds studied by Burdon (1951). From then on, it is possible, in the Gialias valley sectors, to apply erosion rates by soil occupation type deduced from the reservoir filling levels. These data are summed up in Table 13.

- Moreover, on these areas, the erosion is weak or inexistent as shown by the Kouklia reservoir where the deposit processes are dominating (Burdon 1951, Thirgood 1987). The very weak slopes of the Holocene deposit summital layer (0.7° on average) and various stratigraphical sections where signs of reworking are rare illustrate the same lines of reasoning.

3.2.6 Summary The calculated numbers (Table 13) bring to the fore that sedimentation is prevailing in the median sector (32%) and more predominently upstream the watershed (60%). More than the slopes and the precipitations, the vegetation and consequently the rain wash coefficients play a determinant role in explaining soil erosion in Cyprus. Rain washcoefficient modelling for different cypriot watersheds

Based on the available data, each class of the basic classification of soil use or coverage in the Gialias watershed follows geological and topographical characteristics 33

3. Modern Climate, Hydrology and Erosion

Surface area Sector Km² All BV 936 Upstream 116 Median 175 Downstream 341 Holocene 302

Raw Erosion m3/yr 251589 23205 77532 150765 -

Erosion Rate m3/Km²/yr 397 199 442 441 -

t/yr 402542 37128 124051 241224 -

t/km²/yr 635 318 707 706 -

mm/yr 0.397 0.199 0.442 0.441 -

Prod. B.V. % 100 9 31 60 -

Table 13 : Erosion estimate values in the Gialias watershed

Sector

EHa

EHmax

EHmin

Variation

Upstream

2.32

5.17

.88

+122.8%

-61.9%

Median

7.75

7.77

1.33

+0.2%

-82.9%

Downstream

15.08

15.12

2.59

+0.3%

-82.8%

All BV

25.16

28.06

4.80

+11.5%

-80.9%

Table 14 : Variations of erosion potentials in the Gialias watershed during Holocene (extrapolated from the Cypriot reservoir filling) (Hessling 1999) shows also the strong dependency of these two variables. Hessling showed that rain wash coefficients taken on five cypriot watersheds ranged from 10 to 23%. These coefficients were changed by about 50% after that the destruction of 90% of the vegetation cover occured due to fires caused by the bombardments during the Turkish invasion in 1974. We cannot translate directly rain wash coefficients into erosion rates, however we can underline that these two phenomena respond to the vegetation cover disparition in a comparable manner and magnitude.

The Holocen erosion in the present conditions (EHa) is produced in the watershed during 11500 years. It is deduced from the reservoir filling (see Table above) based on a distribution of soil conditions in the watershed and in its four sub-parts identical to today’s conditions. We must recall here that this chronological time scale transfer is not significative of Holocene erosion. It must be noted that the period of time that covers the Holocene as defined by Alley et al. (1993) corresponds really to the first Holocen deposits met in the stratigrapic maps. The morphological and paleoclimatic meanings of this date are thus significative in the watershed study case.

The sedimentation that is contrasted and varies by more than 300% between the sectors recall that the morphogenesis of an hydrological system is spatially heterogeneous with intermediate accumulation areas and a diachronic transit of sedimentary load.

The maximal Holocen erosion (EHmax) is calculated for maximal soil stripping conditions, i.e. for bared or cultivated soils. We did not integrate here the values of highest possible erosion, recorded for wine yards, since they constitute a particular case with respect to the period considered. Indeed, the earlier pieces of evidence of viticulture refer to Bronze Age. The minimal Holocen erosion (EHmin) is the calculation of the erosion for a dense forest in the watershed.

At the present time, the spatial distribution and the denudation of the mountain sides delineate the potential anthropic influence on erosion. It is possible to make an estimate of the changes that have occured in soil conditions and areas covered by trees, not in function of physiographic zones but, according to changes in human occupation between two periods of time. The extreme variation, in the Troodos range, between the dense forest and the wine yards leads to an erosion increase of 800% and of about 600% for the bared (or stripped) soils. The soil clearing, the deforestation, and a too high frequency of clear forest fires would result in a 130% increase of erosion (180% if a wine yard is planted). In the climate conditions of today the anthropic impact is defined by these values. For each section of the watershed, the Table below (Table 14) summarizes the maximal potential of erosion variations in the watershed due to the modifications of the tree cover in the present time climatic context.

These calculations are performed with the assumption that the only variable explaining erosion is the human occupation. It assumes that the hydro-climatic conditions susceptible of influencing the erosive processes are invariant. In the perspective of the Holocene, the presence of a ‘primitive’ forest and the possibilities of biological resurgences are factors of prime importance to assess the potential anthropic impact on erosion. The presence of a dense tree cover during the ancient and medium Holocene seems very logical and is indirectly proved by the presence of ancient forests on the Troodos (Jones et al. 1958), where 34

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

etc.). Therefore, the objective here is to calculate an accommodation space which is truly representative of the possible maximum. The volume can be calculated as follows:

residual remains of ancient forests have been described. However, it is more questionable for the Messaoria plain (Jones et al. 1958, Thirgood 1987) where precipitations are lower than 300 mm/year. It can never be evidenced by pollen analysis performed till now on archeological sites oftenvery poor in pollens (Bottema 1976, RenaultMiskovsky 1989, Burnet 1997). Its possible disappearing, prior to travellers recount, is generally ascribed with proof to the Neolithic deforestation (Poore 1956, Jones et al. 1958).

EAmax = Surf × H For the minimum accommodation space, the measured thickness of the Holocene deposits is considered on the ground level as maximal (Figure 23). The volume is formed by the Holocene deposit surface in the context of V-shaped valley. This geometrical configuration of the deposit accommodation space is more realistic and is expressed by the formula:

A first reflection on the Gialias hydrosystem functioning can be conducted. The Holocen flooding surfaces are known due to our observations and due to the geographic mapping of the island (GSDC 1995). In this way, we also know approximatively the thickness of the valley filling during the Holocene in the median and downstream sectors. It therefore possible to calculate the maximal adaptable space, i.e. the maximal Holocen sediment volume, for each sector and to compare it afterwards to the produced sediment volumes. Matching these two pieces of information allows to deduce the possible solid transports from the median and upstream sectors towards their respective downstream counterparts (Figure 23 A).

EAmin = Surf × H/2

The deposit surface areas as well as thier thickness is known by sectors. The paleotopography of the preHolocene valleys is much less known. A few sections and probing enable to delineate its geometry. Nevertheless, we will use here simple geometrical forms which maximize the adaptation space by simulating flat bottomed valleys, or conversely, which minimizes it taking into account V shaped valleys (Figure 23 B). The available data indicate ususally that the mother rock geometry fluctuates between these two limiting models. The limits of the external envelop could be better defined by a futur drilling campaign or by geotechnical investigation means (electric probing, etc...).

Sector

EAmax

EAmin

Unit

km3

km3

Upstream

neg.

neg.

Median

2.05

1.03

Downstream

85.53

42.77

All BV

87.58

43.79

Table 15: Accommodation space for sections of the Gialias watershed As we have seen before, with an amount of 40% for the cumulated data in the upstream and median sectors and 60% in the downstream sector (Table 13), the sediment supply is relatively balanced across the watershed. This is strongly contrasting with retention capacities of about 3% and 97% for the upstream, median and downstream sectors respectively. Transfer dynamics between units seems to play a significant role.

The maximum accommodation volume is the volume formed by the exposure surface areas of the Holocene deposits multiplied by their thickness. This volume is maximal since the topography of the substrate would then, in that case, form a canyon with vertical sides (Figure 23) a situation which is not observed in reality and represents unrealistic modelling in soft rock context (marl, sandstone,

In the table above, Holocene sediment volumes can be compared. The estimate is deduced from the present erosive conditions and from the minimum and maximum accommodation space geometry recorded in the different sectors. A negative volume thus shows that erosion is more important than accommodation. The difference

EAmax EHa km3

EAmax EHmax km3

EAmax EHmin km3

Transfer >

Transfer >

Median sector only

-8

Median + upstream erosion

-6

Downstream only Downst. + median not stored All watershed

Sector Unit Upstream

km3

EAmin EHmax km3

EAmin EHmin km3

Transfer >

Transfer >

Transfer >

Transfer >

-13

-5

-11

-17

-2

-8

-4

-9

-12

-1

70

70

83

28

28

40

76

78

87

37

40

41

62

60

83

19

16

39

EAmin - EHa

Table 16: Comparison between erosion and accommodation space during the Holocene 35

3. Modern Climate, Hydrology and Erosion

Figure 23 A and B: Erosion in the watershed (A) and accommodation space (B) represents sediment transport towards the neighbouring downstream sector. A positive volume means that erosion is not sufficient to fill the accommodation space. It must be noted that the accommodation space in the upstream sector is considered as negligible. The sediment volumes supplied by this sector (Table 14) are then transferred towards downstream (median sector). The median sector accommodation space being smaller than the amount of erosion volume, the difference between erosion volume and accommodation space is calculated downstream with or without the sedimentary surplus deduced from the upstream sector.

underlines the importance of the longitudinal transfer process. Conversely, the numbers calculated for far-east Messaoria (downstream) and taking into account transfers within the watershed indicate that solid filling is in high deficit with respect to the accommodation space volume. Several other observations can be gleaned from Table 16 as well: The accommodation space of the median sector is in no case sufficient to stock erosion supplies cumulated by both the median and upstream sectors where accumulation is almost inexistent. Transfers are therefore of about 100% between upstream water and the median sector. It is at least 7% (in the improbable case where EAmax is allowed for) and at most around 90% between the median and the upstream sector. Solid transfer from the median sector

From a general point of view, and in agreement with the diversity in sediment supply modes and accommodation spaces present within the watershed, we can state that the upstream and downstream sectors are in excess which 36

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

towards downstream water thus fluctuates approximately in a 1 to 2 ratio during the Holocene.

times. We then note that the climate morphogenic capacity is more marked during long periods of time for 11,500 years with respect to those corresponding to reservoirs functioning during the first half of the 20th century.

In the downstream sector, the accommodation space is 84% larger when closer-to-reality estimates are taken into account for EAmin and EHa, and 1100% larger when the variables EAmax and EHmin are taken into account. Error margins in our calculation cannot be easily evaluated, but they rather tend to underestimate accommodation space capacities and overestimate erosion in the downstream sector by ignoring the volumes of sediments evacuated towards the marine domain.

This first element is in contradiction with the erosive paroxysm caused by the 20th century agriculture and pastoral activity described by the engineers (Tsiourtis 1993). It also raises questions on the climate erosive capacity during the Holocene. However, the method we described above raises numerous problems. The first is that the data available do not let error margins be accurately calculated. However they seem to be large. Determination of the accommodation space could be refined with the progress of the geotechnical investigations carried out in the region. Although it is meant to largely improve, the approach based on comparing eroded volumes, contrarily to strictly numerical models, presents the advantage of being based on reliable first measurements: i.e. the filling levels of retention dams. In this regard, it must be considered as a first step towards a general reflection on sediments deposited during the Cypriot Holocene.

Consequently we observe that the watershed erosive activity, even when extrapolated to maximal erosion conditions (bared soils and cultivated lands) is inferior to the Holocene accumulations reported downstream. The maximal possible consequences can only be imputed to anthropic degradation of the tree cover and thus cannot alone explain the Holocene sediment accumulations in the downstream sector of the watershed. Sediment volumes are calculated according to variable soil conditions in a constant climatic context equal to the one of the present

37

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

4. METHODOLOGY: DEFINING AND IDENTIFYING CHRONOSTRATIGRAPHIC FACTS

Figure 24: Acquiring environmental and detrital signals 4.1 Methodology This work was based on missions performed in Cyprus with the objective of: (1) to map the extent of the Holocene deposits; and (2) to define the sites where in-depth chronostratigraphic and sedimentary studies could be most beneficial. These missions have been complemented by the available cartographic maps and satellite imagery (Figure 24). The archaeological sites, their surroundings, as well as more remote areas have been covered. Privileging linear water courses has allowed us to work on natural sections. Mechanically excavated trenches have been made as well, and the emerged littoral plains and lagoons have been dr The study of the landscape mobility in a watershed, from upstream to downstream, requires the presence of many environmental markers to allow the determination of the different fluvial, littoral, coastal, and marine environments susceptible of being affected by the Gialias sedimentation.

alleviate problems due to the specificities of different sedimentary environments when studying erosion and sediment transport phenomena. In this regard, the sediment granulometry is not a good marker of detritism relatively to an upstream-downstream perspective because it is too much influenced by environmental factors. We have ourselves developed methods to characterize the soils and deposits using micro-morphology and we have looked for indications to identify sediment sources and land use modifications across physicochemical investigation methods, in particular across the magnetic mineralogy techniques. Some other classical analyses have allowed us to connect sedimentary and stratigraphic characteristics with sediment environments. Collaborations with paleo-ecologists (M. Bourcier & H. Bruneton) have let these interpretations be supported by the ecological characterization of paleo-aquatic environments.

The multiplicity of sedimentary environments leads us to look for common mineral denominators in order to 39

4. Methodology: Defining and Identifying Chronostratigraphic Facts

Figure 25: Terminology used to study alluvial terraces Finally, dating the events relies before all on radiocarbon dating and archaeological data.

consist of a summital layer represented by the cartographic extent of the formation exposure surface (Figure 25). The flank is the escarpment of the terrace. The flank results from an incision process posterior to the terrace formation, and determines its morphology. A fossilized terrace flank, or more generally a fluvial incision identified within a stratigraphic sequence is here referred to as a gullying discordance. It is materialized by a stratigraphic joint between two formations.

4.1.1 Definition of stratigraphic units, morphologic elements and sampling strategy Continental deposits: methods of identification. A global approach: stratigraphy and geomorphology The deposits are divided into elementary sequences, series and formations (Macaire 1990, Campy & Macaire 2003). The elementary sequence represents the deposit corresponding to a morphogenic event, as for instance, a flood. A series is constituted of N elementary sequences of the same type (Figure 25). For convenience and to remain consistent with the archaeological terminology, series are sometimes called sedimentary units. A formation is composed of several superimposed or placed side by side series resulting from the same type of morphogenic agent, particularly in the case of water streams.

The primary criterion to identify fluvial environment and dynamics is based on facies identification which depends on the sedimentary environment. Sampling strategy and basic concept of pedosedimentary facies In this work, the unit scale that we adopt in our analysis is the series. It is sometime called layer or stratigraphic unit. Although the fossil time can be very short in some series compared to non-sedimentation time, series are homogeneous with respect to their sedimentary environments and their hydrodynamics.

The time inferred from the elementary sequence is called the ‘fossil’ time, i.e. the period of time necessary to set up the deposit. Based on a series, the sedimentary record conveys the dynamic time, that is to say the ‘fossil’ time added to the time separating two elementary sequences. The latter is present in the form of a hiatus and can be sometimes marked by ablation thus creating erosional microsurfaces (Reineck & Singh 1980). It has no stratigraphic signification and must not be confused with discordance.

Thus, they are representative of the type and frequency of the morphogenic events. The definition of different series relies on facies identification (see below). However, the facieses cannot be distinguished from each other based uniquely on deposit dynamics. The mineral composition (lithofacies) is also an element of importance to identify facies, like the presence of fossil fauna (biofacies), but it is absent form the continental deposits studied here.

Alluvial terraces are incised formations and thus are composite objects which designate both a deposit and morphology type. From the morphologic point of view, they

The pedological activity (pedofacies) is also at the origin of facies differentiation. Any deposit constitutes a topographical surface for a very variable duration, and 40

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

therefore at least has pedological microfeatures the nature and extent of which are also very variable. As a consequence, facies identification is made based on the pedosedimentary properties of the accumulations. This could be the source of ambiguities in the definition of stratigraphic units. Nevertheless the paleosoils we encounter here are for a large majority alluvial paleosoils on which pedological features have developed over relatively short periods of time of the magnitude of one century. They directly come after the setting up of elementary flood sequences. It results that acquiring surface pedological features (horizons A) is an intrinsic step in the formation of series or stratigraphic units.

This is the case for the rythmites which were collected in the plain of Famagusta and for some particularly well developed elementary sequences from the Gialias in Potamia. Identifying sedimentary environments : sediments and sedimentary structures The different sedimentation environments are identified by investigating the different components constituting the deposits (mineralogy and granulometry), their organization (bedding, granulometric classification, etc.), the geometry of the sedimentary units and the stratigraphic arrangement (Campy & Macaire 2003). The lack of fauna remains in this semi-arid fluvial environment makes the facies identification particularly important. For the littoral and marine deposits, the sedimentary environment identification also strongly relies on the paleoecological study.

The particular case of paleosoils which are constituted of several horizons must be noted. It is not systematically observed for all the Cypriot alluvial soils which are embrionic. When necessary, it is possible to separate at both the macro- and microscopic scales elements acquired during the formation of a series and elements imputed to migration from a horizon or summital layer. Enrichment of a pedological structure in organic material, i.e. of a sediment structure produced by pedogenesis, is acquired very near the surface. The underground redistribution of carbonates in the form of nodules or fossilized roots usually mark a B(t) horizon differentiation in the Cypriot paleosoils. The presence of this type of pedological feature with lower horizon is therefore not an environmental marker of the sediment deposit. In practice they do not constitute a key element to define a stratigraphic unit (Table 17).

Continental deposit typology: Mountain side deposits Colluvial structures are usually weakly organized, with a sandy loam texture, and with angular gravels in the PotamiaAgios Sozomenos sector. They can also be distinguished by their mineral composition which favours vicinal sedimentary sources of carbonated origin (calcareous rocks, marls and sandstones), opposed to endogenous rocks which originate from the watershed upper part. The sediment granulometry is related to the transport process operating on the mountain sides. The presence and individualization of sand lenses and sandy beds, i.e. their granulometry, the classification of grains and the lens size are related to the possibility of building a hierarchy of discharges on the mountain slopes. For some given substrates and slopes, the hierarchy will depend on the vegetation cover, the land use and the meteorogical events (Neboit 1983, Van Vliet-Lanoë et al. 1992, Devillers et Provansal 2003). The hierarchy of discharges ranges from diffuse water trickling, leading to colluvial sheet formation characterized by a fine granulometry without sedimentary facies, to concentrated rain washes generating rills filled with coarse material (sands and granules forming lenses) which are relatively well sorted (Coque 1998). The studied rills contain more or less calibrated and blunted ballast. The rill size rarely exceed one meter.

The micro-morphological description of the deposits (Bullock et al. 1985) is an efficient tool used to discriminate the characteristics specifically pertaining to deposit dynamics and pedological processes as well as the characteristics inherited from the mother rock or from an upper horizon. It also allows the validation of the pedological process uniformity within a stratigraphic unit.

Stratigraphic unit

The sampling strategy relies on identifying series using their pedosedimentary facieses. One sample is taken for each series in the limit of allowed weight for shipping sediments to France and of accumulation thickness. A subsampling taking into account the elementary sequences can be adopted in order to answer a particular problematic.

Definition Deposit feature (bedding etc.) Litho-facies

Upper and lower limits other unit Series Uncomformity

Pedo-facies (Horizons A) Facies

Topographic surface

Table 17: Stratigraphic unit definitions 41

4. Methodology: Defining and Identifying Chronostratigraphic Facts

Fluvial environments in semi-arid context Unlike temperate environments, the hydrosedimentary dynamics originating the fluvial plain aggradation in arid and semi-arid environment are still not extensively studied (Thomas 2000, Daniels 2003). For this reason, no exhaustive and reliable alluvium facies typology can be directly applied to this work. We therefore propose an original classification of this type of deposits, for the Cypriot water streams, the facieses and their hydrodynamic significations being discussed on a case-to-case basis.

This type of facies results from the incapacity of the water course to carry fine material towards downstream when the flood ends. The competency is sufficient to carry the gravel bank fraction as a whole over short distances but the briefness of the flood episode and the return to normal condition is sufficiently fast to make it possible for the sedimentary materials to be deposited without being sorted. The presence of large loam sheets on the channel bottom, whereas ballast and sands are in large quantities in the watershed, reveals that settling in the main water bed occurs very probably at the end of each flood episode.

After having envisaged the current forms and deposits as well as the validity of using them as Holocene reference framework, we intend to establish a typology for the different Holocene alluvial facieses explored during the drilling campaign in the Gialias watershed. The hydrodynamic interpretation of the deposits is discussed as well. In order to do so, we will study the channel deposits where discharges are most concentrated and perennial. The river bank deposits, indicating the limit between the river and the floodplain, will be addressed afterwards. Finally, the flooding deposits which are characteristic of the floodplain will be addressed at the end of the typological study. These deposits are the most abundant in the stratigraphic sequences and provide us with the best possible chronological resolution for the fluvialenvironment detrital records.

A bank incised in a regular 1.5 meter thick layer of pebbles encloses the active channel when the Gialias is not bordered by a more ancient and of consequent size alluvial terrace. When the bank is lower (lateral erosion, dismantlement), it is recovered by a fine layer of flooding loams (about 10cm thick). The loams mark the Gialias floodplain the lateral extent of which is small (a few meters). The spatial recurrence is also highly reduced. Most of the discharge occurs in the middle of the channel. Overflows are currently quasi inexistent. Inquiring the farmers and neighbouring inhabitants confirms this fact. Similarly in the sector of Potamia we have verified that the centennial precipitations of the year 2000 (more than 90mm in a few hours, the highest record in Cyprus since 1936, Cyprus Mail of 11 October 2000) did not generate real flooding of the alluvial plain. The flooded zones were then very limited and linked to land management artefacts, the road to Dhali and the Agios Sozomenos bridge.

The Gialias current hydrodynamic functioning The cartography of the active channel of the Gialias, which was made in May 2002 as it was dewatered, allows us to describe the current fluvial-sedimentary environments between Dhali and Potamia. The different morphosedimentary elements identified and their distribution result from linear discharges occurring within the channel. The recent incision of the river is materialized by the pools and the scarps within the channel that cut the marl substratum (Figure 26). It is also revealed by large surfaces from where the substrate emerges. The submersible dams that are present are progressively exposed by the incision dynamics. Thus, incision is measured to be more than 2 meters deep since 1958. The distribution of the incised surfaces occurs in function of the flow rates. The concave meander banks, the narrowing of the channels and the dam zones are environments where incision is the fastest (Figure26). The incision scarps appear more numerous as we progress downstream the Gialias.

The deposits which are present in the channel (Figure 26, Figure 27) also attest to the current intermittence of the water course. This underlines the lack of spatial discrepancy of the sedimentation processes between the Gialias main plain and floodplain. The flooding deposits (loams) and the deposits in the main plain (pebbles of MS facies) all gather in the Gialias active channel. As a consequence, the main plain and the floodplain deposits are in turns deposited in the active channel (Figure 27). This attests to the lack of real floodplain and explains the particular deposits found in the current channels. The granulometric and textural analysis of the different deposits within the channel show a bad sorting profile (I about 1.5), with medium sand mode (GA1.1, 2 and 3, Figure 27). From the viewpoint of textural analysis, this weak sorting capacity is found as well in places where all categories of granulometric fractions are encountered (ballast, sands and fine sands). Only loam samples from the pool filling up (GA1.4) show better sorting along with a strong negative asymmetry (Skewness –0.39), characteristic of decantation/settling processes. The sands within the alluvial terraces running along the Gialias border (Figure 27) exhibit a strong positive asymmetry revealing less sudden conditions of discharge than those observed today.

The locations of incision, where the streams are concentrated, are devoid of deposits because sediments can be transported further downstream. However, most of the pools are partially silted up by loams deposited by floods. The deposits are preferentially distributed in places where the stream is theoretically the weakest as in convex meander banks and channel enlargements. The deposits are of two types: (1) non-contiguous pebble banks mixed with sands; and (2) sometimes sandy loam sheets paved with pebbles form the second type. The pebbles attest to the discharge briefness. 42

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

Figure 26: Detailed Cartography of the Gialias active channel between Dhali and Potamia 43

4. Methodology: Defining and Identifying Chronostratigraphic Facts

(Figure 28, Picar et High Jr. 1973, Reading et al. 1986, Harvey 2000). The permanent water discharge efficiently transports fine materials (eluviation). This facies type is denoted ‘OF’ and is the type most commonly found in sediment stratigraphic records. The presence of a sandy or loamy framework in combination with pebbles at the head of a series can be explained by flood-end or particular postsedimentary processes.

The sediment facieses of the active channel of the Gialias, in particular the Matrix Supported Frameworks (MSF) of the pebble banks are unique with respect to all the Holocene deposits identified in sediment stratigraphies. The particularity of these deposits can be explained by the rainfall decrease which was recorded in Cyprus for more than 50 years (Hadjiioannou 1987). The water reservoirs built on the Gialias (upstream and downstream Lythrodhondas), in 1945 and 1952 have a cumulated capacity of 64 000 m3 (Water Development Staff 2001). This amount is more than 100 times lower than the mean flow rate of the Gialias in Nissou (upstream, close to Dhali). At the very most, the use of them can contribute to the river intermittent regime. It can also accelerate flooding events by attenuating peak floods at their beginning and end. But, the reservoirs must have a limited impact if we consider their current volume capacity. Finally, the exponential increase in drinkable water needs has resulted in the multiplication of well drilling. In the sector of Potamia-Agios Sozomenos only, we have established the cartography of 469 active wells the 2/3 of which has been created since 1960. The Cypriot reservoirs’ capacity has increased from 6 up to 303 millions m3 (Water Development Staff 2001). That capacity is still insufficient and the water consumption increase is mostly due to domestic and tourist needs (Water Development Staff 2001). The phreatic levels are constantly decreasing (Toufexis 1968) and concrete wells have been constructed in the middle of the Gialias channel. The water table is measured 6m deeper outside the rainy month period. At the beginning of the century, the construction of pools, that is to say, 1 to 2 meter-deep large excavations in the Gialias water bed have made possible the cultivation of fields. The phreatic table is sub-exposed during the whole year beside the summer period which begins at the end of June (Reid 1909, Hutchinson 1909). During this period water springs existing in the Gialias bed were still active (Reid 1909).

2) When pebbles rest on each others and interstitial spaces are filled with smaller pebbles or granules (Figure 28, filled framework), the deposit is then interpreted as corresponding to a reduction of the discharge power (Reading et al. 1986). This evolution is either said modal or tendential. These low flow channel facieses (FF) are just as well frequent. 3) Pebbles wrapped in a sandy or loamy frameworks in which coarse material elements do not rest on each others (Figure 28, matrix supported, isolated framework or bimodal), result from important discharges characterized by large loads of solid materials occurring over short periods of time and preventing the leaching and subsequent transport of fine grained sediments. This conveys a multimodal character to the processed and deposited forms of sediments which are sometimes described as being loose (Ramos & Sopena 1983, Steel & Thompson 1983, Harvey 2000). In semi-arid environments, these facieses can be present when powerful discharges of limited duration and carrying a strong load of solid materials occur (Flash flood). They can noticeably be compared to mud discharges (Mud-flow) and torrential lava. Karcz (1972) observed in Israel where a positive correlation between the morphosedimentary development of the low-flow channels and the duration of flood events. During flash-floods this type of structure develops only weakly. It is described only for desert fluvial environments when the rare activation of the wadis occurs (Glennie 1970, Reineck & Singh 1980). Their presence in Cyprus will be discussed further in this work.

The hydromorphological transformations which have occurred during the 20th century are certainly of multiple origins (climate, reservoirs and water consumption). They imply a more marked separation between the phreatic groundwater and the river thalweg. Thus, the recent deposits do not allow us to establish a valid reference framework for the fluvial environments at the Holocene. It cannot be inferred from the granulometric curves of the low-flow channel (Figure 27) when compared to the Holocene sediment facieses which we can envisage (see below). Combined together, all these factors contribute to the formation of a peculiar fluvial landscape.

The first two types of sediment facieses compare closely with each other and attest to the concentrated and competent nature of the low-flow channel discharges. The energy drop of the discharges, which makes possible the deposition of fine sediment in interstitial spaces between pebbles and originates the second facies, could denote a strong hydrological irregularity. Nevertheless, when the low-flow channel pebbles are coated by a fine layer of deposits after long term geographic or dynamic variation of the river, then the resulting facies is identical. As a consequence, it is very difficult to interpret the differences between the facieses of the low-flow channels 1 and 2 respectively when they have been at a certain period of time covered with another sediment formation. The third type of facies represents the ‘arid pole’ of the low-flow channel sedimentation. Postsedimentary perturbation can be the cause of it. The intensity and short duration of water discharges make it closer to an hydrodynamic functioning which is characteristic of arid environments.

Holocene deposits in the low-flow channel

Three main sediment facieses for the low-flow channel deposits are encountered in the Gialias watershed (Figure 28). 1) When the amount of sediment material available in the watershed makes it possible for us to observe the presence of low-flow channel deposits, they usually consist of open framework pebbles that are laid in banks or layers 44

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

Figure 27: Section profile and sedimentary study of the Gialias active channel 45

4. Methodology: Defining and Identifying Chronostratigraphic Facts

the river shows incised formations prior to the Holocene, exposing for a limited duration paleosoils rich in newlyformed gypsum. The thin layers of loams present in the embankments rearrange gypsum crystals and pedological aggregates which is not the case for layered loam-sands (Figure 29). The relatively good preservation of the aggregates, only the facets are rounded, reliably attests that they were not carried down to here by a flood. These deposits cannot be attached to flooding phenomena. They are therefore interpreted as resulting from lateral erosion of the floodplain which fossilizes the inward walls of the embankments during long low-flow episodes when water does not reach the embankments. This simple typology of the embankment facies characterizes two types of hydrologic functioning. The sand beds presenting with low dip angles and belonging to the second type B are formed by the same processes as those belonging to type A. However, their comparatively more reduced granulometry might attest to the presence of weaker hydrodynamic processes. These beds with low dip angles are separated by loams related to a relatively long period of low water height (Reid & Frostick 2000). Consequently the B type facies must be attributed to liquid flows much less abundant and sustained (intermittent). For a rank-5 water stream such as the Alykos in Potamia the facies is characteristic of a semi-arid Mediterranean environment.

Figure 28: Facies of the low-flow channel Sand and granule lenses of metric and centimetric sizes are interpreted as characterising intermittent and concentrated discharges of small magnitude on the slopes or in the context of secondary discharges in the alluvial plain. The difference between these two environment types can be easily established based on petrology and both the topographic and stratigraphic context of the sediment deposits. When the sand lenses are contiguous they can be interpreted as an indication of the presence of braided flows in the water stream.

Embankment deposits at the Holocene

The Holocene floodplain

Embankment deposits are rare. They can be identified by the presence of oblique sand banks (Figure 29, figure 30). Two types of embankments can be identified. The first type (type A, Figure 30) is constituted of parallel slanted sand beds resulting from both lateral and vertical displacements of the channel edges (Collinson et al. 1978, Reineck & Singh 1980, Reading et al. 1986). This type of structure can be the consequence of intermittent streams. However, in this case the water column must be high and the interannual variability relatively reduced (Picard & High J. 1973).

The Holocene flood deposits are abundant in the Gialias valley (Figure 31). They can be recognized by their loamy-sand or sandy-loam texture and by the presence of beds or horizontal lamina of sands and loams. The relative differences between the lamina can be tiny. At the macroscopic scale, they are sometimes underlined only by the sediments platy parting deposited on exposures. These laminas can possibly be perturbed by biological (bioturbation) or human activity (anthropoperturbation), but remain recognizable when observed under the microscope. They can be affected by pedogenic phases. The relatively good preservation of sand and/or loam beds during the entire Holocene underlines the low level of biological activity pertaining, in general, to semi-arid and fluvial environments, and in particular, to the Cypriot environments.

It follows that sedimentation occurs on embankments during floods which are afterwards affected by erosional microsurfaces as it is showed by the sometimes visible interlaced sand beds. The sand beds and the erosional surface are directly linked to the water column variations in the channel (Reading et al. 1986). In view of the studied bioclimatic environment, this type of facies therefore results from a relatively abundant and sustained hydrodynamic regime.

Downstream the watershed, in the plain of Famagusta, the 9m thick alluvium layer crossed-examined by many drillings exhibits a quasi exclusive loam-clay texture. This texture can be attributed to decantation processes and also, but not uniquely, to a reduction of deposit granulometry size downstream the watershed. As it is shown by the drilling locations within the low-flow channel, the valley enlargement in this region makes possible the development of a genuine floodplain. A small dislevelment of about one meter delineates the main floodplain (exceptional or distal) from the main floodplain (modal or proximal). Because of the small level drop, differentiating the alluvial plain on 1/50

The second type of embankment deposits (B) is characterized by oblique beds of finer granulometry (sandloam texture). Another difference is the presence of long and thin layers of loams tightly covering the embankment walls (Figure 30). These loams are mineralogically and granulometrically similar to the sediments directly outward the embankment in the floodplain proximity. For the Alykos upstream Potamia (section AY7), for instance, 46

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

Figure 29: The paleo-embankments of the Gialias

Figure 30: Embankment types and hydrologic dynamics 47

4. Methodology: Defining and Identifying Chronostratigraphic Facts

Figure 31: Flooding deposits in the median sector of the Gialias watershed

48

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

Figure 32: Fluviatil deposit microfeatures

49

4. Methodology: Defining and Identifying Chronostratigraphic Facts

Figure 33: Facies and hydrology dynamics of the Gialias median-sector floodplain 000th scale topographic maps is a difficult task. Only a rapid reconnaissance limited to the British military enclave of Dhekelia, and the oblique aerial imagery performed during the spring 1990 flood (Hennesy 1991) makes possible to define the floodplain separation limits. Some 19th and 20th century engineers (Lt. Kenyon 1879, Hoberhummer 1903, Hutchinson & Cobham 1909) have described proximal sandy deposits near Famagusta. This is therefore another indication that sand-loam sediments obtained from the cores truly mark the decantation processes at the edges of the alluvial plain and not a granulometric decrease of the sediments from upstream to downstream.

Figure 31). However, in most cases, flood sequences cannot exhibit granulometric sorting without being attributed to post-sedimentary processes such as bioturbation (example 3, Figure 32). For flooding deposits in the sector of Agios Sozomenos-Potamia, three main facieses are identified: (1) the positively sorted alluvium; (2) those presenting parallel bedding; (3) and the sandy loam deposits which do not exhibit sedimentary or microsedimentary features. These facieses can be identified at the macroscopic scale (Figure 31) and/or at the microscopic scale (Figure 32). The fine distinction between the different fluvial environments is usually based on comparing sediments originating from a gradual suspension process with sediments originating from a uniform process. The first category is deposited according to a positive granulometric sorting whereas the second category is not structured and usually contains finer particles (Passega 1957 and 1963 for marine environments, Bravard 1986, Salvador et al. 1993, Arnaud-Fassetta 1998 for the adaptation to fluvial environments). The argument based on the water column height is often put forward in order to explain the difference between the gradual suspension and the uniform suspension (Passega 1963). In the first case, the height is large enough to allow the vertical separation of the different granulometric fractions. The fine materials are suspended

Upstream Potamia-Agios Sozomenos, the floodplain deposits are never uniquely composed of clayey loams. The sands are always present in variable proportions in relation with the floodplain reduced lateral- extent. The extent of the alluvial valley delimited here by morphogenic inheritances does not let a genuine distal plain exist. The deposits, only resulting from decantation processes, are sporadically present under the forms of beds or laminas (Figure 32). These beds or laminas more often represent the part (Figure 32) of an elementary flood sequence (Macaire 1990). When an elementary sequence is totally or partially preserved, the loams and sands can be distributed according to a positive vertical granulometric sorting (example 3, 50

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

in the upper part of the water column where the laminary flow prevents the transport of coarse grained particles at that level (Visher 1969, Woodroffe 2002). On the lower part of the water column, the turbulences generated by the friction of water on the channel bottom allow the suspension of the sands (Visher 1969). The column height is of lesser importance for a uniform suspension in the energetic flow mode because of turbulences being present over the entire height of the water column (Passega 1963). In the case of a floodplain in semi-arid environment the facies spatial and temporal distribution cannot be uniquely imputed to the water column height.

is conditioned by the morphogenesis of the alluvial valley. Thus, Blair (2000) observes coarse-grained sediment deposits originating from flash floods upstream Californian watersheds whereas the same type of floods generate loam deposits with differing sedimentary features (Sheet flood) downstream where the alluvial plains considerably widen. This could be the case as well for the Gialias watershed where discharges in the median sector are laterally restricted by Holocene and pre-Holocene morphogenic inheritances. Parallel bedding deposits A large part of the deposits in the Gialias floodplain, as in other similar hydrodynamic systems (McKee et al. 1965, Picard & High 1973), is composed of an accumulation of parallel and horizontal beddings made of sandy loams. For a system exhibiting strong hydrological irregularity and according to the nature and the granulometric profile of the sediment deposits, this type of sedimentary feature can be attributed to different fluviatil sedimentation environments. The topographic and stratigraphic location as well as the high percentage of fine material particles allows one to distinguish easily the flooding deposits (Daniels 2003). This type of sediment facies is often encountered in semiarid environment and compared to flash floods (Reading et al. 1986, Daniels 2003). The hydrologic characteristics leading to this type of deposit seem nonetheless less stringent than those necessary for uniform suspension. This situation seems to form an intermediate case between two types of hydrological functioning represented by the uniform and gradual suspension respectively (Figure 33).

The deposits resulting from these different transport modes and environments are detailed in the following sections through the description of their sedimentary microfacies. Uniform suspension deposits As shown in the example of the uniform suspension in Figure 32 (3rd example), the weak granulometric sorting but also the abundance of loams and clays in combination with sandy grains, sometimes of larger dimensions, bring to the fore other flood related phenomena. The mixing of a large fraction of fine materials with relatively coarse grained sands occurring during the entire flood event tends to prove that the streams are either too loaded with materials in suspension, or too short-lived to be noticeably able to evacuate the fine sands downstream. The mixing of the sandy and loamy phases prevents us from finding a unique explication for the weak competence of the discharges. On another scale, but similarly to the low-flow channel deposits (Figure 28), the presence of fine particles between the sand grains can be explained by invoking post-sedimentary phenomena (fine materials are deposited after sand deposit). It can be explained here by the sudden arrest of discharges, all the particles present in the water column being brutally deposited thus preventing any true structure from forming. The hydrological characteristics which can be deduced from the facies descriptions are brought together by various authors (Reineck & Singh 1980, Thomas 2000, Phillips 2002) who have depicted the flash flood phenomenon. These characteristics are thus of importance as far as the solid load, the short duration of the flood event and the hydrological intermittence are concerned (Figure 33).

Positive-vertical granulometrically sorted deposits For the deposits resulting from gradual suspension (Figure 32, examples 1 & 2), the hydrological parameters (water column height, periodicity and discharge energy drop) cause the transport of fine materials downstream at the beginning of the flood sequence. The loams and clays are here comparatively less present. At the top of the elementary sequence, the discharges are not anymore energetic enough to be able to collect the sands. Then, the fine material deposits amount for almost the whole of the elementary sequence. In order to produce this type of facies, for a same watershed the period of discharge must be longer, the drop of energy less brutal and the water column higher (Reading et al. 1986, Blair 2000, Jones et al. 2001, Campy & Macaire 2003) than for the uniform suspension facies of the floodplain (Figure 33).

In the limits imposed by numerous static (sediment sources, topography, etc.) and dynamic (climate, vegetation, human impact, etc.) parameters which substantially vary from a hydrodynamic system to another, the uniform suspension facies can be also compared to overflow deposits occurring during flash floods (Picard et High 1973, Reineck et Singh 1980, Blair 2000). In some particular contexts, they are utilized as markers of semi-arid fluvial environments (Smith et al. 1993). However, it must be noted that a facies type for flash floods in the context of alluvial plains does note exist (Daniels 2003). If this type of flood flow is of climatic origin (convective storms in arid environment, hydrologic intermittence), its pedosedimentary expression

Ordering and facies of littoral and marine deposits The sequential stratigraphy model The littoral deposits situated downstream the fluvial system have been studied by carrying out drillings in the basal plain of the Gialias near Famagusta. The deposits are ordered according to the classic model used in sequential stratigraphy which synthesizes the stratigraphic architecture of the continental margin deposits during a eustatic cycle. The sequential stratigraphy technique allows 51

4. Methodology: Defining and Identifying Chronostratigraphic Facts

Figure 34: The sequential stratigraphy model for facies lateral variations in isochronous continental deposits (alluvium) up to the continental margins (distal marine deposits). In the context of a sea level rise for the last 18000 years, three sedimentary bodies have been individualized (Figure 34). From the position of a low marine level, the prograding sediments form a low level prism. During the sea level rise the sediment suite move towards the continent (retrogradation) and give rise to what is called the transgressive interval. Finally, when the sea level stabilizes, the sediments prograde again and thus form a high level prism (Cojan & Renard 1997).

surface takes place within the paleovalleys according to their morphology (Baeteman 2004). Thus, the low level sediment prism which is absent and the transgression surface is materialized by the discordance of the Holocene deposits in the valley. In Cyprus, these deposits directly rest on Tertiary or Pleistocene rocks (Figure 35). This configuration was also evidenced for several rias in the Mediterranean basin (Dubar & Anthony 1995, Brückner 1997, Kayan 1997, Kayan 1999, Borrego et al. 1999, Brückner 2003, etc.). The distribution geometry of the Holocene deposits depends strongly on the paleovalley shape. In the case of the Gialias ria, the rocks of the Pliocene and Pleistocene which are close to the exurgence of the present time form ripples (altitude between 5 and 10m a.s.l.) oriented north to south perpendicularly to the river axis and opened by narrow passes at the ria outlet (Figure 35).

It must be noted that the two surfaces are of particular interest from the chronostratigraphical and paleogeographical point of view. The transgression surface is a topographic surface on which sediments from the transgressive interval are deposited. The term ‘transgressive’ implies an ambiguity in the model. Indeed, even though this surface is progressively submerged by marine sediments it is however only partially. The alluvium from the transgressive suite also fossilizes this surface. Thus, the transgression surface comprises a marine section as well as a continental section (Figure 34), and therefore materializes the discordance of the retrograding deposits. The maximal flooding surface separates the transgressive interval from the prograding sediment suite. It thus corresponds to an inversion of the transgression and progradation dynamics and represents the maximal extent of water, most often in the form of a ria.

The regressive erosion which takes place during the low level period (Stage 5, Würm) did not last sufficiently long to let the water streams reach their equilibrium profile (Figure 35). It results that the ria catchment basin exhibits a pronounced slope on which the transgressive interval develops. Another important consequence is the presence of a discontinuity in the slope along the fluvial valley where the alluvia are shaped in terraces elevating several metres above the littoral plain. This abrupt elevation drop of the fluvial system is more pronounced for the isochronous detrital sediments of the transgressive interval (Figure 35).

Once the model is described, it is appropriate to determine certain restrictions pertaining to the concerned period (Holocene) and the morphogenic context (ria). The Holocene deposits in the Famagusta plain do not rest on the continental escarpment or ramp but on the bottom of the ancient fluvial valley (Figure 35) formed during the last low-sea level period (stage 5). The transgression

Finally, the rapidity of the Holocene eustatic rise (5th order cycle) involves a limited number of the so- called parasequences with respect to the EXXON model (Vail et al. 1977) which refers to the geological time scale (1rst and 2nd order cycles, Cojan & Renard 1997). 52

Holocene Morphogenesis and Anthropisation of a Semi-Arid Watershead, Giallias River, Cyprus

Figure 35: The sequential stratigraphy model applied to the Holocene ria of the Gialias

These observations are confirmed by data obtained from the core drilling campaign carried out in the littoral plain (see further below) and allow us to better define the different sedimentation environments present in that sector. These data can be gathered in 4 large families:

The sediments found in lagunary environment are polymorphic. Their colour can attest to the presence of either reductive or oxidative processes in relation with estival dewatering of the lagoon. When this drying is marked, the presence of halites can be recorded. The presence of organic matter can also result in a brownish colouring. Here, the sediment texture and granulometry must be also related with the decantation processes (negative asymmetry, fine texture). The reliable determination of this type of environment in the geographic context of our study can be carried out via the fauna study (ostracods and macrobenthos).

(1) the fluviatil facieses which characterize the typography before the burying of the valley and possible reworking; (2) the alluvia from the trangressive or low level prism; (3) the littoral deposits present in the sedimentary suite and which can be materialized in various sedimentation environment (littoral zone, lagoon, etc., Dalrymple et al. 1992, Woodroffe 2002); (4) the infralittoral marine deposits of detrital origin.

The littoral ridges are formed by rather coarse grained sands which exhibit good sorting and positive asymmetry (Skewness) resulting from marine leaching processes (Visher 1969, Fan et al. in press). The marine fauna is there also present but often reshuffled (Carter & Woodroffe 1994, Vella 1999).

The Holocene transgressive (or retrograding) deposits are identified according to several criteria. From a stratigraphic point of view, they are positioned between a continental Pleistocene topography and deposits strictly of marine origin. From a sedimentologic point of view they are close to usually reworked fluvial sediments which contain pebbles and granules and often rich in marine fauna.

4.1.2 Sediment granulometry Methodology The granulometric analyses are performed on the sandy fraction. Their aim is to characterize the deposit environment and its dynamic relative variations. It must be noted that pedogenic processes do not affect much of the sediment texture and granularity at the scale of a whole series. Observation under the microscope has never revealed the presence of illuviation phenomena in the analysed samples. These phenomena are related to the vertical circulation of water in soils (vadose environment) which is not a marked phenomenon in semi-arid environments.

The marine silts of detrital origin quasi-exclusively exhibit a loam clay texture of dark greenish colour (olive 5Y 5/3, olive grey 5Y 4/2). This shows the prevalence of reductive processes, the marine fauna being largely present. In some sections, a succession of light beds, rich in calcium carbonates alternating with dark and more organic beds can be compared to the seasonal rhythmicity known for this type of environments (Dubar et al. 2002, Fan et al. in press). 53

4. Methodology: Defining and Identifying Chronostratigraphic Facts

The processes of pedogenesis which help the carbonated encrustation of fine materials are present at various levels in the sediments. The fine particles form aggregates in a carbonated cement framework which forbids detrital dynamics analysis in fine material granulometric sequences. Similarly, the great variability of the encrustations as well as the randomized partitioning during the screening and the destruction of organic matter did not allow us to evaluate this bias in the granulometric analysis sequence. The detrital material of carbonated origin is abundant in the watershed (see above). The carbonate destruction by acidic attack (HCL) does not seem then an adequate hypothesis here. The granulometric analysis of the loams and clays cannot be therefore interpreted easily and has not been systematically carried out.

In this work, the mean grain is expressed in m, i.e. (P84+P16+P50)/3 where P16 represents the percentile 16 in m. Sorting Index (i) = (84- 16)/4 + ( 95 - 5)/6,6 Skewness symmetry index (Ski) = ((84+ 16)-2 50)/2(84- 16)+(( 95+ 5)+2 50/2(95- 5) These indices assume a Gaussian distribution of the particle size (Folk & Ward 1957). As a consequence, it must be noted that the indices reliability is less important for the overall sample granulometry than for the sole sand samples. Indeed, utilizing two different methods (dry sieving and laser granulometry) artificially leads to a bimodal sample distribution.

The sample treatment is carried out in several phases. Humid sieving enables one to separate the coarse material fraction (ballast >2mm), the sandy fraction ([2mm;50m]) and loamy clay fraction (