International Geological Congress "Report of the Eighteenth Session, Great Britain, 1948" Some outlines on the tectonics of the Upper Amazon embayment

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INVENTARIO

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W. RUEGG and D. FYFE Peru

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Repritited from International Geological Congress " Report of the Eighteenth Session, Great Britain, 1948/' Part VI

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SOME OUTLINES ON THE TECTONICS

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OF THE UPPER AMAZON EMBAYMENT '

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By W. RUEGG and D. FYFE •

Pero

ABSTRACT

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That portion of Peru lying East of the Andes was affected by various tecton1c movements at different times, resulting in the present picture of a widely folded, thrusted and probably block-faulted region. Chief among the orogenies were the Palaeozoic and late Tertiary paroxysms which generally led to strong tilting and thrusting in this sector of the present Andean Cordilleras, and to more gentle surface crumpling on conjectured mobile blocks or block-systems in the easternmost spurs of the Cordillera Oriental. An important feature of this basin is the occurrence of large stratigraphic gaps revealing both conformable and angular contacts between the Lower Cretaceous beds and underlying formations which are usually much older. An attempt is made to explain such phenomena, by the difference in age, mobility and position of the individual blocks. There are also many data on the lithological characters of the formations present and brief notes on their respective facies types. •

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I. UPPER UCAYALI-URUBAMBA REGION

(a) Boquerón del Padre Abad.-This region is situated west of the Río Ucayali, lying on tbe highway between Tingo Maria and Pucalpa. As a whole· it is stFongly folded and faulted, and in the Boquerón gorge the main fold is a complex zone consisting of a s· equence of Upper Triassic and Liassic limestone (Santiago Forn1ation) forming the core of the structure, followed by the Boquerón sandstone and conglomerate series of Jurassic, very probably Kimmeridge-Tithonian age, by Cretaceous sandstones and shales, and the Tertiary Red Beds (Rüegg, 1947). The entire section is steeply tilted and overturned towards the east, the core showing strong thrusting in the same direction with slices and wedges squeezed away. On the eastern limb there are various steep to vertical faults with. displacements presumably of c.onsiderable stratigraphic throw, involving all formations, viz., revealing fault or thrust contacts of the Santiago fü-rmation with the Tertiary Red Beds, etc. Strong tectonic distortion and disintegration conceals the relationship between the individual series, but it is likely that there exists sorne angularity between the .Mesozoic formations though these depositional contacts were not examib.ed in detail. (b) Ganso Azul Field-Sira Mountains.-As known from the Ganso Azul Well No. 1 there exists apparent concordance bet�een the Permian fusulinid limestone and the overlying Cretaceous sediments, the dip being absolutely unaltered and flat throughout. This may hold good for the nearly level cul... miríation plane of the dome while conditions might be different downflank. · In the northern portion of the Sira Mountains (Aymeria creek, etc., east of the lower Pachitea river) the investigations corroborated the presence of the supposed structural and stratigraphical concordance found in the Ganso Azul field. Here thick and coherent exposures of greyish Carboni­ ferous limestone are normally overlain by blue and black limestone layers of very probably Jurassic age which in}urn are followed by the Agua Caliente sandstone and s�aJe ªssigned to the Neocomian. There is no évident change in dip and strike in �this section, except tl�ai· ·á\irery thin pebble bed is observed resting on top of the blackish lünestone indicating a short erosiona! stage before the Cretaceous vvas· deposited. . (e) Río Urubamba . Very striking is the section of vertical beds marvellously exposed in the Pongo de Mainique where Devonian (?) graywacke is in straight, clean-cut contact with Permian

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PART VI:

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THE GEOLOGY OF PETROLEUM

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PUNO'

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FIG. l. 78

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_ AYMEN T UPPER AMAZON EM B

RÜEGG AND. FYFE:

limestone, qnd this jn turn with sorne Ctetaceous . ·sandstone and the Tertiary Red Beds (Heim, 1948). Here the different gaps are by far greater than·at ,Ganso Azul and definite field evidence excludes any ps��do... concordance or faultíng, and may indicate a one-time folding. of the entire sequence in youngest Tertiary times. Such a hiatus wíth no distinct structural discon.formity would thus tend to prove' that no decided movement ·of the· area or block covering the region between Ganso Azul and the·; Río Urubamba� except perhaps gentle emergence and submergence, had taken place befote the violent · Pliocene or " Q.uechua" folding. * ""'

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CONTAMANA HILLS AREA

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T4e Contamana Hills lie sorne 25 kilometres east of the town of that name and range notth-westsouth-east, approximately parallel to the Uc�yali river. They form a sharp westward facing ridge with elevations of 300 to 450 metres above the level of the surrounding low rolling jungle pl:ains. They extelid from about 7°S. Lat. to 7°30'S. Lat., and from about 75° 05'W. L