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The basin of Lake Titicaca in the high plateau of the Central Andes lies across the border between Peru and Bolivia. This memoir based mainly on the author’s observations during approximately 1 year in the field describes in reconnaissance detail the geology of the basin. The area is complex and forms a geological province distinct from adjoining Andean areas to the south and northwest.

A general areal map covering about 58,000 square kilometers was prepared (scale 1:500,000) and a small area of some 200 square kilometers was mapped in detail (1:30,000) at Pirín, highest oil field in the world, northwest of Lake Titicaca. Significant observations have been documented in numerous measured stratigraphic sections and structural profiles.

The sedimentary sequence aggregates more than 20,000 meters exclusive of a nearly unknown Lower Paleozoic succession (Cambrian, Ordovician and Silurian) on the east flanks of the Andes north of Lake Titicaca. The studied sequence includes dominantly marine rocks of the Lower and Middle Devonian (3,000 meters), Lower Permian (1,800 meters), Upper Jurassic (1,200 meters), mixed marine and continental Cretaceous (4,000 meters), Lower Tertiary continental clastics (7,000 meters), and younger Tertiary and Quaternary volcanics (4,000 meters). The volcanic rocks are divisible into two series, (1) a thick sequence of older folded and faulted rocks, and a younger group of essentially horizontal beds contemporaneous in origin with the great volcanoes of the Western Cordillera.

Six orogenic cycles are inferred from the stratigraphic record. These were (1) near the close of the Paleozoic (?), (2) in the Late Jurassic, (3) near the close of the Medial Cretaceous, (4) near the close of the Late Cretaceous, (5) in the Miocene (?), and (6) in the Pliocene. The present elevation of the Andes is the result of Pleistocene—Recent epeirogenesis.

Structural effects of the first and fourth orogenies are most marked in the high areas of the Cordillera Occidental. The second and third appear to be limited mainly to the middle ranges and the Cordillera Oriental. The fifth was responsible for plication of a broad belt extending from the Pacific coast to the Brazilian shield, and the sixth is expressed mainly in structures of the-area that drains into the Altiplano. It appears that the coastal geanticline (Cordillera Costanera) is pre-Car-boniferous in age. The Cordillera Occidental first came into existance near the close of the Paleozoic or no later than the Late Jurassic, and the Cordillera Oriental appeared in the Cretaceous.

Although large scale overthrusts generally have not been recognized in the central Andes and their existence has categorically been denied by Arnold Heim, on conclusive evidence the basin of Lake Titicaca lies within two opposed sets of overthrusts. Compression was from the northeast and southwest. The evidence for these opposing movements is expressed not only in overthrust faults but also in abundant overturned and recumbent folds in which the axial planes commonly dip away from the axis of the basin at angles of 45° and less. The Devonian, Middle Cretaceous, and Tertiary rocks vary greatly in structural competency because soft shales, gypsum, and, locally, some salt occur in the Cretaceous. Thrusting has been localized along the base of the resistant Tertiary and near the upper surface of the resistant Devonian rocks; consequently, the Cretaceous rocks are folded and faulted into extreme disorder with extensive overturned isoclinal folds having been mashed between competent Tertiary and Devonian beds of much simpler structure.

Four regional thrust zones and several local thrusts have been recognized and mapped.

The present topographic basin now occupied by Titicaca appears to be the result of block faulting and stream erosion. Studies of the configuration of the lake floor based on published bathymetric maps considered together with the surface geology, suggest that a high-angle fault divides the lake along a northwest axis into two approximately equal parts and brings Devonian and Tertiary rocks in contact. South-westward-facing escarpments on the lake floor near the northeast margin of the lake have the aspect of fault scarps.

Lake Titicaca is very deep, 281 meters, in front of the submerged scarp as compared with 40 meters and less in the drowned rock gorge at the Strait of Tiquina, outlet for the main basin of the lake. Very recent faulting on the floor of Titicaca is suggested. Extensive erosion of Pliocene or early Pleistocene lake clays around the lake indicates that comparatively recently the water level fell more than 100 meters and, subsequently, several cubic kilometers of these clays were transported to the deeper part of Lake Titicaca.

Most of the igneous activity of the region occurred during the Tertiary. Diorite dacite, trachyte, andesite, and basalt are well represented, with very minor amounts of more acid types. Most intrusives are hypobyssal and, judging from the field relations, appear to be deeply eroded stocks and necks. These probably date from the older stages of vulcanism.

Underlying a thin mantle of glacial outwash, and therefore older than the late Pleistocene, extensive nearly horizontal lake clays rise topographically more than 100 meters above the present surface of Lake Titicaca. These beds are best developed west and north of Lake Arapa. They are interpreted as relics of an ancestral lake, late Pliocene or early Pleistocene, which formerly occupied a much more extensive area than Lake Titicaca.

Terminal moraines and small cirques are first encountered about 200 meters above the lake on the northeast side of the basin, and considerably higher on the south-west side. Even outwash deposits do not generally reach the shore of the present lake although they do reach the highest shore line of the ancestral lake. Probably the lake has stood near its present level since the Pleistocene, as is shown by a well defined strand line about 8 meters above the lake surface. The lake level has fluctuated a maximum of 3 meters within the past 15 years.

A chapter is devoted to the history of Pirín, the highest petroleum field in the world, and analysis is made of petroleum prospects in the region.

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