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AAPG Bulletin

Abstract


Volume: 54 (1970)

Issue: 5. (May)

First Page: 735

Last Page: 757

Title: Outline of Tectonic History of Bolivian Andes

Author(s): H. H. Lohmann (2)

Abstract:

Stille's chronologic sequence of events for the "typical" tectonic cycle does not apply in the Bolivian Andes because (1) the deep-sea sedimentation phase was limited to the Ordovician through Devonian, (2) the batholiths are Triassic, and (3) the principal folding phase was late Tertiary.

Cambrian and older rocks are poorly known in Bolivia, but an Ordovician-Devonian cycle of marine sedimentation is well established. During this cycle, up to 10,000 m of mud, silt, and sand was deposited in the Eastern Cordillera. A Silurian glacial episode took place on the Altiplano massif west of the Eastern Cordillera. The marine Upper Devonian is overlain by another glacial sequence--the Carboniferous Gondwana Group; as the age of the oldest Gondwana beds is not known, the beginning of this glacial cycle cannot be dated precisely. The Gondwana cycle terminated in Bolivia with a marine transgression that deposited the Copacabana Group of Early Permian (Wolfcampian) age.

Salt stocks, presumably containing salt of Permian age, are present along the eastern margin of the Altiplano, between it and the Eastern Cordillera.

Triassic time is represented by a sandstone sequence in the high Andes and by a sandstone-limestone-salt sequence in the southeast. Granite intrusion took place in Late Triassic time (180-199 m.y. ago), but was not preceded by any important folding as it should have been if Stille's tectonic cycle were valid. There are similarities between the granite and salt intrusions.

Jurassic strata are scarce in the Bolivian Andes. They might be represented partly by nonfossiliferous sandstone and conglomerate. The post-Jurassic section consists mainly of Cretaceous through Miocene continental deposits, although two marine incursions are known to have occurred during Cretaceous time.

The principal folding of the Bolivian Andes was completed by Miocene or Pliocene time, and the vertical uplift which formed the present ranges began. Almost all structural evidence shows a history of compression in the area until the Pliocene uplift.

Holocene uplift movements are described from the La Paz and Santa Cruz areas (western and eastern edges of the Bolivian Andes respectively).

Petroleum, tin, and copper prospects are discussed.

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