Landlocked Bolivia, fourth largest country in South America with an area of 420,000 square miles, extends from approximately 10° to 22° south of the equator and from 58° to 70° west of Greenwich.

End_Page 408------------------------------

Main morpho-structural elements in Bolivia from west to east are the following.

1. The Cordillera Occidental which exposes Tertiary and younger extrusives.

2. The Altiplano depression with thick Tertiary and Quaternary sediments and extrusives in a probable downthrown block.

3. The Cordillera Real, "backbone of the Bolivian Andes," with Tertiary or Mesozoic granitic and quartz monzonite intrusives forming peaks exceeding 21,000 feet and containing the rich tin-mining districts of Bolivia. A southern continuation of this element (without granite) is called the Cordillera Central.

4. The Cordillera Oriental consisting mainly of Ordovician and Devonian sediments. Carboniferous, Cretaceous, and Tertiary sediments are preserved in synclinal cores.

5. The Subandean zone or Andean foothills generally consisting of marine Devonian and continental post-Devonian sediments. The producing oil fields of southern Bolivia are located within this zone.

6. The open country called the Beni plain and Chaco plain in the north and south of Bolivia, respectively, present site of oil exploration activity.

7. The Brazilian shield exposing Precambrian basement.

The Paleozoic section in Bolivia includes rocks from all periods and has a maximum thickness on the order of 40,000 feet. Cambrian, Ordovician, Silurian, and Devonian dominantly shallow marine shales and sands were deposited throughout the Andean and Pampa areas and along the Brazilian shield. At the top of the Ordovician is an unconformity with apparent southward truncation of upper, middle, and lower Ordovician beds. A thin but widespread layer of glacial origin occurs at the base of the Silurian. Sediments of Lower, Middle, and Upper Devonian age conformably overlie the Silurian.

The Permo-Carboniferous sediments consisting of alternating glacial and interglacial deposits of continental origin in southern Bolivia. These become increasingly more marine toward the northwest. Lower Permian limestones known in Peru extend into Bolivia as far as the Subandean zone.

Post-Permian continental type deposition follows; however, age distribution is difficult to establish. A large gap in sedimentation probably exists between Permo-Triassic and Upper Cretaceous. Age of the Vitiacua limestone is questionable.

Thick continental Tertiary deposits fill parts of the Subandean zone and the Chaco and Beni plains, and are known in the synclines in the southern part of the Eastern Cordilleras. A Tertiary sequence of a different type has been found beneath the Quaternary of the Altiplano.

Basalts of Cretaceous, late Tertiary, and Quaternary age are present. Permian basalts found in Peru are rare or absent in Bolivia.

The conspicuous elbow of the Andes in the Arica-Santa Cruz corner possibly had its origin in Precambrian time and has guided the tectonics ever since.

A transcurrent fault zone is believed to have influenced Bolivia tectonics between Corumba-Santa Cruz and probably continuing through the Cochabamba-Oruro areas to the Chilean coast near Arica.

The present Bolivian Andes are the product of a late Tertiary orogeny. By comparison, the earlier orogenies of the Mesozoic and Paleozoic exerted but a mild effect on the structure of Bolivia.

Although lateral compression is generally believed responsible for the folding and faulting of the Andes and of their eastern foothills, some geologists now strongly postulate mostly vertical uplift created the present picture of this impressive mountain system in Bolivia.

End_of_Article - Last_Page 409------------