SEMPERE, THIERRY, Convenio YPFB-Orstom, Santa Cruz, Bolivia

ABSTRACT: Phaneorozoic Sequence Stratigraphy of Bolivia and Adjacent Regions

Bolivia has been in relation with the nearby Pacific margin of western South America throughout the Phanerozoic, and presents the prime interest of possessing an almost complete Phanerozoic record, which is now relatively well known thanks to numerous new data, mainly unpublished. Sequence stratigraphy of these strata permits to propose new interpretations and hypotheses.

The Paleozoic margin was initially passive (late Cambrian-Llanvirn, "Puna aulacogen"), but became active during a middle Ordovician compressional episode. Most of late Cambrian to early Triassic Bolivian rocks are of marine origin, with dark shale units recording sea level rises, whereas middle Triassic to Recent rocks were mainly deposited in continental environments (except six restricted-marine ingressions in the late Cretaceous-Danian, and one in the late Miocene, all with hydrocarbon potential). A noteworthy similarity exists between the Devonian to Jurassic stratigraphies of Bolivia and the Parana basin, suggesting that Bolivia behaved as part of the Brazilian craton from late Cambrian to late Jurassic, when it was captured into the Pacific margin geotectonic system.

Organic-rich units correlate with Paleozoic highstand deposits and younger ingressions. The Bolivian Phanerozoic record is characterized by high thicknesses, partly due to middle Ordovician-Carboniferous and late Cretaceous-Cenozoic foreland basins. Paleozoic foreland geometries include northeastern onlaps, and hence possible stratigraphic traps. Generation, migration and trapping of hydrocarbons depended mainly on Cenozoic structural loading and burying, and hence on propagation of Andean deformation, which used several Paleozoic shale units as decollements. Precise knowledge of the evolution of the Phanerozoic geodynamic contexts and basin geometries through sedimentation and subsequent deformations is crucial for hydrocarbon exploration strategies in these regions.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.