Phanerozoic Evolution of Bolivia and Adjacent Regions

Abstract

The geologic evolution of Bolivia and the central Andean system during the past 500 m.y. was largely controlled by the geodynamics of the South American margin of western Gondwana. The Phanerozoic strata were deposited in mainly marine environments until the Early Triassic, after which continental environments predominated. However, there were six restricted marine transgressions in the Late Cretaceous-Danian and one in the late Miocene. The Late Cambrian-Early Ordovician margin was initially a passive margin. It became an active one during a Middle Ordovician compressional episode and was controlled by large-scale transtensional or transpressional conditions from the Late Ordovician to the Triassic. The Late Ordovician-Mississippian evolution was characterized by vigorous subsidence of the marine foreland, which was filled with thick, shallowing-upward sequences showing northeastward onlaps. Ashgill and latest Devonian-Mississippian glaciomarine and fluctuating sea level processes are recorded in the succession. Shallow marine carbonates, marls, and sandstones, as well as some evaporites and eolianites, were deposited during Pennsylvanian-Early Triassic time. After Middle Triassic rifting was aborted, the Bolivian basin behaved in a cratonic way until it was caught up in the Andean system due to the onset of transtension along the margin in the Late Jurassic. It became part of the Andean foreland domain in early Senonian time. Andean thrust deformation propagated into Bolivia from the west in the late Oligocene and progressed eastward through Neogene time. Organic-rich units correlate with Paleozoic highstand deposits and younger transgressions. Generation, migration, and trapping of hydrocarbons depended mainly on Cenozoic sedimentary burial and tectonic loading and hence on propagation of Andean deformation.