Late Paleozoic carbonates and glacial deposits in Bolivia and northern Argentina: significant paleoclimatic changes

Abstract

In Bolivia, a marked climatic paleogradient (from west northwest to south) is visible in the Carboniferous depositional systems. In the northwest is the Pangean trend, a warm-water Pennsylvanian and Permian succession (preceded by a Late Devonian glacially derived rock assemblage). To the south is the cold climate Gondwanan trend, a succession of Late Devonian and Pennsylvanian cold-water siliciclastics with glacially influenced deposition. Whereas Devonian through (limited) Mississippian strata are comparable in overall character, a sharp climatic gradient in western South America is established by the earliest Pennsylvanian. The Pangean trend in northwestern Bolivia and Peru continues with warm-water Pennsylvanian and Permian carbonates, evaporites, and mixed siliciclastics of a semiarid, open seaway association (Copacabana Formation). This unit was deposited by marine transgression north (northern Bolivian subsurface and Lake Titicaca area), reaching central Bolivia by the Early Permian (Early Cisuralian). Regionally, the warm Pangean pattern continues into the younger and more restricted overlying Cisuralian and younger Permian and Triassic rocks characterized by restricted marine deposits of both humid and arid association (including red beds). To the south, Early Pennsylvanian rocks in the Gondwanan trend record continental and lacustrine glacial deposition as far north as central Bolivia, with glacial influence strongest in southern Bolivia and northern Argentina. By the Late Pennsylvanian, glacial influence has waned and is restricted to southern Bolivia near the Argentine border. The Copacabana Formation is enigmatic because of the following: (1) its autochthonous succession over cold-water, glaciogenic deposits of the Late Devonian and Mississippian and (2) its apparent coeval deposition with Pennsylvanian (and Permian) glacial diamictites. Although the former can be attributed to paleolatitudinal shift, or a clockwise rotation of Gondwana, what is not easily explained (and much discussed) is the autochthonous continuity of northeastern and central Bolivian carbonate deposits of the northern Peru–Bolivia Basin with southern Pennsylvanian and Permian glaciogenic deposits, which accumulated in the Tarija–Chaco Basin. Given that these cold and warm-water deposits were coeval in time, a severe climate gradient must have existed across Bolivia beginning in Pennsylvanian time. Western Gondwana records steady movement from high latitudes (~558S) in the Late Devonian to midlatitudes (~408S) by Pennsylvanian time. Glacial deposits seen in the northwest during the Late Devonian become restricted to the southern Tarija–Chaco Basin by the Late Pennsylvanian. By Early Pennsylvanian (Bashkirian) time, carbonates, evaporites, and siliciclastics were deposited in northwest Bolivia. In central Bolivia, Mississippian diamictites, undated Pennsylvanian siliciclastics, Copacabana lithofacies, and carbonates of the Vitiacua Formation are vertically stacked at a few locations.