ACTIVE BASIN CARBONATE-SILICICLASTIC SEQUENCES OF THE PENNSYLVANIAN AND PERMIAN COPACABANA FORMATION, TITICACA GROUP, BOLIVIA

Depositional history of the Pennsylvanian-Permian Titicaca Group comprises part of a central Andean marine through regressive red bed megasequence. In Bolivia, this 1st-order Pangean sequence records inherited basement controls behind a western arc and ephemeral pericratonic seaways. Marine deposits of the Copacabana Formation were subject to local tectonic controls, gradual changes in climate (northward paleolatitudinal shift), and high-amplitude eustasy. Detailed stratigraphic sections of the Copacabana and associated formations were measured at over 20 localities in the Eastern Cordillera and Altiplano. Two Madre de Dios Basin well sites to the east and established Peruvian depocenters to the north were considered in developing a sequence stratigraphic model. Calcareous foraminifera, fusulinid, conodont, palynomorph data were complied and significantly refined correlations of otherwise very similar lithostratigraphic stacking patterns.

Diverse carbonates, compositionally immature but texturally mature arkosic, lithic and volcaniclastic sandstones, shales, tuffs and evaporites characterize the 30 to 400m+ Copacabana Formation in Bolivia. Multiple uplifted sources provided abundant siliciclastic material that was reworked and mixed with shallow water carbonates. Up-dip stacked TST/HST systems tracts with significant hiatuses represent open and restricted marine to semi-arid coastal depositional systems. About twelve 3rd- to 2nd- order 30 to 100 m sequences are separated by sequence boundary zones with paleosols. Proximal ramp sequences include remnant eolian sands with calcrete and cross-bedded, fossiliferous marine sandstones, grainstones with microkarst. Subtidal ramp sequences maybe shale cored and occur without subaerial exposure of fossiliferous packstone/grainstone cycle caps. Thick accumulations of progradational carbonates and minor sandstone characterize HSTs. Complex overprinting of 2nd-, 3rd-, and 4th-order excursions in sea level are suggested. Accumulation patterns and sandstone/carbonate cyclicity are indicative of combined tectonic and glacio-eustatic depositional controls. Icehouse cyclicity persisted until the Artinskian when stacking patterns changed with both the regional tectonic regime and changes in global climate.