PENNSYLVANIAN AND PERMIAN SEQUENCES IN BOLIVIA: DIRECT RESPONSES TO GONDWANA GLACIATION

Western Gondwana exhibits a steady progression from mid- (~50^oS, Early Carboniferous) to lower latitudes (< 40^oS) by Late Carboniferous time. Glacial deposition ended in Bolivia by early Pennsylvanian (Morrowan) time, when carbonates, evaporites and siliciclastics were being deposited. The Titicaca Group comprises part of a central Andean transgressive carbonate and regressive-restricted red bed megasequence (Cuevo Supersequence or Subandean Cycle). The Pennsylvanian and Permian Copacabana Formation records many large and small scale cycles that form composite sequences in the central Andes. The Pangean 1st-order transgressive sequence in the Perú-Bolivia Basin records inherited basement controls and ephemeral pericratonic seaways to the interior of a western landmass. Diverse carbonates, compositionally immature but texturally more mature arkosic and lithic sandstones, shales, tuffs and evaporites characterize Copacabana Formation lithologies, which have been dated using foraminifera, fusulinids, conodonts and palynomorphs. Stacked TST/HST systems tracts with significant hiatuses formed in open and restricted to semi-arid coastal and marine depositional systems. Twelve 2nd and 3rd-order, 30 to 100 m sequences have paleosols developed in marine limestone of the underlying sequence and extensive siliciclastic lowstand/transgressive shoreline facies above sequence boundaries. Thick accumulations of progradational carbonate characterize HSTs. More distal subtidal ramp sequences (well-developed in the Cochabamba and northern Subandes areas) are shale-cored with fossiliferous packstone/grainstone caps lacking evidence of subaerial exposure. Small, meter-scale shallowing-upward parasequences and internal autocyclic, icehouse facies mosaics make up the large Copacabana sequences. Overall accumulation patterns and sandstone/carbonate cyclicity are indicative of combined tectonic and high-amplitude, high-frequency glacio-eustatic depositional controls in Pennsylvanian through Early Permian time. Sequence boundaries, responding to major glacial advances, occur at 318, 311, 309, 308 and 306.5 Mya (Pennsylvanian), and 299 (Penn/Perm boundary), 293 and 283 Mya (Permian) times.