Pennsylvanian and Permian sequences in Bolivia: Direct responses to Gondwana glaciation
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Western Gondwana underwent a steady drift from mid-latitudes (50S, Early Carboniferous) to lower latitudes (<40S) by Late Carboniferous time, and glacial deposition had ended in Bolivia by the early Pennsylvanian (Morrowan). At this time, carbonates and evaporites were being deposited across the Per-Bolivia Basin. The Pennsylvanian and Permian Titicaca Group represents an Andean transgressive marine to restricted carbonate and regressive red bed megasequence (Cuevo Super-sequence or Subandean Cycle). The transgressive part of this Pangean first-order sequence records inherited basement controls and ephemeral pericratonic seaways into the interior of a western landmass. The well-dated Copacabana Formation records many high-frequency sequences and meter-scale cycles that form larger, third- and second-order composite sequences in the central Andes. Diverse carbonates, compo-sitionally 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. Estuarine barrier sands and cross-bedded, fossiliferous marine sandstones with limestone litho-clasts were derived by reworking of semilithified Copacabana rocks during lowstands and transgressive flooding events. Sedimentation rates in Bolivia were relatively low (7-25 m/m.y.) compared with the thicker and shale-rich Copacabana Formation in Per. Stacked transgressive systems tracts and highstand systems tracts with significant hiatuses formed in open-marine and restricted to semiarid coastal depositional systems. Twelve second- and third-order, 30-100 m composite sequences have incise-ment or protosol development above marine limestone of the underlying sequence; extensive siliciclastic lowstand and/or transgressive shoreline facies occur above these sequence boundaries. Thick accumulations of progradational carbonate characterize the highstand systems tracts. More distal subtidal ramp sequences (well-developed in the Cochabamba and northern Subandes areas) are shale-cored with fossiliferous packstone-grainstone caps, but they lack evidence of subaerial exposure or discon-formity. Small, meter-scale shallowing-upward parasequences and internal auto-cyclic, icehouse facies mosaics comprise the larger Copacabana composite sequences. Pennsylvanian sequence boundaries (occurring as a result of glacial drawdowns) occurred at ca. 318 Ma, 311 Ma, 309 Ma, 308 Ma, and 306.5 Ma. Permian drawdowns occurred at 299 Ma, 293 Ma, and 283 Ma. 2008 Geological Society of America.
