PERMO-CARBONIFEROUS COPACABANA FORMATION IN BOLIVIA: CARBONATE-CLASTIC SUPERSEQUENCES IN A BACK-ARC SETTING

Late Carboniferous and Permian marine and continental rocks of Bolivia (with cool- and warm-water fauna) were deposited in Gondwanan back-arc basins above glacially-influenced Late Devonian and Early Carboniferous siliciclastic units. Fieldwork at 23 isolated sections along the Cordillera Oriental is incorporated with earlier published biostratigraphic data to produce a working stratigraphic synthesis. Small- to large-scale facies patterns and time-transgression are recognized in the carbonate/clastic ramp deposits of the Copacabana Formation (Titicaca Group).

Over 400m of Bashkirian to Kungarian strata in the Lake Titicaca region thin and young along tectonic strike and up regional depositional dip to the southeastern subandes. Brachiopod-dominated, storm-influenced muddy open marine carbonates are common, particularly in Pennsylvanian deposits. These rocks are interbedded with restricted marine, Crurithyrid-rich, burrowed wackestone and black shales, and are overlain by siltstone and cross-bedded sandstone. Thin siliceous beds and nodule-bearing dolomudstone (silcretes equivalent to anhydrite in the northern subsurface) are common and represent up-dip facies elements of sabkha-homoclinal ramp depositional mosaics. Blue-green ash beds are present in all strata but gravel-sized deposits occur only in Peru. Meter-scale, shallowing-upward successions with evidence for repeated subaerial exposure suggest cyclothemic depositional style.

Abundant Permian skeletal grainstones include locally thick, cross-bedded pelmatozoan-dominated beds which represent high-energy environments and cover all areas of Pennsylvanian non-deposition (paleohighs?). Ubiquitous Permian deposits contain many bryozoans, fusulinids and corals (some in situ). Some Asselian/Sakmarian deposits can be organized into widespread decameter (3rd-order?) stacking cycles. Cycles consist of interbedded lime mud/dark shales with tempestites (subtidal) and grainstones and clastics with subaerial features (peritidal).

Implications for climatic, tectonic and eustatic conditions for sediment accumulation are preliminary. Sub-tropical(?) Permo-Carboniferous deposition occurred within a system of troughs and paleohighs in cool-subtidal, warm-photic and peritidal conditions. Early restricted Pennsylvanian basins were flooded in the Permian and evolved into open, higher-energy ramps with aggradational facies stacking patterns. Sediment was accommodated by pre-rift, plate margin transtension, although Early Permian decameter depositional cycles suggest a global eustatic overprint.