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

Late Carboniferous and Permian marine and transitional 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 for the carbonate/clastic ramp deposits of the Copacabana Formation.

Over 400m of Bashkirian to Kungurian 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 marine subtidal carbonates are common. In Pennsylvanian deposits these facies are interbedded with Crurithyrid-rich wackestone and black shale and are overlain by cross-bedded sandstone and siltstone. Thin siliceous beds and nodule-bearing dolomudstone (silcretes equivalent to anhydrite in the northern subsurface) are abundant and represent up-dip facies elements of sabkha-homoclinal ramp depositional mosaics. Meter-scale successions show evidence for repeated subaerial exposures and episodic deposition of ash beds.

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. Asselian/Sakmarian deposits can be organized into widely correlated decameter stacking cycles. Cycles consist of interbedded lime mud/dark shale with tempestites (subtidal) overlain by grainstones and clastics with subaerial features (peritidal shoals).

Sub-tropical Permo-Carboniferous deposition occurred within a system of troughs and paleohighs in cool-subtidal, warm-photic and evaporitic conditions (especially interesting with regard to underlying/coeval Gondwanan glacigene environments). Early restricted Pennsylvanian basins were flooded in the Permian and evolved into open, high-energy ramps. Sediment was accommodated by pre-rift, plate margin transtension. Analysis for cyclothemic eustatic overprint is contingent on higher resolution biostratigraphy.