Provenance of Lower Pennsylvanian Pottsville Formation, Cahaba Basin, Alabama

Abstract

The Cahaba basin, also known as the Cahaba Synclinorium, of north-central Alabama contains a thick (>2.5 km) synorogenic clastic wedge belonging to the Lower Pennsylvanian Pottsville Formation. Sediment composition and detrital geochronology reveal a provenance history for these fluvial-deltaic to shallow marine sediments. Pottsville clastics are cyclothemic, consisting of interbedded sandstone, siltstone, claystone, shale, and numerous coal beds, with orthoquartzite sandstone and conglomerate at the base. Basal Pottsville is dominated by quartz–rich sediments; feldspars increase upsection. Plagioclase feldspars dominate over k-spars. No obvious trend in lithic fragments was observed. Compositions of these sandstones reflect a “recycled orogenic” provenance. Heavy minerals are dominated by rutile, tourmaline, and medium-grade almandine garnets. Overall lack of chromium in the Pottsville Formation indicates that ophiolitic belts or terranes with abundant mafic igneous rocks were not a common source for these sandstones. Mudrock geochemistry of the Pottsville units indicates a mixture of an island arc provenance and an active continental margin dominated by metamorphic rocks. 40Ar/39Ar study of detrital muscovites data reveal ages typical of all three Appalachian orogenic events (Taconic, Acadian and Alleghenian). The Western Blue Ridge, the Talladega belt, and the Cat Square terranes of the Inner Piedmont are the most probable source areas for these detrital grains. This study confirms the suggestion that sediments of the Pottsville Formation were primarily derived from the uplifted Appalachian orogen to the east and northeast.