2008 Joint Annual Meeting (5-9 Oct. 2008): Detrital History of Lower Pennsylvanian Pottsville Formation, Cahaba Basin, Alabama

The Cahaba Basin (also known as the Cahaba Synclinorium) of north-central Alabama contains >2.5 km of Lower Pennsylvanian clastic wedge sediments of the Pottsville Formation. Sandstone composition and detrital geochronology reveal a new provenance history of these deltaic to shallow-marine sediments.

Pottsville strata are cyclothemic, consisting of indurated interbedded sandstone, siltstone, claystone, conglomerates, and numerous coal beds. Quartzolithic modes increase upsection in the Pottsville sands showing increasing dominance of both sedimentary and metamorphic lithic fragments. Plagioclase feldspars dominate over K-spars. The Pottsville sandstones are typically within the “recycled orogenic” provenance field of QtFL. Heavy-mineral assemblages include opaques, ZTR, chlorite/chloritoid, and micas, but are dominated by low- to medium-grade almandine garnets. General lack of chromite (based on microprobe data) indicates that ophiolitic belts were not a common source for these sandstones. Mudrock chemistry of the Pottsville units reveals an active continental margin and island arc provenance. 40Ar/39Ar study of detrital muscovite grains reveals cooling ages of source terranes representing all three Appalachian orogenic events (Taconic, Acadian and Alleghenian). Most probable source areas were the eastern Blue Ridge, the Talladega Slate belt, and possibly the Inner Piedmont (Tugaloo and Cat Square terrane).

Based on sandstone composition, heavy-mineral contents and sediment geochemistry, the Pennsylvanian Pottsville sands from the Cahaba Basin resemble sands of the upper Tertiary Himalayan clastic wedge of the Bengal Basin. This composite study reinforces the revised suggestion that sediments of the Pottsville Formation were primarily derived from the rising Appalachian orogen to the east and southeast.