A PETROGRAPHIC AND GEOCHRONOLOGICAL ANALYSIS OF THE POTTSVILLE FORMATION IN THE SOUTHERN APPALACHIAN CAHABA FORELAND BASIN

The Pennsylvanian synorogenic Pottsville Formation was deposited in the southernmost Appalachian foreland basin in Mississippi and Alabama during the Alleghanian orogeny of the Appalachian. The southernmost Appalachian foreland basin consists of three sub-basins, and the Cahaba Synclinorium contains the thickest (> 2.5 km) deposits of the Pottsville sediments. A petrographic comparison of sediment composition and detrital geochronology reveals the provenance history of the fluvial-deltaic siliciclastic basin fill in Cahaba.

Sandstones from the Cahaba synclinorium are compositionally immature and dominated by litharenite. The overall composition of the sandstone in lower and upper section is similar though the lower unit of the Pottsville has higher quartz content, and feldspar content increases toward the up section. The higher feldspar content in upper section indicates less chemical weathering or closer to source. Conglomerates in upper sections of the basin contain clasts similar in composition to the rocks of the nearby Valley and Ridge, Piedmont and Blue Ridge provinces. The composite modal analysis of sandstone composition in conglomerates of the conglomerate measures in the Cahaba indicates a “recycled orogenic” provenance, which is compatible with the sandstone composition in quartzarenite measures.

⁴⁰Ar/³⁹Ar age dating of detrital muscovite from arenitic samples from the lower, middle and upper quartzarenite measures in Cahaba yield Taconian, Alleghenian and Acadian ages respectively suggesting an Appalachian source. The three different ages in the different stratigraphic level of the quatrzarenite measures suggest a shifting of sources with time within the Appalachian mountain belts. The ⁴⁰Ar/³⁹Ar age dates of a metatonalite clast from Cahaba `provide a complex age spectrum with a minimum of 300 Ma; which also indicates that the rocks in Cahaba section is older than the minimum age of metatonalite age spectrum. A miospore Schultzospora rara also constrains the age of the youngest Pottsville strata which is ~310 Ma. The petrographic and detrital geochronologic data therefore suggest that the Pottsville sedimentation in the Cahaba Synclinorium was consistent throughout the stratigraphic column, sourced from the Appalachian during the late Paleozoic.