DETRITAL HISTORY OF THE CONGLOMERATE MEASURES OF THE POTTSVILLE FORMATION IN THE CAHABA SYNCLINORIUM, SOUTHERN APPALACHIANS, ALABAMA

The Cahaba Synclinorium of Alabama contains >2.5 km of Pennsylvanian synorogenic clastic wedge sediment in the Pottsville Formation. The upper 800 m of the formation consists of coarse conglomerate with lesser amounts of sandstone, shale, and coal. Known as the conglomerate measures, this magnafacies was deposited in a series of stacked braidplain-anastomosis cycles, possibly at the distal fringes of giant piedmont fans. Aggradation of widespread bedload-dominated fluvial systems may have led to the development of anastomosed fluvial systems with peatlands, which are represented by thick, low-sulfur coal seams. A total of 36 conglomerate and 10 sandstone samples were collected from cores. Petrographic analysis reveals the source of this coarse-grained, synorogenic sediment.

Clasts within the conglomerate units consist mainly of chert, sedimentary and metamorphic lithoclasts and lesser amount of volcanic lithoclasts. Medium-to-high grade metamorphic lithic fragments are dominant. Carbonate clasts are present, along with large fragments of chert that appear to be derived principally from the Cambrian-Ordovician Knox Group. Both rhyolite and basalt clasts are present in the conglomerate, as are argillaceous lithic fragments. Clast composition indicates a proximal orogenic source that includes elements of the Appalachian thrust belt and the crystalline core of the Appalachians. The low abundance of heavy minerals and abundance of ultra-stable minerals in the upper Pottsville sandstone reflects intense chemical weathering, which is consistent with an equatorial paleolatitude. The presence of rutile and garnet further suggests a medium- to high-grade regionally metamorphosed source in the southern Appalachians. Ongoing research on detrital geochronology and mineral chemistry will provide additional information on provenance of the upper Pottsville Formation.