COARSE CLAST PETROGRAPHY AND GEOCHRONOLOGY OF THE UPPER POTTSVILLE CONGLOMERATE MEASURES, CAHABA SYNCLINORIUM, SOUTHERN APPALACHIAN THRUST BELT, ALABAMA

The uppermost Pottsville Formation, known as the conglomerate measures in the Cahaba synclinorium, consists of conglomerate interbedded with lesser amounts of sandstone, shale, and coal. These were deposited as the product of synorogenic braidplain-anastomosis cycles that formed by aggradation on an Early Pennsylvanian alluvial plain. The abundance of extraformational conglomerate suggests major unroofing of active thrust sheets in the proximal part of the Alleghanian foreland basin.

Clasts in conglomerate of the upper Pottsville Formation consist mainly of, sedimentary lithic fragments (chert, sandstone, and mudrock) and metasedimentary lithic fragments (phyllite, schist, quartzite), and volcanic and plutonic rocks. Sedimentary lithic fragments are the dominant clast types (>50%). However, those become less abundant up-section as the contribution of metamorphic clasts (up to 30%) increases.

Dynamic and static state recrystallization textures in metamorphic clasts suggest a high-to-medium grade metamorphic source in the Appalachian orogen. Undulose extinction in quartz also suggests retrograde metamorphism in the source area. Carbonate clasts along with large fragments of chert that appear to be derived principally from the Cambrian-Ordovician Knox Group. The composite modal analysis and ternary plot of sandstone composition in conglomerate indicates a recycled orogenic provenance. The similarity of the clasts to the Cambrian to Pennsylvanian formations in the thrust belt and Piedmont of Alabama and Georgia also suggests a source to the south and southeast in the Appalachians.

⁴⁰Ar/³⁹Ar ages of ~323 Ma for a felsic clast confirm a peak Alleghanian volcanic source in the southern Appalachians. A metatonalite clast provides a complex spectrum with a minimum age of 300 Ma; if this result is verified by further work it could indicate the Cahaba section continued to receive Alleghanian metamorphic detritus later than previously thought.