INTEGRATING SANDSTONE PETROGRAPHY AND SEQUENCE STRATIGRAPHY AT UPPERMOST SECTION OF THE POTTSVILLE FORMATION, CAHABA SYNCLINORIUM, ALABAMA

The Pottsville Formation (~2.5 km in thickness) in the Cahaba Synclinorium of Alabama is a Lower Pennsylvanian synorogenic clastic wedge, which is composed of interbedded sandstone, siltstone, claystone, shale, conglomerates, and coal beds.

Subsurface core-logging and outcrop sections of the upper member (~350 m in thickness) of the Pottsville Formation revealed several facies boundaries on the basis of grain size assemblages and sedimentary structures that formed in response to changes in base level associated with glacial eustasy and rapid tectonic accommodation. Facies assemblages include stacked and isolated channel deposits indicating fluvial-deltaic, beach-barrier, and tidal shoal deposits. Parasequences mark repeated progradation of prodelta through alluvial plain environments. Parasequence-bounding flooding surfaces are readily recognized, whereas complex, multi-storey channel architecture makes the positions of Vail-type sequence boundaries uncertain.

Thin section petrography reveals a relationship between distinct facies and compositional assemblages. Both isolated and stacked channel patterns show representative petrofacies suites. Quartz contents (70-76%) in both the facies are similar but there is a change in feldspar contents which may indicate both a shift in source area tectonics and/or changes in detrital routing paths.