SEQUENCE STRATIGRAPHY OF THE POTTSVILLE FORMATION, CAHABA BASIN, ALABAMA: INFLUENCE OF CLIMATIC CHANGE DURING THE LOWER PENNSYLVANIAN

Thick (2.5+ km) lower Pennsylvanian clastic wedges of the Pottsville Formation accumulated in the Cahaba Basin, Alabama. In response to high magnitude sea-level changes or interaction of eustasy and rapid tectonic accommodation, a number of flooding surfaces have been developed which are used as bounding surfaces of parasequences. These Parasequences fall within very high frequency depositional cycles, supporting predominance of glacial-eustatic influences.

The lower Pottsville unit in the Cahaba Basin contains transgressive ravinement surface bounded cycles. These cycles mostly contain thick sand-shale interval with a heterogeneous coal zone on the top. Few cycles have locally developed quartzose sandstone units which represent tidal sandbank and beach deposits and served as hydrocarbon reservoir. The middle Pottsville formation is composed of marine flooding surface bounded parasequences containing thinner beds of marine shale. Coarsening upward patterns indicate these parasequences as highstand system tracts. Sandstones in this part of the formation are more micaceous rather than quartzose. Marine influence is lower in this sequence. Transgressive surfaces are used to demarcate parasequences boundaries in the upper Pottsville unit though few localized flooding surfaces have been identified within this unit. Repetitive fining upward sequences in this depositional pattern provide a wide array of prograding deltaic and alluvial facies in highstand system tract. Tidal facies is insignificant in this part rather than the lower and middle Pottsville Formation.

Eustasy and tectonic interaction gave rise to a diverse range of depositional environments for the Pottsville Formation, starting from a tidal-beach through delta-estuarine settings towards an alluvial sedimentation pattern. This diversity in deposition may be caused by significant climatic changes during the lower Pennsylvanian, when glacial eustasy and consequent subsidence in Cahaba and Black Warrior foreland basins ahead of Appalachian uplift had a major influence.