SEQUENCE STRATIGRAPHIC ANALYSIS OF THE UPPER POTTSVILLE FORMATION, CAHABA SYNCLINORIUM, ALABAMA
Subsurface core-logging and outcrop sections revealed several facies boundaries on the basis of grain size assemblages and sedimentary structure that formed in response to changes of base level associated with glacial eustasy and rapid tectonic accommodation. Facies assemblages include 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. Repetitive fining upward sequences in this depositional pattern provide a wide array of aggrading tidal and alluvial channel facies within highstand and transgressive systems tracts.
Thin section petrography shows very minor or no imbalance between the stable and labile minerals at major stratigraphic contacts. Also, heavy mineral content remains consistent (~1.8%) in the section, which supports a very little change in the syn-depositional paleoclimate, which has been interpreted as everwet. Strong marine-nonmarine cyclicity shows the influence of eustatic control on sedimentation, and tectonic subsidence was the dominant control on sediment accommodation. In addition, the evolving Appalachian orogen provided abundant coarse-grained sediment that facilitated a transition from tide-influenced deltaic sedimentation to bedload-dominated fluvial sedimentation.