INTEGRATING FACIES ANALYSIS AND SANDSTONE PETROGRAPHY OF THE POTTSVILLE FORMATION IN THE CAHABA SYNCLINORIUM, ALABAMA: RECOGNITION OF A SEQUENCE STRATIGRAPHIC MODEL

The Alleghanian orogeny and the Pennsylvanian glacial-eustasy play a vital role for deposition of the Lower Pennsylvanian Pottsville Formation in the Cahaba synclinorium. These two major events result in a wide array of depositional environments both under marine and terrestrial settings. Study of about 1300 m continuous drilled cores shows numerous fining upward and coarsening upward successions.

Facies analysis of sedimentary sequences of the cores reveals a cyclic succession from tide-dominated marine, and shore zone to alluvial plain environments. Sandstone modal composition suggests a similar trend, which shows high feldspar and lithic content in the alluvial facies, and high quartz, low feldspar and lithic contents in the marine and shore-zone facies. This major transition in sandstone composition and cyclic appearance of tide dominated deltaic sublitharenite indicate sedimentary and metasedimentary provenance from a collisional orogen like the Appalachians. Widespread marine horizons associated with extensive condensed sections correspond to eustatic high sea level stands. Modal composition and facies analysis indicate that unroofing from the rising Appalachian mountain contributed for accumulation of thick terrestrial sediments, whereas marine successions were deposited due to base level changes resulting from glacial eustasy. Based on continental and marine effects on deposition, a depositional sequence model has been implemented to the non-marine upper part. On the contrary, the marine dominated lower part of the drilled cores is better suited with a genetic sequence model.