A sequence stratigraphic synthesis of the Lower Pennsylvanian Pottsville Formation encountered in two drill cores in the Cahaba synclinorium, Alabama

Abstract

The Lower Pennsylvanian Pottsville Formation is a classic clastic wedge of the Appalachian mountain system composed of interbedded sandstone, siltstone, claystone, shale, and coal beds with orthoquartzitic conglomerate at the base. The formation developed in response to thrust and sediment loading on the convergent margin of the Laurussian craton during the Alleghanian orogeny. The greatest preserved thickness of Pennsylvanian strata, more than 2500 m (8000 ft), accumulated in the Cahaba and Coosa coalfields of the greater Black Warrior Basin. The Carboniferous coalfield, which hosts the Pottsville Formation in the greater Black Warrior Basin, occurs in large synclinoria separated by anticlinoria where Cambrian-Ordovician rocks are exposed at the surface. Eight lithofacies have been identified within two studied drill cores from the Cahaba basin on the basis of dominant rock types, color, grain size, and sedimentary structures. A transition from a tidal through delta-estuarine to fluvial environments has been identified with the aid of facies analysis. Conformable contacts indicate a gradual change in the depositional environments from delta front to the delta plain. Laterally extensive coal beds have been used to demarcate sequence packages in the terrestrial part, whereas a few localized flooding surfaces have been identified in the marine-dominated strata. Repetitive fining upward sequences show a wide array of prograding iii deltaic and alluvial facies in the highstand system tracts. Conglomerate layers associated with fluvial scour surfaces reflect the influence of eustacy on sedimentation. Thin-section petrography shows incongruence between mudstone and conglomerate measures. Two distinct petrofacies assemblages have been identified within the sampled intervals of cores, and these may be indicative of changes in base level, depositional environment, climate, and possibly source terrain. The lower marine intervals of the studied core may have been sourced from the distal northern Appalachian Mountains via the Pennsylvanian mega-drainage system. The proximal southern Appalachian Mountains may have contributed for the deposition of the upper terrestrial section. A combination of depositional and genetic sequence stratigraphic models has been applied to the Pottsville Formation in the study area. The depositional sequence model is better suited in the terrestrial-dominated upper part of the core due to abundance of major fluvial erosional discontinuities. The genetic sequence model is applicable to the marine-dominated lower part, which is characterized by the presence of marine flooding surfaces and associated condensed sections.