Carboniferous of the Southeastern United States
The Pennsylvanian rocks of the central Appalachians record a progressive change in paleogeography and paleoenvironment, from extensive sea (Pottsville time), to relatively small bay (Allegheny–lower Conemaugh time), to entirely river-influenced lowsalinity bay-lake (upper Conemaugh–Monongahela time), to relatively small lakes of fluvial plain (Dunkard time). Sediments derived mainly from the southeast were dispersed into the elongate sea-bay-lake by prograding deltas. Sediments first filled the unstable geosynclinal trough-basin of southern West Virginia, and then the northeast-trending Dunkard basin of northern West Virginia which developed as a depression in a relatively stable platform. Sea transgression during Conemaugh time was in part tectonically controlled, but shifting delta lobes influenced the distribution of marine shell beds.
The West Virginia deltaic complex evolved from a wave-dominant delta with fringing barrier islands (Pottsville time) to a fluvial-dominant delta (Conemaugh and Monongahela time), and facies of Conemaugh and Monongahela rocks are similar to those found in modern shallow-water deltas. Major anticlines were growing structures influencing northeast-trending drainage and facies. Tectonic warping of plateau nearly normal to hingeline orientation of N. 50° E. explains the vertical stacking of sandstone belts approximately 30 mi. wide trending northwest. However, supply frequently over-whelmed basin subsidence burying growing structures under sediment. Consequently, channel sandstones commonly trend across present fold axes and exhibit an offset stacking arrangement resulting from differential compaction.