Thick, low-sulfur coal beds are intercalated with a sequence of stacked delta-lobe deposits in the Pocahontas Formation of southwestern Virginia and southern West Virginia. Coal thickness and coal quality are related to the distribution and thickness of the underlying deltaic sandstone beds.

During the Early Pennsylvanian, the northwestward progradation of deltaic lobes on the southeastern shoreline of a regressive Carboniferous seaway was interrupted by periodic stillstands. Sand at the delta front was reworked and segregated by coastal currents and waves to form a system of fringing barrier bars. Vegetation flourished in swamps behind these protective barriers, mostly on platforms created by the abandoned delta lobes. The growth and accumulation of plants on the inactive, sand-dominated delta lobes formed domed deposits of ombrogenous peat that were low in ash and sulfur. In contrast, interlobe areas accumulated high-ash mucks consisting of organically rich clay and silt. In modern analogous peat-forming environments in tropical southeastern Asia, accumulations of low-a h and low-sulfur domed peat are dependent on high rainfall, which contributed little if any mineral matter to the peat. The ombrogenous origin of the Pocahontas coal beds is similarly indicated by the distribution of thick, low-sulfur and low-ash coal over the thick central area of the sandstone lobes, whereas thin, impure, and discontinuous coal beds occur in the shale-dominated interlobe areas. This analysis demonstrates that knowledge of the geometry, thickness, and orientation of delta lobes can be useful in the exploration for thick, high-quality coal.

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