Sandstone distribution patterns in the Pocahontas Formation of Southwest Virginia and southern West Virginia

The Pocahontas Formation is a clastic wedge of sandstone, siltstone, shale, coal, and underclay that is transitional between underlying marine strata of Mississippian age and overlying continental beds of Pennsylvanian age. It attains a maximum thickness of 750 ft. at the southeastern edge of the Appalachian coal field and thins northwestward by the tonguing out of lower beds and by the truncation of upper beds at an overlying unconformity. Sandstone, which composes about 70 percent of the formation, occurs in lenticular bodies that have two distinct distribution patterns--lobate and linear--of contrasting orientation and composition. The lobate bodies are elongate to the northwest and contain sandstone having a quartz content of about 50 to 65 percent. Within a lobe, thickness lines show a northwestward bifurcating pattern similar to the pattern of channels in an alluvial distributary system of a modern delta. Laterally, lobate bodies merge to form northwestward-thinning sandstone wedges as much as 300 ft. thick. The linear pattern is shown by 3- to 8-mi.-wide sandstone lenses that extend north-northeast for about 45 mi. They are near the distal ends of the lobate sandstone and consist of 0 to 140 ft. of relatively pure quartzose sandstone. The well-washed character and orientation of the linear bodies, normal to the lobate bodies, indicate a barrier-bar origin. These sediment distribution patterns show that the Pocahontas Formation was deposited mainly in deltaic complexes built out from the southeast during marine regression to the northwest. Brief periods of transgression and stable shoreline conditions are recorded by tongues of marine strata and by the barrier bars.