Sandstone Distribution Patterns in the Pocahontas Formation of Southwest Virginia and Southern West Virginia

Abstract

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. 31 32 KENNETH J. ENGLUND

Coal-Bearing Depositional Systems

“…Notwithstanding the preference of many coal geologists for the delta environment as a model for coal formation, indications are increasing that a high proportion of coal has been formed on strand plains behind barrier beaches either within delta complexes, between deltas (interdeltaic coastal plains of Englund 1974;Vaninetti 1978;Ayers and Kaiser 1984), or completely separate from any deltaLc influence. This new awareness is probably due to the realisation that the delta model has often been invoked without much geological evidence.…”

Linking late Paleozoic sedimentary provenance in the Appalachian Basin to the history of Alleghanian deformation

“…By comparison, much of the rest of the Pennsylvanian succession is characterized by more widespread depositional systems. The majority of the Pennsylvanian strata are comprised of interbedded medium-to coarse-grained sandstones, mudstone and coal; and deposition occurred in a fluvial deltaic plain to marginal marine environment (Englund, 1974;Miller, 1974;Thomas and Cramer, 1979).…”

Systematically variable geometry and architecture of incised‐valley fills controlled by orbital forcing: A conceptual model from the Pennsylvanian Breathitt Group (Kentucky,USA)

“…The Breathitt Group, formally defined in the early 1990s (Chesnut, , ), comprises Lower to Middle Pennsylvanian strata overlying the basin‐wide Mississippian–Pennsylvanian unconformity in eastern Kentucky (Ettensohn & Chesnut, ). The largest volume of deposits can be defined as ‘coal‐measures’, characterized by associations of sublitharenite to litharenite sandstones, mudstones and extensive coal seams (Davis & Ehrlich, ; Englund, ) deposited in fluvial to shallow‐marine environments. Repetitive coarsening‐upward and fining‐upward patterns of ‘cyclothemic’ character with thicknesses of several tens of metres occur within individual formations (see below), superposed to a progressive transition to coarser and increasingly terrestrial facies associations expressed over hundreds of metres upward through the group.…”