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The Catskill deltaic sequence (Upper Devonian-Lower Mississippian) is inadequately mapped and subdivided in the subsurface of northern West Virginia (central Appalachian basin). Stratigraphic interpretations are often hindered by the liberal use of drillers' names for noncontemporaneous gas-bearing sandstones.
Base-line analysis of 275 gamma-ray logs permits the resolution of lithostratigraphic units in this sequence. Five regionally mappable units are recognized, within which the position, geometry, trend, and distribution of reservoir sandstones are interpreted. Three units are within the Upper Devonian Chemung Formation, and two are considered equivalents of the Hampshire (Upper Devonian) and Pocono (Lower Mississippian) Formations. Stratigraphic cross sections graphically display relative thickness variations of reservoir sandstones and document the inter-tonguing facies of the Catskill deltaic sequence that are attributable to "cyclic" marine transgression and regression, and/or to changing sedimentation or subsidence rates in the basin.
Five lithofacies (A-E) are identified by the study of equivalent strata on outcrop. Facies A consists of gray shales and thin-bedded siltstones, interpreted as interbedded hemipelagic shales and turbidites. Facies B is comprised of thick-bedded and amalgamated siltstones and sandstones containing hummocky cross-stratification, representing composite storm deposits of the shelf and slope. Facies C consists of thick-bedded to massive clean sandstones and interbedded red and gray shales, interpreted as barrier-beach and back-barrier deposits. Facies D is dominated by red beds (silty mudstones and siltstones) with some interbedded sandstones, representing vertical accretion and channel deposits of an alluvial plain. Facies E consists of gray sandstones containing marine fossils and greeni h-gray shales (Pocono Formation), interpreted as marine and marginal marine deposits developed during net transgression of the sub-aerial "Catskill delta."
Contrary to some previous studies, this study indicates these strata are best interpreted within the context of deltaic sedimentation. More specifically, stratigraphic and sedimentologic evidence suggests deposition in a wave- and storm-dominated deltaic complex.
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