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Strata of the Devonian deltaic complex are preserved in 110,000 sq mi of the Appalachian basin. Petroleum production occurs only in the western half, principally in higher permeability sandstone, but some in prodelta dark shale. Oil is produced from Upper Devonian strata only in the westernmost part of the basin. Absence of petroleum production in the east presumably is because hydrocarbons have escaped from Ridge and Valley province outcrops. Sedimentary properties can be extrapolated to the subsurface to better understand the factors controlling petroleum occurrence.
The deltaic clastics were derived from an eastern quartzose source in the latitude between New York and Virginia. Depositional patterns of petroliferous sands resulted from gradual infilling of a subsiding basin, interrupted by rather abrupt eastward transgressions, which produced nearly synchronous, winnowed sands traceable across many counties.
The oldest prodelta sediments (Needmore Shale) were derived from the Baltimore area. Immediately after the Tioga volcanic event prodelta deposition spread abruptly westward nearly to the Ohio River. Cazenovia Stage deltaic siltstones and sandstones occur in Pennsylvania and New York. In late Tioughnioga Stage the zone of winnowing extended to West Virginia (Clearville Siltstone) with terrestrial redbeds accumulating in the Catskill Mountains. Early in the Taghanic Stage hundreds of feet of abrupt eustatic sea-level rise or basin deepening shifted the shoreline scores of miles eastward.
Throughout the rest of Late Devonian time the shoreline encroached generally westward with fairly sudden eastward shifts of tens of miles during times of sea-level rise. These shifts isolated 5 distinct clastic pulses in the outcrop region during the Finger Lakes and Cohocton Stages. In the western area younger sands of the Cassadaga Stage and Bradford Stage were deposited during winnowing accompanying shoreline shifts. Maximum westward encroachment of the subaerial Catskill delta was in the Bradford Stage, followed by Early Mississippian transgression which produced the Pocono Group sandstones.
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