Upper Devonian Catskill Formation outcrops in west-central Pennsylvania were studied to develop a regional depositional model, and a better understanding of the sedimentologic controls on distribution of the petroleum sandstone reservoir types in the subsurface part of the basin. The Catskill Formation is characterized, from base up, by the Irish Valley, Sherman Creek, and Duncannon Members, whose thicknesses and proportions

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markedly change by lateral intertonguing along northeast-southwest depositional strike. The Irish Valley Member, in much of the area, consists of alternations of marine and nonmarine sediments which are arranged in several motifs. Each motif consists of a fining-upward facies assemblage, showing a variety of structures attributed to subtidal and intertidal flat sedimentation. The Sherman Creek Member consists of nonmarine mudstone and siltstone and subordinate amounts of sandstone, organized into a succession of fining-up cycles, deposited on a low relief and inactive coastal plain with small and high sinuosity meandering streams and broad flood basins. The Duncannon Member consists of interbeds of nonmarine, thick to massive and complexly cross-bedded sandstone with subordinate amoun s of mudstone and siltstone, believed to represent low sinuosity to possibly braided river systems. The Duncannon facies dominates the Catskill Formation in the central parts of the area and intertongues laterally with the Sherman Creek Member and parts of the Irish Valley Member which comprise the main bulk of the Catskill to the south and north. In the same area, the Irish Valley facies, while attaining its tidal origin, is not characterized by repetition of motifs. Paleoenvironmental synthesis suggests that the Catskill shoreline was fed by a tide-dominated delta in central Pennsylvania (Centre County) and was flanked to the south and north by broad tidal flats which graded landward to an inactive coastal plain environment.

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