Roger G. Walker

The "Catskill Delta": A Prograding Muddy Shoreline in Central Pennsylvania

The Upper Devonian Catskill Formation contains at its base the Irish Valley Member, which is about 600 m thick, and consists of alternating marine and nonmarine strata. Below the Irish Valley is a sequence of turbidites and slope deposits, and above, a sequence of dominantly nonmarine alluvial plain sediments. The Irish Valley Member contains about twenty-five repeated facies sequences, varying in thickness from about 4 to 45 m. From bottom to top, the sequences contain: (1) a sharp basal surface; (2) green bioturbate fine sandstone, with scattered quartz granules and brachiopods or crinoidal debris; (3) olive green fissile shale; (4) green siltstones with thin wave-rippled sandstones and sparse marine fauna; (5) red siltstones with root traces, desiccation cracks, and very thin wave-rippled sandstones; and (6) red siltstones with root traces, desiccation cracks, tan calcareous nodules and, in some cases, fining-upward alluvial sequences. We suggest phases 1 and 2 indicate marine transgression, phases 3 and 4 indicate shoaling by progradation of a muddy shoreline (sand bodies thicker than 50 cm are rare), and phases 5 and 6 represent mainly vertical accretion on a coastal plain. Because of the absence of major sand bodies in phase 4, we believe wave action was slight, the tidal range was low, and the rate of mud supply by longshore drift was high. Shifting in position of the major streams during regional subsidence is the probable cause of repetition of the sequences.