INCISED-VALLEY FILLS AND OTHER EVIDENCE OF SEA-LEVEL FLUCTUATIONS AFFECTING DEPOSITION OF THE CATSKILL FORMATION (UPPER DEVONIAN), APPALACHIAN FORELAND BASIN, PENNSYLVANIA

An exceptionally complete and extensive new exposure of the Catskill F ormation (Upper Devonian) in the Susquehanna Valley of central Pennsyl vania demonstrates that fluctuations of relative sea level of at least two orders in the Milankovitch range affected its depositional histor y well into the Famennian Stage, Seven incised-valley-fill (IVF) units , ranging in thickness from 10 to 33 m, are present up to about 600 m above the base of the Sherman Creek Member, Where fully developed, the y have the three-part vertical sequence recently proposed as the signa ture of IVFs: (1) above a basal scour surface, cross-stratified sandst one and conglomerate deposited in environments that were initially hig h-intensity braided fluvial but that became increasingly affected by m arine storm processes; (2) a medial unit of green-gray fissile mudrock containing symmetrical sandstone ripples in flaser and linsen forms a ccumulated in the anoxic calm of the central zone of an estuary; and ( 3) an upper unit of wave-rippled sandstone and/or channelform sandston e representing progradational bayhead filling of the estuary, Other ma rine influences on deposition of the Sherman Creek Member are indicate d by a diverse suite of brachiopods and by wave rippled sandstone, A r ough estimate suggests that these marine incursions into the Catskill coastal-alluvial plain represent fourth-order fluctuations of relative sea level. Features of the Irish Valley Member confirm that depositio n of fissile mudrock took place on the marine shelf under low-energy, anoxic conditions only intermittently interrupted by storms, and that thicker (> 2 m) sandstone units represent sharp-based shelf sand bars, Fifth-order fluctuations of relative sea level produced numerous, rep etitious shallowing-upward sequences in both paralic and shelf facies of the Irish Valley. These interpretations provide the basis for seque nce-stratigraphic analysis of both members of the Catskill Formation a s a unified depositional system, In lowstand phases, Catskill streams erosionally incised their courses into the coastal-alluvial plain, byp assing coarser sediment to the shelf, where it accumulated as lowstand shelf fans, In the transgressive phase, incised valleys were drowned as transgressive surfaces moved diachronously upvalley, changing braid ed fluvial systems to storm-wave-influenced marginal-marine sand envir onments, followed by the establishment of anoxic, deeper estuarine con ditions, On the shelf, transgressions restored muddy conditions of low -energy and anoxia. Highstand conditions resulted in filling of the in cised valley by aggradation in the estuary and progradation at the bay head, gradually returning the area to pre-incision coastal-alluvial pl ain conditions.