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Upper Devonian strata, totaling 1,000-3,000 m in Pennsylvania and New York, record the shift from marine turbidity current to alluvial-plain deposition. Few detailed descriptions of the interval of transition have been published, and there is no adequate discussion of depositional environments based on modern sedimentologic principles.
In the Susquehanna Valley, south-central Pennsylvania, the turbidite sequence is followed by several hundred meters containing numerous thin marine-nonmarine alternations. These alternations, commonly as thin as 6 m, are composed from bottom to top of (1) sharp basal surface; (2) green, bioturbate, fine sandstone with brachiopods or crinoids; (3) green-gray fissile shale; (4) red, thin-bedded to massive mudrock or very fine sandstone with root marks, cracks, and small wave ripples; and (5) red massive mudrock with root marks, and tan calcitic nodules. Cross-stratified sandstone beds a few meters thick and with erosional bases are found rarely within units 4 and 5. Above the highest band of marine fossils is a thick sequence of red alluvial-plain cycles, composed of sediments which bec me finer toward the top of each cycle (Catskill). These characteristics suggest that the shift from marine to nonmarine deposition was broken by many abrupt transgression (1 and 2). Shorelines prograded mainly by mud deposition (3 to 4), implying that wave energy was slight. Because coarser sediments with channeled bases are absent in unit 3 and rare in unit 4, tidal channels and tidal fluctuations must have been absent. The large volume of mud represented by each alternation must have been supplied mainly by longshore drift or overbank flooding from large, widely spaced streams, and the alternations probably are controlled by local tectonic pulses or tectonically influenced sedimentation rates.
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