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Nearshore and Shelf Systems
Storm- and Tide-Influenced Prograding Shoreline Sequences in the Middle Devonian Mahantango Formation, Pennsylvania
Thickening- and coarsening-upward depositional cycles are present in the Middle Devonian Mahantango Formation, Pennsylvania. They are found in fossiliferous, shallow marine deposits (terrigenous shale and mudstone, quartzose siltstone and sandstone, and quartz-pebble conglomerate) displaying various physical and biogenic structures. These cycles record shoaling associated with shoreline progradation; they are bounded by erosional disconformities (transgression surfaces) overlain by reworked lags. Three orders of superimposed coarsening-upward cycles are developed: large cycles (30-80 m thick), typically are composed of several small cycles (2-20 m thick) and a third order of cycles is recognized in proximal exposures.
An idealized summary sequence for Mahantango facies records an upward transition from: 1) abundantly fossiliferous mudstone with thin sandstone beds, interpreted as distal offshore storm deposits; to 2) similar mudstone interbedded with hummocky cross-stratified sandstones, interpreted as proximal offshore, subtidal storm deposits; to 3) amalgamated, hummocky sandstone bodies, interpreted as lower shoreface, subtidal storm deposits; to 4) swaley sandstone, intepreted as wave-dominated nearshore deposits from the upper shoreface; to 5) thin, flat-bedded sandstone exhibiting a wide variety of physical and biogenic structures, interpreted as shallow subtidal channel-mouth shoals or ebb-tidal deltas from the upper shoreface; to 6) trough crossbedded, medium to very coarse grained sandstone and pebbly sandstone displaying reversing flow directions, reactivation surfaces, and lateral accretion bedding, interpreted as shallow subtidal to possibly intertidal deposits of tide- and river-influenced estuaries/distributaries; to 7) relatively thin and laterally extensive, sandy conglomerates with erosional bases and abundant trace and body fossils, interpreted as reworked “transgressive lags”.
This succession records shoaling from a shallow, muddy, storm-dominated sea to a sandy, barrierless shoreface. The lower shoreface was storm-dominated; the upper shoreface was tide-dominated and wave-influenced. Paleocurrent data reflect a straight shoreline (oriented roughly northeast-southwest to north-northeast - south-southwest) dissected by sinuous, shore-normal tidal channels that probably received coarse sediment from braided streams on the lower coastal plain. A mesotidal range is inferred, with channels incised to subtidal depths of about 10 m. The coast and shelf of the central German Bay represents a partial modern analogue for this system.
Episodic progradation to the northwest produced the superimposed cyclicity. Thinner cycles can be traced only a few tens of kilometres; they likely resulted from autocyclic processes or tectonic events of restricted spatial and temporal scale. Thicker cycles, traceable for many tens to hundreds of kilometres, are best attributed to major tectonic events in the adjacent Acadian highlands.
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