Storm-dominated shelf sedimentation marginal to a delta: an example from the Devonian of southwestern Virginia and eastern West Virginia

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Virginia Polytechnic Institute and State University


The Chemung Formation in the south-central Appalachians constitutes part of the thick Devonian - lower Mississippian Acadian clastic wedge. Sediment in the Chemung was derived from a progressively-uplifted source area to the east and accumulated in the proximal environments of an epeiric sea. Sandstones within the dominantly muddy formation are found in a variety of sequence types characterized by hummocky cross stratification. Individual beds usually have sharp erosive bases and gradational tops. A hydrodynamic interpretation of hummocky sequences suggests that they are the result of extremely large but infrequent storm events and are surrounded by fair-weather deposits of mud rock. The Chemung shelf is envisaged as a very low-gradient (<0.5 ft./mile) clastic ramp extending hundreds of miles toward the craton interior. Transport of sand from the nearshore onto the shelf over distances up to 200 miles was facilitated by storm generated gradient currents. Coarsening/thickening- upward sequences of storm beds represent progradational shelf sand lobes. Three facies associations represent environments of deposition and record changing styles of storm sedimentation on the shelf. The distal inner shelf environment is characterized by discrete hummocky sandstone sequences deposited by only the largest of storms. The proximal inner shelf environment is a much higher energy environment in which periodic storms and fair-weather waves shoal on the low gradient sea floor producing thicker and amalgamated storm sequences. A low-energy shoreline environment is situated landward of the proximal inner shelf and contains only minor storm and fair-weather deposits. The widespread occurrence of hummocky sequences within the Chemung illustrates the importance storm processes can have in the transportation and deposition of sediment in shallow-marine settings.