Abstract

The shapes and surface textures of quartz sand and silt grains from the mid-Atlantic continental shelf were measured and analyzed with the Fourier grain-shape technique, and examined with the scanning electron microscope, in order to determine the sources and distribution of late Pleistocene and Holocene sediment on this shelf. The study indicates that there are two distinct sources of sediment for the mid-Atlantic shelf: the Atlantic Coastal Plain and the Appalachian Highlands. The sediment contribution of the Atlantic Coastal Plain is represented by abundant spherical, rounded quartz grains with surface textures created by the weathering of the quartz grains in organic-rich soils. The sediment contribution of the Appalachian Highlands is represented by abundant nonspherical, angular quartz grains with crystalline nodes, grain embayments, fractures, and euhedral quartz overgrowths created by the crystallization, recrystallization, and diagenesis of quartz. The distribution of coastal-plain and Appalachian sand on the mid-Atlantic shelf is controlled to a great extent by the late Quaternary paleogeography. The highest percentages of Appalachian sand are generally found within the estuaries and on the surfaces of the shelf-valleys and deltas of the Hudson, Great Egg, Delaware, Susquehanna, and Roanoke River systems, which drain the Appalachian Highlands. The highest percentages of coastal-plain sand are generally found in areas most removed from the depocenters of the Appalachian river systems and in areas that are proximal to exposures of coastal-plain strata. On the other hand, the distribution of coastal-plain and Appalachian silt is controlled to a great extent by the prevailing Holocene coastal and shelf currents. These currents are very capable of transporting silt across the shelf and have effectively mixed Appalachian silt from the depocenters of the Appalachian river systems with coastal-plain silt from the intervalley areas. Five sedimentary petrologic provinces are defined for the mid-Atlantic shelf on the basis of areal variations in the abundances of coastal-plain and Appalachian sand. Three provinces (Hudson, Delaware, and Hatteras) are the deposits of the large Appalachian river systems of the mid-Atlantic seaboard; the two remaining provinces (southern New Jersey and southern Delmarva) are deposits of fluvial and littoral sediments that were eroded from local outcrops of coastal-plain strata.

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