MINERALOGY OF THE SILT FRACTION IN SURFICIAL SEDIMENTS FROM THE UNITED-STATES MID-ATLANTIC SHELF, SLOPE AND RISE

An analysis of the abundances and distributions of silt-sized heavy minerals from the U.S. mid-Atlantic outer continental shelf, slope, and rise shows that heavy minerals constitute a substantially greater weight percent of the silt fraction than that of the sand fraction regardless of environment and sediment texture. Concentrations of silt-sized heavy minerals progressively decrease from the shelf where they average 6.94%, to the slope and rise where they average 4.45% and 3.45%, respectively. A mixed amphibole-garnet + staurolite-epidote-pyroxene association dominates the silt-sized heavy mineral assemblage on the slope and rise; an ilmenite-amphibole-epidote association predominates on the shelf. Downslope trends in detrital nonmicaceous silt-sized heavy mineral abundances are related to hydraulic sorting rather than to chemical weathering. Elevated concentrations of the authigenic pyrite, siderite, dolomite + ankerite, and, possibly, phosphorite in the surficial slope sediment suggests that formation of silt-sized heavy minerals by diagenetic processes is relatively more important there than on the continental shelf or rise.