ABSTRACT

Mineralogical studies of the silt fraction have been made on surficial sediments from the U.S. North Atlantic outer continental shelf off southeastern New England. The silt-sized heavy mineral assemblage, which is predominantly detrital, has been concentrated in this fraction by hydraulic factors and ranges between 11.8 wt. % of the silt fraction in the sandy sediments near Georges Bank to 3.4% in the clayey silt deposit south of Martha's Vineyard. By contrast, the sand fraction averages only 1.5% heavy minerals. Corundum and the rutile + leucoxene + brookite + anatase, epidote, and apatite + monazite + vivianite groups are relatively enriched in the silt fraction, whereas the amphibole and garnet + staurolite groups are relatively concentrated in the sand fraction. "Opaque" minerals verage 27.8% ilmenite, 4.8% magnetite + hematite + goethite + siderite, 1.6% ferromanganese-coated grains, and 0.9% pyrite of the silt-sized heavy mineral fraction.

Lateral variability within the silt-sized heavy mineral assemblage is considerable. Zircon and ilmenite abundances progressively decrease from east to west on the continental shelf off southeastern New England. Conversely, abundances within the amphibole and the epidote groups increase from east to west. Quartz dominates the light mineral + layered silicate fraction, where it averages 70.3%. The K-feldspar/plagioclase ratios and glauconite + mica/chlorite ratios average 0.38 and 2.61, respectively. The layered silicates are concentrated in the finer-grained sediments. Although the heavy mineral assemblage is ultimately a function of provenance, the spatial trends observed in the heavy mineral populations are due primarily to sorting that is associated with westwardly-flowing bottom cu rents and are related to the specific gravity and the characteristic size of the mineral grains.

Elevated concentrations of titanium in the silty sands south of Nantucket Shoals are related to the abundances of ilmenite, sphene, and the TiO₂ group minerals and average 0.67 wt. % of the bulk sediment. This deposit may constitute an exploitable resource.