Sediments of the Continental Margin off the Eastern United States
The character and distribution of surficial sediments on the continental shelf and slope between northern Maine and southern Florida have been denned on the basis of more than 6,000 bottom samples. Most shelf sediments were deposited in shallow water (probably littoral depths) during the last lower stand of sea level. Modern sediments are accumulating only in estuaries, in certain nearshore areas and on the continental slope. A large portion of the shelf sediment north of lat 41° N. was deposited by Pleistocene glaciers that covered the area. Sediments on the middle Atlantic shelf consist of predominantly arkosic to subarkosic fluvial sands. The inner shelf off the southeastern United States is covered by suborthoquartzitic fluvial sands, derived mostly from Piedmont rivers; carbonate-rich sands occur over much of the outer shelf and upper slope. Residual detritus, reworked from underlying mid-Tertiary formations, is an important sedimentary component on Georges Bank and Nantucket Shoals, Onslow Bay, the Florida-Hatteras slope, and the Blake Plateau. Although most sediments are not in compositional equilibrium with the present-day shelf environment, there is considerable evidence to suggest that many may be in at least partial textural equilibrium. Holocene reworking has removed most fine-grained sediment, leaving only coarse to medium sand. Some fine-grained fluvial sediment escapes the estuaries and near-shore during floods and storms, but this influx is not sufficient to offset the effect of winnowing by currents and waves. A significant portion of the modern near-shore sediment, in fact, may be derived from landward transport of fine-grained sediment from the central and outer shelf. Despite the evidence of active sediment movement on the shelf, however, the net lateral transport of sand and gravel appears to be limited. Because of the lack of present-day terrigenous sedimentation, a primary source of modern shelf sediment is calcareous skeletal material. If the present-day surface were preserved in the geologic record, much of it probably would be a carbonate-rich layer, containing altered and reworked skeletal material representing a variety of depositional environments.