Present-day Shelf Research Articles

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.

Geosynclinal Compositions and the New Global Tectonics

ABSTRACT The lithological composition of several geosynclinal sequences estimated from published measured sections in the Appalachian and Cordilleran belts shows that sedimentary rock type is not a diagnostic attribute of geosynclinal assemblages. However, though no sedimentary rock types are excluded from geosynclinal assemblages, some are generally rare (evaporite), others are particularly common in either the miogeosyncline (carbonate) or eugeosyncline (graywacke), and still others are common throughout (shale). The chemical composition of individual sequences, estimated using analyses of specific stratigraphic units and average compositions for major sedimentary and volcanic rock types reveals chemical contrasts between miogeosynclinal and eugeosynclinal assemblages, and between ancient geosynclinal assemblages and the sedimentary successions which have accumulated in the deep ocean basins and along the present-day continental margins. Miogeosynclinal and eugeosynclinal assemblages differ chemically because of the respective predominance of carbonate and volcanic rocks; both contain chemically similar, highly siliceous clastics. If miogeosynclines and eugeosynclines are the respective ancient analogues of present-day continental shelf and rise areas, their contrasting compositions may reflect long-term secular changes in the mode and locus of sedimentation as a result of organic evolution and changes in the relief, volume, and location of continental blocks, ocean basins, and plate margins during and since the Paleozoic.

Aspects of Carbonate Sedimentation on the Atlantic Continental Shelf off the Southern United States

ABSTRACT The carbonate fraction of over 1,000 samples from the Atlantic continental shelf off the southern United States was investigated. The main purpose of this study is to determine the usefulness of the carbonate fraction in determining various aspects of shelf history in a so-called non-carbonate area. Particle characteristics of the carbonate fraction are emphasized. These include determination of the degree of particle rounding, percent fragmental, percent blackened, and percent of worn versus fresh components. The abundance of CaCO3, is mainly a function of the amount of dilution by terrigenous materials from Piedmont rivers. Calcium carbonate is least abundant on the Georgia shelf and increases greatly to the south and slightly to the north. The abundance of black shells, oolites, and shallow water fauna over large portions of the shelf attest to a shallow-water origin of much of the shelf surficial material. The abundance of worn and broken shells reflects the low rate of sedimentation in this area. Much of the present-day shelf sediment cover was deposited in the nearshore zone of the last transgressing sea but the sediment was not derived from the beach environment itself, as indicated by particle roundness considerations. The areal distribution of certain carbonate fraction arameters can be interpreted as linear patterns paralleling the present shorelines. These may reflect ancient shorelines.