Sand Bodies on Coastal Plain Shelves: Holocene Record of the U.S. Atlantic Inner Shelf Off Maryland

ABSTRACT The inner continental shelf of Maryland, Delaware, and northern Virginia has been examined with high-resolution seismic reflection equipment and vibracores to develop an understanding of Quaternary processes and history on a coastal plain shelf. Morphology of the study region is dominated by the large shelf valley of the ancestral Delaware River and estuary and a linear-ridge field. Shallow subsurface strata consist of gently seaward-dipping Neogene sedimentary units showing no evidence of tectonic deformation. Eleven major acoustic surfaces, including the presumed Tertiary-Quaternary nonconformity at about -30 m to -60 m, are present within the upper 120 m of the shelf subbottom. Buried channels are common to the seafloor of the entire region; in the Delaware Bay entrance, most are c t to 45 m below sea level and were filled laterally by split plalform progradation from both the New Jersey and Delaware shelves. Many small channels on the Maryland shelf are aligned with existing onshore drainage or historical inlet sites and display a linear relationship between maximum thalweg depth and distance from shore. The upper 6 m of the sedimentary sequence of the inner shelf consists of terrigenous sand and silt derived from the adjacent Coastal Plain and Piedmont Provinces. Four major sediment types are recognized: three of these are subarkosic arenites varying only in modal grain size and sorting; the fourth is a slightly sandy mud. Environments of deposition preserved on the present shallow shelf are: modern marine, back-barrier, lagoonal, and fluvial. Gray-brown, fine to coarse, well-sorted quartz sand is the dominant type of surface sediment and its relative abundance decreases in the subsurface. Increases in sand thickness occur locally in ridge areas and correspond directly to topographic relief. The ridge sand unconformably overlies poorly sorted fine sand and mud remnant from Holocene b ckbarrier and lagoonal deposition; coarse constituents of the unit are commonly incorporated into the base of the ridge sand. Linear ridges are a dominant topographic feature of the U.S. mid-Atlantic shelf, and they are particularly well-defined on the Maryland shelf. Marked similarities in geometry and sediment relations of these features provide evidence of their origin on the Holocene shoreface and later segmentation and isolation on the shelf. Individual ridges commonly display a progressive south to north change from a well-defined, narrow, single-crested shape to a poorly defined, broad, multi-crested shape. This axial trend and the variation in coastal intersection angle are inherited from the ridge's origin on the shoreface where growth and bifurcation occur along the northeastern tip. Shelf sand bodies off the central Delmarva Peninsula have formed by wave and current processes acting on previously deposited sediments, and these sand bodies are being formed and modified at present. The inner shelf of this region represents the trailing edge of a marine transgression; as such, it is the coastal sedimentary facies most likely to be preserved in the rock record.