Investigation of isolated sand shoals on the inner shelf of Southern Virginia : final report

Offshore of Sandbridge Beach, Virginia, the surface of the inner continental shelf is a generally featureless, gently sloping plain, broken by several isolated sand shoals. The westernmost shoal, commonly called the Sandbridge Shoal, is located approximately 5.5 km from the shoreline in 1 O - 12 m of water. An analogous feature is located approximately 15 km northeast of the Sandbridge Shoal at depths greater than 15 m. During a pretiminary study conducted in 1987, 534 km of trackline were surveyed with acoustic subbottom and side-scan sonar systems. Geophysical data were recorded for an additional 318 km of trackline between 1988 and 1990. Genetic similarities between the two shoal features were analyzed and conceptual models of development were proposed. In addition to the geophysical data, 11 vibracores with a maximum length of six meters and 18 surface grab samples were acquired. Shell materials in the cores were dated using amino acid racemization and radiocarbon methods. Correlation of seismic data with vibracores and surface grab samples indicate the Sandbridge Shoal is approximately 6 x 8 km in areal extent and has a horseshoe shape in plan view. The shoal contains at least 8 x 107 m3 of clean, well-sorted, medium to coarse sand, and tapers to the north and east. The offshore shoal has a larger areal extent, but its relief above the surrounding seabed is less than half that of the Sandbridge Shoal. Both shoals are associated with large paleochannel systems, and inferred lagoonal or estuarine sediments are located below and landward of the sand bodies . ., Sediments within the shoals fine downwards, have little evidence of an aeolian overprint, lack high concentrations of heavy minerals, and contain remains of only high-salinity organisms