Conclusion

The geologic framework and surficial morphology of the shoreface and inner shelf off Wrightsville Beach, North Carolina, were mapped using high-resolution sidescan-sonar, bathymetric, and seismic-reflection surveying techniques, a suite of over 200 diver-vibracores, and extensive sea-floor observations by divers. The inner shelf is a sediment-starved, active surface of marine erosion; modern sediments, where present, form a thin, patchy veneer blanketing Tertiary and Quaternary units. The lithology of the underlying units exerts a primary control on the distribution, texture, and composition of surficial sediments, as well as inner shelf bathymetry.

The shoreface is dominated by a linear, cross-shore morphology of rippled scour depressions (sea-floor depressions floored with rippled, coarse shell hash and gravel [RSDs] ) extending from just seaward of the surf zone onto the inner shelf. The RSD sediments consist primarily of shell hash and quartz gravel. On the upper shoreface, the RSDs are incised up to 1 meter below surrounding areas of fine sand, and have an asymmetric cross-section that is steeper to the north. On the inner shelf, the RSDs have a similar but more subdued profile that is below the resolution of the bathymetric surveys. Their morphology here has been mapped exclusively by diver surveys.

The formation and maintenance of RSDs is probably related to processes at several spatial and temporal scales. At the meter- and sub-meter scale of bedforms and sediment grain size, roughness-induced variations in boundary layer structure and turbulence over rough substrates may further enhance winnowing and result in the concentration of coarse material in the floor of the depressions. At a scale of tens to hundreds of meters, subtle variations in topography may enhance downwelling currents within the RSDs, resulting in scour and winnowing of fine sediments. At the kilometer-scale of the entire shoreface, variations in island shape (e.g., concave vs. convex seaward) and the presence of a large jetty and ebb-tidal delta complex at the downdrift end of the island may enhance seaward-directed flows during storm events, as water accumulates in the bight formed by the jetty/delta and island.

Several low-amplitude, crescentic, barchan-like dunes are present on the inner shelf. Their composition, location and orientation suggests they have formed in response to localized sediment surpluses on an otherwise sediment-starved shelf sector. Local areas consist primarily of adjacent marine hardbottoms. Additional sources include outcropping Quaternary sediments or possibly beach nourishment material from the shoreface. The dunes are oriented obliquely offshore, which is transverse to the probable mean direction of storm-induced inner shelf flows.

The mapping results and interpretations presented here suggest that both the surficial morphology and underlying geologic framework play a critical role in modern sedimentation patterns that couple the beach and the inner shelf.