Discussion

Grain Size Variations: The formation of rippled scour depressions (sea-floor depressions floored with rippled, coarse shell hash and gravel [RSDs] ) is probably related to grain size variations that result in preferential erosion of fine material within substrates having a substantial coarse fraction, leaving a winnowed lag deposit on the seafloor. For example, coarse lags of Quaternary paleo-fluvial channel-fill (QPFCF) sediment would provide an initiating substrate that is coarse-sediment rich, compared to the surrounding fine-to-silty Oligocene Orb-A (sandy silt) sediment. Significant changes in boundary layer structure have been measured over an abrupt transition from fine to coarse substrates (Sleath 1987; Fredsoe and others 1993) or have been simulated numerically (Laursen and others 1994). These studies suggest an abrupt increase in shear stress and turbulence at the sediment textural boundary that would enhance the winnowing process and inhibit the deposition of fine sediment. This process should be very effective in both forming and maintaining RSDs in this sediment-starved environment, since there is insufficient (fine) sediment to bury the exposed coarse material. The presence of coarse-grained sediment in general may also play an important role, since small RSDs appear to form where rows of mobile, coarser grained sediment have been aligned by bottom currents (Flood 1981). The RSDs may then develop as a result of enhanced erosion created by the greater surface roughness, with localized abrasion or scour around sediment particles on the bed (Allen 1969). A final observation is that the sharp, northern edges of the RSDs are on the updrift side of the predominant longshore drift direction (a ragged or "wispy"edge characterizes the downdrift side). We speculate that this may be indicative of southward, shore-parallel sediment transport (the dominant longshore transport direction), resulting in minor infilling of the RSD on the northern edges and minor erosion on the southern edges. The predominant orientation of the RSDs transverse to this shore-parallel transport suggests they may be part of a larger, regional system of sand waves or similar bedforms oriented roughly perpendicular to larger-scale along-shelf flows. This configuration was proposed by Swift and Freeland (1978) for similar features off the northern Outer banks of North Carolina. Certainly this is a possibility at Wrightsville Beach; coast-parallel currents probably play a critical role in maintaining the observed asymmetric RSD morphology as fine material moves alongshore. In this case, infilling occurs on the northern side of the RSDs. Current-induced asymmetry of rippled scour depressions has also been observed in other shoreface and shelf environments (Aubrey and others 1984; Schwab and Molnia 1987; Schwab and others 1997).

Based on E. Robert Thieler, William C. Schwab, Mead A. Allison, Jane F. Denny, and William W. Danforth, Sidescan-Sonar Imagery of the Shoreface and Inner Continental Shelf, Wrightsville Beach, North Carolina: U.S. Geological Survey Open-file Report OF 98-616.

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