Discussion

Rippled Scour Depressions: Vibracore and grab sample data (Thieler 1997) indicate that the coarse shell hash and gravel in the rippled scour depressions (sea-floor depressions floored with rippled, coarse shell hash and gravel [RSDs] ) is derived from two sources:

1) in-place winnowing and/or local erosion and transport of Quaternary paleo-fluvial channel-fill sediments; and
2) reworking and seaward transport of beach nourishment material from the subaerial beach and upper shoreface.

Figure 8. Click for larger image with caption.

Pearson and Riggs (1981) were the first to document the presence of beach nourishment sediment on the shoreface off Wrightsville Beach, which is recognizable by its gray-stained quartz fraction and black-stained carbonate fraction. Subsequently, Thieler(1997) identified the geographic extent of this sediment on the shoreface and inner shelf. In addition to the gray-black sand and shell material, Thieler (1997) also identified a brown-orange stained nourishment sediment on the shoreface and inner shelf that reflected a change in dredging borrow areas from the backbarrier lagoon (gray-black) to the tidal deltas and navigation channel of Masonboro Inlet (brown-orange). Both of these sediment types are recognizable in the cores shown in Figure 8 (at right).

Two explanations for the formation of RSDs off Wrightsville Beach are presented in the following pages listed below. It is likely that both act in concert to form and maintain these features.

Grain-Sized Variations

Cross-shore Transport