SPATIAL CORRELATION OF PALEO-CHANNELS WITH GRAVEL OUTCROPS AND SHORE-OBLIQUE SANDBARS IN THE NEARSHORE

There is growing evidence that surface sediment characteristics in the nearshore, and possibly beach and sandbar morphodynamics, may be influenced by framework geology. A research cruise was conducted to determine if there is a correlation between stratigraphically preserved paleo-channels, gravel outcrops and shore-oblique sandbars. These latter morphological features have been observed together in various locations in the surf zone of southern Virginia and the Outer Banks of North Carolina. Further, they seem to be associated with areas of anomalously high beach erosion, or hotspots. The gravel outcrops are characterized by non-sandy fine sediments and gravel, and appear to be part of an underlying substrate, possibly a ravinement surface. The shore-oblique sandbars are typically found adjacent to, and on the south side of, the outcropping gravel patches. Time-series data have shown remarkable spatial stability in both features over several years.

Chirp seismic was used to investigate the presence of relict channels. Study sites were chosen based on the location of previously mapped shore-oblique bars in the nearshore and paleo-fluvial channels in the offshore. Side-scan sonar was employed to highlight the precise outcrop locations, while the presence of bars was ascertained from both the Chirp record and side-scan data.

Preliminary results indicate the presence of large relict channels at three of the sites, in the vicinity of previously identified bars and outcrops. The fourth site, a control with no bars or outcrops, shows some evidence of small, but poorly defined channels. Analysis is ongoing to spatially correlate the paleo-channels to transverse bars and gravel outcrops, as well as to areas of elevated beach erosion.