STRATIGRAPHIC HETEROGENEITY: IMPLICATIONS TO MODERN SHOREFACE PROCESSES

On the sediment-starved inner shelf near Kitty Hawk, NC, seafloor sediment is heterogeneous. The lithological variability has been attributed to the exposure of a coarse-grained, poorly sorted underlying stratum that is exposed at the surface of the seafloor. It has been interpreted as a transgressive ravinement surface based on its lithology and regional continuity (40 km). We have used the surface to define the volume of sediment in the nearshore and positively correlated that volume to decadal shoreline change. Because the layer appeared to be coarser grained than the overlying nearshore sands, we assumed that it would prohibit the mining of sediment from below it. In 2005, 19 vibracores were collected from the nearshore to determine the down core variability in grain size and its implications for the evolution of modern shoreface processes. In all of the cores, a coarse layer was identified which differed from the nearshore marine sands above it and corresponded to the continuous seismic reflection surface previously identified. In contrast to the nearshore sands, the coarse layer is comprised of greater than 50% coarse sands and gravels and has a peak at 0 to -1 phi. Furthermore, the D50 of the coarse layer is almost an order of magnitude higher than that of the nearshore sands, implying that the sediment is more difficult to mobilize. Because these stark lithologic differences generate roughness gradients on the modern seafloor, our interpretation of the stratigraphic variability has important implications to sediment transport and the evolution of the littoral system as sea level continues to rise.