Post-Transgressive and Modern Erosion on the New Jersey Outer Shelf

Recent erosion is evident on the outer New Jersey shelf (> 50 m water depth) based on analysis of multibeam bathymetry, backscatter, and chirp seismic reflection data, as well as grab samples and short cores. Truncation at the seafloor of the transgressive ravinement surface indicates that erosion occurred in the post-transgressive environment, i.e. after passage of the shoreline and development of the surficial sand sheet that caps the ravinement. Apparently moribund oblique sand ridges are also truncated by large, erosional swales oriented along the primary modern current direction, indicating that erosion likely post dates sand ridge evolution, which is known to persist to water depths of 40 m. Post-transgressive erosion has exposed a variety of strata at the seafloor, including: shallowly buried, fluvial channel systems, formed during or somewhat after the Last Glacial Maximum and filled during the transgression; the outer shelf wedge, likely deposited during falling sea level conditions; and a regional reflector, "R", that likely represents erosion during the last regression , i.e., exposing material >40 kyr. Depths of erosion range from a few meters to >10 m. A "ribbon" seafloor morphology marks much of the eroded regions. Ribbons are observed in the backscatter data as alternating bands of low and high backscatter elongated in the direction of bottom flow. Samples from the high backscatter regions are a mixture of shell hash, mud and sand; the latter exhibit populations of both abraded and unabraded grains. The shell hash is likely an erosional lag, perhaps remnants of the transgressive ravinement surface. The muds and unabraded grains are, because of negligible modern sediment input, evidence of newly eroded, previously undisturbed sediment. The lower-backscatter areas of the ribbon morphology consist of a well-sorted medium sand unit only a few tens of cm thick, overlying the shelly-muddy-sands. Well-rounded gravels and cobbles have been found in areas with very high backscatter; seismic data through one gravel mound indicates that it is likely derived from the base of an eroded fluvial/estuarine channel. Reworking of seafloor sediment in the post-transgressive regime appears to change from sand ridge evolution in inner to middle shelf depths to more predominantly erosional modification at outer shelf depths. We speculate that this change may be related to the reduction in the effectiveness of wave resuspension of sediment with increasing water depth.