Evolution and Preservation of Sand Ridges from the New Jersey Middle and Outer Shelf
Sand ridges on the northeast US Atlantic shelf form in the near shore environment, most likely in response to storm driven flows. With Holocene sea level rise, sand ridges are detached from the shoreface and transitioned into an offshore hydrodynamic regime. This paper reports on the results of more than a decade of high-resolution survey work over sand ridges on the New Jersey middle and outer shelf. The morphology and seismic structure of offshore sand ridges and smaller-scale features are investigated in an effort to place constraints on bedform evolution and sand bed preservation in these deeper waters. The gross morphology of ridges (height, width, length) does not change with depth beyond ~20 m water depth, indicating that sand ridges do not grow in the offshore hydrodynamic regime. However, several lines of evidence indicate that sand ridges on the middle shelf (~20-50 m water depth) continue to substantively evolve: (1) ridge shape, which has an asymmetry opposite to that seen near shore; (2) a complex backscatter response, which indicates textural variations that are not consistent with near shore ridge development;
3 phase shifts between modern mid-shelf sand ridges and undulations in the underlying transgressive ravinement surface, which indicate substantial translation of the ridges from their formative locations; and
4 the presence of smaller-scale (~100-500 m wavelength, <1 m rms height), N-trending transverse bedforms on the seaward flanks of ridges, indicating sediment transport on the upcurrent flanks to a shoreward-directed flow. In contrast, sand ridges on the outer shelf (~50-150 m water depth) have higher backscatter at the crests, lower backscatter in the swales, and have no secondary bedforms on their flanks, implying that these are largely winnowed, relict features. Longitudinal bedforms (ribbons) on the outer shelf inhabit large NE-oriented swales between clusters of ~ENE-trending ridges. Morphologic and seismic evidence suggests that the ribbon-floored swales represent erosional intrusions into the otherwise relict ridge morphology. Truncation of both the transgressive ravinement and the moribund sand ridges place the erosion in a marine setting, post-dating the passage of the shoreface ravinement and the evolution of sand ridges that form initially in the near shore environment. These conclusions may have implications for understanding the preservation potential of transgressive sand sheets in the stratigraphic record.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009