High-resolution swath sonar investigation of sand ridge, dune and ribbon morphology in the offshore environment of the New Jersey margin

Sand ridges on the northeast US Atlantic shelf form in the near-shore environment, most likely in response to storm-driven flows. As the Holocene transgression has continued, the ridges have been transferred to an offshore hydrodynamic regime, where currents are not constrained by the coast and storms do not influence bottom currents as frequently or as strongly. Here, we qualitatively and quantitatively investigate the morphology of offshore sand ridges and smaller-scale features in an effort to place constraints on bedform development in these deeper waters. A recent high-resolution swath sonar survey mapped a portion of the New Jersey shelf from similar to 20 m water depth to the shelf break (similar to 120 m), imaging both sand ridges and smaller-scaled dunes and ribbons in far greater detail than has been previously possible. Using a robust statistical analysis, we find that the gross morphology of ridges (height, width, length) does not change with depth beyond similar to 20 m water depth, and changes in ridge orientation generally mirror changes in regional contour orientation. Hence, ridges have not continued to grow since transgression has brought them into the offshore hydrodynamic regime. However, on the inner shelf from (similar to 20 m water depth to the Mid-Shelf shore), we do find evidence in the ridge shape, which has an asymmetry opposite to that seen near shore, and in the complex backscatter response that some important modifications to ridges are taking place at these water depths. In contrast, on the mid-shelf (from the Mid-Shelf shore to the Franklin shore), ridges tend to have higher backscatter at the crests, implying that these are largely winnowed, relict features. Lineated, smaller-scale (similar to 100-500 m wavelength, <1 m rms height) features are also present in the seafloor morphology, On the inner shelf these an similar to North-trending, and are evidently transverse to an onshore-directed flow (dunes), whereas on the mid shelf these are similar to NE/SW-trending and are parallel to a SW-directed flow (ribbons). The inner shelf dunes frequently exist on the seaward flanks of ridges, whereas the mid-shelf ribbons only inhabit large NE/SW-oriented swales between clusters of similar to ENE-trending ridges. Morphologic and seismic evidence suggests that the ribbon-floored swales represent erosional intrusions into the otherwise relict ridge morphology. Also present on the mid-shelf are asymmetric blow-out pits and transverse dune packets, both of which are consistent with a SW-directed flow. Relict iceberg scours are present on the outer shelf Hudson Apron, evidently preserved by outcropping stiff clays, and are overlain in some places by similar to ENE-trending ridges. (C) 1999 Elsevier Science B.V. All rights reserved.