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 envir onment, most likely in response to storm-driven flows. As the Holocene tran sgression has continued, the ridges have been transferred to an offshore hy drodynamic regime, where currents are not constrained by the coast and stor ms do not influence bottom currents as frequently or as strongly. Here, we qualitatively and quantitatively investigate the morphology of offshore san d ridges and smaller-scale features in an effort to place constraints on be dform development in these deeper waters. A recent high-resolution swath so nar survey mapped a portion of the New Jersey shelf from similar to 20 m wa ter depth to the shelf break (similar to 120 m), imaging both sand ridges a nd smaller-scaled dunes and ribbons in far greater detail than has been pre viously possible. Using a robust statistical analysis, we find that the gro ss morphology of ridges (height, width, length) does not change with depth beyond similar to 20 m water depth, and changes in ridge orientation genera lly mirror changes in regional contour orientation. Hence, ridges have not continued to grow since transgression has brought them into the offshore hy drodynamic 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, whic h has an asymmetry opposite to that seen near shore, and in the complex bac kscatter response that some important modifications to ridges are taking pl ace at these water depths. In contrast, on the mid-shelf (from the Mid-Shel f shore to the Franklin shore), ridges tend to have higher backscatter at t he crests, implying that these are largely winnowed, relict features. Linea ted, smaller-scale (similar to 100-500 m wavelength, <1 m rms height) featu res are also present in the seafloor morphology, On the inner shelf these a n similar to North-trending, and are evidently transverse to an onshore-dir ected flow (dunes), whereas on the mid shelf these are similar to NE/SW-tre nding and are parallel to a SW-directed flow (ribbons). The inner shelf dun es frequently exist on the seaward flanks of ridges, whereas the mid-shelf ribbons only inhabit large NE/SW-oriented swales between clusters of simila r to ENE-trending ridges. Morphologic and seismic evidence suggests that th e ribbon-floored swales represent erosional intrusions into the otherwise r elict ridge morphology. Also present on the mid-shelf are asymmetric blow-o ut 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.