Sylvia Nordfjord

Basal inflection-controlled shelf-edge wedges off New Jersey track sea-level fall

A key marker for examining the latest Pleistocene history of the New Jersey margin is the high-amplitude, long-recognized R-horizon reflector. This time-transgressive surface was formed 47-33 ka and represents the integrated topographic and bathymetric surface developed during the complicated sea-level oscillation associated with the regression that preceded the Last Glacial Maximum (LGM). From landward to seaward across the outer shelf, the R reflector changes from subhorizontal, in most locations <9 m beneath the modern seafloor, to seaward dipping, forming the base of two offlapping sediment wedges: the previously described outer-shelf wedge and a deep-shelf wedge seaward of it. This transition occurs across two inflection zones, where the dip of the R reflector steepens seaward, that can be traced for tens of kilometers along strike and mark the landward limits of these wedges. These inflection zones are possibly former wave-dominated shorefaces; these represent the primary topographic elements present during the last regression. We speculate that these inflections dictated both the locations for deposition of the two prograding, offlapping wedges that developed during the complex sea-level fall prior to the LGM and their successive erosion before and after the LGM. We suggest that such inflection zones and their associated wedges are important markers of regression in clastic-dominated outer-shelf settings along passive margins.

Chirp seismic surveying of shallowly buried fluvial systems on the outer New Jersey continental shelf: Mapping channel morphology and internal stratigraphy in 3D

The outer New Jersey continental shelf has been the focus of Office of Naval Research‐sponsored acoustic studies for more than a decade. As part of this effort, ONR has funded a number of field programs to characterize the geologic history and sedimentary properties of the shallow seabed. We report here on a chirp seismic campaign conducted in 2001, with particular focus on characterizing the morphology and internal stratigraphy of shallowly buried fluvial channel systems that are widespread in this region. These channels were formed during the last glacial lowstand, and then filled and buried by sediments during sea level rise. Mapping the channel morphology in 3D presented a significant challenge. Although coverage (200‐m line spacing) was dense, application of standard interpolation techniques resulted in poor representation of channel continuity along sinuous pathways. We developed a technique for transforming the seismic horizon data into a channel‐oriented reference frame, wherein the channel is eff...