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C.S. Fulthorpe, J.A. Goff, J.A. Austin, Jr., S.P.S. Gulick, S. Nordfjord

High-resolution (1-15 kHz), shallow-penetration (~50 m), deep-towed chirp seismic profiles were collected on the New Jersey mid- and outer-shelf in August-September 2001 (R/V Endeavor cruise 359) in support of ONR's Geoclutter program. Geoclutter goals are to understand and predict acoustic scattering from the seafloor and the shallow sub-seafloor in order to distinguish man-made objects from naturally occurring features ("geological clutter") on sonar returns. The 2001 chirp survey focused on previously identified buried channels, of presumed fluvial origin, which may contribute to geoclutter. These channels are inferred to have formed during the sea-level lowstand of the last glacial maximum, and their fill likely constitutes a high-resolution record of the latest transgression. This survey represents the most comprehensive mapping to date of shallowly buried fluvial channels in a continental shelf setting.

To allow robust mapping of channel systems, the survey maintained dense seismic profile spacings, mainly 200-900 m, but with some at 50 m. We focus on the southern portion of the survey area, where three channel systems are identified within an 8.6 x 10.2 km grid. The northernmost of these lies sufficiently within the survey area to be mapped in detail. Its channels are mainly V-shaped, up to 350 m wide and 12 m deep. Present water depth is ~80 m and channel flanks are truncated by the seafloor, except in a small area where an overlying unconformity truncating channel flanks is preserved. Some channels are truncated by erosional seafloor scour pits, mapped earlier with high-resolution multibeam data. Channel fill stratigraphy indicates multiple stages of incision and filling. Interfluves comprise up to ~20 m of a seismically transparent facies overlying stratified sediment. Grab samples contain medium-to-coarse sands, but such surficial sediments may not be entirely representative of the transparent facies. The contact with the stratified sediment is irregular and resembles an incised surface. However, stratified blocks can occur within the transparent material, suggesting post-depositional disruption. Regional reflector "R", previously attributed to pulses of erosion during regression, underlies the channels throughout this area. "R" is mainly smooth, but is sparsely incised by its own small (0.5-2 m deep), leveed channels.

Flow in the trunk channels was E to NE and is therefore not perpendicular to the NNE-trending shelf. Incision depth and channel width both increase eastward. Channels are both meandering and dendritic: several tributary channels feed the main trunk. The three channel systems are ~-4 km apart, but the survey area is too small to ascertain whether these are independent systems or converge basinward. These channels are among the smallest in the Geoclutter survey area. Ongoing interpretation and mapping to the north reveal channel systems of a range of scales, up to the buried paleo-Hudson channel (~40 m deep and 2 km wide).

Sampling is essential to provide sediment ages, paleoenvironments and lithologies. In September-October 2002, R/V Knorr deployed the lake drilling GLAD-800 system, modified with active heave compensation, to sample channels and stratigraphic targets identified using the chirp data.