Assessing the 'GEO' in GEOCLUTTER: New Chirp Sonar, Sampling, and Compressional Wave Veolcity Results from the New Jersey Shelf

Abstract

Geoclutter is a multi-year initiative supported by ONR to understand potential surficial and shallow sub-seafloor geologic causes for artifacts in tactical sonar returns. Following up a detailed acoustical characterization of a portion of the outer New Jersey continental shelf, two cruises have collected sediment samples, measured in-situ compressional wave velocities at the seafloor, and collected detailed 2D (200/400 m profile spacing) and pseudo-3D (50 m profile spacing) deep-towed chirp sonar profiles. In Area 1, ~20 (E-W) by ~26 km (N-S) in water depths 60-100 m, two distinct, shallowly buried, dendritic drainage systems trend generally NW-SE. Channels as small as 25 m wide and only 1-2 m deep can be resolved. Trunk channels are up to ~2 km wide and 12-15 m deep. Tributaries meander; N-S and SW-NE incisions are observed in both systems. Both of the drainage systems appear to outlet toward a broad gravel patch on the outer shelf, which was mapped by grab sampling and acoustic backscatter. Compressional velocities at the seafloor were measured by a probe recently developed at UNH. Observed values varied by sediment type. In Area 2, SW of Area 1, ~8 (E-W) by ~10 km (N-S), a less well-developed set of incisions appears to trend ~E-W. Some incisions in both areas correlate with ``clutter'' targets previously mapped. To the north of Area 1, a series of E-W lines at 0.5 nmi spacing covers the outer shelf between the Hudson mid-shelf valley and the head of Hudson Canyon. A spectacular, buried, apparently meandering incision more than 1 nmi across may link the two seafloor features. Incision fill suggests early, rapid emplacement of debris/rubble(?), followed by more gradual(?) deposition of stratified material. We suspect that changes in filling style correlate with changes in the flow regime through the channel system through time, during the early part of the Holocene transgression. Total thickness of fill is 30 m or more. Relationships between drainage systems in Areas 1 and 2 and the larger ``proto-Hudson'' channel system are not obvious, but must be complicated. Timing relationships among these features should be elucidated by drilling efforts planned for the New Jersey outer continental shelf in September 2002.

Department

Center for Coastal and Ocean Mapping

Publication Date

2001

Volume

2001

Journal Title

Fall Meeting, American Geophysical Union (AGU)

Conference Date

Dec 10 - Dec 14, 2001

Publisher Place

San Francisco, CA, USA

Publisher

American Geophysical Union Publications

Document Type

Conference Proceeding

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