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Sylvia Nordfjord^1,2, Sean P. S. Gulick², James A. Austin, Jr.², John A. Goff², and Craig S. Fulthorpe²

¹The University of Texas at Austin, Department of Geological Sciences, C1140, Austin, TX 78712-1101, United States
² The University of Texas at Austin, Institute of Geophysics, 4412 Spicewood Springs Rd., Building 600, Austin, TX 78712-1101, United States

ONR’s Geoclutter program is a multi-year initiative to investigate systematic relationships between geologic phenomena and discrete acoustic signals ("geoclutter"). The New Jersey middle and outer shelf is the natural laboratory for Geoclutter because of the abundance of surficial geologic structures, including complex dendritic networks of fluvial (?) channels shallowly buried below the seafloor. Their geophysical characterization includes collection of high resolution seismic data in August-September 2001, using a deep-towed chirp sonar aboard the R/V Endeavor. These data image the upper ~30 m of sediments at ~2 m horizontal resolution and ~10 cm vertical resolution. Track spacing is typically 200 m. This resolution allows us to resolve subsurface structures in greater detail and over a wider range of spatial scales than previous imaging efforts using boomer sources. We present preliminary results from the central portion of the survey, a ~20x30 km area covering two separate dendritic systems.

Principal drainage axes of buried channels are oriented NW to SE. Multiple incisions include cut-and-fill morphologies that characterize the channel fills, suggesting a complicated history since the last eustatic lowstand, ~20-22 ka. Both flat-lying and chaotic reflections are imaged within the fills, suggesting that infilling occurred episodically, in response to high-order relative base-level variations, and under fluctuating energy conditions. Channel-fills are often capped with a thin veneer of sand deposited after channel-filling; this has been confirmed by recent grab sampling geoclutter aboard R/V Cape Henlopen.

These sediments appear to have been deposited during the last glacio-eustatic cycle (~100 kyr). A regionally important seismic reflector mappable throughout the middle to outer shelf, "R", has been interpreted as an erosional surface created during the last regression. This hypothesis is supported by the few age constraints currently available (AMS Carbon-14 dates), which show the age of "R" to be between ~43.1 kyr and 47.8 kyr. "R" appears at the base of the outer shelf sediment wedge as a high-amplitude, continuous reflection and shoals landward to the middle shelf, where it is in places truncated by the seafloor. While some channels are capped by and therefore older then "R", the majority of observed buried channels either incise "R" or lie stratigraphically above "R".

The variety of buried channels on the New Jersey shelf suggest that late Quaternary channelized flow occurred over a large range of spatial scales, from widths of hundreds of meters and thicknesses of a few meters to widths of kilometers and thicknesses of tens of meters. Generally, trunk (main) channels have box-like cross-sections, with flat floors and high width/depth ratios. Smaller, tributary channels have v-shaped cross-sections with lower width/depth ratios. We believe that the observed range of channel types is likely influenced by both the magnitude of discharge and the character of the flow regime.

The deepest channel fills have never been sampled, despite repeated attempts using both vibra- and piston coring. However, in September/October 2002, the GLAD-800 system, modified with heave compensation, was used aboard the R/V Knorr to determine lithologies and physical properties at depths previously unattainable.