U.S. Geological Survey Open-File Report 94-152

Geologic Features of the Sea Bottom Around a Municipal Sludge Dumpsite near 39鑞, 73鱒, Offshore New Jersey and New York: U.S. Geological Survey Open-file Report 94-152

DESCRIPTION OF THE SEA FLOOR

A regional view of the continental slope and rise off the northeastern U.S. depicted by the GLORIA survey (EEZ-SCAN 87 Scientific Staff, 1991) shows that, in comparison with neighboring areas to the northeast and southwest, the dumpsite area is not traversed by major submarine channels. To the northeast lies Hudson Canyon, the largest canyon of the U.S. east coast continental slope, and its well defined meandering channel on the continental rise. To the southwest the continental slope is intricately dissected by a number of canyons and valleys which feed into a complex system of channels and less well-defined, probably debris-choked, flow pathways on the rise.

GLORIA sidescan-sonar mosaic of slope and rise off the mid-Atlantic coast of the U.S. shows the study area in relation to features of neighboring areas of the sea floor Bathymetric contours at 200, 1000, 2000, and 3000 meters. The areas of site study, Sea Beam bathymetry, and municipal sludge dumpsite are outlined.

Overlying the GLORIA images of the dumpsite area with the Sea Beam bathymetric contours shows a subdued but complex surface of the continental rise in the dumpsite area.

Center: Composite illustration of GLORIA imagery and bathymetric contours of the dumpsite study area. The central part of this area was mapped using Sea Beam data, and is shown using a 10-m contour interval; contours of the peripheral area, at a 100-m interval, from Escowitz and others (1988); of the northeastern part, kindly provided by A. Shor). Note the congruence of the GLORIA backscatter pattern and the bathymetric contours. Central dark colored deposit separates lower areas, which finer scale imaging (below) shows to be furrowed. Left image: GLORIA imagery of study area without overlay. Right image: Contour lines (Robb, 1992) and location of deep-towed 120-kHz sidescan-sonar imagery. Rectangle shows location of data illustrated below.

While the adjacent continental slope, to landward, has an average declivity of about 3.5o, the average declivity of the continental rise is about 0.45o, changing from about 1.9o in the uppermost part near the continental slope to about 0.29o in the seaward part of the study area. There is an intermittant trough at the base of the continental slope along a band of sedimentary mounds on the upper continental rise (see A on the perspective diagram below).

Perspective image of Sea Beam bathymetric data set. View from southwest, looking northeast, across upper rise. Lower part of continental slope on upper left. Brightness of each grid cell in this image is varied with respect to a vertical sun angle and the viewer's position. Note the terrace or trough at the base of the continental slope (A), the slope-parallel wrinkles(?) (B), an area of probable slide deformation on the upper rise (C), and depressions (D). Slight north-south bands of noise show where data swaths abut. Image courtesy of Robert C. Tyce, University of Rhode Island.

There are hillocks and depressions having relief of 20-30 m on the rise surface, especially in the southern part of the mapped area. Continuous, well-defined channels are absent on the rise in the dumpsite area. Instead, broad flat low areas that are generally distinguished on the GLORIA images as high backscatter regions extend seaward from the debouchment of continental slope canyons or chute-like valleys.

Subbottom profiles show faulting and displacement of subsurface strata on the rise near the base of the slope as well as fracture and tilting of tabular blocks of surficial sediments; uneven sea-floor surfaces are observed at greater distances offshore, implying that blocky debris traveled long distances.

Above: Airgun seismic-reflection profile along upper continental rise near base of slope shows strata of uppermost rise, slump structures, and faults. Labeled scarp is shown in higher resolution profile below.

Above: High-resolution 3.5-kHz subbottom profile shows faulting and tilting of surficial acoustically laminated strata on upper rise; graben created by slumping of upper rise sediments. Labeled scarp is crossed by airgun profile above.

In places acoustic profiles show subsurface reflecting surfaces that are truncated at the sea floor. In other places there are smooth and nearly acoustically transparent mud-flow(?) deposits or deposits of blocky debris.

High-resolution 3.5-kHz subbottom profile across the seaward part of study area on upper continental rise. Arrow marks transition between area of smooth surface to the northeast, and rough, acoustically impenetrable surface, probably of blocky debris-flow deposits to the southwest.

Boulders derived from mass wasting of the continental slope are widely distributed on the surface of the continental rise. They were observed during a number of Alvin dives off the New York Bight (Hanselman and Ryan, 1983; Rawson and Ryan, 1983; Ryan and Farre, 1983). A boulder sampled during Alvin dive 2163 (1989) was identified to be of middle Eocene age (C. W. Poag, personal commun., 1990). Many boulders, commonly meters in diameter, which visually resemble the Eocene mudstone, were observed on the continental rise 12 to 45 km from the base of the slope.

Boulders observed during dives of RV Alvin in 1989. Boulders are about 2 m in diameter. A (left). Dive 2163, water depth about 2485 m; B (right). Dive 2165, water depth about 2405 m.

Smaller fragments (&gt10 cm in diameter) of semiconsolidated Eocene mudstone were recovered from benthic trawls on the upper continental rise as far as 11 km from the base of the slope. Fields of tens of boulders were observed with a short-range sonar system used by Alvin during dive 2165. Some boulders on the rise are associated with channels, where they may have been transported by turbidity currents or debris flows. Boulders observed during Alvin dives 2162, 2164, and 2165 near the sludge-dump area are not associated with channels. The 120-kHz sidescan-sonar surveys revealed offshore-trending ridges and furrows commonly associated with boulder-like targets, in the regions of high backscatter on the GLORIA mosaic, near the central part of the dumpsite area (Robb, 1991).

Left: 120-kHz Sidescan-sonar tracklines (vehicle tracks) in relation to bathymetry. Rectangle locates next image.

Center: Mosaicked images over furrowed bottom area; 10-m contour interval. Closed-contours show depression (upper right quadrant: 2500-m contour identified) and the neighboring, downrise, elliptical high (near upper center of image). Rectangle locates the swath shown in the next image.

Right: A single swath of AMS-120 sidescan-sonar imagery, 900 m wide x 3300 m long. See location to left.

To continue, click on:: Discussion...; Conclusions...; References Cited...; Illustrations Summary
Or, to review, click on:: Abstract...; Introduction...; Data and Methods...; (Bathymetric map); (GLORIA mosaic); Description of the sea floor...

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