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A dense grid of near-bottom Sea MARC 1 30-kHz sidescan and 4.5-kHz sub-bottom sonar data, high-resolution air-gun seismic reflection profiles, seafloor photographs, aind drill data have been used to study the near-surface geology of a 30 × 30 km segment of the New Jersey continental margin containing DSDP Sites 612 and 613. On the basis of age and acoustic properties, 16 surficial sedimentary units have been defined and mapped. The continental margin is separated into three distinct provinces: the upper and middle continental slope; the lower continental slope; and the upper continental rise. The upper and middle continental slope is covered by terrigenous Quaternary sediment and is incised by Uto Vshaped (in cross section) canyons and valleys which die out on the lower slope. Areas of the upper and middle slope not incised by canyons also show evidence of significant, geologically recent, downslope-directed erosion. The surficial lower continental slope consists almost exclusively of rugged exposures of middle Eocene biosiliceous limestone. In the southwestern part of the lower slope, two lower slope canyon systems begin in water depths of 1500-1600 m. They have steep, joint-controlled bounding walls and broad floors that coincide with bedding planes in the Eocene section. We argue that these canyons formed mainly by the removal of jointand bedding plane-bound slabs of strata by downslope-directed currents during the Pleistocene. The eastern portion of the lower slope is composed of gently seaward-dipping surfaces, which coincide with Eocene bedding, offset by steep, fewmeter-high scarps, which step down deeper into the stratigraphic section. Also present in this area are numerous downslope-trending grooves, a few meters deep and wide, which we interpret to be mega gouge marks cut into the exposed limestones during rock and sediment slides. Terrigenous deposits of the upper continental rise onlap the erosional surface of the lower continental slope. An abrupt decrease in seafloor gradient (4.5 to 1.5°) marks the continental slope/rise boundary. Three oblong topographic depressions, 30-70 m deep and a few kilometers wide and long, are excavated into the upper continental rise. They range from being empty to nearly filled by horizontally bedded strata which are interpreted to be mass flow deposits derived from the slope. Immediately landward of one depression, on the lowermost slope, are a series of rock-slide scars which appear fresh on sidescan and sub-bottom profiles. Seaward of this depression are downslope-trending shallow channels, allochthonous debris and one large block (several meters in diameter) which has skidded > 4 km downslope across the 1.5° gradient of the upper continental rise, leaving a clearly defined trail. The depressions are apparently eroded as downslope-directed mass flows encounter the abrupt gradient reduction at the slope/rise boundary. Prior to the late Miocene, significant erosion exposed semi-lithified middle Eocene rocks that have apparently remained exposed to the present day. Although this event (or events) cannot be precisely dated, we present two possible mechanisms for the initiation of lower slope erosion during the Oligocene. Subsequent slope erosion has prevented significant sediment from accumulating above this major erosional surface. Only through the process of continental rise onlap has sediment accumulated to significant thickness above the major unconformity. Approximately two-thirds of the study area is now draped by a 1to 3-m thick hemipelagic deposit of late Pleistocene-Holocene age. While the magnitude and frequency of downslope-directed sediment transport and erosion may have decreased since the late Pleistocene, the remotely-sensed data indicate that these processes have not altogether ceased. Results from our detailed study indicate that sites for stratigraphic sampling on continental slopes must be chosen very carefully, if the most complete stratigraphic sections are desired. Margins of submarine canyons may be the preferred locations, because most downslope-directed and potentially erosive flows tend to follow down canyon thalwegs, leaving canyon margins relatively undisturbed.

Surficial Geology of the Continental Margin offshore New Jersey in the Vicinity of Deep Sea Drilling Project Sites 612 and 613