We have interpreted and mapped multichannel (MCS) and single-channel (SCS) seismic data from the middle Atlantic margin, offshore New Jersey, encompassing Ocean Drilling Program Leg 150 drill sites on the present upper slope, to reveal three-dimensional depositional geometries and slope morphologies and provide insight into slope evolution. Interpretation and mapping focused on six seismic reflections from the upper Oligocene to upper Miocene section, which constitute a subset of a large number of seismic horizons identified during Leg 150. The reflections are considered probable sequence boundaries, although the defining geometries are found beneath the shelf, not in the region of the slope where the drill sites are located.
  Structure and isochron maps reveal how slope depocenters evolved as the shore-connected shelf/slope sediment prism prograded toward the study area. Post-upper Oligocene depositional fabrics are commonly oriented along strike (i.e., along slope). During parts of the middle and late Miocene, sediment accumulated in accommodation space seaward of secondary slope breaks, which formed seaward of the primary clinoform breakpoints associated with the prograding shelf/slope sediment prism. Isochrons of upper Miocene sediments illustrate the arrival of the toe of the shelf/slope sediment prism at the southwestern edge of the study area.
  Pre-Pleistocene canyons are rare within the study area. A large, buried, middle Miocene (reflection m3) canyon differs markedly from modern slope canyons in its broad, planar floor. We infer that the canyon formed as a result of slope failure, though later canyon fill exhibits evidence of downslope sediment transport. Younger, middle and upper Miocene canyons mapped on reflections m1 and m0.5 are narrower and V-shaped. Both slope failure (probably coupled with headward erosion) and downslope erosion may have contributed to canyon formation at different times and on different parts of the slope. The presence of canyons together with lithological evidence of mass-flow deposits highlights the role of downslope sedimentary processes, confined and unconfined, in controlling erosion and deposition during the Miocene. Hemipelagic deposition appears to have been the dominant sedimentary process between mass-flow and canyon-cutting events.

Date of initial receipt: 28 February 1995
Date of acceptance: 14 July 1995

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