The styles of soft-sediment deformation, mineralogy, and biostratigraphy of mass-transport deposits recovered from Leg 150 drill sites (902-906) provide important new information on the erosional and sedimentary history of the New Jersey continental slope and rise, and on the relationship of mass-wasting processes to changes in relative sea level. Nine distinct sedimentary facies are recognized within these mass-transport deposits, which mainly represent muddy slumps and debris flows, and to a lesser extent, sandy mass flows. These sedimentary facies, coupled with the ages of their clast and matrix material, reveal source areas, transport mechanisms, and depositional processes, and the relative contribution of downslope transport to the evolution of the continental rise.
Mass-wasting deposits compose 10% to 15% of the total sediment recovered at Sites 902-904 beneath the continental slope. Upper Miocene and Pleistocene debris flows and slumps frequently correlate to sequence boundaries beneath the upper paleoslope at Site 903, and to a lesser extent, beneath the middle paleoslope at Sites 902 and 904. In contrast, upper Oligocene to upper Miocene sands and sandy mass flows correlate to sequence boundaries beneath the middle paleoslope at Sites 902 and 904 and not beneath the upper paleoslope at Site 903. Mass flows at these sites document shelf and coastal plain erosion and sediment transport to the basin during glacioeustatic lowering. These observations suggest a correlation between mass-transport deposits and sequence boundaries. However, this correlation is complex because preservation of mass-wasting deposits appears to have been significantly influenced by the morphology and gradient of the existing paleoslope. In addition, mass-transport deposits also occur within sequences especially during the upper Miocene and Pleistocene.
Site 906 was drilled in the floor of modern Berkeley submarine canyon. The sedimentary facies recovered from the middle Miocene section document an episode of canyon excavation and infilling. The Berkeley and middle Miocene canyon fill facies are different from the mass-transport deposits that characterize intercanyon regions of the slope (they contain debris flows with clasts of diverse ages and multiple turbidites). The middle Miocene canyon cutting event apparently occurred very rapidly (~1.1 m.y.) during a single third-order sea-level cycle (13.5-12.4 Ma) and the mass-transport deposits at the base of the canyon fill rest upon a Type I sequence boundary, which forms the canyon floor.
Downslope transport of sediment via mass movements has episodically contributed to the evolution of the upper continental rise. At Site 905 on the continental rise, approximately 30 m of mass-transport deposits were accumulated during the middle Miocene. Benthic foraminifer assemblages indicate that some deposits originated in the upper slope. The lithology of the clasts in these deposits indicates they were derived from canyon excavation on the continental slope and the age of the matrix material (~13.5 Ma) suggests these deposits may correspond to initiation of canyon cutting events as the middle Miocene canyon at slope Site 906. During the upper Miocene sediment transported to the continental rise was derived from intercanyon regions as revealed by similar age of the clasts and matrix material. Downslope transport to the upper continental rise was especially significant during the lower Pleistocene when ~215 m of mass-transport deposits were delivered to Site 905. The lithologies and ages of clasts and matrix in some slumps and debris flows indicate that these deposits resulted predominantly from canyon-cutting episodes, and to a lesser extent from localized slope detachments in intercanyon areas.
Date of initial receipt: 2 March 1995
Date of acceptance: 28 November 1995
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