The relative importance of gravity-induced versus current-controlled sedimentation during the Quaternary along the Mideast U.S. outer continental margin revealed by 3.5 kHz echo character
A detailed echo-character map based on more than 50,000 km of 3.5 kHz echograms reveals the relative importance of contour currents, turbidity currents and mass-wasting processes for the upper 50-100 m of sea-floor sediments along the U.S. continental margin between Southern Georges Bank and Cape Hatteras. Nineteen individual echo types are mapped and interpreted as deposits produced either by current-controlled or gravity-induced processes based on echo-type distribution, analysis of selected sediment core samples, and the published results of other workers. Turbidity currents and mass wasting have apparently been the primary depositional agents throughout 60% of the region. In contrast, contour currents alone appear to have influenced only 10% of the region. However, combined processes, i.e., non-separable interaction between downslope and parallel-to-contour processes have influenced an estimated 25% of the margin. The remaining 5% of the study area constitutes the outermost continental shelf; echo types here reflect the influence of fluvial-deltaic and shallow marine sedimentary processes. Changes in the relative distributions of various echo types with sea-floor depth indicate that the dominant sedimentary processes change downslope and are significantly different within each physiographic province. Deposits formed by mass wasting dominate the continental slope where 50% of the sea floor is eroded and gullied by slides, slumps and the heads of debris flows. Deposits developed under the influence of combined processes are most wide-spread on the upper continental rise and cover 40% of the sea floor, primarily in the form of deep-sea channel levees modified by contour currents. On the lower continental rise, turbidites form more than 50% of the sea-floor deposits. The downslope deposits (turbidity currents, slumps, debris flows, etc.) were emplaced primarily during the Pleistocene. Glacioeustatic sea-level lowering moved beaches and rivers across the exposed continental shelf to the shelf edge, where massive fluxes of terrigenous sediment could be delivered to the continental slope and rise by turbidity currents. Significant amounts of this sediment were transported through submarine canyons and leveed, deep-sea channels to the lower continental rise and formed the lower continental rise terrace, a flat turbidite basin ponded behind the Hatteras Outer Ridge. Southwest of the Wilmington Channel, this glacially derived influx was diminished; however, depositional patterns indicate that downslope processes continued to be dominant at least as far south as Cape Hatteras. Where rapid deposition on the continental slope and uppermost continental rise created instabilities, slope failure produced slumps, slides and debris flows and provided an additional, volumetrically important redistribution of sediment downslope. The influence of contour currents is clearly observed only in those areas topographically shielded from the masking effects of downslope processes, in particular along the seaward side of the buried Chesapeake Drift and the Hatteras Outer Ridge. © 1989.
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