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Seismic Stratigraphic and Seismic Geomorphologic Case Study of Deep-Water Slope Deposits: Processes and Products


Detailed seismic stratigraphic and seismic geomorphologic study depositional features on a continental slope using 3D seismic data has revealed a variety of geomorphic elements. These include canyons, deflation hollows, contour-parallel erosional furrows, mass transport deposits, mass transport erosional grooves, pock marks, turbidite channels, sediment waves, and sediment drifts. The principal geomorphic factors include contour-parallel currents and contour-normal gravity flows. Evidence for uni-directional, contour-parallel currents that persist through time lies in the presence of erosional tails extending down-current from pockmarks and other sea-floor irregularities as well as dune fields indicating sediment transport direction. Sediment gravity flows comprising mass transport material both confined within canyons as well as unconfined on the slope, as well as turbidites confined within canyons are observed. Pock marks likely associated with fluid expulsion are observed in the subsurface and are concentrated along specific horizons. They are commonly associated with long erosional furrows originating at the pock marks and extending in a contour-parallel direction. Another set of erosional grooves is observed on canyon interfluves and are inferred to be associated with canyon-confined flows (tidal currents or dilute sediment gravity flows) that have overtopped canyon walls. They typically are best observed where canyons take subtle bends. The groove orientation suggests that flows are directed down-canyon and the upper part of the flows are “stripped” and decoupled from the lower part of the flows at these bends. These grooves are of lesser magnitude than those associated with contour-parallel currents. Large (~600m wide and 1km long) elliptical hollows that seem to nucleate on pre-existing fluid-escape pock marks are observed. These features, which can be described as “deflation hollows”, contain very-fine sand to silt at the base and in some instances are associated with small dune fields characterized by contour-parallel migration direction emanating from the basal fill. The surrounding sections appear to be devoid of any sand-prone deposits. Consequently, the sand at the base of these hollows appears to have been the product of winnowing by contour-parallel currents where rare disseminated grains of sand, derived from erosion of the substrate by wall erosion associated with eddying of contour-parallel currents, are concentrated.