PALEOGENE NEOGENE DEPOSITIONAL HISTORY OF THE MIDDLE UNITED-STATES ATLANTIC CONTINENTAL RISE - MIXED TURBIDITE AND CONTOURITE DEPOSITIONAL SYSTEMS

The construction of the middle U.S. Atlantic continental rise from late Paleogene to late Pliocene time included downslope sediment gravity flows and alongslope contourite deposition. Some previous studies have suggested that contour-following bottom currents were the dominant control on depositional patterns. Depositional relief from two large contourite drifts, largely built in late Miocene to late Pliocene time, continue to dominate present-day lower rise morphology. Detailed seismic sequence mapping presented here suggests that significant turbidite (fan) systems also were active and are preserved in the stratigraphic section. A single thick depositional sequence is mapped for Oligocene to middle Miocene sediments deposited when inferred fan systems off the Norfolk-Washington canyons were reworked by bottom currents along the lower rise to form the proto-Hatteras Outer Ridge. Detailed mapping of three late Miocene to late Pliocene depositional sequences indicates that fan-channel systems were active at the same time large contourite drifts were being constructed. Sediment delivered to the lower rise by these elongate turbidite systems undoubtedly was an important local source for construction of the Hatteras Outer Ridge and, through entrainment and resuspension, was an important source for regional contourite deposition controlled by the deep Western Boundary Undercurrent. We suggest that submarine fans and current-controlled sediment drifts may develop simultaneously as companion systems. Types of fan/drift interaction include current-only modification of fans, transitional fan-to-drift, and adjacent or overlapping submarine fan and sediment drift deposition.