Abstract Previous investigations of the East Breaks slide interpreted this site as having been formed by a single mass-wasting event at the terminus of a late Wisconsinan shelf-edge delta. According to these interpretations, after initiation, this slide/slump propagated downslope and divided into two major lobes as it encountered a bathymetric high formed by a salt diapir. This study used a large, new data base of high-frequency 3.5-kHz acoustic reflection profiles and piston cores that shows that the two lobes were genetically separate. Their formation involved the two major types of downslope sediment transport mechanisms that dominate continental slopes: gravity-driven slide/slump/debris flows and turbidity currents. As previously recognized, slumps and debris flows formed the western lobe, but the eastern lobe was formed by channelized turbidity currents. A locus of salt tectonism might have been involved in creating the turbidity current system in the eastern lobe. This large deposit may provide a useful model for subsurface exploration in a portion of the deep water frontier which is less dominated by salt tectonics than much of the central Gulf Coast offshore slope.

ABSTRACT The mudline, the depth of substantially increased silt and clay content and the level below which deposition prevails on continental margins, often occurs near, but is only rarely coincident with, the shelf-to-slope transition. An evaluation of the mudline off the U.S. Mid-Atlantic States and northern Gulf of Mexico highlights marked differences between depth and position of this horizon and those of the shelfbreak, and is summarized in four relationships. Type I = Off Cape Hatteras and portions of the west Florida shelf, offshelf transport of sand-size material results in a mudline position well below the shelfbreak. Spillover at Cape Hatteras, where the mudline occurs at 800–1000 m, is a response to the powerful NE flow of the Gulf Stream that tangentially crosses the narrow, shallow shelf. Type II = The shallower depth of the mudline (200–400 m, or distinctly below the shelfbreak) off the Mid-Atlantic States between Norfolk and Wilmington canyons, and off Panama City, Florida, margin identifies the long-term signature of energy concentrated on the seafloor; erosion results from the interplay of several mechanisms, including fronts, tides, and internal waves. The mudline at these localities thus defines the position where, over time, shear-induced resuspension has largely exceeded the threshold required for sediment transport. Type III = The near-coincidence of the mudline (130–175 m) with the shelfbreak at the head of Hudson Canyon is a response to physical oceanographic parameters and to offshelf spillover; involved are the intersection of density fronts separating Shelf and Slope Water, and the channelizing effect of the canyon head cut deeply into the outer shelf. Type IV = Considerable shoaling of the mudline arid a marked departure between this level and the shelfbreak occur on margins where large amounts of sediment are supplied. Broad asymmetric shoreward swings of the mudline on the Gulf of Mexico margin west of DeSoto Canyon record Mississippi and other river input and its extensive lateral dispersal by regionally important water mass flow. Along many continental margin segments, the mudline is an erosion-deposition boundary whose position relative to the shelfbreak on a margin is the long-term resultant of several factors including sediment supply, offshelf spillover by a plexus of fluid-driven processes and gravity flows, shelfbreak morphology, structural framework, sediment stability and eustacism.