Abstract Six computer-generated seismic depth sections over the outer continental shelf and upper slope reveal that subhorizontal Lower Cretaceous reflectors continue 20 to 30 km seaward of the present shelf edge. Extensive erosion on the continental slope has occurred primarily during the Tertiary, causing major unconformities and retreat of the shelf edge to its present position. The precise age and number of erosional events is not established, but at least one major erosional event is thought to be Oligocene and related to a marine regression in response to a worldwide eustatic lowering of sea level. Velocities derived from the multichannel data reveal distinctive ranges and lateral trends as functions of sediment age, depth of burial, and distance from the coastline. Seismic units beneath the shelf and slope of inferred Tertiary age range from 1.7 to 2.7 km/sec, increasing with age and depth of burial. Units interpreted as Upper Cretaceous rocks beneath the shelf range from 2.3 to 3.6 km/sec and show a distinct lateral increase across the shelf followed by a decrease beneath the present continental slope. Inferred Lower Cretaceous and Upper Jurassic rocks beneath the shelf increase from 3.7 to 4.8 km/sec from nearshore to offshore and indicate a change in facies from clastic units below the inner shelf to carbonate units beneath the outer shelf and upper continental slope. Both reflection and refraction data suggest that thin, high-velocity limestone units (5.0 km/sec) are present within the Lower Cretaceous and Upper Jurassic units beneath the outermost shelf edge, but that these change lithology or pinch out before reaching the middle shelf. Although lateral changes in velocity across the shelf and local velocity inversions appear, the interval velocities along the length of the margin show excellent continuity between Cape Hatteras and Cape Cod. The high-velocity horizons within the Lower Cretaceous and Upper Jurassic shelf-edge complex indicate the presence of a carbonate bank or reef. The continental shelf off New Jersey is underlain by a trough approximately 150 km wide with up to 12 km of sedimentary fill. The oceanic basement beneath the upper continental rise is usually at a depth of 10 to 11 km and is overlain by 6 to 8 km of sediments. The rise sediment trough is separated from the shelf trough by an acoustic basement ridge 25 to 75 km wide where penetration never exceeds 6 km beneath sea level, although faulting and carbonate bank or reef complexes frequently limit penetration to 3 to 4 km in this zone. The acoustic basement ridge coincides with the East Coast Magnetic Anomaly and is interpreted as thick oceanic crust formed during the initial phase of seafloor spreading between North America and Africa. Rapid differential subsidence occurred on opposite sides of the basement ridge during the Jurassic and Early Cretaceous. Differential subsidence beneath the shelf also occurred along the margins, with narrower and shallower shelf troughs occurring off platform areas such as Cape Cod and Cape Hatteras.

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.