Sedimentary Responses to Processes
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.
Figures & Tables
The Shelfbreak: Critical Interface on Continental Margins
The shelfbreak is that point where the first major change in gradient occurs on the outermost edge of the continental shelf. Although this environment delimits the boundary between two principal and well-defined provinces, the continental shelf and slope - and thus is of the first order of importance on continental margins - it has received surprisingly little specific attention in either modern oceans or in the rock record. This volume, the first compendium dedicated specifically to the shelfbreak, was derived from an SEPM Research Symposium convened at the joint Annual Meeting of SEPM and AAPG on June 2, 1981. The material is organized in a manner to illustrate examples of the shelfbreak in both modern oceans and the rock record.