Shelfedge Dynamics and the Nepheloid Layer in the Northwestern Gulf of Mexico
David W. McGrail, Michael Carnes, 1983. "Shelfedge Dynamics and the Nepheloid Layer in the Northwestern Gulf of Mexico", The Shelfbreak: Critical Interface on Continental Margins, Daniel Jean Stanley, George T. Moore
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An investigation of shelfedge sedimentary processes in the Gulf of Mexico has been underway for the past five years. It has consisted of in situ bottom boundary layer (BBL) experiments, time series observations using moored instruments, and hundreds of hydrographic stations. A ubiquitous nepheloid layer exists over the outer continental shelf in the BBL. It reaches a maximum thickness of 30 m when offshore flow near bottom stacks detached bottom boundary layers at the shelfedge. The shear stresses which maintain the sediment suspension are contributed by a superposition of many modes of motion. In the northwestern Gulf of Mexico, surface gravity waves, high frequency internal waves and tides do not appear to contribute significantly to the sedimentary processes at the shelfbreak. However, diurnal inertial oscillations do resuspend silt and clay at the shelfedge and transport that sediment to the offshore. Winter storms produce three types of phenomena that influence sediment transport: (1) direct, energetic, cross-shelf wind-driven flow; (2) production of dense, cool, saline bottom water that flows offshore under the influence of gravity; and (3) inertial oscillations which propagate to the bottom. The mean shelfedge flow was found to be west to east in the interior, with bottom waters oriented more southeasterly. The latter should contribute to a long term advection of sediment off the shelf. Flow on the bottom of the upper slope has been observed to be oriented to the northeast, suggesting a convergence in the BBL near the shelfbreak.
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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.