Modern Sediment Dynamics at the Shelf-Slope Boundary Off Nova Scotia
Philip R. Hill, Anthony J. Bowen, 1983. "Modern Sediment Dynamics at the Shelf-Slope Boundary Off Nova Scotia", The Shelfbreak: Critical Interface on Continental Margins, Daniel Jean Stanley, George T. Moore
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Long term current-meter data from outer shelf, shelfbreak and slope sites off Nova Scotia have been compared with sediment textures in the same area to assess whether they are in equilibrium. Currents on the shelf and shelfbreak are strong with maximum velocities exceeding 50 cm s−1. Sediment grain-size distributions were dissected into near-Gaussian medium sand subpopulations and non-Gaussian tails. These subpopulations were interpreted dynamically as representing bed-load (coarse tail), suspended load with “dynamic settling” (central subpopulation) and suspended load with “passive settling” (fine tail). Below 500 m water depth, only the fine-tail subpopulation is seen. The modal size of the central subpopulation corresponded well, in most cases, to u* estimates from Shields' criterion and to the observed maximum currents.
Sediment textures can be explained by modem dynamic conditions. Sand transport is dominantly in suspension and in an alongslope direction with a small downslope component. Medium sand is transported only during short periods of high flow, whereas fine sand transport is during more continuous weaker flow. Permanent deposition occurs at a point downslope where the currents rarely exceed the suspension criterion for the size of particle concerned. Slight differences between inferred u* gradients at two slope areas, separated by 150 km, are tentatively interpreted as reflecting the effects of topographic Rossby waves, formed by Gulf Stream eddies impinging on the slope.
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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.