Infaunal-Sediment Relationships at the Shelf-Slope Break
Norman J. Blake, Larry J. Doyle, 1983. "Infaunal-Sediment Relationships at the Shelf-Slope Break", The Shelfbreak: Critical Interface on Continental Margins, Daniel Jean Stanley, George T. Moore
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Infauna changes dramatically across the shelf-slope break, along with the physical and chemical parameters of the sediments and overlying water column. Grain size across the transition first increases slightly, then rapidly changes from sand to mud with concomitant increase in clay mineral and organic matter content. Light penetration decreases and there occurs a damping of seasonal temperature fluctuations. Infauna! assemblages change from those characterized by filter feeding organisms to those dominated by deposit feeders. Of the animals with hard parts likely to be preserved in the fossil record, the molluscan order nuculoida, composed of deposit feeders, is heavily represented seaward of the mudline. Biomass and density of organisms first decrease as grain size gets larger near the shclfedge, then increase as the mudline is crossed, then decrease again in the mud downslope. Winnowing recycles fecal material from the shelf infaunal assemblages back into the water column, this contributes to the generally high productivity of shelf waters. Much of the feces seaward of the mudline is incorporated as part of the sediment, contributing to the relatively high organic content. Deposit feeders downslope of the mudline are the primary source of sediment reworking, while physical winnowing processes are more important at and adjacent to the sheifedge.
In the sedimentary record, a sudden change in fossils from groups dominated by filter feeders to groups dominated by deposit feeders may indicate proximity to the shelf-slope break. Such a diagnostic change is associated with a decrease in fossil content of a sand layer and concurrent increase in grain size, followed by a facies change from sand to mud with rapid increase in fossil content, and finally followed by a decrease in fossil content in mud away from the zone of facies change.
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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.