Seismic Facies of Shelfedge Deposits, U.S. Pacific Continental Margin
Michael E. Field, Paul R. Carlson, Robert K. Hall, 1983. "Seismic Facies of Shelfedge Deposits, U.S. Pacific Continental Margin", The Shelfbreak: Critical Interface on Continental Margins, Daniel Jean Stanley, George T. Moore
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Pacific-style continental margins, such as that of western North America, are marked by large contrasts in the type of shelfedge sedimentary deposits and the processes that form them. The Pacific shelves of the United States are generally much narrower than the Atlantic shelves, and the source areas exhibit more relief. The greater relief of Pacific coast source terranes results in a relatively high rate of sedimentation in humid areas and fluctuating (areally and seasonally) sedimentation patterns and rates in semiarid areas. Sediment shed from the adjacent landmass is discharged, generally seasonally, onto the Pacific Continental Shelf at point sources. Many of the sediment sources of the northwestern United States and southern Alaska feed directly onto swell- and storm-dominated shelves. On such narrow unprotected shelves, sediment has a short residence time in submarine deltaic deposits before being remobilized and dispersed to outer-shelf and upper-slope environments.
Through study of high-resolution seismic-reflection profiles, we have identified four principal types of shelfedge deposits: (1) starved, (2) draped, (3) prograded, and (4) upbuilt and outbuilt. Each type of shelfedge deposit results from a characteristic balance between sedimentation rate and distributive energy (waves and currents) and is, therefore, characterized by distinctive seismic facies and bedding patterns. A special type, the cut-and-fill shelfedge, and a composite type consisting of two or more of the main depositional styles supplement the four principal types of shelfedge. Incorporated within each of these facies, especially on the upper slope, are chaotic deposits formed by slumps or slides, which are common along technically active margins.
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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.