Recognition of the Shelf-Slope Break Along Ancient, Tectonically Active Continental Margins
Raymond V. Ingersoll, Stephan A. Graham, 1983. "Recognition of the Shelf-Slope Break Along Ancient, Tectonically Active Continental Margins", The Shelfbreak: Critical Interface on Continental Margins, Daniel Jean Stanley, George T. Moore
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Tectonically active continental margins include transform, protoceanic and convergent settings. In transform settings, numerous small basins develop on oceanic, transitional and continental crust. Protoceanic gulfs may be formed by orthogonal or oblique rifting; both types are characterized by segmented crust with consequent juxtaposition of deep basins and active uplifts. Convergent margins include the following types of basins: intramassif basins, major forearc basins and accretionary basins (trench-slope basins). Extensional backarc margins have structural styles and histories similar to protoceanic gulfs.
Shelf-slope breaks in these settings tend to be transient in time and space because of rapid vertical movements; abrupt facies changes are the result. Two types of shelf-slope breaks are common: elastic-starved and progradational. The former type is characterized by unconformities or biostratigraphically compressed intervals separating shallow-marine/nonmarine from slope/basinal deposits. Commonly, glauconitic and phosphatic lithologies mark the clastic-starved shelf-slope break. Progradational shelf-slope breaks are characterized by deltaic outbuilding, which results in coarse shallow-marine, shoreline and fluvial deposits prograding over slope mudrocks. Clastic-starved shelf-slope breaks tend to predominate in transformal and protoceanic settings due to the segmented nature of crust and the structural control on shelfedges. Most detritus bypasses outer-shelf and upper-slope environments. Deltaic progradation is the dominant process in forearc basins, with abundant detritus supplied by neighboring magmatic arcs. During the latest stages of filling of forearc basins, shelf-slope breaks may correspond with the structural boundary between forearc basins and subduction complexes. Shelf-slope break deposits involved in continental collision in general are destined to be destroyed during continental suturing.
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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.