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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 939

Last Page: 940

Title: Recognition of Shelf-Slope Break Along Tectonically Active, Ancient Continental Margins: ABSTRACT

Author(s): Raymond V. Ingelsoll, Stephan A. Graham

Article Type: Meeting abstract


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Tectonically active continental margins include transform, protoceanic-rift, and subduction settings. Shelf-slope breaks in these settings tend to be transient in time and space. Wrench and fault-block systems feature irregularly shaped basins and uplifts that have abrupt vertical movements and facies changes. Subduction systems form elongate basins and ridges in response to the interplay of tectonics and sedimentation.

Because of the nondepositional character of narrow, tectonically controlled shelves, shelf-slope breaks commonly are expressed as unconformities separating shallow-marine/nonmarine and slope/basinal deposits. Alternatively, shelf and basinal deposits may be separated by a biostratigraphically compressed interval, including glauconite and phosphorite, which represents the clastic-starved shelf-slope break. Subtly expressed shelf-slope break deposits of active margins commonly are masked by abrupt sedimentation in adjoining areas along with abrupt facies migration. Consequently, bracketing deeper and shallower marine facies is the usual key to locating ancient shelf-slope breaks.

Basinal facies are distinctive in many wrench and fault-block systems, with two common sedimentary motifs: (1) ponded submarine-fan deposits displaying little proximal-to-distal facies segregation, and (2) clastic-starved sections of laminated (commonly biogenous) sediment reflecting anoxic conditions in isolated silled basins. These facies, together with slump-dominated slope facies, adjoin shelf-slope break deposits.

Variable geometries characterize subduction-controlled shelf-slope breaks, but generally, ancient deposits overlie highly deformed, deep-marine components (trench-slope and trench sediments) and are overlain by relatively undeformed fore-arc deposits. Underlying deposits may have exotic sources and may contain recycled materials. Overlying deposits consist of predominantly land-derived components.

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