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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 1008

Last Page: 1008

Title: Unstable Progradational Clastic Shelf Margins: ABSTRACT

Author(s): Charles D. Winker, Marc B. Edwards

Article Type: Meeting abstract


Ancient shelf margins have generally been overlooked in some progradational clastic systems such as the northwestern Gulf of Mexico and the Niger delta. Apparently the contemporaneous structural deformation, particularly growth faulting, obscures depositional dips and foreset-topset geometry, making recognition of shelf breaks from these criteria virtually impossible. Nonetheless, their positions can be estimated from their association with characteristic micro-faunal assemblages, with initiation of growth faulting, with facies changes, and with geopressure.

Rapid subsidence of progradational shelf margins results primarily from three processes: isotatic depression of the basement due to sedimentary loading, extensional thinning of the sedimentary wedge due to gravity tectonics, and compaction. Instability of the continental slope causes substantial basinward mass transport by deep-seated gravity sliding. This is manifested as down-to-basin listric growth faults originating at the outer shelf and upper slope (extensional regime), and shale and salt ridges and domes originating at the lower slope (compressional regime). The rapidly subsiding shelf margin acts as a major sediment trap, leading to accumulation of thousands of feet of shallow-water sediments, including deltaic sandstones, along a growth-faulted trend that may be hundreds of m les long.

Shelf-margin deltas differ substantially from shallow-shelf deltas in that they show thicker and better differentiated progradational units and steeper clinoforms. Sand geometry of shelf-margin deltas is influenced by two competing factors: absence of a broad shelf to attenuate wave energy, thus favoring wave dominance, and high sand continuity, versus rapid subsidence, which prevent lateral reworking and thus favor river dominance and low sand continuity. Rapid downfaulting of shelf-margin deltaic sandstones against dewatering slope shales leads to the accumulation of excess fluid pressure in deep fault-bounded reservoirs. Mapping of geopressure trends can therefore provide a generalized picture of shelf-margin progradation in Cenozoic basins.

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Copyright 1997 American Association of Petroleum Geologists