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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 924

Last Page: 924

Title: Deposition on Pacific Shelf Edge: Zone of Contrasts: ABSTRACT

Author(s): Michael E. Field

Article Type: Meeting abstract


Pacific-style continental margins, such as in western North America, are marked by large contrasts in shelf-edge sedimentary deposits and processes. The United States Pacific shelves are generally much narrower than Atlantic-style shelves, and the source areas exhibit more relief. The result is a generally high rate of sedimentation in humid areas, and fluctuating (areal and seasonal) patterns and rates in semiarid areas.

Sediment shed from the adjacent landmass is discharged onto the U.S. Pacific continental shelf at point sources; intervening zones of the shelf edge between point sources are commonly sediment starved. Where submarine canyons intercept the shelf, sediment bypasses the shelf and slope to fan and basin environments. Spillover from channelized transport in canyons results in local sedimentary accumulations at the shelf break.

Major sediment sources of the northwestern United States and the Gulf of Alaska feed directly onto swell- and storm-dominated shelves. On narrow unprotected shelves, the sediment has a short residence time in submarine deltaic deposits before remobilization and dispersion to outermost-shelf and upper-slope environments. In these environments, prograding sequences of shelf-edge sedimentary deposits form, commonly with a high potential for preservation in the geologic record. On broad or protected shelves, however, prodelta deposits have a longer life expectancy, and only a small amount of sediment escapes to the shelf edge.

The high seismicity and active tectonism that characterize the strike-slip and underthrusting regimes of western North America are important in forming sedimentary sequences on the outer shelf-upper slope. Failure of rapidly accumulating mud, rich in organic material on the outer shelf of the Gulf of Alaska and northern California, is triggered by large-magnitude earthquakes and local uplift. Repeated failures over long periods result in unique sedimentary packages that have potential for becoming both source beds and stratigraphic traps for hydrocarbons.

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Copyright 1997 American Association of Petroleum Geologists