--> Abstract: Slope Readjustment: An Alternative to Sea-Level Lowstands as a Control on Submarine Canyon and Fan Development, by J. A. May and W. C. Ross; #90937 (1998).

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Abstract: Slope Readjustment: An Alternative to Sea-Level Lowstands as a Control on Submarine Canyon and Fan Development

MAY, JEFFREY A., Consultant, and WILLIAM C. ROSS, Interpretive Imaging

Subsurface and outcrop data, combined with numerical simulations, support a new model for the formation of submarine canyons and fans. Falling sea level does not have to be invoked. Instead, changes in basin physiography can lead to erosional truncation of continental slopes, sediment bypass, and marine onlap of submarine-fan complexes.

Two kinds of basin margins are recognized. (1) Progradational margins represent the basinward advance of graded depositional profiles that form when diffusive processes and sediment-gravity flows are in equilibrium with sediment supply, basin subsidence, and basin physiography. (2) Erosional margins result when upperslope gradients exceed an equilibrium profile, and are characterized by slumping, erosion, and sediment bypass to the lower slope. Progradational margins can develop bathymetric escarpments and become erosional in response to a rapid rise in sea level, structural deformation of the basin margin (e.g., faulting, diapirism), and/or a transition from carbonate to siliciclastic deposition. Erosional margins can be transformed into progradational margins when the bathymetric escarpment (i.e., oversteepened margin) is buried by onlapping and aggrading fan deposits.

Falling sea level can play a major role in the formation of submarine canyons and fans along some basin margins, by forcing the source of sediment input to the shelf edge. Basinward shifts in facies and subaerial-exposure surfaces that continue basinward as submarine unconformities result. However, many other margins display regional surfaces of slope truncation that do not extend updip into subaerial unconformities. Such margins also exhibit abrupt landward shifts in facies, with canyon and fan deposits unconformably overlying shallow-water to nonmarine facies.

Examples include the Eocene of San Diego, California, and the Oligocene and Miocene of the northern Gulf of Mexico. In these cases, changes in basin physiography due to syndepositional faulting triggered slope readjustment, leading to slope erosion and submarine-fan deposition. Thus, every submarine-fan system does not reflect a drop in sea level.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah