Submarine Fans in a Sequence Stratigraphic Framework

Henry W. Posamentier, Rodney D. Erskine, Robert M. Mitchum, Peter R. Vail

Submarine fans are fan- or cone-shaped turbiditic deposits formed in upper bathyal or deeper water depths. Within a sequence stratigraphic framework, these basin-floor turbidites can occur during lowstand-fan or lowstand-wedge systems tract time. During lowstand fan time, streams are rejuvenated and depocenters shift from the coastal plain to the upper slope, causing retrogradational slope failure and canyon formation. The sediment delivered here bypasses the canyon and continues down the slope as a succession of gravity flows and is deposited as fan-shaped turbiditic deposits at the base of the slope. Seismic and outcrop evidence suggest that these sand-prone deposits are abruptly introduced into the basin and are generally characterized by subtle external mounding and internal bidirectionally downlapping seismic reflections where seismically resolvable. Deep-water sediment deposited during this interval has no coeval shelf equivalent.

During lowstand wedge time, streams cease downcutting and valleys which have been freshly incised begin to fill. Because coarse sediment will preferentially be deposited within these incised valleys, the sand-to-mud ratio delivered to the upper slope will be decreased and, consequently, there is an inherent difference between submarine fans deposited at this time and those deposited during lowstand fan time. Deposition during lowstand wedge time is characterized seismically by slope front fill or wedge-shaped geometries downlapping the earlier submarine fan (i.e., deposited during lowstand fan time). These shale-prone deposits are largely comprised of thinner-bedded turbidites as well as the occasional leveed channel.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.