Abstract

Submarine fans are fan- or cone-shaped turbiditic deposits formed at upper bathyal or deeper water depths. Within a sequence stratigraphic framework, these deposits are most likely to occur within the lowstand systems tract. The early part of this systems tract is characterized by an interval of relative sea-level fall, resulting in stream rejuvenation and depocenter shift from the coastal plain to the upper slope. This leads to retrogradational slope failure and canyon formation at the shelf-slope break. 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 on the basin floor (i.e., basin-floor fans). Seismic and outcrop evidence suggest that these sand-prone deposits are introduced abruptly into the basin and are typically characterized by subtle external mounding and internal bi-directionally downlapping seismic reflections. Deep water sediment deposited during this interval has no coeval shelf equivalent.

During the subsequent stillstand and slow relative sea-level rise, streams cease downcutting and valleys which have been freshly incised begin to fill. Because coarse sediment will be deposited preferentially 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 (i.e., slope fans) and those deposited during the time of relative sea-level fall. Deposits of these relatively sand-poor (slope) fans are characterized by slope-front fill or wedge-shaped “geometries” downlapping the earlier submarine (basin-floor) fan. They are typically composed of thinner bedded turbidites as well as the occasional leveed channel deposit. Seismically they often are characterized by a chaotic/contorted seismic facies resulting from mass movement processes active on the continental slope at the time.