Models for Submarine-Fan Deposition within a Sequence-Stratigraphic Framework

Abstract

Submarine-fan deposition may occur any time sediment gravity flows transport sediments across the slope and onto the basin floor. These events, however, are considered most likely to occur and have the highest sand-to-mud ratio during lowstands of relative sea level During these intervals, depocenters shift to the outer shelf and beyond, increasing the likelihood of initiating sediment gravity flows. The stratigraphic units deposited during this time interval comprise the lowstand systems tract, using sequence stratigraphic terminology (Posamentier and Vail, 1985, 1988). This systems tract is subdivided into two component units: the lowstand fan and the lowstand wedge The lowstand fan is deposited during intervals of relative sea-level fall, whereas the lowstand wedge is deposited during the subsequent relative sea-level stillstand and slow rise. Both the lowstand fan and lowstand wedge contain turbidite deposits that comprise submarine fans. However, there are significant differences between submarine fans occurring within the two units. Submarine fans within the lowstand fan typically are deposited farther into the basin than those within the lowstand wedge. Consequently, where local physiography allows, these fans can be referred to as basin-floor fans and those within the more proximal lowstand wedge as slope fans, indicative of the typical locations of their depocenters. The lowstand fan is typically more sand prone and underlies the younger lowstand wedge. The lowstand wedge is composed of both the slope fan as well as coeval shelf-edge deltas and incised valley fills. In contrast, the lowstand fan has no shelf equivalent and is composed almost entirely of basin-floor fan deposits. Consequently, the lowstand fan is time-equivalent to a shelfal/upper slope unconformity developing at this time.