Seismic Stratigraphic Criteria for Recognition of Submarine Fans
Authors: Mitchum Robert M. 
Published: October 2019

This paper presents criteria for recognizing ancient submarine fans from seismic data and illustrates examples of seismically defined fans as well as common “pitfalls” of fan interpretation. Submarine fans usually can be recognized seismically by morphology and position in the overall paleogeographic setting. Submarine fans are commonly related genetically and spatially to canyons or other sediment transport paths across the outer shelf and slope, although some “canyons” may be poorly developed or far removed from the fan. Most sands in fans are deposited as turbidites, although grain-flow, inertia-flow, slump, and other mass-transport processes probably contribute to the fan. The mounded fan shape is due to concentration of material by the funneling action of the canyon system and to changes in gradient along the depositional path. Differential compaction may accentuate mounding. Bathymetry of the depositional site strongly controls fan shape.
On seismic reflection profiles, the canyon-fan system can be separated into the canyon, lower fan, upper fan, and later canyon fill. The canyon is an elongate, commonly erosional, notch that serves as a sediment transport path across the shelf edge. After fan deposition, the canyon is filled with a prograding shale-prone configuration that has an onlap-fill transverse profile. The seismic character of the sand-prone lower fan is a mound shape with a convex-upward upper surface. The internal reflection pattern is basically that of convex-upward reflections that downlap and terminate toward the fan edge and define internal lobes and depositional axes within the fan. The upper fan consists of one to a few thick, sand-prone channels flanked by large, mostly silt- and shale-prone levees that become smaller downdip. Upper-fan seismic criteria include concave-upward upper surfaces of levees sloping from the central channel, bidirectional downlap of concave-upward internal reflections, and “bow-tie” diffractions at channel edges. An irregular moundy reflection pattern is commonly interpreted as a complex of slumped leveed channels. Although current modern sedimentological models assume contemporaneity of upper and lower fans, seismically much of the upper fan commonly appears to be younger than the main part of the lower fan. The surface separating upper and lower fans is characterized by downlap or onlap of the upper fan onto the lower fan. This superposition probably occurs because early phases of fan deposition are most likely to be dominated by coarse clastics deposited in the lower-fan facies, while later phases are likely to be characterized by silty to shaly clastics deposited in upper fan levees.
Eustatic falls and lowstands of sea level are times of preferential deposition of submarine fans. During eustatic falls, canyons are cut, shelf areas are bypassed by sediments, and coarse clastics are deposited in sand-prone lower fans at canyon mouths or directly on the slope or basin plain. During lowstands, upper fans and other slope-fill sequences follow. With rise of sea level, canyons are filled predominantly with fine-grained material.