Robert M. Mitchum, Jr., 1985. "Seismic Stratigraphic Expression of Submarine Fans", Seismic Stratigraphy II: An Integrated Approach to Hydrocarbon Exploration, Orville Roger Berg, Donald G. Woolverton
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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 by 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 m aterial by the funneling action of the canyon system, and to changes in gradient within the canyon. 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 can y on 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 onlapfill transverse profile. The seismic character of the sand-prone low e r 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 u p p er fa n consists of one or 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 leveed channels. Although current modern sedimentological models assume contemporaneity of upper and lower fans, much of the upper fan commonly appears seismically to be younger than the main part of the lower fan. The surface separating upper and lower fans is commonly 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 elastics deposited in the lower-fan facies, while later phases are likely to be characterized by silty to shaly elastics 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 elastics 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.
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The publication of AAPG Memoir 26 opened up a whole new field to the exploration community. The material contained in that publication demonstrated the relationship of seismic data to the geology of the subsurface in greater detail than previously thought possible. In the years that have followed great effort has gone into refining and advancing the principles and methods of seismic stratigraphy. The goal of this Memoir 39 publication is to provide 15 papers which present and document the advances and refinements in seismic stratigraphic princples and methods that have occurred since the publication of Memoir 26. These articles help the geologist, geophysicist, and manager to keep abreast of this rapidly developing field and provide some guidelines for the evaluation of stratigraphically controlled prospects.