Eight seismic reflectors within the Pleistocene Mississippi fan, Gulf of Mexico, have sufficient areal extent to allow the construction of isopach and structure contour maps. These maps show that the fan was constructed by at least seven elongate fan lobes that moved slightly eastward and basinward with time. A fan lobe is basically a channel-overbank complex that can be divided into (1) an upslope erosional canyon formed by retrogressive slope failure; (2) an upper fan characterized by a major channel that acted as a conduit for sediment transport to deeper water; (3) an aggradational middle fan, convex in cross section, with a sinuous, migratory channel running along its apex; and (4) a lower fan where the channel frequently shifted position and often bifurcated prior t termination with the depositional mode changing from channelized to nonchannelized (sheet sands).

Drilling on the middle fan revealed that the channel fill contains a fining-upward series commencing with gravel, overlain by pebbly mud and sands, which gradually change into laminated silty muds and fine muds. The overbank deposits are very sand-poor and contain thin-bedded, fine-grained turbidites. Sedimentation rates inside and outside the channel range from 11 to 12 m/1,000 years. Fauna is sparse and indicates an upper and middle neritic origin.

The channel complex on the lower fan shows a decrease in size and sinuosity of the youngest channel. Drill holes reveal alternating "channel" and "overbank" deposits, supporting the concept of frequent shifting of the channel. At the distal end in the sheet-sand depositional area, sands are up to 9 m (30 ft) thick. Total net sand, based on gamma-ray logs, for the youngest fan lobe was calculated to be 47%; for the underlying one, 65%. Average sedimentation rates are 5-6 m/1,000 years.

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