The Mississippi fan is a broad, arcuate Pleistocene accumulation of displaced shallow-water sediments. The fan consists of elongate fan lobes that shift position at the onset of each active sedimentation event, probably related to the lowering of sea level and the outbuilding of the shelf. Isopach and structure maps, based on eight acoustical reflectors of regional extent, demonstrate this shifting as well as changes in the location of the source area, and a progradation basinward.

The youngest fan lobe is a suitable model for the underlying ones. It can be divided into four major morphologic units: (1) canyon--the Mississippi canyon resulted from retrogressive slumping during the late Wisconsin and was nearly filled thereafter; (2) upper fan lobe--a large, nearly filled erosional channel with low levees and a recent active central channel; (3) middle fan lobe--convex upward in cross section with a sinuous 3-km (2-mi) wide migratory and aggradational channel on its apex; (4) lower fan lobe--the central channel becomes smaller, less sinuous, shifts position periodically (as indicated by the indistinct abandoned channels), bifurcates, and terminates.

Fan lobes are primarily channel-levee-overbank complexes, erosional in the upper fan and aggradational basinward. The channel is an active conduit; deposition in the channel took place during and after an active transport period. Accumulation rates on the middle and lower fan are high, ranging from 6 to 12 m/1,000 yr (20 to 40 ft/1,000 yr). A major portion of the sand is transported to the lower fan area.

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