Seismic Stratigraphic Analysis of the Mississippi Fan

Examination of extensive multichannel and single-channel data across the Mississippi Fan, Gulf of Mexico reveals that at least eight seismic sequences comprise the Pliocene/Pleistocene section (sequences I-VIII, oldest to youngest). The sequence boundaries are basin-wide unconformities identified on the basis of truncation and reflector terminations. In general, each sequence is lens-shaped in cross-section, thinning laterally from an area of maximum thickness. Isopach and structure maps compiled for each of the sequences indicates a seaward and eastward migration in the fan depocenter during its development. Mapping of the orientation of the axis of maximum thickness for each sequence indicates a major eastward shift in the depocenter during the Late Pleistocene. This shift suggests a major influx of sediment from sources other than the Mississippi River Embayment, possibly from the DeSoto Canyon. Seven seismic facies have been identified within the fan unit, each with one or more facies types. A depositional mechanism has been proposed for each based on the reflection pattern, position within each sequence, and lateral facies relationships. Channel, overbank, mass transport, and turbidity flow deposits have been interpreted within each sequence. Each facies occupies a unique position in the evolution of each sequence, reflecting a succession of depositional regimes. This succession may be related to cycles of sea level fluctuations. Analyses of facies distributions and interpreted depositional mechanisms suggest that the development of an individual sequence can be described in four stages. Stage 1: Initial deposition is characterized by thick, mounded chaotic units, probably the result of mass transport depositional processes. Initiation of canyon development may occur during this stage. Major deposition is possibly triggered by a fall in sea level, when rapid progradation of the shelf edge occurs with associated failure of unstable sediment. Stage 2: Stage 1 deposits are capped by higher amplitude, moderate continuity reflectors which fill across the irregular upper surface of the mass transport deposits. The reflection patterns suggest deposition from predominantly turbidity flows, possibly associated with the middle to late stages of a fall in sea level. Canyon development continues. Stage 3: "Classic" channelized lobe deposition occurs during this stage. The canyon controls deposition, channelizing the flow of material reaching the fan. During this stage, other sediment sources to the fan are reduced as major deposition is confined to the canyon and associated channel on the fan itself. Stage 4: During the last stages of the rise and a highstand of sea level, hemipelagic and pelagic deposition dominates. High amplitude, high continuity reflectors drape the sequences.