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The Mississippi Fan is a broad, arcuate accumulation of Pleistocene deep-water sediments deposited in the eastern Gulf of Mexico. Multiple acoustical sub-bottom reflectors can be traced regionally across the entire fan, dividing this sediment body into sediment packages called fan lobes. Seven of these acoustical reflectors have been mapped and they divide the fan into seven fan lobes. Structure and isopach maps of these fan lobes reveal a general shift during the Pleistocene from west to east and toward deeper water. Each fan lobe is basically a channel-overbank complex with differing morphology and channel characteristics that allow it to be divided into upper, middle, and lower fan physiographic regions.The upper fan region of the modern fan lobe connects to the Mississippi Canyon and is characterized by a nearly filled, large erosional channel with broad levees. A smaller, central channel is cut into the large channel fill.The middle fan region starts near the major break in slope. It is convex in cross section with a sinuous channel at its apex. The size of the channel as well as its sinuosity decrease downfan. An acoustical high-amplitude zone near the base of the channel fill coincides with coarse-grained material.The lower fan region starts where the single sinuous channel pattern changes into a set of near-parallel surface images, interpreted to be former channel courses. Only one channel was active at a given time and its life span was geologically short before it shifted to a new position. At the distal end, these channels may bifurcate before becoming unrecognizable on high-resolution seismic records and sediment deposition changes from a confined to an unconfined mode. The latter is characterized by sheet-sand deposition containing a significant amount of sand that originated from upper slope and shelf environments.Major objectives for drilling the Mississippi Fan were to place the fan lobes into a time-stratigraphic framework, to determine if the midfan channel is migratory in nature, to establish the lithological characteristics of the acoustical high-amplitude zone present near the bottom of the channel fill, to analyze if sand is transported to the lower fan and in which depositional mode it is emplaced, to confirm or modify existing fan models, and to determine the physical and chemical characteristics of these deep-sea fan deposits.Nine sites were occupied on the Mississippi Fan, four (Sites 621, 622, 617, and 620) on the middle fan, four (Sites 623, 624, 615, and 614) on the lower fan, and one (Site 616) on the flank of the youngest fan lobe and within a surface slump deposit. Most of the holes penetrated only the youngest fan lobe.In addition to these nine sites, two sites (618 and 619) were occupied on the continental slope off Louisiana in intraslope basins formed as the result of active salt diapirism. Orca Basin (Site 618), characterized by a 200-m-thick anoxic, high salinity layer of bottom water, was expected to provide a complete upper Neogene stratigraphical and chemical record. Howev...

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Pleistocene sediments were cored at nine middle and lower Mississippi Fan sites, in water depths from 2500 to 3300 m. Radiography, thin section, scanning electron microscope, and X-ray diffraction studies provide data to describe the fan's major depositional environments.Sands and minor gravels are concentrated in middle and lower fan channel fills, and in lower fan channel-mouth deposits. Silts and clays occur in overbank deposits, passive channel fills, and interbeds associated with coarser facies. Graded bedding and lamination, both of varying thickness, are the dominant sedimentary structures in all environments.Granule and pebble gravels are composed of well-rounded chert and polycrystalline quartz, with minor metamorphic and igneous rock fragments. Moderately to well sorted sands are mainly fine and very fine feldspathic litharenite, sublitharenite, and subarkose. Sands commonly have thin-section porosities between 20 and 35%; woody organic contents range from 0.7 to 7.9% total organic carbon.Authigenic minerals occur in sands and muds, but are most common in fine-grained silts and clays. Smectite, dolomite, calcite, pyrite, and gypsum are the main authigenic phases.At this stage in their depositional history, the sands are relatively clean, have high porosities, show only minor porereducing diagenetic effects, and thus have excellent hydrocarbon reservoir potential.

The Mississippi Fan comprises a minimum of seven individual depositional units, called fan lobes. The two most recent fan lobes, penetrated during DSDP Leg 96, were deposited during the late Wisconsin glacial. The morphology of the youngest fan lobe is dominated by a channel-overbank complex which traverses nearly its entire length. Systematic downfan changes in the nature of the channel system, distribution of the seismic fades, and seafloor gradient suggest division of the youngest fan lobe into four major regions: canyon, upper fan, middle fan, and lower fan. Two of these regions-the middle fan and the lower fan-were drilled during Leg 96. The seismic character of the continental slope adjacent to the canyon is similar to that of the upper fan because of the influence of diapiric activity and the erosional/depositional processes within the channel complex. South of the diapirs, however, the seismic character of the upper fan becomes more compatible with the middle fan. This change reflects the transition from an erosional mode in the canyon area to a predominantly aggradational mode downfan. The middle fan is characterized by an aggradational channel located along its apex. This leveed channel system is highly sinuous and migratory in nature. The channel fill, drilled at Sites 621 and 622, consists of a zone of high-amplitude reflectors (channel-lag deposits) in its lower part overlain by semitransparent to transparent reflections (silts and muds). Seismically, the fine-grained overbank sediments (Sites 616, 617, and 620) are characterized by laterally extensive, structureless, transparent zones (north of the channel) or by predominantly discontinuous, parallel reflectors (south of the channel). The lower fan region can be subdivided into two areas. In its upfan area (Sites 623 and 624), randomly scattered, buried channels suggest frequent shifting, infilling, and abandonment. The extension of discontinuous, parallel reflectors from these channels suggests lateral distribution of sediment by nonchannelized turbidity currents. In its downfan portion (Sites 614 and 615), the modern channel is barely discernible morphologically and bifurcates before terminating. Seismically, this area consists of continuous parallel to discontinuous parallel reflectors.