This chapter summarizes the results of Leg 96 sedimentologic studies of the Mississippi Fan. These studies principally describe the youngest fan lobe (deposited during late Wisconsin time) and indicate that a substantial amount of sand was transported through a leveed channel system to the lower fan during that period. Significant amounts of coarsegrained material were also left as a channel lag in the middle fan region. Recovery of these sands was poor, however, and a direct correlation of the Mississippi Fan sections with classic Mutti-Ricci Lucchi facies could not be made. Major submarine slide and debris-flow deposits suggest that a significant portion of the total sediment contribution to the fan might come from the margins of the Gulf of Mexico and not necessarily follow the existing channel system. Good recovery of fine-grained sediment in the upper 50 to 100 m at most sites provides substantial insight on depositional processes of muddy turbidity currents and other density flows in a passive margin, large-scale submarine fan setting. In general, the drilling results on the Mississippi Fan are compatible with seismic stratigraphic interpretations.
During Leg 27 five sites were drilled in the eastern Indian Ocean-four in abyssal plains near the western margin of Australia and one in the Timor Trough. Abyssal plain sediments are divided into two major units: (1) a lower, acoustically transparent layer of relatively uniform thickness draped over basaltic basement and (2) a horizontally layered sequence of highly variable thickness filling low areas on the surface of the transparent layer. The transparent layer, ranging in age from Upper Jurassic to Upper Cretaceous, consists chiefly of dark gray, siliceous clays and claystones with minor zeolitic clay and nannofossil ooze. Sedimentation rates are between 5 and 30 m/m.y. being highest in the Lower Cretaceous. Upper Cretaceous sediment are sparse or absent. The relatively high sedimentation rates for pelagic clay are probably due to the proximity of the Australian continent. The layered unit consists of calcareous oozes and lesser zeolitic clay and radiolarian ooze, all of Cenozoic age. Many of the oozes contain shallow-water foraminifers and graded sequences suggesting deposition by mass transport. Calculated sedimentation rates for the upper Cenozoic range from 5 to 15 m/m.y. Very few lower Tertiary sediments were recovered because drilling sites were selected near crests of highs in the transparent layer where Cenozoic sediments are thin. The great influx of carbonate ooze in the Cenozoic is probably related to changes in climate and oceanic circulation associated with the separation of Australia and Antarctica in the Eocene.In the Timor Trough approximately 400 meters of nannofossil and foraminiferal ooze containing abundant reworked fossils and graded sequences unconformably overlie shallow-water dolomitic limestone and dolomitic calcarenite of middle to lower Pliocene age. Accumulation rates are roughly 185 m/m.y. in the Quaternary and 25 m/m.y. in the upper Pliocene. The rapid sedimentation rates are presumably due to high pelagic production and to introduction of sediment from the flanks of the trough. The trough apparently formed in middle Pliocene and has undergone relatively little deformation since.
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.
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