ABSTRACT

Pleistocene sediments were cored at nine middle and lower Mississippi Fan sites, in water depths from 2500 to 3300 m (8200 - 10825 ft). Radiography, thin section, SEM, and XRD studies provide data from which the fan's major depositional environments are described.

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 of varying thickness is the dominant sedimentary structure 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% TOC.

Authigenic minerals occur in sands and muds, but are most abundant in silts and clays. Smectite, illite, dolomite, calcite, pyrite, and gypsum are the main authigenic phases.

At this stage in their depositional history, the sands are clean, have high porosities and permeabilities, show only minor pore-reducing diagenetic effects, and thus have excellent hydrocarbon reservoir potential.