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A considerable portion of fine-grained sediments of the Mississippi Delta can be found in large-scale interdistributary basins that are located between distributary channels of different delta lobes of the same delta system. Because of their size, these basins are prone to filling with fine-grained material. Thus, the central basin is devoid of coarser influxes from the surrounding levees. Barataria basin (about 150 × 50 km in size) is such a basin bordered by the modern Mississippi River and the abandoned Bayou Lafourche. Ninety-five vibracores, 3-10 m deep, form the basis for this study. Correlation between cores along cross sections is based on lithology and organic matter content. X-ray radiographs show common occurrences of massive homogeneous and fine parallel- aminated sequences. Small-scale cross-lamination, lenticular bedding, and faunal bioturbation are rare, the latter most likely because of oxygen-starved conditions. Subtle facies differences can be detected between levees, basin drainage channels, lacustrine areas, submerged marsh bottoms, and various peats. Early diagenesis occurred throughout the basin in the form of heavy pyritized and/or calcite-rich zones. From examination of thin sections, both by light microscopy and SEM, it appears pyrite is most abundant in marsh sediments with intermediate (about 30-70%) organic matter content. These marshes contain sufficient iron (from clays) and sulfur (associated with decaying plant matter) to give rise to pyrite formation.
Detailed analyses of a basin of this kind are important in predicting and understanding geometry, continuity, and diagenetic features of deltaic shales. In addition, when reworked, these materials form possible source beds for hydrocarbons found in continental shelf and slope settings.
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