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Recent Sedimentary Facies in Interdistributary Basin, Mississippi Delta

Il Yi Hi, Elisabeth Kosters, Thomas F. Moslow

Five sedimentary facies have been recognized from 23 vibracores in an abandoned interdistributary basin of the St. Bernard delta lobe, 15 km southeast of New Orleans. They are: (1) detrital clays containing shell fragments and lenticular laminations, interpreted as a bay facies; (2) laminated to massive-appearing, fine grained sandbeds averaging 10-20 cm in thickness, of possible overbank or crevasse splay origin; (3) thinly interbedded, parallel laminated and ripple laminated, sandy and clayey silts forming 50 to 70-cm-thick sequences that increase in clay content upwards, interpreted as flood events during overbank deposition; (4) extensively rooted detrital clays with less than 10% organic matter as disseminated plant material, representing a transitional, brackish-to- aline marsh facies; (5) organic-rich clays (35-75% organic matter) and peats (> 75% organic matter), interpreted as marsh and swamp facies. Major depositional environments observed on the present deltaic plain include irregular-shaped lakes, distributary channels, natural levees, overbank splays, small meandering channels, oyster reefs, and swamp, brackish and saline marshes.

Radiocarbon dates and facies correlations indicate that the major factors influencing differential subsidence of the delta lobe during the past 3,000 years include: (1) tectonic setting; (2) sea-level changes; and (3) sediment supply, loading, and dewatering. X-ray radiographs show details of internal physical sedimentary structures, microorganisms (such as pelecepods, gastropods, and foraminifera), and biogenic structures such as burrows and rooting, as well as early diagenetic features such as authigenic minerals, inferred as pyrite. The maximum development of pyritized forms are not in peats and organic-abundant clays (15-75% organic matter), but in clay with less than 10% organic matter. Analysis of the average ash content of true peats indicates the low potential of peat accumula ion in these marsh environments, which is thought to be related to the rapid subsidence rates and resulting marine inundation of the delta plain.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.