Abstract: Mississippi River Delta Facies Framework

By

David E. Frazier

The coastal depositional complex prograded by the Mississippi River in the past 6,000 years was subdivided into sixteen separate delta lobes by detailed analysis of several hundred sediment cores. Definition of the delta lobes was accomplished through identification of vertical sequences of environmental facies which were determined by their sedimentary textures and structures as well as the environmental significance of incorporated fauna and flora.

Sand constitutes 20 to 25 percent of the sediment load of the Mississippi River and silt and clay the remainder in varying mounts. These proportions are readily apparent in the regional stratigraphic sections which have been constructed through the deltaic deposits.

The typical facies sequence of the Mississippi River delta lobes consists of a basal progradational unit of fine-grained prodelta silty clay, overlain by sand-prone delta front deposits which together form the delta platform; a central, heterogeneous, aggradational delta-plain unit, which comprises interfingering organic-rich marsh-bay deposits overlapped inland by natural levees of the distributary network; and an uppermost transgressive unit consisting of reworked shoreline sands, offshore bars and their associated bay deposits.

Several shorter sections have been constructed more recently through two eastern lobes which thicken basinward from less than 50 to more than 190 feet in an attempt to define the geometry of sand bodies incorporated delta sequences.

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