Fifteen separate delta lobes have been formed by the Mississippi River in the past 6,000 years. Fourteen are included in the Teche, St. Bernard, and Lafourche deltaic complexes; the fifteenth includes the present birdfoot delta, which is an extension of the initial lobe of the Plaquemines-Modern complex. Each deltaic complex is genetically related to a major Mississippi River trunk stream, whereas delta lobes within each complex are the result of different distributary networks extending from the trunk stream.

The delta lobes were defined by detailed facies analyses of sediment cores from hundreds of shallow borings combined with lithologic and faunal data from several hundred additional borings. Each lobe consists of a basal fine-grained prodelta facies, an overlying sandy delta-front facies, and an uppermost fine-grained delta-plain facies. The latter deposits include peat accumulations and nonorganic flood-basin and natural-levee deposits.

More than 100 radiocarbon age determinations have been used to establish the chronology of major deltaic complexes and their subsidiary delta lobes. These data, together with the facies relations, indicate that growth of each deltaic complex was sporadic; progradational phases were interrupted by transgressions resulting from local shifting of sediment influx and from subsidence.

Similar deltaic sequences, prevalent in Tertiary outcrops along the northern flank of the Gulf Coast geosyncline, extend basinward as massive subsurface clastic wedges, which constitute a major part of the peripheral basin fill.

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