Recent Deltaic Deposits of the Mississippi River: Their Development and Chronology

ABSTRACT Sixteen 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 delta complexes; the remaining two include the present birdfoot delta, which is an extension of the earlier formed initial lobe of the Plaquemines-Modern complex. Each delta complex is genetically related to a major Mississippi River course. Individual delta lobes within each complex are the result of the successive distributary networks of a major river course. 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 uppermost fine-grained delta-plain facies. The latter deposits include peat accumulations and nonorganic floodplain and natural-levee deposits. More than one hundred radiocarbon age determinations made on discrete delta-plain peats have been used to establish the chronology of the 16 delta lobes. These data, together with the facies relationships, indicate that the development of each delta complex was not a continual process; instead, river shifting from one major course to another caused the temporary cessation of development in one delta complex as progradation occurred in another. 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 portion of the peripheral basin fill.