The Mississippi delta complex is one of the largest and probably the best known of the world's major delta systems. Its subaerial surface--the Mississippi deltaic plain--extends from an apex at the mouth of the Mississippi alluvial valley to a base of 200 miles along the Louisiana coast. Underlying this plain and the adjacent continental shelf and slope is a huge mass of Late Quaternary river-mouth deposits which make up the deltaic complex. These sediments occupy a seaward deepening, trough-like depression in the underlying surface that developed contemporaneously with deposition and which is localized in the depositional area. Downwarping of this segment of the continental margin, together with the eustatic rise in sea level during Late Quaternary time, was sufficient t accommodate the great thicknesses of Late Quaternary deposits. These deposits reach a known maximum of about 1,000 feet under the continental shelf at the seaward limit of control.

Beneath the deltaic plain and inner shelf, the Late Quaternary deposits consist of a thick onlapping sequence, grading upward from basal fluvial and strandplain sands and gravels to deltaic and marine silts and clays with local sand lenses. This is overlain by a thinner sequence of offlapping deltaic sands, silts, and clays exposed on the deltaic plain. The onlapping sequence records the eustatic rise of the sea from its last low stand while the offlapping sequence represents progradation after the sea reached its present level some 3,500 to 4,000 years ago. At that time the Gulf shore coincided approximately with the present coastwise Pleistocene-Prairie contact and subsequently has been advanced far seaward by construction of the deltaic plain. In building this plain, the Mississipp River occupied and abandoned two courses and several deltas prior to establishing its present course and active delta. In order of decreasing age, these are the Teche, St. Bernard, LaFourche, Plaquemines, and modern birdfoot delta.

Except for the birdfoot delta, which is advancing into deep water of the continental slope, each of the deltas was built forward onto the shallow inner margin of the continental shelf. Sediments delivered to the deltas through leveed distributaries were deposited in a variety of environments. Most of the sand was laid down in distributary-mouth bars of the delta front and in distributary channels as they were abandoned. Silts and clays were swept further seaward into the marine prodelta zone and, during floods, into swamps, brackish marshes, bays, lakes, and channels of interdistributary and delta-flank depressions of the deltaic plain. Following river abandonment of an active delta, subsidence, caused principally by compaction, resulted in the encroachment of peats and organic mucks f the swamp-and-marsh environments across the delta, leaving only the levees exposed. Continued subsidence permitted local transgression of these deposits by marine sediments. Regional downwarping and compaction during and following delta building have depressed the deposits far below original depositional levels and account for their great thickness.

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