New Depositional Model for Mississippi River Delta Plain

Shea Penland, Elisabeth C. Kosters, John R. Suter

The current Mississippi River delta plain model depicts a single Holocene delta plain consisting of six delta complexes sequentially deposited over the last 7,000 years by the classic delta switching process. In order of increasing age, these complexes are the Atchafalaya, Balize, Lafourche, St. Bernard, Teche, and Maringouin. Between 1981 and 1986, the Louisiana Geological Survey has acquired more than 10,000 km of high-resolution seismic profiles, 248 offshore vibracores, 397 onshore vibracores, 234 soil borings, and 226 new radiocarbon dates throughout south Louisiana. Analysis of this data set led to the development of a new, more precise stratigraphic model which depicts the Mississippi delta plain as actually two individual, imbricated shelf-phase delta plains depos ted at different sea level stillstands. Termed the Modern and Late Holocene, these two delta plains are separated by a ravinement surface several hundred kilometers in extent that can be traced updip to a relict-transgressive shoreline, termed the "Penchant Shoreline."

The Late Holocene delta plain consists of a set of delta complexes 15-20 m thick deposited during a sea level stillstand 6 m below the present, 4,500-7,000 y.B.P. This unit consists of the exposed Maringouin and Teche delta complexes offshore of south-central Louisiana and an unnamed delta complex buried by the Modern delta plain in southeast Louisiana. A relative sea level rise between 2,800-4,500 y.B.P. to about present sea level led to the transgressive submergence of the Late Holocene delta plain, generating Trinity Shoal, Ship Shoal, and the "Penchant Shoreline," which represents the subsurface eastern extension of the Vermilion Bay shoreline. The 10-15-m thick Modern delta plain began building seaward of the "Penchant Shoreline" about 2,800 y.B.P. The St. Bernard and Lafourche d lta complexes and associated transgressive shorelines represent the abandoned portions of the Modern delta plain, separated from the underlying Late Holocene delta plain by a regional ravinement surface. The active portions of the Modern sequence consist of the artificially leveed, thick, deep-water Balize complex and the thin shelf-phase Atchafalaya delta complex building onto the transgressed Late Holocene delta plain.

This new model emphasizes the importance of sea level as a control on deltaic deposition. Additionally, in this interpretation, shallow-water shelf-phase deltas, which differ considerably from the traditional deep-water Mississippi delta model, are seen to be the primary depositional constituents of Mississippi River delta plains.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.