Abstract

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 241 new radiocarbon dates throughout south Louisiana. Analysis of these data sets 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 deposited at different sea-level stillstands. Termed the Modern and Late Holocene, these two delta plains are separated by a ravinement surface several hundred kilometres in extent that can be traced updip to a relict-transgressive shoreline, termed the Teche Shoreline.

The Late Holocene delta plain (3,500-7,000 ka) consists of a set of delta complexes 15 to 20 m thick deposited during a sea-level stillstand 6 m below the present. This unit consists of the exposed Maringouin and Teche delta complexes offshore of south-central Louisiana and an eastern delta complex exposed offshore of the Chandeleur Islands in southeast Louisiana. A relative sea-level rise caused submergence of the Late Holocene delta plain, generating Trinity Shoal, Ship Shoal, and the Teche Shoreline, which represents the subsurface eastern extension of the Vermilion Bay shoreline. The 10 to 15 m thick Modern delta plain began building seaward of the Teche Shoreline about 2,800 ka. The St. Bernard and Lafourche delta complexes and associated transgressive shorelines represent the abandoned portions of the Modern delta plain, separated from the underlying Late Holocene delta plain by the regional Teche 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 the Mississippi River delta plain.