Abstract: Subsidence and Eustatic Controls on the Holocene Development of the Mississippi River Delta Plain

S. Penland, Harry H. Roberts, S. J. Williams

Subsidence and eustacy are recognized as important process variables controlling the development and stability of the Mississippi River delta plain during the Holocene transgression (HT). A rapid rise in relative sea level (RSL) within the delta plain, driven by subsidence and eustacy is also viewed as a major cause of our nation's worst coastal land loss conditions found in Louisiana. Since 1986, the U.S. Geological Survey and Louisiana State University have conducted studies to understand the framework geology and processes of coastal land loss in Louisiana. Using seismic profiles, shallow and deep cores, radiometric data, landscape change analysis, and tide gage data, this research has examined the rates of RSL change and the delta plain response patterns over the last 18,000 years The objective of this paper is to discuss the temporal variability in the rates of RSL change and the response of these delta plain to the changing values.

One of the earliest accomplishments of this research program was the recognition that the Mississippi River did not build just a single delta plain at the end of the Holocene transgression. New geophysical data indicates that the Mississippi river built a series of backstepping delta plains at different eustatic positions while in filling it's incised valley during HT. Each delta plain represents a parasequence set that is bounded by flooding surfaces (FS) generated by a rapid rise in RSL that caused regional coastal land loss conditions and backstepping to occur. The sequence stratigraphic framework of the Late Quaternary valley fill suggests that RSL rise must exceed 2-3 cm/yr to create a FS and produce a retrogradational, back stepping shoreline such as the Teche shoreline which re resents the maximum flooding surface (MSF) of the HT. Under these conditions of a rapid rise in RSL primary driven by eustacy, the delta cycle process stops and a major transgression ensues producing a backstepping relic shoreline deposit on the continental shelf.

Three major stillstands are recognized in the HT, each is sequentially associated with the Outer Shoal, Ship Shoal, and Modern delta plains. During stillstand, subsidence of the deltaic sediments within the Holocene section is the primary process driving RSL rise. Rates of RSL are less than 2 cm/yr and the delta process operates building a delta plain comprised of a parasequence set of delta lobes separated from the underlying delta plain by a FS. Rates of subsidence are controlled by Holocene thickness, sediment age, gas and water contact and lithology.

The recognition of the 2-3 cm/yr threshold value for delta plain stability improves our capability to predict the influence of sea-level fluctuations on stratal stacking patterns and future coastal conditions.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994