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GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 49 (1999), Pages 23-24

EXTENDED ABSTRACT: Freshwater Depth in the Chicot Aquifer of Southwestern Louisiana During the Holocene Sea Level Lowstand

Dale J. Nyman

Nyman & Associates, Baton Rouge, LA.

ABSTRACT

About 10k years ago sea level was approximately 300 feet lower than it is today. The shoreline of the Gulf of Mexico was near the continental shelf and the freshwater-saltwater interface in the Chicot aquifer was deepened and pushed southward by the increased southward groundwater gradient. Remnants of the depth to which the freshwater-saltwater interface was deepened probably occur beneath southern Iberia Parish and near Marsh Island, see Figure 1. In these areas Nyman (1984) found a large region where base of freshwater (BFW) occurs to the unusual depth of more than 600 feet below sea level, with isolated depressions where the BFW is greater than 700 feet. The deepest depressions may approximate the depth at which freshwater occurred during the last sea level lowstand. The elevation of the freshwater-saltwater interface (BFW) was interpreted from geophysical logs of oil tests. Most of the oil tests were drilled between 1950 and 1980.

As sea level rose during late Holocene time, the saltwater wedge in the Chicot aquifer was pushed northward and moved northward most rapidly in the most permeable portions of the aquifer, e.g., through the Atchafalaya-Mississippi alluvium. A series of east-west trending faults, offshore from Marsh Island and its current shoreline, slowed northward saltwater encroachment and partially protected the deep freshwater basin as sea level rose and freshwater gradients were reduced. The current configuration of the base of freshwater also suggests that the Five Islands salt dome chain slowed the westward saltwater movement from the Atchafalaya Basin. These factors provide a plausible explanation for the presence of these unique pockets of deep freshwater.


REFERENCES

Nyman, D.J., 1984, The occurrence of high concentrations of chloride in the Chicot aquifer system of southwestern Louisiana: Louisiana Department of Transportation and Development, Office of Public Works Water Resources Technical Report no. 33, 75 p.

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Figure 1. Contour map showing the elevation of the base of freshwater (BFW) and pockets of unusually deep freshwater.

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