NYMAN, DALE J., Nyman & Associates, Baton Rouge, LA

Abstract: Freshwater Depth in the Chicot aquifer of Southwestern Louisiana during the Holocene Sea Level Lowstand

About 10k years ago sea level was approximately 300 ft 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 lowered probably occur beneath southern Iberia Parish and in the Marsh Island area. In these areas there is a large region where freshwater occurs to the unusual depth of more than 600 ft below sea level, with isolated depressions where the depth of freshwater is greater than 700 ft. The deepest depressions may approximate the depth at which freshwater occurred during the last sea level lowstand. The elevation of the freshwater-saltwater interface was interpreted from geophysical logs of oil tests.

As sea level rose during Holocene time the saltwater wedge in the Chicot aquifer was pushed northward and moved 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 the 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 fresh water. 

AAPG Search and Discovery Article #90924©1999 GCAGS Annual Meeting Lafayette, Louisiana