SAND DISTRIBUTION OF THE CHICOT AQUIFER IN SOUTHWESTERN LOUISIANA

The Chicot Aquifer, which lies in the southwestern corner of Louisiana is an aquifer composed of a mixture of sand and gravel with clay layers to lenses separating portions of the aquifer. The more continuous clay layers have lend to the nomenclature of Upper and Lower Chicot for the aquifer in eastern portion and 200 foot, 500 foot and 700 foot sands in the western portion, especially Calcasieu and Cameron Parishes. However from the examination of approximately 1,000 electrical resistivity logs, which include most of this aquifer, it appears that the aquifer is more complex than nomenclature indicates. It appears the percentage of the Chicot Aquifer that is sand varies greatly depending on depth and lateral position.

In general, sand content decreases from north to south across the aquifer. In addition it appears that in general the Chicot is less sand rich in the western portion than the eastern portion. This could be a result of differences in source material. In general, the ancestral Mississippi River supplied the sediment to the eastern portion of the Chicot, while the ancestral Red River supplied sediment to the western portion of the Chicot. In the vertical direction the top quarter is the least sand rich and lower quarters are more sand rich. If the aquifer system is divided into top, two middle and bottom quarters the variation of sand as a result of the historical variation of deltaic systems becomes apparent. The bottom quarter of the Chicot appears to have a fairly uniform sand percentage from west to east. By comparison for the top quarter of the Chicot is very different. The western portion has very little sand, often less than 20%, while the eastern portion is far more sand rich, sometimes over 80%. What these results appear to indicate is that at the start, bottom quarter of Chicot deposition, late Pliocene-early Pleistocene, all of southwestern Louisiana was part of a deltaic system that is mixture of Red River and Mississippi River deposits. However, by the top quarter of Chicot deposition, late Pleistocene, the focus of deltaic deposition is in the eastern portion of the aquifer towards Vermilion Bay after the Mississippi River permanently captures the Red River, and becomes a large amalgamated fluvial-dominated delta system.