ABSTRACT

Lakes Pontchartrain and Maurepas are large bodies of brackish water. The chemical components of this saline environment influence the lateral distribution of clay minerals. Clay minerals are supplied to the lakes by various rivers and bayous and by erosion of the shoreline. The water level in the lake system fluctuates rapidly; and, as a result, the salinity of any given area is quite variable over a given period of time. However, the trends of the ionic gradients are fairy constant. Clay minerals flocculate and settle in reasonably consistent patterns in response to these compositional gradients. Kaolinite flocculates first and settles in areas of lowest salinity. Montmorillonite has its greatest concentration in the higher salinity portions of the lake and is the most resistant to s ttling.

Illite also appears to be concentrated in the sediments of the higher salinity areas; however, some of the illite may be authigenic. The potassium to hydrogen and sodium to hydrogen ratios in the higher salinity portions of the lake approach the ratios which define the hydrochemical stability field of K-mica, which suggests that illite may be formed as an alteration product of other clay material.