ABSTRACT

Bottom sediments from the Louisiana continental shelf were collected from near the mouth of the Mississippi River at Southwest Pass to about 150 miles west. Clay minerals in the shelf sediments are present in a fairly homogeneous suite composed of illite, kaolinite, and montmorillonite. The relative abundance of kaolinite decreases 16 percent to the west. Chlorite is present in variable but minor amounts. Samples from near Southwest Pass and in the Atchafalaya River are characterized by greater relative amounts of kaolinite and lesser amounts of illite and montmorillonite.

Total cation exchange capacity of the less-than-2 micron fraction of the sediment from the shelf is also quite uniform but decreases to the west corresponding to the decrease in kaolinite. The clay exchange position cations have equilibrated with the Gulf of Mexico waters before their deposition. Sediments near the Mississippi River and in the Atchafalaya River are slightly different chemically and mineralogically than the other shelf clays, indicating the influence of coarser grained material and the lack of equilibration with Gulf waters. Near the mouth of the Mississippi River, exchangeable calcium and potassium are less concentrated than in the other shelf sediments, exchangeable magnesium is about the same, and exchangeable sodium is greater. A sample from the Atchafalaya River had the greatest kaolinite content and least cation exchange capacity.