Late Quaternary Mississippi River deltaic sediments off the coast of Louisiana were studied to obtain information on possible clay-mineral alteration in a marine environment. The samples utilized were cores from four foundation test borings and included clays which had been deposited in several environments. For comparison, studies were also made of the clay minerals transported by the Mississippi and Red rivers.

The results support the conclusion that clay minerals do not alter appreciably as a result of diagenesis in a marine environment and that the clay-mineral composition of source sediments determines the clay-mineral content of marine sedimentary rocks. All the deltaic samples contained montomorillonite, illite, kaolinite, and chlorite. The most noticeable variation among the samples was in montmorillonite content. Montmorillonite was the most abundant clay mineral in most of the deltaic samples but was much less concentrated in some of the remaining samples. Variations in montmorillonite content did not correspond with differences in depositional environment. Clays with high montmorillonite content occurred in both essentially marine and fresh-water facies.

Analyses of samples of Mississippi River and Red River sediments indicated that these rivers carry the same suite of clay minerals as is present in deltaic sediments. Furthermore, samples of Red River sediment, characteristically reddish-brown, were predominantly low in montmorillonite content, whereas samples of the gray Mississippi River sediment were high in montmorillonite content. These differences in clay-mineral composition and color correlate substantially with corresponding variations in samples of deltaic sediment.

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