Three major rivers supply the bulk of the clay mineral matter to the northeastern Gulf of Mexico. The type of clay supplied is a function of the clay minerals in the soils of the source basins and appears to be largely climatically controlled. The westernmost major river, the Mississippi, is supplying a very large amount of richly montmorillonitic clay. In an easterly direction kaolinite gradually becomes more abundant in the soils and rivers; and the Apalachicola River, at the eastern extremity, is contributing a clay mineral suite very rich in kaolinite with much less montmorillonite. The Mobile River is geographically and clay mineralogically midway between the Mississippi and Apalachicola rivers.
As clay from the Mississippi and Apalachicola rivers enters saline water, a very slight relative increase in the proportion of kaolinite to montmorillonite occurs. As yet this slight increase has not been explained satisfactorily. In the Apalachicola River, it may be the product of a minor size separation combined with a loss of swelling properties by some of the stripped mica portion of the clay. This slight change, which may be in part a response to changing chemical environment, is of a much smaller order of magnitude than the river to river change in the relative abundance of montmorillonite and kaolinite. The relative magnitude of the two effects should be kept in mind in deciphering paleogeography on the basis of clay minerals.
Once the clays have been delivered to the Gulf of Mexico, the distribution of that portion of the clay not flocculated by the saline waters at the river mouths is governed first by the shallow water, wind-driven currents, and then by the semipermanent oceanic currents. The eventual distribution of the clays follows a gradational facies pattern in which the sources of supply and the distributional directions are clearly evident. When viewed in regional aspect, the study of clay mineral distributions in ancient unmetamorphosed sediments should form a valuable adjunct to conventional paleogeographic methods.