An integrated stratigraphic, paleontologic, and petrologic study shows the presence of 3 general environments of deposition for the Cretaceous and Tertiary sediments of the upper Mississippi embayment: 1) fluviatile, 2) inner neritic, and 3) outer neritic. Prior petrologic studies have shown similar provenance features for all stratigraphic units involved. X-ray diffraction analyses of 199 clay samples from the 3 environments and from all the stratigraphic units studied show kaolinite, illite, and montmorillonite as the dominant clay minerals, with small amounts of mixed-lattice and chloritic materials in a few samples. There are 3 clay mineral assemblages - one dominantly kaolinite, a second dominantly montmorillonite, and a third having nearly equal amounts of kaolinite, illite, and montmorillonite. The clays of the Eocene (undifferentiated) formations, and of the Cretaceous McNairy and Tuscaloosa formations and the northern part of the Coffee formation are dominantly kaolinite; those of the Porters Creek, Clayton, and Owl Creek formations and the southern part of the Selma formation are dominantly montmorillonite; and those of the Coon Creek formation, the northern part of the Selma formation, and the southern part of the Coffee formation are kaolinite, illite, and montmorillonite in nearly equal amounts. Clays deposited in the fluviatile environment are dominantly kaolinite, those in the outer neritic environment are dominantly montmorillonite, and those in the inner neritic environments are composed of nearly equal amounts of kaolinite, illite, and montmorillonite. Segregation of clay minerals in the depositional environments is believed responsible for these variations in clay mineralogy. Diagenesis and variations in contributions from the source area are believed to be insignificant factors.