Three Upper Triassic (Rhaetian) formations which crop out in the Hoyada de Ischigualasto, San Juan Province, Argentina, have been analyzed partly for clay-mineral content. These continental units are conformable. From oldest to youngest, they are known as Estratos de los Rastros, Estratos de Ischigualasto, and Estratos de los Colorados. More than 80 samples representing observable changes in lithologic characteristics were taken at irregular intervals from 30 meters below to 30 meters above the two interformational boundaries. In the section studied, the boundary between the two lower formations is not well defined, either on lithologic or paleontologic characteristics, and that between the upper two formations is transitional in nature (through about 30 meters).

The Estratos de los Colorados (redbeds with abundant gypsum) have illite as their principal, and in most cases only, clay mineral component. The two underlying formations contain an assemblage of kaolinite, montmorillonite, and illite. It is probable that the clay mineralogy reflects the environment in which it developed. The Estratos de los Colorados are thought to represent a desert environment in which alkaline conditions and the presence of potassium would be propitious for the formation of illite. The two underlying units, however, are dominantly composed of lacustrine deposits, commonly carbonaceous or bearing thin coal seams, and represent an environment different from that of the Estratos de los Colorados, i.e., a humid, reducing environment in which cations could be leached a ay and kaolinite would develop.

The clay mineralogy may be useful for locating more exactly the interformational boundary between the Estratos de los Colorados and the Estratos de Ischigualasto. However, there does not appear to be any significant difference in the clay mineralogy of the Estratos de Ischigualasto and the Estratos de los Rastros which would indicate the presence of an interformational boundary.

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