Abstract

The clay minerals of the Gypsum Spring and Lower Sundance Formations exposed along the eastern flank of the Big Horn Mountains were analyzed to determine their significance in interpreting the geologic history and paleoenvironment of the area during Middle and Upper Jurassic time.

Illite was the dominant clay mineral and constituted over 50 percent of the clay component. Montmorillonite and mixed-layer clay (generally mixed-layer montmorillonite-illite) were second in. abundance and have a complimentary relationship to one another. These clay minerals generally comprised 30 percent or less of the total clay content of the sample, although montmorillonite was more abundant in some samples. Chlorite was least abundant of the major clay components and formed 20 percent or less of the total. Kaolinite was present in trace amounts in only five samples.

The deposits are thought to have formed in lagoonal or marginal conditions where, periodically, circulation was so restricted that gypsum deposits formed.

The almost total absence of kaolinite in the clay fraction, the evaporites present and the high carbonate to elastics ratio in nearly all the lithic units suggest an arid or semi-arid climate prevailed in the region.

As primary silicates will not alter readily in the presence of carbonate, and in view of the prevalence of calcium and magnesium carbonate in the Gypsum Spring and Lower Sundance Formations it is believed that the illite-montmorillonite-chlorite suite of clay minerals found in the formations is principally residual in origin, having been transported to the site rather than authentically formed.