Abstract

Semi-quantitative X-ray analysis of 75 samples of Morrison and Kootenai shales, collected at 14 localities in southwest Montana, indicates that the clay mineralogy of the two formations is entirely compatible with earlier conclusions about the provenance and environment of deposition of the two units.

Illite is the most abundant clay mineral in both the Morrison and Kootenai Formations. It was probably derived from Paleozoic and Lower Mesozoic marine rocks in western Montana and eastern Idaho. Kaolinite is the subordinate clay mineral in the Morrison and chlorite subordinate in the Kootenai. This stratigraphic zoning of kaolinite and chlorite may reflect a significant change in source rock during deposition of the formations or it may be related to a rather sudden change to a more arid climate near the start of Kootenai deposition.

The paucity of montmorillonite and its concentration in extreme southern Montana is a function of the direction of volcanic ash dispersal in the Late Mesozoic. East or east-southeast dispersal of ash from known Late Jurassic or Early Cretaceous vents in California and Oregon would only slightly influence deposition in Montana.