Abundant analcime in the purple and ocher units of the Pogo Agie Member of the Chugwater Formation and associated montmorillonite are interpreted as the result of alteration of volcanic material in sodium-rich water. The Seven Devils and Casto volcanics of Permian and Triassic age are the nearest apparent source for the volcanic material. Chemical analyses and normative minerals are given for two analcime-rich samples (analcimolites) and a clayey siltstone. Thin section and X-ray mineralogy, modal analyses, stratigraphy, and paleontology are utilized to give an integrated interpretation of the genesis of the Popo Agie Member.

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