ABSTRACT

The Popo Agie Member of the Chugwater (Triassic) Formation is underlain by an unnamed red bed unit and overlain unconformably in some places by the Nugget (Triassic?) Sandstone and in others by the Gypsum Spring (Jurassic) Formation. The Popo Agie Member is herein informally divided into the following units of regional extent: lower carbonate unit; purple unit; ocher unit; and upper carbonate unit. The upper and lower carbonate units are characterized by silty and sandy limestones, silty dolomites, and limestone microcongolomerates. The purple and ocher units are characterized by spherulitic analcimolites, analcimic, silty claystones, and analcimic siltstones. Mineral zonation of the spherules (cryptocrystalline silica (?) center, clay rim, analcime outer shell) is common.

On the basis of five measured and sampled sections of the Popo Agie Member in west-central Wyoming, there are two main mineral assemblages: 1) quartz-illite-feldspar, and 2) quartz-analcime-montmorillonitegoethite. These assemblages occur in three definite mineral zones. The upper and lower zones are characterized by the first assemblage and correspond to the upper and lower carbonate units. The middle zone, characterized by the second assemblage, includes the purple and ocher units.

The analcime and the montomorillonite are believed to have originated from the alteration of volcanic ash by water rich in dissolved salts. Both the volcanic ash and the water furnished the necessary Na ⁺ ions in the analcime. The structure of the zoned spherules in the analcimolites, and their intimate relation to a montmorillonite matrix, indicate that the spherules and the montmorillonite grew in place before burial. The cryptocrystalline centers of the zoned spherules probably represent part of the original volcanic material which was altered and has nucleated the spherules. Possible relict shards and volcanic textures occur in the rocks of the purple and ocher units. The association of analcime and montomorillonite also suggests that the silica and alumina were derived from volcanic material. The heavy mineral assemblage indicates a volcanic origin with admixtures from weathered igneous or metamorphic and sedimentary terrains. The sudden appearance of the analcime and montmorillonite, and the general parallelism of the boundaries between the mineral zones and the rock units, indicate that the appearance of the analcime and montmorillonite is an event of major importance in the history of the Popo Agie Member.

The principal environments of deposition are believed to have been: 1) lacustrine (with volcanic sources) during formation of the analcime and some of the montmorillonite; 2) lacustrine (no volcanic material) during formation of the carbonates; and 3) fluvial during formation of some of the montmorillonite, and the rocks of the lower and upper carbonate units (excepting the carbonate beds). Associated tracts and minor modifications of the main environments also occurred.