PETROGRAPHY OF AN ANOMALOUS CARBONATE FROM THE PERMIAN GOOSE EGG FORMATION OF WYOMING

The Permo-Triassic sedimentary record of northcentral Wyoming is represented largely by the Permian Goose Egg Formation and Triassic Chugwater Group. These formations are characterized by their abundant red bed siliciclastics and bedded gypsum. However, they also contain thin, anomalous carbonate units, which remain undescribed. Petrographic observations were made for one carbonate unit from the Permian Goose Egg Formation near Lovell, Wyoming. We observe ooid- and peloid-like grains, graded bedding, microbial mats, root casts, and fenestral pores. We observe at least three generations of pore-filling cements: first, a pendant blocky calcite cement; second, a blocky, isopachous saddle dolomite cement; and third, radiaxial fibrous pendant calcite cement. Timing of cementation is interpreted to be syndepositional to very early and reflects a fluctuating water table soon after deposition. The depositional and diagenetic textures observed are interpreted to have formed in a shallow or ephemeral saline lake or spring in an otherwise arid, desert environment. Permo-Triassic red beds and evaporites elsewhere in the midcontinent have a paucity of depositional and diagenetic carbonates and contain evidence of extremely acid saline lakes and groundwaters. This anomalous carbonate unit in the Goose Egg Formation, suggests that there may have been localized and short-lived alkaline continental waters at times amid the red desert soils of Pangea.