Continental saline environments interpreted from bedded gypsum of the Triassic Red Peak Formation (Chugwater Group), northcentral Wyoming

Bedded evaporites and associated red bed siliciclastics record saline lake and groundwater systems from Permo-Triassic Pangea. A major component of these red bed and evaporite systems is bedded gypsum. However, little attention has been paid to the textures of ancient gypsum. Observations of gypsum textures can refine interpretations of depositional environment and diagenetic history. This project describes textures of bedded gypsum from an outcrop of the Triassic Red Peak Formation (Chugwater Group) near Greybull, Wyoming. This thesis uses fieldwork, petrography, and x-ray diffraction to describe an outcrop of the upper Red Peak Formation, with a focus on textures of bedded gypsum, to make interpretations about depositional environments. The study outcrop is comprised of alternating units of bedded gypsum and red bed siliciclastic mudstone. The red mudstone units are massive, rich in blocky peds, host abundant cross-cutting gypsum veins, and are interpreted to be paleosols. Three distinct lithologies of bedded gypsum are described and identified: bottom- growth gypsum, laminated gypsum, and clastic gypsum. Bottom-growth gypsum is interpreted to have precipitated at the bottom of shallow saline surface water bodies. Laminated gypsum likely formed in shallow saline lakes and mudflats; here, gypsum cumulates precipitated and were later reworked. Clastic gypsum units are composed of eolian-reworked bottom growth gypsum crystals deposited in sandflats and dunes. The study section of the Red Peak Formation was formed in shallow saline lakes and associated mudflats, sandflats, dunes, and desert soils

Gypsum lakes, sandflats and soils revealed from the Triassic Red Peak Formation of the Chugwater Group, north‐central Wyoming

Abstract Bedded gypsum is relatively common in bedded evaporites associated with red bed siliciclastics of Permo‐Triassic Pangea. However, little attention has been paid to the textures of ancient gypsum, which can be used to refine interpretations of depositional environment and diagenetic history. This project describes the textures of bedded gypsum from an outcrop of the Triassic Red Peak Formation (Chugwater Group) near Greybull, Wyoming. Fieldwork, petrography and X‐ray diffraction reveal three distinct lithologies of bedded gypsum: bottom‐growth gypsum, laminated gypsum and clastic gypsum. Bottom‐growth gypsum precipitated at the bottom of shallow saline surface water bodies. Laminated gypsum probably formed in shallow saline lakes and mudflats. Clastic gypsum units are composed of aeolian‐reworked bottom‐growth gypsum crystals deposited in sandflats. Red siliciclastic mudstones are characterised by their massive nature and abundant blocky peds. Detailed study of this outcrop of the Red Peak Formation shows that it formed in shallow saline lakes and associated mudflats, sandflats and desert soils