GYPSUM TEXTURAL RECORDS OF CONTINENTAL SALINE ENVIRONMENTS FROM THE TRIASSIC RED PEAK FORMATION, WY

Evidence from bedded evaporites and associated red siliciclastics show that acid saline lake and groundwater systems existed in Pangea during Permo-Triassic time. 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 gypsum from outcrops of the Triassic Red Peak Formation (Chugwater Group) near Greybull, Wyoming.

We used fieldwork, petrography, and x-ray diffraction to describe the upper Red Peak Formation, with a focus on gypsum textures. Outcrops are comprised of alternating units of red mudstone and bedded gypsum. The red mudstone is massive and ped-rich with abundant cross-cutting gypsum veins. The bedded gypsum is friable and massive at the bottom of the section and becomes increasingly hard, laminated, and thinly bedded towards the top of the section. Sedimentary structures include ripple cross-bedding and cross-laminations, planar laminations, wavy bedding, mudcracks. Nine discrete petrographic textures of gypsum are described, including bottom-growth gypsum, displacive gypsum, clastic gypsum, and flaser and lenticular laminations. Bottom-growth gypsum is interpreted to have formed in shallow saline lakes. Displacive gypsum crystals precipitated in brine-saturated mudflats. Clastic gypsum grains were the product of reworking of bottom-growth gypsum; these grains were transported by wind and deposited in sandflats, mudflats, and lakes. Lenticular and flaser bedding are products of shallow water wave action in shallow lakes. Some late-stage diagenesis is easily distinguishable, such as cross-cutting veins of gypsum. Our preliminary results suggest that the gypsum of the Red Peak Formation was deposited in shallow saline lakes, brine-saturated mudflats, and sandflats in a dry climate.