EPHEMERAL SALINE LAKES AND MUDFLATS FROM THE PERMIAN CLOUD CHIEF FORMATION, TEXAS: EVIDENCE FROM TEXTURES IN GYPSUM

Bedded gypsum, gypsum that precipitates or is otherwise deposited at the Earth’s surface, forms a multitude of textures indicative of specific depositional conditions. Detailed sedimentological observation of these textures is critical in interpreting the depositional environments of ancient bedded gypsum. Two outcrops of bedded gypsum from the Permian Cloud Chief Formation near Quitaque, Texas were studied through centimeter-scale measured sections and thin section petrography. Lithologies include bottom-growth gypsum and laminated gypsum, both interbedded with red siliciclastic mudstone to fine sandstone. Bottom-growth gypsum units contain beds of vertically oriented, bladed and swallowtail-shaped gypsum crystals with thin, draping laminations of gypsum mudstone. Laminated gypsum units contain planar laminations, wavy/undulating laminations, doming laminations, convolute laminations, and teepees. Red siliciclastic deposits are commonly cross-cut by fibrous gypsum veins and exhibit either thinly laminated, ripple laminated, or massive bedding; soft sediment deformation and abundant blocky peds also commonly occur. The depositional gypsum textures and their association with red beds indicate precipitation and clastic reworking of gypsum crystals in ephemeral saline lakes and associated mudflats. Although anhydrite was not observed in the study outcrops, a “cloudy,” blurry appearance of many gypsum textures in the Cloud Chief Formation suggests a history of dehydration to anhydrite and subsequent rehydration to gypsum. Despite this likely neomorphism, gypsum crystals retain depositional morphologies that are remarkably recognizable in some outcrops. The gypsum textures and interpreted depositional environments presented here are compared to those of the Triassic Red Peak Formation (Chugwater Group) of Wyoming and some modern acid-saline lakes in Western Australia. Permo-Triassic bedded gypsums and associated siliciclastic red beds from across equatorial Pangea may represent a continent-wide system of ephemeral acid-saline, alkaline-saline, and neutral-saline mudflats, sandflats, dunes, and desert soils strongly influenced by a dynamic groundwater table.