Refining the sedimentology and geochemistry of the mid-Permian Blaine Formation of the Rebecca K. Bounds core from Greeley County, Kansas

Red siliciclastics and evaporites of Permian Nippewalla Group in mid-continental US were deposited in western equatorial Pangea. Previous studies of these rocks record Permian air temperatures as high as 73 °C, saline lake waters with a pH as low as ~1, and sedimentology that shows arid conditions. This current study conducts detailed sedimentological observations and geochemical analyses of the Blaine Formation of the Nippewalla Group. The Rebecca K. Bounds core from west-central Kansas is used to interpret the depositional environments and diagenetic history of the Blaine Formation. The lithologies of the Blaine Formation, listed in order of most to least abundant, are displacive halite, bedded gypsum/anhydrite, bedded halite, siliciclastic mudstone, and crystalline carbonate. These units represent a neutral-to-acid, perennial-to-ephemeral, saline lake-mudflat-desert soil system. The displacive halite lithology, consisting of randomly orientated halite crystals in a red mudstone matrix, represents a mudflat environment with shallow saline groundwater. The bedded gypsum/anhydrite lithology has three major textures: bottom-growth, laminated, and massive textures. The bottom-growth and laminated gypsum/anhydrite formed by chemical precipitation in a shallow saline water environment. The massive gypsum/anhydrite texture was possibly deposited as physically reworked grains. Bedded halite has cumulate and chevron crystals, dissolution pipes, and efflorescent crusts. These features indicate that evapoconcentration, flooding, and desiccation events occurred in a shallow ephemeral saline lake environment. Additionally, the iron-oxides, presence of “hairy blobs”, and the paucity of carbonates in the bedded halite shows that these were likely acid saline lakes. The siliciclastic mudstones are massive, red, and represent dry mudflats which experienced infrequent flooding and displays pedogenic overprinting. The thin crystalline carbonate unit is composed of diagenetic dolomite and halite. Original deposition may have been as detrital grains or as a chemical sediment that underwent diagenesis. The Blaine Formation has a complicated diagenetic history. Many of the early diagenetic features are related to dissolution, desiccation, and pedogenesis. Later diagenetic processes formed pseudomorphs, cements including magnesite, dolomitic interlocking crystal mosaic, and quartz veins and overgrowths. Dehydration and rehydration of gypsum and anhydrite plausibly occurred throughout the paragenetic sequence. Detailed sedimentology of the Blaine Formation marks an arid time consistent with the other formations of the Nippewalla Group. Similarities with Permian and Triassic red beds and evaporites from other well-preserved cores reveal that these acid saline lake systems were widespread across much of Pangea.