LAKE SYSTEMS OF PANGEA: THE DEPOSITIONAL AND DIAGENETIC HISTORY OF THE PERMIAN BLAINE FORMATION FROM THE REBECCA K. BOUNDS CORE OF KANSAS

Red siliciclastics and evaporites are common in the Permo-Triassic record of Pangea. However, the only well-preserved examples of these rocks are from cores drilled with salt-sensitive methods. These high-recovery, well-preserved cores are rare, so few detailed sedimentological and stratigraphic observations have been made for Permo-Triassic red beds and evaporites. Previous studies of the Nippewalla Group in Kansas, focused mainly on incomplete cores, have indicated low pH lake and groundwater brines. Here, we describe core observations and thin section petrography, as well as fluid inclusion petrography and x-ray diffraction analysis, to refine depositional conditions and diagenetic history for the Permian Blaine Formation (Nippewalla Group) in the Rebecca K. Bounds core of Greeley County, Kansas.

The Blaine Formation lithologies are displacive halite, bedded gypsum/anhydrite, bedded halite, siliciclastic mudstone, and minor amounts of crystalline carbonate. These units represent a lake system of shallow perennial and ephemeral acid saline lakes, surrounded by mudflats and desert soils. The displacive halite lithology, consisting of randomly orientated halite crystals in a red mudstone matrix, represents saline mudflats. The bedded gypsum/anhydrite lithology has three major textures: bottom-growth, laminated, and massive textures, suggesting chemical precipitation at a lake bottom, chemical precipitation in a lake water column, and physical reworking of lake crystals, respectively. Bedded halite has cumulate and chevron crystals, dissolution pipes, and efflorescent crusts. These features indicate deposition in shallow ephemeral saline lakes. Laminated siliciclastic mudstones were likely deposited by floods on mudflats, with circumgranular cracks, craze plane cracks, and suspect root features indicating pedogenesis. The sole, thin crystalline carbonate unit of diagenetic dolomite and halite may have been deposited as detrital grains or as a chemical sediment before undergoing diagenesis.

Detailed sedimentology of the Blaine Formation marks both an arid time and an extreme water chemistry similar to other Permo-Triassic red beds and evaporites. Acid saline lake systems were widespread across much of Pangea