A PERMIAN HYPERSALINE LAKE WITH BACTERIAL SULFATE REDUCTION AND DOLOMITE PRECIPITATION IN THE MINNEKAHTA MEMBER, BIGHORN BASIN

During the late Paleozoic, western Pangea was a widespread desert of windblown silt and ephemeral saline lakes, which resulted in a thick succession of red beds and evaporites. Across Wyoming, thin (<15 m), enigmatic, aerially extensive carbonate units punctuate the red-bed succession. The middle Permian Minnekahta Member is one such unit as it is purportedly correlatable across much of central and eastern Wyoming, an area of approximately 100,000 km². Detailed investigation of the Minnekahta in the eastern Bighorn Basin of north-central Wyoming provides new insights into its origin. Within the basin, the Minnekahta is predominantly purple and grey laminated dolomicrite with low δ¹³C values (-0.5 to -4.5‰) overlain by bedded gypsum. Based on sedimentologic, stratigraphic, petrographic, and geochemical data the Minnekahta in the basin is postulated to have accumulated in a stratified hypersaline lake with anoxic bottom water and bacterial sulfate reduction that led to the precipitation of organogenic (microbial) dolomite. These conditions produced laminated, pyritic dolomicrite with thin, bedding-parallel chert nodules that formed as primary silica gels in the lake. The later oxidation of pyrite gave the Minnekahta its purple colouration. Detailed correlation reveals a number of sub-basins separated by antecedent highs within the lake. Each sub-basin had a unique depositional history through two cycles of replenishment. The lacustrine origin is consistent with recent study and interpretation of the Minnekahta in the Black Hills of South Dakota, ca. 300 km away. This suggests that the lacustrine deposits in this study formed in a few of a large number of extensive, partially connected or coeval lakes across central and eastern Wyoming, which together produced the Minnekahta Member, and potentially relate to climatic cyclicity in the Pangean desert. The Minnekahta evaporitic lakes (basins) thus provide a new and intriguing record of widespread lake development across the vast and inhospitable western Pangean desert. This includes the lakes in the Bighorn Basin, which were orders of magnitude larger than any modern lake with bacterial sulfate reduction and dolomite precipitation, further substantiating the extreme and atypical nature of the late Paleozoic western Pangean desert.