Paleoceanographic and Paleoclimatic Controls on Biochemical Sedimentation in the Permian Phosphoria Rock Complex in the Bighorn Basin, Wyoming

The Phosphoria Rock Complex of the western US contains a wide array of biochemical sedimentary rocks that accumulated in an oceanographic system with no modern analogue. The group includes phosphorites, chert, marine and lacustrine carbonates, red beds, and evaporites that accumulated in and around the subtropical epeiric Phosphoria Sea of western Pangea. Most intriguing are the phosphorites, which formed in environments spread across 100+ km for reasons that remain poorly understood. The well-exposed Phosphoria Rock Complex succession in the Bighorn Basin of northern Wyoming has been underexploited in investigations of the anomalous paleoceanography of the sea. This study utilizes field and subsurface data in the basin combined with petrographic and geochemical analyses to 1) investigate the climatic and oceanographic controls on deposition, 2) reconstruct the temporal evolution through the Phosphoria Rock Complex succession, and 3) provide insight into the unique paleoceanographic conditions of the Phosphoria Sea. Within the Bighorn Basin, the Phosphoria Rock Complex contains three retrogradationally stacked sequences (the Grandeur, Franson, and Ervay cycles) with interfingering terrestrial and marine deposits. Terrestrial deposition records windblown silt with periodic coastal salinas that formed during sea-level highs. The terrestrial record is interrupted by two periods of development of widespread lakes east of the Phosphoria Sea, including a large hypersaline lake in the eastern Bighorn Basin in which bacterial sulfate reduction lead to organogenic dolomite precipitation (Minnekahta Member). The marine Ervay cycle accumulated at the maximum extent of the Phosphoria Sea. It contains three disparate lithologic members that record the evolution from an area of phosphogenesis (Retort Member) to a glass ramp with siliceous sponge meadows (Tosi Member) to a carbonate ramp (Ervay Member). These deposits provide further substantiation for upwelling in the Phosphoria Sea and record its unique temporal evolution. Intense upwelling led to phosphogenesis, whereas the interaction of sea level, upwelling intensity, oxygenation, and dissolved silica concentrations led to the successive faunal turnovers recorded in the Ervay cycle. These deposits provide further insight into the controls on Phanerozoic glass-ramp deposition and a unique example of the potential disparity in the sedimentary deposits of pre- and post-Cretaceous upwelling systems