PALEOCEANOGRAPHIC AND PALEOCLIMATIC CONTROLS ON SILICEOUS VS. CARBONATE BENTHOS, SKELETAL MINERALOGY, AND EOGENESIS IN THE PHOSPHORIA ROCK COMPLEX OF THE BIGHORN BASIN, WYOMING, USA

The Phosphoria Rock Complex accumulated in a western Pangean epicontinental sea during the middle Permian, following the acme of the Late Paleozoic Ice Age and the aridification of western tropical Pangea. Within the group, the presence, composition, and eogenesis of carbonate benthos record systematic variation caused by changing paleoceanography and influence of adjacent continental climate. These variations are both lateral and stratigraphic in extent. In the well-preserved Ervay cycle, thin basal phosphorites are overlain by cherty siliciclastics and carbonates with widespread siliceous spicules interpreted to record the landward migration of a biogenic silica factory during transgression. Redox and nutrient levels were such that carbonate factories were suppressed; instead, siliceous sponges dominated. Subsequently, oceanographic conditions became conducive for carbonate production. The most basinward facies are brachiopod-bryozoan carbonates with widespread authigenic cementation and replacement by phosphate, glauconite, and chert. Landward, calcitic heterozoan deposits lacking authigenic minerals transition to aragonitic molluscan facies and peritidal carbonates. Supratidal to terrestrial aeolian silt and evaporitic salinas of the western Pangean desert bordered the sea. The carbonate facies are evidence of variance in water temperature, nutrient content, salinity, and carbonate saturation state caused by the interaction of cool, mesotrophic waters further offshore with the hot and arid western Pangean desert. The carbonate facies distributions are a direct reflection of the presence of thermo- and chemoclines. Eogenetic alteration was also directly related to the paleoceanography. Offshore, mesotrophic conditions led to widespread intraskeletal cementation by authigenic minerals. The arid and hot continental climate caused warmer and more saline shallow water, which favored aragonitic biota, marine carbonate cementation and penecontemporaneous dolomitization. Relative-sea-level fluctuations also systematically contribute to the oceanographic and facies variability. These deposits allow reconstruction of the interaction of the late Paleozoic climate with regional oceanography and their control on the resulting carbonate and bioelemental deposits.