THE RELATIONSHIPS BETWEEN PALEOCLIMATE, PALEOCEANOGRAPHY, DEPOSITION, AND EOGENESIS IN THE PHOSPHORIA ROCK COMPLEX OF THE BIGHORN BASIN, WYOMING

The Phosphoria Rock Complex is an enigmatic group of mid-late Permian sedimentary rocks that accumulated in the Phosphoria Sea, an epicontinental sea along western Pangea. The group includes phosphorites, organic-rich siltstones, cherty carbonates, open-marine to restricted carbonates, red beds, and evaporites. These diverse deposits are interpreted as forming under oceanographic conditions with no modern analog. They also accumulated at a time of extreme climate following the end of the Late Paleozoic Ice Age, warming global climate, and the aridification of western Pangea. The objectives of this study are to use field and core study combined with petrographic work to 1) better understand the role of the global and regional climate on the atypical oceanography, and 2) investigate the climatic and oceanographic controls on deposition and eogenesis. The Bighorn Basin of north-central Wyoming is an ideal location to address these questions as it contains interfingering terrestrial and marine deposits. The studied deposits record a transition from open-marine calcitic heterozoan carbonates to shallower-water molluscan carbonates to peritidal carbonates with coated grains and significant microbial influence. Landward of the carbonate facies were terrestrial environments with aeolian silt and evaporitic salinas. The distribution of these facies is directly a result of water temperature, salinity, nutrient content, and carbonate saturation state. Variability in these oceanographic controls record the interaction of the hot and arid western Pangean desert with cool and nutrient-rich waters in deeper parts of the sea. Furthermore, eogenetic alteration in these deposits is directly related to the facies and their oceanographic controls; water temperature and carbonate saturation state controlled marine carbonate cementation, nutrient content determined the amount of authigenic replacement, and the proximity of peritidal deposits to the arid climate of western Pangea led to contemporaneous dolomitization and synsedimentary cementation. The Phosphoria Rock Complex within the Bighorn Basin thus provides a unique example of the interaction of the extreme late Paleozoic climate with regional oceanographic conditions and the influence on the resulting sedimentary deposits.