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The Terrestrial-Marine Transition in the Phosphoria Rock Complex of the Bighorn Basin, Wyoming


The Phosphoria Rock Complex (PRC) is a widespread succession of bioelemental, carbonate, evaporite, and siliciclastic rocks that contain a prolific petroleum system and economic phosphate deposits. The PRC accumulated during the Permian, a time of significant environmental change from the Late Paleozoic Ice Age to the Mesozoic hothouse. The phosphatic sediments of the PRC are interpreted to have formed in a unique oceanographic system with no modern analog. Several contrasting models have been proposed to explain the atypical oceanography. They have focused on the origin of the biolemental rocks and the roles of upwelling, intermediate water masses, and temperature and salinity stratification. Most studies have focused on the areas and times of prolific phosphorites; the evolution of the oceanography through the entire PRC stratigraphy is not well understood. This study utilizes 20 stratigraphic sections within the Bighorn Basin to examine the oceanographic evolution through the complete PRC. The deposits range from entirely marine in the west to entirely terrestrial in the east. Terrestrial sabkha and salina deposits include red beds, meter-scale gypsum beds, and carbonate mudstones that are structureless, microbially laminated, and contain gypmoldic porosity. Open-marine deposits are predominantly subtidal bryozoan and brachiopod wackestone, packstone, and floatstone containing phosphate and glauconite. Restricted marine deposits straddled the coastal transition and are peritidal and lagoonal dolostones with coated grains, fenestral fabric, and microbial laminations. These deposits compose 3 retrogradationally stacked depositional sequences. The characteristics of the facies belts are laterally and stratigraphically variable through each sequence. These complexities record the interaction of an arid landmass to the east, typical of equatorial Pangea during the Permian, with a nutrient-rich and productive epeiric sea. Stratigraphically, the terrestrial and marine deposits evolved concurrently through each of the depositional cycles with changing oceanographic and climatic conditions impacting depositional motifs. Stratigraphic heterogeneity is partially a result of changing sea level, the major control within marine facies, but also internal autogenic controls, more important in terrestrial facies. These stratigraphic and geographic complexities are an important control on the nature of the prolific PRC reservoir facies present within the Bighorn Basin.