THE DOGMA OF UPWELLING, TIME FOR A CHANGE: PHOSPHATE ACCUMULATION IN A SHELTERED INLAND SEA, THE PERMIAN PHOSPHORIA FORMATION, USA

The late Permian Phosphoria Formation (late Leonardian to Guadalupian) in the northwestern United States is comprised of marine sedimentary units of significant economic and scientific importance due to the presence of the world’s largest known sedimentary phosphate deposit. Deposition occurred along a westward-deepening carbonate ramp within the Phosphoria Sea, an epicontinental marine embayment along the northwestern margin of Pangea (~ 20°N paleolatitude). Throughout most of the basin the Phosphoria Formation is composed of the Meade Peak and Retort Members, two mixed carbonate-phosphorite-siltstone-chert sequences. Deposition of these two members has been attributed to marine upwelling based on the presence of abundant phosphate, organic matter, and silt, coupled with modeled basin hydrography and wind patterns. Traditionally, the occurrence of phosphorite alone is often used as the basis for identifying upwelling systems as most modern phosphate deposits involve this process. However, there are no modern analogs for the Phosphoria Sea or sedimentary systems comparable to the Phosphoria Formation. In contrast to previous, mostly geochemical and basin modeling approaches to characterize the Phosphoria Formation, this study focuses on fine-scale stratigraphic relationships and microfacies analysis within the Meade Peak member in order to better understand controls on deposition and the mechanisms responsible for phosphorite accumulation. Our results show that the silt-sized quartz, previously attributed to eolian input into the basin, was more likely transported by marine processes as it shows traction structures such as ripples and intense marine bioturbation. Associated phosphate deposits are often coarse-grained and characterized by significant reworking, indicating sedimentation in a relatively shallow-water, high-energy regime. It therefore seems difficult to apply the traditional upwelling model to these Permian sediments, as it relies on a tranquil oxygen minimum zone to accumulate significant amounts of phosphate. Upwelling as the prime cause of phosphate deposition for the Phosphoria Formation seems also unlikely due to the position of the basin on the continent surrounded by land on three sides, and semi-detached from the open ocean to the west by an island arc system.