DEPOSITIONAL ANATOMY OF A PHOSPHATE DOMINATED SOURCE ROCK SUCCESSION: THE MEADE PEAK MEMBER, PHOSPHORIA FORMATION, PERMIAN, USA

The late Permian Phosphoria Formation (late Leonardian to Guadalupian) is comprised of a sequence of marine sedimentary units of significant economic and scientific interest due to the presence of the world’s largest known phosphorite deposit. Additionally, the Phosphoria Formation has been reported to contain up to 32.9% total organic carbon locally, sourcing several oil fields in Wyoming. Deposition occurred along a westward-deepening ramp within the Phosphoria Sea, an epicontinental marine embayment covering approximately 400,000 Km² of west-central Pangea (~ 20°N paleolatitude). Throughout most of the basin the Phosphoria Formation is composed of mixed carbonate-phosphorite-siltstone-chert sequences. Deposition of the organic-rich phosphorite members has been linked to marine upwelling based on the presence of phosphate, organic matter, and silt, in addition to modeled basin hydrography and wind patterns. The occurrence of phosphorite alone has often been used as the basis for identifying upwelling systems as most modern phosphate deposits involve this process. However, no modern upwelling systems act as satisfactory analogs for the Phosphoria Sea or sedimentary systems comparable to the Phosphoria Formation. This study focused on fine-scale stratigraphic relationships and microfacies analysis within the Meade Peak Member in order to illuminate the physical controls on deposition and the mechanisms responsible for phosphorite accumulation. Our results reveal that the abundant quartz silt was likely transported into the basin by marine processes due to the presence of traction structures such as ripples and intense marine bioturbation; rather than the previously suggested wind controlled mechanisms, postulated to drive upwelling. The organic-rich phosphate deposits are often coarse-grained and characterized by significant reworking, indicating sedimentation in a relatively shallow-water, high-energy regime. The shallow nature and evidence of oxic conditions renders application of the traditional upwelling model difficult at best.