PHOSPHOGENESIS UNDER EXTREME PALEOCEANOGRAPHIC CONDITIONS: CHEMOSTRATIGRAPHIC EVIDENCE FROM THE LATE PERMIAN RETORT MEMBER OF THE PHOSPHORIA FORMATION, WESTERN USA

The Phosphoria Rock Complex (Phosphoria, Park City, and Goose Egg formations) is a succession of phosphorite-siltstone-chert-carbonate-evaporite lithologies deposited on the western margin of North America during the Late Permian (latest Leonardian through Guadalupian series). Deposition of the PRC occurred in the Phosphoria Sea, which formed in a shallow marginal foreland basin on the western margin of North America at a paleolatitude of about 20° N. The Phosphoria Rock Complex is composed of two distinct intervals of organic matter- and phosphate-rich rocks (the Meade Peak and Retort members) that represent periods of oceanic upwelling. The focus here is the Retort member (0.5-35 meters total thickness in the study area); high-resolution sampling and chemostratigraphic analysis along a seaward to landward transect reveals that phosphogenesis in the Retort began during the lowstand systems tract (LST) and continued into the transgressive systems tract (TST), and occurred over a wide range of paleoceanographic conditions. The boundary between the LST and the TST is best developed in the midramp setting where a bioturbated hardground surface marks a major chemostratigraphic change. Although the organic carbon (TOC) content is consistent in both the LST and TST (3.1 and 3.8 wt %, respectively), total sulfur (TS; as pyrite), TOC/TS and Cd are distinctly different. In the LST mean values (n=23) for TS, TOC/TS, and Cd are 1.7 wt %, 2.5, respectively, and 5 ppm, and 4.4 wt %, 1.0, and 25 ppm, respectively for the TST (n=34). These data suggest that the LST was dominated by dysoxic to anoxic conditions and that the shift to the TST was marked by the presence of free hydrogen sulfide (at least in bottom water) that led to euxinic conditions. The abundance of siltstone facies in the Retort suggest that iron supplied by eolian input played a major role in the preservation of sulfide, and iron fertilization may have been a factor in contributing to overall productivity and phosphogenesis. These data provide evidence that phosphogenesis in the marginal, epicontinental Phosphoria Sea occurred under paleoceanographic conditions not seen in any modern phosphogenic system and further suggests that a simple actualistic interpretation of the Phosphoria system is not valid.