Geochemistry of Permian rocks from the margins of the Phosphoria Basin: Lakeridge core, western Wyoming

Abstract

The Permian Phosphoria Formation and interbedded units of the Park City Formation and Shedhorn Sandstone in western Wyoming represent deposition along a carbonate ramp at the eastern margin of the Phosphoria Basin, with portions of the Phosphoria units reflecting periods of upwelling and widespread phosphogenesis. Thickness-weighted slab-samples of these units were collected at a maximum interval of 3 m along an 80+ m-length of unweathered core and analyzed for major-, minor-, and trace-element contents. Interpretations of geochemistry were made within the confines of a previously recognized sequence stratigraphy framework. Major shifts in element ratios characteristic of terrigenous debris that occur at sequence boundaries at the base of the Meade Peak and Retort Members of the Phosphoria Formation are attributed to changing sediment sources. Inter-element relationships in the marine fraction indicate that bottom waters of the Phosphoria Basin were predominantly denitrifying during deposition of the Ervay, Grandeur, and Phosphoria sediments, although sulfate-reducing conditions may have existed during deposition of the lower Meade Peak sediments. Oxic conditions were prevalent during deposition of a large part of the Franson Member, which represents sedimentation in a shallow, inner- to back-ramp setting. Variations in sediment facies and organic matter and trace element contents largely reflect changes in Permian sea level. Changes in sea level in basin-margin areas, such as represented by the study section, may have affected the oxidation of settling organic matter, the foci of intersection of upwelling bottom waters with the photic zone, the rate of terrigenous sedimentation, and, ultimately, the overall environment of deposition. Our study suggests that phosphogenesis can occur under lowstand, transgressive, and highstand conditions in marginal areas, assuming water depths sufficient for upwelling to occur. Formation of phosphorite layers under upwelling conditions appears to have been most dependent on a lack of dilution by terrigenous sedimentation and carbonate shoaling. Differences in the geochemistry between two similar environments represented by the upper and lower Phosphoria units are largely attributed to higher rates of diluting terrigenous sediment during deposition of the upper unit. This is consistent with prior interpretations of a more shoreward setting for the upper Phosphoria.