--> Chemical Diagenesis in Stratigraphic Context: The Ervay Cycle of the Phosphoria Rock Complex (Permian), Wyoming and Montana

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Chemical Diagenesis in Stratigraphic Context: The Ervay Cycle of the Phosphoria Rock Complex (Permian), Wyoming and Montana

Abstract

Occurrence of authigenic minerals and organic matter vary systematically both regionally and across stratigraphy in the Ervay cycle of the Phosphoria Rock Complex (PRC) (Late Permian), in western Wyoming and Southwest Montana. The transgression is characterized by organic-rich phosphatic mudrocks, phosphorites, carbonate mudrocks, and phosphatic calcite-cemented bryozoan buildups. Transgressive fauna are dominantly calcitic and include bryozoans, brachiopods, and crinoids. Widespread silty spiculitic cherts occur at maximum flooding and into the distal highstand. The highstand is characterized by subtidal and peritidal dolomites, bioturbated dolomitic sandstone, siltstones, and evaporites. Peritidal highstand carbonates are pervasively dolomitized and consist of fenestral peloid boundstone, fenestral algally-bound ooid grainstone, and incipient algal-laminated peloidal wackestone to packstone. Shallow subtidal highstand carbonates are almost completely dolomitized and consist of ooid grainstone; bioturbated peloid mudstone to packstone; and sucrosic biomoldic mollusk (scaphopods, pelecypods, and gastropods) packstone, rudstone, and floatstone. Partially to wholly dolomitized bryozoan, brachiopod, and crinoid packstone, floatstone and rudstone occur in deeper, more distal setting in the highstand. Authigenic silica, phosphorite, carbonate, and evaporite, as well as organic matter occur across stratigraphy, varying in distribution, composition, morphology, and origin. These heterogeneities reflect changes in biogeochemical paleoenvironments regionally and across systems tracts, corresponding to changes in biota and depositional facies. Paleoenvironmental changes occur as a function of changes in sea-level and circulation patterns, temperature, organic and inorganic geochemistry, and the resulting biotic response.