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Chemical Diagenesis and Biota in Stratigraphic Context: The Phosphoria Rock Complex (Permian), Rocky Mountain Region USA


Chemical diagenesis, biota, and sedimentation vary systematically both stratigraphically and regionally in the Phosphoria Rock Complex (PRC) (middle to late Permian), Rocky Mountain region, USA, in response to dynamic paleoenvironmental conditions. These environmentally sensitive and interrelated responses coevolved and resulted in systematic development of prolific organic-matter (OM)-rich phosphatic deposits and reservoir porosity. This study integrates stratigraphic, geochemical, and petrographic techniques to determine the evolution of biota, chemical diagenesis, and paleoceanography, in a unique biochemical sedimentary system deposited during a pivotal climatic shift in Earth history. Deposition of the PRC occurred over a major second-order transgressive-regressive sequence (~8-14My duration) and its accompanying biochemical shifts, overprinted by three (Grandeur, Franson and Ervay) third-order cycles (~2-5My duration). On a second-order scale, abundant and widespread OM-rich phosphatic sediments dominate the Roadian-aged transgression. That changed to silicisponge-spicule cherts and calcitic biota, and finally to pervasively dolomitized aragonitic biota and microbialite communities of the late second-order highstand. This evolution was driven by basin configuration and circulation patterns, global changes in sea-level (causing progressive restriction) and sea-water chemistry, and by a major transition from icehouse to greenhouse climatic conditions. Third-order Franson (latest Kungurian to Wordian) and Ervay (Wordian to Capitanian) cycles show pronounced biochemical trends across systems tracts. Accumulation and microbial breakdown of sapropelic OM and resulting phosphatization dominated sedimentation in lowstands and early transgressions in distal settings. Silicisponge-spicule cherts and calcite-cemented calcitic biota (bryozoan, brachiopod, and crinoid) dominate sedimentation through late transgressions and early highstands seaward of bioturbated dolomite muds and peritidal sediments. Authigenic silica is abundant in late transgressions and early highstands, but is common throughout the PRC. Late highstands were dominated by well-developed peritidal microbial communities, bioturbated muds and dolomitic sandstones, ooids, aragonitic molluscs and phylloid algae that occur landward of silicisponge and calcitic biota. Late-highstand carbonates are pervasively dolomitized and host abundant porosity.