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Biochemical and chemical controls on sedimentation, sequence stratigraphy, and diagenesis, in the Phosphoria rock complex (Permian), Rocky Mountain region, USA
Pommer, Maxwell
Pommer, Maxwell
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2018
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2018-11-08
Abstract
Biochemical sedimentation, near-surface diagenesis, stable isotopes, and porosity vary systematically stratigraphically and regionally in the Phosphoria Rock Complex (PRC), Rocky Mountain region, USA, in response to dynamic paleo-environmental conditions spanning the Middle Permian. Environmental, biochemical, and isotopic trends mimic diagnostic trends of the End-Permian Mass Extinction (EPME), occurring through Kungurian-Wordian time (~274Ma to 265Ma), ~13MY before the EPME and ~5MY before the end-Guadalupian crisis. These indicate PRC trends are of a similar genesis to the EPME, and EPME dynamics were driven by locally modified global processes spanning the Middle to Late Permian. Biochemical, isotopic, and environmental trends are heterogeneous across the PRC, a second-order (~9MY) cycle, and the third-order (2-5 MY) Franson (latest Kungurian - Wordian) and Ervay (Wordian) cycles. During transgressions, influx of cool, acidic, low-oxygen, nutrient-rich waters warmed and interacted with hot, oxygenated marine and evaporitic waters. Flourishing sapropelic algal and anaerobic microbial communities resulted in phosphorites and sulfidic-OM-rich mudrocks seaward of calcitic biota and micritic carbonates, redbeds, evaporites and microbialites. Values of δ18O and δ13C in carbonates and silica are depleted in distal settings due to microbial decay of OM and increase landwards due to evaporative fractionation. Values of δ18O in carbonate fluorapatite are depleted in distal environments, increase landwards and towards maximum transgression, indicating warming. Porosity in transgressions is low due to near-surface cementation and recrystallization, as well as compaction and infill of pore space by secondary OM (bitumen) in OM-rich mudrocks. S-rich OM catalyzed early secondary-OM generation and inhibited OM-hosted porosity generation through burial. During highstands warm, oxygenated, alkaline marine waters dominated in and became increasingly hot, shelf-confined, and evaporitic. With limited nutrient influx, and input of eolian-sourced silica, resulted in widespread spiculites and calcitic-biota carbonates at maximum transgression and in distal highstands. Increasingly restricted and evaporitic conditions through highstands resulted in dolomitized bioturbated muds and sandstones, aragonitic molluscs, ooids, and peritidal microbialites. Values of δ18O and δ13C became increasingly enriched throughout highstands in marine carbonates and widespread authigenic silica. This and moganite-bearing chalcedony suggest evaporitic reflux drove silicification and dolomitization. Porosity is most abundant in dolomites deposited in restricted, evaporitic highstand conditions.
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