PALEOENVIRONMENTAL DYNAMICS IN THE MIDDLE PERMIAN PHOSPHORIA SEAWAY: A RECORD FROM ISOTOPIC AND BIOCHEMICAL TRENDS, ROCKY MOUNTAIN REGION, USA

Biochemical and marine isotopic trends in the Phosphoria Rock Complex (PRC), Rocky Mountain Region, USA, responded to global environmental and oceanographic shifts spanning the middle Permian amplified in a shallow epicontinental seaway on the western margin of Pangea. U/Pb isochron ages of 268.9+3Ma in a brachiopod and 278+12Ma in lime micrite from the youngest (Ervay) highstand confirm a Wordian age for the end of the PRC. 87Sr/86Sr values (0.707378-0.707054) in brachiopods, and stable isotope values reflect global signals at this time modified at regional and local scales.

Integrated stratigraphic, petrographic, and isotopic (δ¹⁸O, δ¹³C, 87Sr/86Sr, U/Pb, and δ³⁴S) analyses indicate a systematic evolution of environmental and biochemical trends through deposition of the PRC linked to global processes. The PRC underwent pronounced and characteristic sea-level cyclicity on tectonic and glacial time-scales evidenced by distinct biochemical and isotopic shifts. These include:

• accumulation and microbial breakdown of sapropelic organic matter in distal settings resulting in phosphatization in lowstands and transgressions;
• deposition of silicisponge-spicule cherts and calcite-cemented or partially dolomitized calcitic biota during late transgressions and early highstands;
• deposition of dolomitized peritidal microbial communities, aragonitic molluscs, bioturbated muds and sandstones, ooids, and landward of silicisponge and calcitic biota and seaward of redbeds and evaporites during highstands;
• depleted δ¹⁸O and δ¹³C values (-12.5‰ - 2.8‰, and -12.0‰ - 3.1‰ [PDB] respectively) in open-marine and organic matter bearing carbonates during transgressions, and enriched values (-8.3‰ - 3.9‰ and -4.3‰ - 5.2‰) in marine and peritidal dolomites during highstands. This is interpreted to be resultant of seawater fractionation through evaporation and bacterial sulfate reduction.

Isotopic and biochemical responses are coincident and characteristic of long-term global environmental shifts associated with the end-Permian mass extinction, and support the interpretation that the biotic dynamics associated with extinction were driven by global scale environmental and oceanographic trends modified in local environments throughout the middle and late Permian.