Chemical and Biotic Responses to Mid-Permian Paleo-Environmental Dynamics, Phosphoria Rock Complex (Permian, Kungurian-Wordian), Rocky Mountain Region, U.S.A.

Abstract

Biota, chemical diagenesis, and sedimentation vary systematically both stratigraphically and regionally in the Phosphoria Rock Complex, Rocky Mountain region, USA, in response to dynamic paleo-environmental conditions spanning the Middle Permian. Biochemical responses evolved as a function of interaction between upwelled oceanic waters (nutrient rich, acidic, and low-oxygen), shallow marine to peritidal waters (oxygenated, alkaline, and saline), and eolian-sourced silica. Interactions were driven by global climate and oceanography, basinal tectonics, and local modification of marine waters. Biochemical responses to environmental trends resulted in systematic development of organic-matter (OM)-rich phosphatic deposits and carbonate-hosted reservoir porosity. Biochemical, isotopic, and interpreted environmental trends on second- (~9 MY) and third-order (2-5 MY) time scales include:

OM-rich phosphatic sediments dominated in distal settings during transgressions, especially through global transgression in the Roadian. These, and carbonate δ¹³C and δ¹⁸O values (mean -8.7‰ and -4.4‰ PDB) suggest anaerobic microbes thrived in distal transgressive environments where upwelling nutrient rich and low-oxygen oceanic waters warmed and interacted with evaporitic and oxygenated restricted marine waters.

During late transgressions on the ramp margin and in early highstands, silicisponge-spicule cherts and calcite-cemented or partially dolomitized calcitic biota (bryozoans, brachiopods, and crinoids) dominate sedimentation seaward of bioturbated dolomite muds and peritidal sediments. These, and δ¹³C, δ¹⁸O, and δ³⁴S values (mean -0.6‰, -4.1‰, and 11.4‰ PDB and CDT in carbonates and sulfates respectively) suggest open marine waters interacted with restricted marine waters across most of the depositional profile. Authigenic, biogenic, and eolian-sourced silica occur throughout highstands. Moganite bearing chalcedony indicates extremely alkaline conditions in many environments.

Late highstand sedimentation was dominated by well-developed peritidal microbial communities, bioturbated muds and dolomitic sandstones, ooids, and aragonitic molluscs landward of silicisponge and calcitic biota. Late-highstand carbonates are pervasively dolomitized and host abundant porosity, especially in the youngest (Ervay) highstand. These, and δ¹³C, δ¹⁸O, and δ³⁴S (mean 0.2‰, -2.4‰, and 11.8‰) indicate increasingly widespread evaporitic and restricted marine waters occur across the ramp.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018