107-11 Stratigraphy, paleoenvironments, and correlation of the Permian Phosphoria Rock Complex, southwest Montana

Session: Sedimentary Geology Division/SEPM Student Research Poster Competition: Dynamics of Stratigraphy and Sedimentation

Presenting Author:

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Abstract:

The Permian Phosphoria Rock Complex (PRC) records highly variable shallow marine paleoenvironments in Montana during the lead-up to the Permian–Triassic mass extinction. The PRC is an economically important phosphate and critical mineral deposit, and in MT is dominated by the Retort, Tosi, and Shedhorn members. This study correlates stratigraphic sections from three localities in MT that parallel the northwestern coastline of the Phosphoria Basin. Evaluating this region in contrast to the well-studied PRC of neighboring ID and WY is integral to reconstructing this unique paleo-upwelling system. 

The Retort Member, composed of phosphorite and organic-rich siltstone with dispersed bivalve, fish, and other macrofossils, reflects a transgressive interval. Nodular and coarse-grained phosphorites provide evidence for deposition under nutrient-rich conditions in a low-energy setting. The overlying Tosi Member, consisting of thin beds of spiculitic to phosphatic chert, is regionally variable. The Shedhorn comprises glauconitic sandstone with hummocky and trough cross-stratification, intraclasts, and abundant bioturbation, interpreted as a regressive, high-energy shallow marine system with periodic local storms.

During the Permian, these localities' proximity to the well-defined northwestern shoreline yielded distinct deposits from the nearby ID and WY PRC. To characterize these differences we correlated stratigraphic sections from previous field data and analyzed more than 75 hand samples and thin sections. Western localities exhibit a discrete succession of facies with sharp transitions, while more eastern exposures reveal multiple alternating facies changes between cherts and sandstones. This suggests that siliceous sedimentary sources of these different facies would have been laterally closer together in the east, with the interfingered facies produced by rapid cyclic sea-level or environmental fluctuations. 

These complex facies trends allow us to reconstruct spatiotemporal variability in water energy and paleoecology across the northern margins of the PRC. Ongoing interpretation will focus on combining these stratigraphic observations with petrographic analysis of thin sections to more accurately characterize lithologies, diagenesis, and fossil content. Since deposition extends conformably up to the end-Permian, evaluating the impacts of global environmental stresses on this paleo-upwelling system will enhance our understanding of responses of life and environments to a warming world.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9913

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