300-6 Determining sediment routing to the Eocene Bridger Subbasin of the greater Green River Basin with geochemical fingerprinting of detrital zircon

Session: Reconstructing Earth Surface Processes in Orogenic Systems (Posters)

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Abstract:

The Bridger Formation of the Bridger Subbasin in the southwest portion of the greater Green River Basin (gGRB) represents a significant episode of the final infilling of Eocene paleo-Lake Gosiute. Strata of the Bridger Formation is characterized as a mix of dominantly volcaniclastic sandstones with intermittent mudstones, lacustrine carbonates, and tuffs. Early research hypothesized that the volcaniclastic sediment originated from the proximal Absaroka volcanic province to the north, while more recent research invokes an ancient river system connecting the gGRB to the Challis volcanic province of central Idaho to the northwest. However, these two potential volcaniclastic sources temporally overlap, rendering provenance interpretations based on age alone nonunique. To test models of sediment routing during the Eocene, we analyzed detrital zircon from the Bridger Formation for U-Pb geochronology, Hf isotope analysis (reported as initial Lu/Hf at the timing of crystallization, εHft), and Rare Earth element geochemistry. We compared these results to potential sediment sources by analyzing Eocene detrital zircon from modern rivers draining the Absaroka and Challis volcanic provinces. Results are placed in the context of newly measured stratigraphy of the Bridger Formation and compositional analysis of sandstones therein. Results show an increase in sedimentation rates during the early Bridger Subbasin deposition, coincident with a transition from supermature sandstones to immature volcaniclastic sediment, in an interpreted mixed fluvial-deltaic depositional environment. The majority of Eocene εHft values for Challis modern river samples range from -20 to 0, while those of Absaroka modern river samples are more negative, ranging from -45 to 7. Bridger Formation εHft is more consistent with the evolved (nonradiogenic) Eocene detrital zircon of Absaroka modern rivers. Rare Earth element geochemistry successfully discriminates between potential sediment sources and further supports dominant sediment sourcing from the nearby Absaroka volcanic province. Collectively, results are inconsistent with a paleo-river connecting the gGRB to central Idaho during sedimentation of the Bridger Formation and the final infilling of paleo-Lake Gosiute. Reactivation of the Paris thrust in the late stages of Sevier deformation likely shifted the regional topographic high to the Sevier orogenic wedge, blocking drainage through paleo-valleys of central Idaho from entering the gGRB. Therefore, we interpret the primary sediment source to the Bridger Subbbasin was a river system linking the gGRB to the Absaroka volcanic province and surrounding region.

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

doi: 10.1130/abs/2025AM-11069

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