Laramide Foreland Extension and Record of Farallon Plate Removal: Insights from the Eocene Wagon Bed Formation, Central Wyoming

The sedimentary record of the North American Cordillera is frequently utilized to infer large-scale mantle processes. Shallow subduction of the Farallon plate beneath the North American plate is proposed as the driving force behind stress regimes responsible for inboard contraction producing the Rocky Mountains and associated basins; however, questions remain about the extent of emplacement, timing of removal, and its influence on the land surface above. Interpretations of Farallon plate removal range from Paleocene to Miocene and include the hypothesis that it was never removed beneath Wyoming. Given this uncertainty, additional surficial data may improve plate removal interpretations. We investigate the Eocene Wagon Bed Formation of central Wyoming to evaluate relationships between its deposits and late-stage Laramide tectonics. The Wagon Bed Formation consists of strata deposited in lacustrine and volcaniclastic-rich alluvial environments. Eight new 40Ar/39Ar radioisotopic dates reveal the Wagon Bed Formation was deposited between 50–35 Ma, a longer interval than previously recognized. Stable isotopic data from carbonates and volcanic glass indicate that paleolakes Hani and Mumbich were closed, consistent with stromatolites and authigenic zeolites. The Wagon Bed Formation accumulated along the Granite and Owl Creek mountains, both rootless uplifts, and it records collapse and extension along pre-existing thrust faults by at least 45 Ma which we attribute to reduction of compressional stresses during Farallon plate removal. Furthermore, drainage disruption, lake formation, increase in volcaniclastic material, and unconformities indicate that the Wagon Bed Formation may record the surficial expression of Farallon plate removal on the overriding North American plate.