Strata of the Late Cretaceous Niobrara Formation and Pierre Shale Group include bentonites that provide a distal record of volcanic activity taking place to the west. Detailed stratigraphic analysis combined with mineralogy and geochemistry of the bentonites indicates the following timing of events: 1) Eustatic sea level fall as a result of the end of the Niobrara Cycle; 2) Tectonic deformation of the Western Interior Seaway coincident with tectonism on the Absoroka Thrust in Wyoming and Late Canyon Range Thrust in Utah; 3) Backarc volcanism in Montana associated with the Little Elkhorn Mountain volcanic complex; 4) Forearc volcanism in the Cascades area indicates subduction of a hot oceanic crust where plagioclase in the oceanic crust is being incorporated into the melt; 5) Cessation of tectonic activity results in a return of sedimentation patterns to north-south trending belts with the Boyer Bay and Burning Brule members of the Sharon Springs Formation deposited to the east and the Mitten Black Shale Formation deposited in the basin.
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