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Regression-Transgression Couplets of the Bootlegger Sandstone (Cretaceous), North-Central Montana — The Possible Influence of the Sweetgrass Arch
The latest Albian — earliest Cenomanian Bootlegger Member, outcropping near the city of Great Falls, Montana, was deposited during the early transgressive stage of the basinwide Greenhorn Cyclothem. Clastic marine strata of the Bootlegger Member were deposited in a maximum of 2 m.y., and make up five regressive/transgressive couplets. Each couplet is composed of a thick coarsening-upward unit bounded above by an erosion surface, which in turn is usually overlain by a transgressive lag. The coarsening-upward portion of each couplet indicates shoaling-upward depositional conditions associated with eastward paleoshoreline progradation. Regressive conditions were terminated by the subsequent transgression which caused the westward migration of the paleoshoreline, development of the erosion surface and emplacement of a transgressive lag deposit.
The repetitive nature of sedimentation during Bootlegger Member time is interpreted as indicating the influence of the episodically reactivated ancestral Sweetgrass Arch. Periodic uplift and subsidence of the arch, most likely related to Cordilleran tectonic events, resulted in the fall and rise of relative sea level, respectively, which ultimately controlled the nature of Bootlegger Member sedimentation.
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