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Late Cretaceous marine deposition in the western interior of the United States occurred in an epicontinental seaway elongate in a north-south direction. In central Montana, the western side of the seaway was characterized by a broad, tectonically active shelf. In eastern Montana and the western Dakotas, an actively subsiding basin was located in the central part of the seaway. In western and central South Dakota, the eastern side of the seaway was a more stable west-sloping ramp. Distinctive facies belts in the Eagle Sandstone and equivalent rocks are found in each of these tectonic settings, and some specific tectonic features have expression in the facies patterns. However, paleotectonism was even more important than suggested by these regional patterns. Selected study reas
show that subtle tectonic features have influenced deposition within each of the regional facies belts.
On the western shelf, coastal sandstones of the Eagle-Sandstone near the Bearpaw Mountains show facies and isopach variations which are controlled by linear features visible on satellite images; the linear features generally trend north-south and east-west. Inner shelf sandstones of the Eagle thin and pass laterally northeastward into marine siltstones and shales across the Cat Creek fault zone near Winnett, Montana. Sandstone lenses in the lower Eagle, which are interpreted to be sand ridges, prograde south and west at approximately right angles to the fault zone. Farther east on the outer margin of the western shelf, areas of sand ridge fields in the Shannon Sandstone Member of the Gammon Shale are delimited by northeast and northwest linear features observed on satellite images nea the northern Black Hills. Within the basin, thick areas of Gammon Shale are delimited by northeast and northwest lineaments interpreted from Landsat linear features. On the eastern ramp, noncalcareous shales of the Gammon Member of the Pierre Shale thin and intertongue eastward with chalks in the upper part of the Niobrara Formation. This facies change occurs across linear features visible on Landsat images in western South Dakota. To the east at the inner margin of the ramp, the degree of erosion on the unconformity between the Niobrara Formation and the overlying Pierre Shale changes systematically across northeast-trending Landsat linear features observed near the Missouri River in central South Dakota.
Based on these studies, we interpret the stratigraphic variations to be the expression of paleotectonism on discrete basement blocks bounded by fault zones which are observed on Landsat images as linear features. On the western shelf, elevated blocks controlled the sites of the winnowing and deposition of sandstones. Within the basins, subsiding basement blocks were filled by deposition of shales. These basin blocks acted as sediment sinks which inhibited the eastward dispersal of terrigenous materials from the west. On the eastern ramp, chalks were deposited and locally eroded on slightly elevated blocks which were relatively free of terrigenous material. Paleotectonism, therefore, influenced deposition not only on the active western shelf and in the basin, but also on the more stabl eastern ramp.
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