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The Sharon Springs member of the Pierre formation and the Smoky Hill member of the Niobrara formation, both assigned to the Upper Cretaceous, are separated by an unconformity in at least five of the six sections studied along the Front Range of Colorado. Onlap of the Sharon Springs and possibly truncation of the Smoky Hill are involved in this unconformity. The basal Sharon Springs shales are believed to have accumulated under stagnant, acid bottom marine conditions, whereas, the Smoky Hill deposits were laid down in a well oxygenated marine environment. The foraminiferal fauna of the Smoky Hill is represented by abundant calcareous, pelagic forms as contrasted to a limited arenaceous fauna of the Sharon Springs. X-ray diffraction analyses of the Sharon Springs and Smoky ill bentonitic-type clays show a prevalence of montmorillonite in the Sharon Springs, and kaolinite in the Smoky Hill. The full significance of these clay minerals as related to environment of deposition of the bentonites is not completely understood at this writing.
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