![]() Petrographic studies of the sediments indicate a sedimentary and plutonic (igneous and metamorphic) source. The siltstones are similar to the sandstones in composition and the c lays are composed entirely of kaolinite. Trace amounts of feldspar, rock fragments, muscovite, biotite, carbonaceous material, iron oxides, and heavy minerals were also observed. The average overall composition of the sandstones is 76% quartz, 18% matrix composed of kaolinite, sericite, and/or illite, and 6% secondary silica overgrowths. A paleocurrent analysis of the sandstones shows that a majority of the samples display either bimodal or bipolar distributions, which may suggest a beach environment. The siltstones and clays were deposited on levees and marshes. These sandstones are generally medium grained, moderately to well sorted, subangular to subrounded, and submature to mature. Stratification, primary structures, and lithologic features suggest that most of the sandstones were deposited in a beach complex, an estuarine channel, or on an inlet point bar. The Dakota Sands tone (Cretaceous) exposed near the town of Beulah, Colorado, is composed of sandstones, siltstones, and shales averaging 55 feet (16. ![]()
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