Date of Award
Master of Science (MS)
Michael E. Nelson
The Dakota Sands tone (Cretaceous) exposed near the town of Beulah, Colorado, is composed of sandstones, siltstones, and shales averaging 55 feet (16 . 76 m.) in thickness. 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. These sandstones are generally medium grained, moderately to well sorted, subangular to subrounded, and submature to mature. The siltstones and clays were deposited on levees and marshes. 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 average overall composition of the sandstones is 76% quartz, 18% matrix composed of kaolinite, sericite, and/or illite, and 6% secondary silica overgrowths. Trace amounts of feldspar, rock fragments, muscovite, biotite, carbonaceous material, iron oxides, and heavy minerals were also observed. The siltstones are similar to the sandstones in composition and the c lays are composed entirely of kaolinite. Petrographic studies of the sediments indicate a sedimentary and plutonic (igneous and metamorphic) source. Paleocurrent analysis of the cross-bedded sandstones indicates that the likely source area was to the west. This source is known as the Sevier Orogenic Belt and is located in western Utah and southeastern Nevada. The effects of a number of diagenetic processes were observed including authogenic quartz overgrowths, kaolinization of feldspar, and replacement of quartz by calcite.
Copyright 1983 Bruce A. Miller
Miller, Bruce, "Depositional Environments of the Dakota Sandstone in Southwest Pueblo County, Colorado" (1983). Master's Theses. 1891.