Date of Award
Master of Science (MS)
The Sentinel Butte ash/bentonite has been used for stratigraphic correlation in and around the North Unit of Theodore Roosevelt National Park in western North Dakota primarily by tracing it along the escarpments of the Little Missouri River. The purpose of this study was to determine the geochemical fingerprint for the Sentinel Butte ash/bentonite, and use this fingerprint to assess the correlation value of this and other terrestrial bentonites. Mapping enlarged the known geographic distribution of this unit and provided an insight into its diagenetic history.
Samples from thirty randomly chosen sites and fifteen grab sites were used to evaluate the geologic and geographic variability of the Sentinel Butte ash/bentonite. Cluster and discriminant analysis were used to categorize and differentiate the upper and lower bentonites, and the ash, on the basis of their elemental concentrations. Factor analysis was used to determine which trace elements had the lowest variability associated with them, and to use those elements as a geochemical fingerprint.
The geographic and geological variability of the Sentinel Butte ash/bentonite can be attributed to leaching of the soluble elements during the glass to bentonite alteration, introduction of cations via ion-rich groundwater, and syndepositional and post-depositional contamination. The leached elements include Na20, K20, MnO, Ba, Pb, Rb, Sr, and Nb and are found in higher concentrations in the parent ash. A silica enriched zone at the top of the underlying unit also suggest that Si02 was leached from the parent ash during the alteration process. Elements that were introduced into the ash/bentonite by ion rich groundwater or as detrital contaminants include Fe203, MgO, TiOz, Gr, and Gu. Aluminum concentration values also tend to be slightly higher in the bentonites compared to the· ash. The upper bentonite exhibits higher concentrations of CaO and Zr, which is interpreted as post depositional contamination. Those elements that remained immobile during the alteration process include Pz05, Go, Y, Ga, Zn, and Ni.
The elements that were found to be useful for correlating the Sentinel Butte ashjbentonite and provided a geochemical fingerprint for this deposit include Fez03, TiOz, Cu, Zn, Ga, Y, and Zr. Using these fingerprint elements samples collected outside of the random sampling area were correlated with the Sentinel Butte ash/bentanite. All but three of those samples were found to be correlative with the unknown ash/bentonite, expanding the known areal extent of the unit.
Larsen, Richard A., "Major and trace element characterization and correlation of the Sentinel Butte Ash/Bentonite (Paleocene), McKenzie County, North Dakota" (1988). Theses and Dissertations. 172.