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Calcareous nannofossils from the Niobrara formation (Upper Cretaceous) Western Interior Seaway, USA
This study presents the first regional chronostratigraphic framework and paleoenvironmental analyses of the Niobrara Formation. Three sites in the Western Interior Seaway, USA (western Kansas, eastern South Dakota, and northern Colorado) provide a combination of biostratigraphic, lithostratigraphic, paleomagnetic, and chemostratigraphic data. By synthesizing multiple data types with high resolution nannofossil biostratigraphy, correlations on a regional scale is achieved with a higher degree of confidence and reliability than through traditional biostratigraphy. Correlation of a new high resolution carbon isotope curve from the Smoky Hill Member type section of the Niobrara Formation to the Bottaccione Gorge section in Italy provides a framework for global comparisons of stratigraphic markers (e.g., paleomagnetic reversals, nannofossil bioevents). Integrating biostratigraphy with other datasets is of particular importance in the Western Interior Seaway where paleoceanographic conditions resulted in unique nannofossil assemblages. Holococcoliths, important marker taxa in the Late Cretaceous, are rare in the Niobrara Formation. A new calcareous nannofossil species, Helicolithus blairiae, is described and its potential biostratigraphic utility in the Western Interior Seaway is evaluated in relationship to holococcolith marker species. Calcareous nannofossil assemblages from the Smoky Hill Member of the Niobrara Formation at two localities in Colorado and Kansas were analyzed to determine relevant calcareous nannofossil bioevents for the base of the Campanian Stage. Nannofossil bioevents were compared directly with two proposed markers for the base of the Campanian (bases of the paleomagnetic Chron C33r and Scaphites leei III ammonite Zone). Five calcareous nannofossil bioevents are useful for recognition of this boundary: last appearance datum (LAD) of Zeugrhabdotus moulladei; first appearance datum (FAD) of Aspidolithus parcus parcus; LAD of Helicolithus trabeculatus specimens larger than 7 µm; LAD of Zeugrhabdotus biperforatus; and FAD of Aspidolithus parcus constrictus. Unique paleoceanographic conditions in the Western Interior Seaway resulted in cyclic lithologies of uncertain origin and the absence of holococcoliths during the Santonian. The stratigraphic distribution of productivity indicator taxa (e.g., Biscutum, Zeugrhabdotus) indicates that primary productivity controlled lithology. The absence of holococcolith species may be attributable to fluctuations in a depth-specific habitat.
Kita, Zachary A, "Calcareous nannofossils from the Niobrara formation (Upper Cretaceous) Western Interior Seaway, USA" (2015). ETD collection for University of Nebraska - Lincoln. AAI3717079.