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Abstract
The Niobrara Formation was deposited along the eastern margin of the Western Interior Basin during the Coniacian through early Campanian (Upper Cretaceous). Nannofossils from four locations in western Kansas and eastern South Dakota are investigated. A total of 108 species and 60 genera are recognized, including one new genus and seven new species/subspecies.
Among the studied sections, western Kansas is the most complete. A total of 6 nannofossil zones and 8 subzones can be recognized within the Niobrara of the study area. Cretaceous coccolith Zones CC15 and CC16 are informally subdivided based on the first appearance datum of Lithastrinus grillii and the last appearance datum of L. septenarius, respectively.
Local paleoenvironmental conditions are thought to be responsible for the premature extinction of Eprolithus floralis within the basin. Specific horizons within the Niobrara Formation exhibit abnormally high abundances of Marthasterites furcatus. These "bloom" events appear to be favored by the nearshore nature of the eastern South Dakota Niobrara Formation paleoenvironments.
The paleoenvironmental applications of the nannofossils indicate that the water temperature was significantly different between the south and north parts of the study area during part of the interval of Niobrara Formation deposition. High-latitude species such as Seribiscutum primitivum and Biscutum notaculum appear first at northern sections and then at the southern section. The data also suggest the water of the basin was gradually cooling throughout the deposition of the Niobrara Formation. Gradual invasion of cold water from the Boreal Ocean is postulated as an explanation for these water temperature variations. These paleoenvironmental conclusions are corroborated by the analyses of nannofossil species diversity, relative abundance, distributions of specific environmental indicators, as well as sedimentary lithology.
