CALCAREOUS NANNOFOSSIL PALEOBIOGEOGRAPHY IN THE CRETACEOUS GREENHORN SEA

• Other Authors: WATKINS, DAVID KIBLER.
• Format: Others
• Subjects: Paleontology
• Online Access: http://purl.flvc.org/fsu/lib/digcoll/etd/3085938

Two distinctive, laterally traceable bentonite beds were used to construct two isochronous time slices through the marine sediments of the Upper Cretaceous Greenhorn Cyclothem of the United States Western Interior Basin. Calcareous nannofossil assemblages from these time slices were examined from more than 40 outcrop localities. Nannofossil presence and assemblage diversity and composition were statistically analyzed to examine the paleooceanographic conditions within the basin. === The lower time-slice (X bentonite) is at the stratigraphic horizon which approximately corresponds to the time at which free communication between the basin and open oceanic systems first occurred. The most striking trend in the nannofossil distribution is delineated by the presence or absence of nannofossils. The presence of common to abundant nannofossils in the center of the basin (i.e., near the hingeline of the basin) and the absence of nannofossils from the eastern and western margins indicate that open oceanic conditions conducive to significant standing crops of calcareous nannoplankton existed only in a narrow, centrally located channel. Conditions at the basis margins were probably unsuitable for large populations due to salinity constraints. Multivariate analyses of this time-slice indicate the presence of two distinct biofacies: a northern biofacies determined by relatively high abundances of Biscutum ellipticum and Zygodiscus sp. 1, and a southern biofacies determined by high abundances of Watznaueria barnesae. === The upper time-slice (HL-3 bentonite) samples the nannofossil distribution at a point at or near the maximum transgression of the Greenhorn Sea. In this time-slice, nannofossils occur beyond the limits of the study area or, in Kansas, beyond the erosional limits of the Cretaceous. Diversities are significantly higher, with the highest values occurring in the northern and southern portions of the study area. Multivariate analyses of this time-slice indicate the presence of three distinct biofacies: a northern biofacies determined by relatively high abundances of Cretarhabdus crenulatus, a southern biofacies determined by high abundances of a diverse assemblages of species including Watznauerla supracretacea and Prediscosphaera columnata, and a western biofacies delineated by high abundances of Biscutum ellipticum. === Source: Dissertation Abstracts International, Volume: 45-09, Section: B, page: 2853. === Thesis (Ph.D.)--The Florida State University, 1984.