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Nannofossil Cyclostratigraphy of the Upper Cretaceous Niobrara Formation

Zachary Kita
Earth and Atmospheric Sciences, University of Nebraska - Lincoln, Lincoln, NE, USA
[email protected]

Abstract

Upper Cretaceous chalks and marls of the Niobrara Formation are currently being explored in Colorado and Wyoming for hydrocarbons. As such, precise dating of these sediments is crucial to exploration and production. Traditionally, Cretaceous sections have been assigned relative ages according to zonation schemes based on calcareous nannofossils with little use of geochronologic dates for first and last occurrences. The problem with the Niobrara Formation is that several nannofossil markers commonly used for zonation schemes are absent or diachronous with respect to typical open ocean biostratigraphic events making correlation difficult. The goal of the current work is to enhance biostratigraphic resolution of Western Interior Seaway sediments while subsequently calibrating the ranges of calcareous nannofossils of the Coniacian to lower Campanian using samples from the Niobrara Formation of Kansas. Biostratigraphic resolution will be improved by correlating the nannofossil data from the outcrop section of the Niobrara in Kansas with an isotope curve constructed from the same outcrop, existing macrofossil data, as well as direct calibration with a subsurface Coniacian through Santonian section in Colorado dated cyclostratigraphically. Providing a detailed series of biostratigraphic events (e.g., species extinctions and originations), as well as absolute ages developed from outcrop and subsurface data, will improve the biostratigraphic potential for the Niobrara and other contemporaneous stratigraphic units.

AAPG Search and Discovery Article #90199 © 2014 AAPG Foundation 2014 Grants-in-Aid Projects