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Upper Cretaceous Nannofossil Assemblages Across the Western Interior Seaway: Implications for the Origins of Lithologic Cycles in the Greenhorn and Niobrara Formations

By
Celeste E. Burns
Celeste E. Burns
Department of Geology, University of North Carolina, Chapel Hill, North Carolina 27599-3315
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Timothy J. Bralower
Timothy J. Bralower
Department of Geology, University of North Carolina, Chapel Hill, North Carolina 27599-3315
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Published:
January 01, 1998

Abstract

Calcareous nannofossil assemblages were investigated in two cyclic stratigraphic intervals from the Late Cretaceous Western Interior Seaway to determine the causes of lithologic variations. Relative abundance data were collected from the Bridge Creek Limestone Member (Cenomanian-Turonian) of the Greenhorn Formation and the transition between the Fort Hays Limestone and Smoky Hill Chalk Members (Coniacian-Santonian) of the Niobrara Formation from two cores, the No. 1 Amoco Rebecca Bounds Core, western Kansas, and the USGS No. 1 Portland Core, central Colorado. Stable-carbon and -oxygen isotopic analyses of fine (<38pm) fractions were carried out on samples from the interval with the best preserved nannofossils, the Fort Hays Limestone-Smoky Hill Chalk transition in the Bounds core.

Preservation of nannofossil assemblages varies in the two cores. Correlations between CaCO3 and the abundance of nannofossil species, which are used as proxies for organic productivity, are rarely significant, which indicates an inconsistent relationship between fertility and lithology. Fine-fraction, oxygen-isotopic values from the Fort Hays Limestone-Smoky Hill Chalk transition of the Bounds core correlate highly with nannofossil fertility markers, indicating a close relationship between organic productivity and the amount of run-off into the basin. However, the lack of consistent correlation between organic productivity (as seen in nannofossil assemblages) and lithology suggests that lithology was influenced by a variety of complex processes including variations in carbonate productivity and dilution with clastic material. Carbonate productivity was likely influenced by multiple water masses, including run-off from mountainous regions to the west, warm waters from the Tethys, and cooler waters from the Arctic. The interplay of the water masses and the additional dilution signal render the lithologic cycles out of phase with the periodicities that control organic productivity.

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Contents

SEPM Concepts in Sedimentology and Paleontology

Stratigraphy and Paleoenvironments of the Cretaceous Western Interior Seaway, USA

Walter E. Dean
Walter E. Dean
U.S. Geological Survey, Denver, Colorado
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Michael A. Arthur
Michael A. Arthur
Pennsylvania State University, University Park, Pennsylvania
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SEPM Society for Sedimentary Geology
Volume
6
ISBN electronic:
9781565762299
Publication date:
January 01, 1998

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