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Ichnocoenoses in the Niobrara Formation: Implications for Benthic Oxygenation Histories

By
Charles E. Savrda
Charles E. Savrda
Department of Geology, Auburn University, Auburn, AL 36849-5305
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Published:
January 01, 1998

Abstract

The vertical stacking patterns of laminites and six oxygen-related ichnocoenoses were used to reconstruct paleo-oxygenation histories for the Niobrara Formation (Fort Hays Member and lower parts of the Smoky Hill Member) as expressed in two cores—USGS #1 Portland from east-central Colorado and Amoco #1 Rebecca Bounds from western Kansas. Oxygenation records for the Fort Hays reflect high-amplitude redox cycles that correspond to decimeter-scale limestone/shale couplets. Ichnofabrics of the lowermost Smoky Hill record a complex history of intermediate to low-amplitude redox fluctuations that are superimposed upon a general deoxygenation trend. The remainder of the Smoky Hill is characterized by low- and high-frequency, low- to intermediate-amplitude redox cycles defined by the decimeter-scale alternation between relatively thick laminite sequences and intervals containing clusters of closely spaced, thin bioturbated beds separated by laminites. Decimeter-scale redox cyclicity throughout the study interval generally corresponds to carbonate rhythms and is tentatively atttributed to astronomically forced climate cycles mediated by axial precession. Lower-frequency cycles (for example, those reflected by varying cluster spacing in the Smoky Hill) may reflect a control by orbital eccentricity. High-frequency cycles, such as those reflected by centimeter-scale ichnofabric variations within Smoky Hill clusters, record periodic or episodic processes that operated at time scales shorter than Milankovitch orbital cycles. Comparison of paleo-oxygenation histories for the two cores indicate that benthic oxygenation levels were lower toward the east at least during deposition of the lowermost Smoky Hill.

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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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