Charles E. Savrda, 1998. "Ichnocoenoses in the Niobrara Formation: Implications for Benthic Oxygenation Histories", Stratigraphy and Paleoenvironments of the Cretaceous Western Interior Seaway, USA, Walter E. Dean, Michael A. Arthur
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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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This volume presents the results of a coordinated, multidisciplinary study of Cretaceous carbonate and clastic rocks in cores collected along a transect across the old Cretaceous seaway that extended from the Gulf Coast to the Arctic by a team of academic, industry and U.S. Geological Survey scientists. The overall goal was to construct a subsurface transect of mid-Cretaceous strata that were deposited in the U.S. Western Interior Seaway. In particular, the papers in this volume focus on the Graneros Shale, Greenhorn Formation, Carlile Shale, and Niobrara Formation and equivalents in cores from six drillholes from western Kansas, southeastern Colorado and eastern Utah.