Ichnology of the Bridge Creek Limestone: Evidence for Temporal and Spatial Variations in Paleo-Oxygenation in the Western Interior Seaway
Charles E. Savrda, 1998. "Ichnology of the Bridge Creek Limestone: Evidence for Temporal and Spatial Variations in Paleo-Oxygenation in the Western Interior Seaway", Stratigraphy and Paleoenvironments of the Cretaceous Western Interior Seaway, USA, Walter E. Dean, Michael A. Arthur
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The Upper Cretaceous (Cenomanian-Turonian) Bridge Creek Limestone is characterized by decimeter-scale alternation of pelagic limestones and marly shales that were deposited under variably oxygenated waters. Vertical stacking patterns of laminites and four oxygen-related ichnocoenoses in the Bridge Creek in two cores—USGS # 1 Portland from east-central Colorado and Amoco #1 Rebecca K. Bounds from western Kansas—provide a record of both temporal and spatial changes in benthic oxygenation within the distal offshore parts of the Western Interior Seaway.
Paleo-oxygenation histories reconstructed for the Portland core reflect: (1) a broad trend towards decreased benthic oxygenation through the entire Bridge Creek interval; (2) a high-amplitude redox cyclicity that corresponds to limestone/marly shale couplets; and (3) a higher-frequency, lower-amplitude redox cyclicity expressed within the marly shale intervals.
Trends (1) and (2) are well expressed in the Bounds core. However, bioturbated horizons in marly shale intervals are less common, thinner, of lower ichnocoenosis rank, or absent altogether. This pattern indicates that paleo-oxygenation levels were lower at the Bounds locality, at least during clastic-dominated phases of depositional cycles, and may reflect higher productivity and oxygen demand in the eastern part of the basin.
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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.