Paleoceanography of the Southwestern Western Interior Sea During the Time of the Cenomanian-Turonian Boundary (Late Cretaceous)
R. Mark Leckie, Richard F. Yuretich, Oona L. O. West, David Finkelstein, Maxine Schmidt, 1998. "Paleoceanography of the Southwestern Western Interior Sea During the Time of the Cenomanian-Turonian Boundary (Late Cretaceous)", Stratigraphy and Paleoenvironments of the Cretaceous Western Interior Seaway, USA, Walter E. Dean, Michael A. Arthur
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The Cenomanian-Turonian boundary interval (93-94 Ma) was a time of rapid oceanographic change in the U.S. Western Interior Sea ("Greenhorn Sea"). Previous studies documented changes in δ18O in carbonates and shifts in macrofossil (molluscan) populations indicating the incursion of a subsaline surface-water mass into the region and dysoxic to anoxic benthic conditions across wide areas of the seaway. These changes were accompanied by an expanding oxygen minimum zone in concert with global-scale burial of organic matter, which was driven in part by elevated rates of marine productivity. The oceanography of the southern seaway was undoubtedly complex with signals of a global anoxic event overprinted by regional influences of water mass stratification, mixing, productivity, changes in relative sea level, and biotic turnover.
Our studies of planktic and benthic foraminiferal assemblages and clay-mineral distribution in calcareous mudrocks and dark marlstones from the southwestern side of the Greenhorn Sea provide further constraints on the paleoceanography of this region. Specifically, there is a major change in planktic foraminiferal population structure ("Heterohelix shift") coupled with an influx of kaolinite and illite at both a neritic site (Lohali Point, Arizona) and a distal basin site (Rock Canyon, Colorado). These characteristics are not observed at a proximal basin site (Mesa Verde, Colorado) in between the other locales. The distinctive clay mineral assemblages suggest two disparate sources. One of these sources most likely was from the Sevier orogenic belt along the western side of the seaway, and another was either the southwestern corner of the seaway or the southern part of the stable craton. The distribution of clay mineral and planktic foraminiferal assemblages provide information on circulation of the upper water column.
The benthic foraminiferal assemblages of Lohali Point and Mesa Verde are very similar and have northern affinities, suggesting the influence of cool bottom waters along the western side of the seaway. We suggest that a submerged tectonic forebulge or bathymetric high near Mesa Verde caused "edge-effect" mixing and upwelling of cool water masses originating from the north. To the west, a foredeep shelf in northeastern Arizona and south-central Utah provided a conduit for northward-flowing, warmer surface water masses over the southward-flowing, cooler waters. These southern waters were bifurcated by the Mesa Verde high. The resultant oceanographic front, or mixing zone, caused the contrast in ecological and sedimentological patterns at the sites. With rising sea level came the incursion of oxygen-poor Tethyan intermediate waters into the Greenhorn Sea during latest Cenomanian-early Toranian time and the development of widespread, low diversity benthic foraminiferal assemblages dominated by Neobulimina.
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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.