Stable Isotopic Studies of Cenomanian-Turonian Proximal Marine Fauna From the U.S. Western Interior Seaway
Mark Pagani, Michael A. Arthur, 1998. "Stable Isotopic Studies of Cenomanian-Turonian Proximal Marine Fauna From the U.S. Western Interior Seaway", Stratigraphy and Paleoenvironments of the Cretaceous Western Interior Seaway, USA, Walter E. Dean, Michael A. Arthur
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Macrofossils were sampled from 650 ft (198.1 m) of core material from the USGS Escalante No. 1 core, consisting mainly of proximal offshore marine calcareous mudstones of the Cretaceous Western Interior Seaway ofNorth America. Samples range in age from Upper Cenomanian S. gracile ammonite zone) through Upper Middle Turonian P. hyatti ammonite zone) and include representatives of ammonites, inoceramid bivalves, and oysters. Diagenetically altered carbonates were identified through visual inspection and trace-element (Sr, Mn, Fe, S, K, Na) analysis of shell material. Carbon- and oxygen-isotopic measurements of carbonate shell material were performed on 170 individual shell samples.
In general, δ18O values of inoceramids are more negative and δ13C values are more positive than those of ammonites. Well-preserved oysters are significantly enriched in δ18O in comparison with inoceramids. Inoceramid shell material is generally more enriched in δ13C than the surrounding carbonate matrix. Inoceramid δ18O is enriched relative to the surrounding matrix in the lower portion of the core, however, this relationship is reversed near the top of the core.
Isotopic compositions of inoceramids and ammonites reflect both variations in primary water column chemistry and biological effects. Overall, oxygen isotopic values appear to vary in concert with patterns of sea level variation reconstructed from basin-scale stratal patterns. Ammonite δ18O, in particular, appears to mirror regional short-term, base-level cyclicity. The amplitude of oxygen isotope variations in ammonites (2%o-3%o) suggests an interplay of at least two distinct water masses with very different isotopic compositions. The dominance of one water mass over the other may be related to sea level variation; a northern component water depleted in δw probably dominates during regressive intervals.
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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.