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Stable Isotopic Studies of Cenomanian-Turonian Proximal Marine Fauna From the U.S. Western Interior Seaway

By
Mark Pagani
Mark Pagani
Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802
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Michael A. Arthur
Michael A. Arthur
Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802
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Published:
January 01, 1998

Abstract

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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Contents

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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