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Isotopic Studies of Organic Matter and Carbonate in Rocks of the Greenhorn Marine Cycle

By
Lisa M. Pratt
Lisa M. Pratt
U.S. Geological Survey
Denver Federal Center Box 25046, MS 977
Denver, CO
80225
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Published:
January 01, 1985

Abstract

Variation in the carbon isotopic composition of organic matter in the Greenhorn Limestone are described for Black Mesa, Arizona; Pueblo, Colorado; Bunker Hill, Kansas; Ponca State Park, Nebraska; and Cone Hill, Montana. There is a 2.5 to 3.5 o/oo positive (heavy) excursion in carbon isotopic values spanning the Cenomanian-Turonian boundary at each of these localities. Carbon and oxygen isotopic compositions of inoceramid bivalve shells and whole-rock carbonate are reported for the Black Mesa and Pueblo sections. Oxygen isotopic values of whole-rock carbonate generally become more positive upward in the Greenhorn from the lower Lincoln Limestone Member through the Hartland Shale Member and into the lower Bridge Creek Limestone Member. This trend is inferred to reflect generally increasing salinity of Western Interior seawater associated with increasing water depths and less restricted oceanic connections as the Greenhorn transgression progressed. Superimposed on this overall trend are intervals with marked isotopic and geochemical fluctuations suggesting rapidly changing proportions of oceanic and riverine water or rapidly changing relative rates of evaporation and input. A particularly severe and rapid paleoenvironmental change is inferred to have occurred just prior to the Cenomanian-Turonian boundary defined by macrofossil extinctions.

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SEPM Field Trip Guide

Fine-Grained Deposits and Biofacies of the Cretaceous Western Interior Seaway: Evidence of Cyclic Sedimentary Processes

Lisa M. Pratt
Lisa M. Pratt
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Erle G. Kauffman
Erle G. Kauffman
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Frederick B. Zelt
Frederick B. Zelt
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SEPM Society for Sedimentary Geology
Volume
4
ISBN electronic:
9781565762794
Publication date:
January 01, 1985

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