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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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