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Abstract

Field measurement of gamma-ray spectra on outcrops of Upper Cretaceous marine shale and limestone that were deposited in the Western Interior Seaway in Utah, New Mexico, and Colorado provides accurate estimates of total gamma radiation and potassium, uranium, and thorium contents. Spectrometer profiles of measured sections were combined with lithostratigraphic and biostratigraphic data to show that during Early Turonian time, potassium and thorium contents, total gamma radiation, Th/U, and K/U decreased offshore. Oxygenated bottom waters predominated within 100 km of the western paleoshoreline, whereas oxygen-depleted bottom waters were dominant more than 600 km offshore. During late Middle Turonian time, well-oxygenated bottom water may have extended across the study area. Peak transgressions are characterized by low Th/U and K/U and can be identified in outcrops and on well logs. Anomalously high Th/U and K/U near the Cenomanian-Turonian boundary and in the Coniacian may correspond to “oceanic anoxic events.” Lithologic/geochemical facies are identified on Th-U plots.

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