Natural gamma-ray spectrometry, lithofacies, and depositional environments of selected upper Cretaceous marine mudrocks, western United States, including tropic shale and tununk member of Mancos shale

In-situ measurements of gamma-ray spectra provide estimates of the potassium, thorium, the pre-weathering uranium contents of outcropping marine mudrocks. Th/U is a sensitive indicator of the oxidation potential of paleoenvironments of deposition of Phanerozoic epi- and pericontinental marine deposits. High (>10 x 10-4) ratios of uranium or organic carbon occur in phosphatic and cherty mudrocks; low (<1.5 x 10-4) ratios occur in nonmarine mudrocks and in epicontinental marine strata that were deposited during unusual paleo-oceanographic conditions, which sometimes accompanied global extinction events. Spectrometric data compliment sedimentologic data in defining five lithologic/geochemical facies in marine mudrocks of the Cenomanian-Turonian Greenhorn marine cycle. The rock units that were studied in Colorado, New Mexico, and Utah are the Graneros Shale, Greenhorn Limestone, Garlile Shale, Mancos Shale, and Tropic Shale. Results of analyses of the different rock units are used to explain various geological structures of the area under investigation. Outcrops and geophysical well logs combine to indicate the distribution of lithologic/geochemical focus of marine mudrocks