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Gamma-ray spectrometry was conducted on outcrops of marine shale and chalk deposited in the Cretaceous Western Interior seaway. Study of the Cenomanian-Turonian Greenhorn cyclothem of Colorado, New Mexico, and Utah showed that profiles of gamma-ray spectra are useful in evaluating organic carbon content, interpreting paleoenvironments, and correlating otherwise homogenous sequences of marine shale. Gamma-ray spectra provide estimates of total gamma radiation and K, U, and Th contents. Th/U and K/U ratios can be used to estimate relative abundances of detrital minerals and organic matter, whereas the Th/K ratio is an indicator of clay mineralogy. Variations in these ratios reflect both local depositional processes and widespread events that can be correlated for hundreds o kilometers across lithofacies boundaries. Profiles of total gamma radiation can be used to help map shale facies. Spectrometer surveys can be used to show directions of sediment transport, to indicate proximities of paleoshorelines, and to aid in estimation of lateral and temporal variations in paleosalinity. Outcrop profiles can be compared directly with well logs of gamma-ray spectra.
Gamma-ray spectrometry of shale outcrops is a versatile technique. Results can be interpreted in the field, the sampling program can be modified as it progresses, and the effects of event deposits such as bentonites and bone beds can be characterized. If lead shielding is used to keep sample mass and geometry constant, reproducible results can be obtained even from shales that contain below-average concentrations of K (< 2%), U (< 2 ppm), and Th (< 10 ppm).
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