Detailed sedimentology and gamma-ray log characteristics of a Namurian deltaic succession II: Gamma-ray logging

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Abstract
Summary: A portable gamma-ray spectrometer was use to measure the total radioactivity, potassium content (% K), uranium content (ppm U) and thorium content (ppm Th) in the Upper Namurian Rough Rock at an outcrop in a road section at Elland, Yorkshire. The total gamma radiation generally decreases upwards in the succession, reflecting an upward increase in grain size. Gamma-ray peaks within the succession are due partly to facies/grain-size changes and partly to concentrations of thorium-bearing monazite in mouth-bar sandstones and basal channel lags. Failure to identify the true nature of these peaks could lead to erroneous correlations in the subsurface. The Th/K ratio decreases upwards in association with the increase in grain size and a decrease in the quartz/feldspar ratio. The Th/K ratio and Th-K cross-plots are effective in distinguishing some of the deltaic facies.