Uranium and other trace elements in Devonian and Mississippian black shales in the central midcontinent area

Abstract

Marine black and dark-gray shales of Late Devonian and Early Mississippian age occupy a prominent position in the stratigraphic column in the central midcontinent region because of their areal extent and relation to widespread unconformities of economic interest. These rock units, the Woodford and Chattanooga shales and the Arkansas novaculite, have been recognized over large parts of Kansas, Oklahoma, Arkansas, and Missouri, and are partial or total equivalents of each other. The Woodford shale was examined at 10 localities in Oklahoma. Uranium content of the samples ranges from less than 0.001 to 0.014 percent, but the modal, median, and arithmetic mean values of the Woodford samples as a whole and the samples of the shale only are 0.001, 0.002, and 0.003 percent uranium, respectively. These values are probably representative of most of the Woodford except the phosphatic nodules that are present in some parts of the formation. The samples of phosphatic nodules and laminae have an average uranium content of 0.006 percent. The Chattanooga shale was examined and sampled at 7 outcrop localities in Oklahoma, 10 in Arkansas, and 1 in Missouri, and samples of cores from 2 drill holes in Kansas were also collected. A total of 95 samples, of which 83 were shale samples, was collected, and the uranium content ranged from less than 0.001 to 0.55 percent. Only one sample contained more than 0.013 percent uranium; that was a selected sample of highly radioactive organic-rich material. No samples of shale contained more than 0.012 percent uranium, and the modal and median values of the Chattanooga samples as a whole are 0.002 percent uranium. The Arkansas novaculite was sampled at 15 localities in Arkansas and Oklahoma. Only one sample contained more than 0.004 percent uranium, and it was from a locality in Garland County, Ark., where the Arkansas novaculite has been contact metamorphosed. The average uranium value of all samples from the formation is 0.001 percent; this is believed to be representative of the uranium content of the formation in the report area. Despite the low uranium contents of these formations as a whole, at one locality the Chattanooga shale contains as much as 0.005 percent uranium in an interval 10.1 feet thick, and the Woodford contains as much as 0.005 percent uranium in intervals up to 20 feet thick. In general, if rock of this grade ever becomes of economic interest, the small amount of overburden on the black shale of the Woodford would make it more amenable to large-scale surface-mining methods than the Chattanooga. Selected shale samples were analyzed for organic-carbon content and oil yield. A positive relation of organic-carbon content to uranium content is suggested. A few samples contained enough oil for specific-gravity determination. The number of samples is not sufficient to allow evaluation of the relation, but in these samples the larger the uranium content, the higher the specific gravity of the oil. 289 ACKNOWLEDGMENTS Many Geological Survey colleagues supplied analyses, data, and gu...