- journal_title：Journal of Sedimentary Research
- Contributor：E. Troy Rasbury ; William J. Meyers ; Gilbert N. Hanson ; Robert H. Goldstein ; Arthur H. Saller
- Publisher：SEPM Society for Sedimentary Geology
- Date：2000-
- Format：text/html
- Language：en
- Identifier：10.1306/2DC4092B-0E47-11D7-8643000102C1865D
- journal_abbrev：Journal of Sedimentary Research
- issn：1527-1404
- volume：70
- issue：3
- firstpage：604
- section：Research Articles
We use petrography combined with fission-track mapping to evaluate qualitatively the phases that concentrate uranium in caliche paleosols from late Paleozoic cyclothems of Texas and New Mexico. We also discuss geochemical analyses of U and Pb concentrations and Pb isotopic compositions. Uranium concentrations in brown caliche calcite range from 1 to 9 ppm. Lead concentrations in the same calcites range from 0.5 to 1 ppm. Lead-isotope compositions range from common (no apparent radiogenic component) to lead with a substantial radiogenic component. Caliches with brown calcite most likely developed near the vadose-phreatic interface—often a redox boundary where organic material (and uranium) is expected to be concentrated. In contrast, uranium concentrations in light-colored caliche nodules from the vadose zone have uranium concentrations that are 1 ppm or less. Fifteen of forty-five samples from this study have a range in U/Pb that would permit precise dating of the rocks, but only the data from three give precise ages. Some of the samples have hematite, which has high concentrations of U and Pb. For one of the precisely dated samples, the two-sigma uncertainty was improved from 10 to 2.6 Ma by avoiding hematite when sampling.