Relationship of uranium to petrography of caliche paleosols with application to precisely dating the time of sedimentation

We use petrography combined with fission-track mapping to evaluate qualitat ively the phases that concentrate uranium in caliche paleosols from tate Pa leozoic cyclothems of Texas and New Mexico, We also discuss geochemical ana lyses of U and Pb concentrations and Pb isotopic compositions, Uranium conc entrations in brown caliche calcite range from 1 to 9 ppm. Lead concentrati ons 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 substa ntial radiogenic component. Caliches with brown calcite most likely develop ed 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 sample s from this study have a range in U/Ph that would permit precise dating of the rocks, but only the data from three give precise ages. Some of the samp les have hematite, which has high concentrations of U and Pb. For one of th e precisely dated samples, the two sigma uncertainty was improved from 10 t o 2.6 Ma by avoiding hematite when sampling.