The Pb isotopic composition of the U-rich dolostones shows considerable spread in ²⁰⁶Pb/²⁰⁴Pb (32-79) and ²⁰⁷Pb/²⁰⁴Pb (16.3-18.7) but little spread in ²⁰⁸Pb/²⁰⁴Pb (38.4-39.5). The high U/Pb and low Th/U ratios implied from these ratios reflect the greater mobility of U relative to Th and Pb under near-surface oxidizing conditions. The spread in Pb isotopic composition of 16 samples from a single core chip yields a Pb-Pb age of 255 ± 47 Ma (2s), which is consistent with the Pb-Pb age of 263 ± 41 Ma for the entire data set (n = 28). A subset of pyrolized samples yields ²³⁸U-²⁰⁶Pb and ²³⁵U-²⁰⁷Pb ages of 26 ± 31 Ma and 274 ± 28 Ma. The Pb-Pb and U-Pb ages are in excellent agreement with the timing of the development of the overlying unconformity at about 270 Ma. The relatively large uncertainties in Pb-Pb and U-Pb ages are the result of scatter of data outside that expected from analytical uncertainties. The scatter can be attributed to a prolonged period of exposure (30-40 million years) associated with the development of the unconformity. Initial isotopic heterogeneity and multicomponent mixing may also contribute to this scatter. Scatter on U-Pb isochron plots has been enhanced by recent U loss. Regardless of the age uncertainties, the data preserve evidence of a U enrichment event during the Permian. We suggest that U was introduced into the sequence by oxidized groundwat r and enrichment occurred in response to local redox conditions at or near the unconformity surface. The Pb isotopic ages, coupled with the observation that anomalous U concentrations are restricted to dolomitic zones within the Upper Wahoo Formation, suggests that U enrichment was concurrent with much of the dolomitization. The U-Pb system was not reset during uplift and exposure in the Late Cretaceous and subsequent burial in the Tertiary, revealing that Pb isotopic ages can be preserved in complex disgenetic systems. This observation may indicate that this part of the upper Wahoo Formation was completely cemented during Mesozoic burial and/or that the youngest phase of porosity development took place under reducing conditions in which U, Th, and Pb were not mobilized. Thus, redox cond tions appear to have been important to both the U enrichment process and the preservation of the Pb isotopic ages.