Application of PB Isotopes to the Absolute Timing of Regional Exposure Events in Carbonate Rocks: An Example From U-Rich Dolostones from the Wahoo Formation (Pennsylvanian), Prudhoe Bay, Alaska

ABSTRACT Pb isotope data from U-rich dolostones from the Wahoo Formation (Pennsylvanian) from the subsurface at Prodhoe Bay, Alaska, demonstrate that the U-Th-Pb system can be a powerful geochemical and geochronological tool in understanding carbonate diagenesis. These U-rich dolostones are developed beneath a major, Late Permian to Early Triassic truncational unconformity. U enrichment is uniquely associated with the mineral dolomite, but anomalously high concentrations of U are not present within the dolomite crystal lattice. Major mineral or fluid phases can be ruled out as U hosts. SEM analyses indicate that U anomalies are present in an unknown mineral phase associated with authigenic clays and are commonly concentrated along stylolites. 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.