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

Pb isotope data from U-rich dolostones from the Wahoo Formation (Penns ylvanian) from the subsurface at Prudhoe 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 a re developed beneath a major, Late Permian to Early Triassic truncatio nal unconformity. U enrichment is uniquely associated with the mineral dolomite, but anomalously high concentrations of U are not present wi thin the dolomite crystal lattice. Major mineral or fluid phases can b e ruled out as U hosts, SEM analyses indicate that U anomalies are pre sent in an unknown mineral phase associated with authigenic clays and are commonly concentrated along stylolites. The Pb isotopic compositio n of the U-rich dolostones shows considerable spread in Pb-206/Pb-204 (32-79) and Pb-207/Pb-204 (16.3-18.7) but little spread in Pb-208/Pb-2 04 (38.4-39.5). The high U/Pb and low Th/U ratios implied from these r atios reflect the greater mobility of U relative to Th and Pb under ne ar-surface oxidizing conditions. The spread in Pb isotopic composition of 16 samples from a single core chip yields a Pb-Pb age of 255 +/- 4 7 Ma (2 sigma), which is consistent with the Pb-Pb age of 263 +/- 41 M a for the entire data set (n = 28). A subset of pyrolized samples yiel ds U-238- Pb-206 and U-235-Pb-207 ages of 267 +/- 31 Ma and 274 +/- 28 Ma. The Pb-Pb and U-Pb ages are in excellent agreement with the timin g of the development of the overlying unconformity at about 270 Ma. Th e relatively large uncertainties in Pb-Pb and U-Pb ages are the result of scatter of data outside that expected from analytical uncertaintie s. 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 co ntribute to this scatter. Scatter on U-Pb isochron plots has been enha nced by recent U loss. Regardless of the age uncertainties, the data p reserve evidence of a U enrichment event during the Permian. We sugges t that U was introduced into the sequence by oxidized groundwater and enrichment occurred in response to local redox conditions at or near t he unconformity surface. The Pb isotopic ages, coupled with the observ ation that anomalous U concentrations are restricted to dolomitic zone s within the Upper Wahoo Formation, suggests that U enrichment was con current with much of the dolomitization. The U-Pb system was not reset during uplift and exposure in the Late Cretaceous and subsequent buri al in the Tertiary, revealing that Pb isotopic ages can be preserved i n complex diagenetic systems. This observation may indicate that this part of the upper Wahoo Formation was completely cemented during Mesoz oic burial and/or that the youngest phase of porosity development took place under reducing conditions in which U, Th, and Pb were not mobil ized. Thus, redox conditions appear to have been important to both the U enrichment process and the preservation of the Pb isotopic ages.