DISCORDANCE AND DISSOLUTION OF RADIATION-DAMAGED DETRITAL ZIRCON IN THE WASATCH FORMATION AND ADJACENT UNITS IN THE POWDER RIVER BASIN, WYOMING

Laramide uplift and unroofing of the Bighorn Mountains led to infilling of adjacent basins including the Powder River Basin (PRB) in Wyoming. Ages of detrital zircons were determined using U/Pb and High Density Fission Track (HDFT) techniques (see Montario and Garver, 2009), from the basal Cambrian Flathead Formation, which rests directly above the Precambrian basement and the Maastrichtian Lance Formation, Paleocene Fort Union Formation, and Eocene Wasatch Formation. The younger three stratigraphic units are from the PRB and were deposited in the key interval that records Laramide tectonism and basement unroofing. U/Pb ages of zircon from the Lance and Fort Union formations are dominated by Cretaceous grains, but they also have a small fraction of Precambrian grains. U/Pb ages of zircon from the Eocene Wasatch Formation have a very different pattern: ~93% are Precambrian with nearly 60% at ~1800 Ma. HDFT dating reveals cooling ages from ~60 to 2000 Ma. The Lance Formation yields a HDFT grain-age population dominated by Mesozoic grains with a prominent population at ~80 Ma, and secondary populations at ~237 Ma and ~600 Ma. HDFT ages in the Wasatch Formation are much older with the majority of grains (80%) having cooling ages of ~950 Ma, but with wide a range and a few grains with Paleoproterozoic cooling ages. The HDFT cooling ages in Flathead are also almost exclusively Neoproterozoic (c. 790 Ma), but this sample also has a few Paleoproterozoic cooling ages. For both the Flathead and the Wasatch formations, two populations show dramatic discordance in the U/Pb system: one array with an upper intercept at ~1800 Ma and one at ~2900 Ma. For the array that best shows discordance (20 grains in Wasatch), the grains define an upper intercept of 2924 ± 10 Ma and a lower intercept or 0 ± 35 Ma, and we conclude that discordance has occurred since deposition in the Eocene. The Wasatch Formation is unique because it has an abundance of Paleoproterozoic and Archean grains with cooling ages that are mainly Neoproterozoic, and thus these gains must have extremely high radiation damage. We have imaged grains with natural dissolution features, and we conclude that discordance and grain dissolution has occurred after deposition and we hypothesize that this dissolution is directly related to roll-front uranium deposits in the PRB.