DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF THE WASATCH FORMATION, POWDER RIVER BASIN, WYOMING

The lower Eocene Wasatch Formation is more than 1500 m thick in the Powder River Basin of Wyoming. The Wasatch is a Laramide synorgenic deposit that consists of paludal and lacustrine mudstone, fluvial sandstone, and coal. U-Pb geochronologic data on detrital zircons were gathered for a sandstone unit in the middle part of the succession. The Wasatch was collected along Interstate 90 just west of the Powder River Basin rest area, which is about 50 km east of the Bighorn Mountain front. The sandstone is lenticular in geometry and is a feldspathic arenite. The detrital zircon age spectrum ranged (n=100) from 1433-2957 Ma in age, and consisted of more than 95% Archean age grains, with an age peak of about 2900 Ma. The 2900 Ma age peak is consistent with the age of Archean rocks at the core of the Bighorn Mountains. The sparse Proterozoic grains were likely derived from the recycling of Paleozoic sandstone units. Three conclusions can be drawn from these data. First, the Wasatch sandstone is a first cycle sediment. Second, the Archean core of the Bighorn uplift was exposed and shedding sediment into the Powder River basin during Wasatch time. Very little recycling of Paleozoic strata is evident. Third, the Powder River Basin Wasatch detrital zircon age spectra are distinct from the coeval Willwood Formation in the Bighorn Basin west of the Bighorn Mountains. The Willwood was derived from the Sevier highlands to the west, which indicated that Laramide sediment transport off the Bighorn uplift was strongly asymmetric and directed largely to the east.