PROVENANCE ANALYSIS OF THE FORT UNION FORMATION IN THE WESTERN BIG HORN BASIN USING U-PB DETRITAL ZIRCON GEOCHRONOLOGY

The Paleocene Fort Union Formation was studied at three localities in the western Bighorn Basin of western Wyoming. Detrital zircon U-Pb geochronology was used to determine the provenance of these rocks to better understand the transition between Sevier thin-skinned and Laramide thick-skinned deformation. Uppermost Fort Union strata were sampled near the WY-MT state line near Belfry, MT, just east of Cody, WY and north of Meteetse, WY. A total of 248 zircons were analyzed at the University of Arizona Laserchron Laboratory. Proterozoic zircons dominated each of the samples, ranging from 48-63% of each sample. Numerous Proterozoic orogens supplied these zircons. Mesozoic zircons were the next most abundant component in each sample, ranging in proportion from 20-23%. Archean zircons comprised 8-15% of each of the samples. Paleozoic zircons were the smallest fraction, accounting for only 2-8% of the zircons analyzed. The preponderance of Proterozoic zircons indicates that the principal source area of the Fort Union in the Big Horn Basin is the Sevier Highlands to the west. These zircons were likely recycled from Neoproterozoic metasedimentary rocks that are common throughout that area. The presence of a significant proportion of upper Cretaceous zircons suggests that areas of the Idaho Batholith were exposed as well and supplying sediment. The paucity of Archean zircons indicates that the Beartooth uplift was not as yet unroofed, and thus not a significant source area during Fort Union deposition. Archean zircons present could also have been recycled from distal westerly sources.