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U-Pb detrital zircon analyses provide a new maximum depositional age constraint on the Uinta Mountain Group (UMG) andcorrelative Big Cottonwood Formation (BCF) of Utah, and signifi cantly enhance our insights on the mid-Neoproterozoic paleogeographic and tectonic setting of western Laurentia. A sandstone interval of the Outlaw Trail formation with a youngest population (n = 4) of detrital zircons, from a sampling of 128 detrital zircon grains, yields a concordia age of 766 ± 5 Ma. This defi nes a maximum age for deposition of the lowermiddle Uinta Mountain Group in the eastern Uinta Mountains and indicates that the group is no older than middle Neoproterozoic in age (i.e., Cryogenian). These data support a long-proposed correlation with the Chuar Group of Grand Canyon (youngest age 742 Ma ± 6 Ma), which, like the Uinta Mountain Group and Big Cottonwood Formation, records nonmagmatic intracratonic extension. This suggests a ~742 to ≤766 Ma extensional phase in Utah and Arizona that preceded the regional rift episode (~670-720 Ma), which led to development of the Cordilleran passive margin. This is likely an intracratonic response to an early rift phase of Rodinia. Further, because the Chuar Group and the Uinta Mountain Group-Big Cottonwood Formation strata record intracratonic marine deposition, this correlation suggests a regional ~740-770 Ma transgression onto western Laurentia.The detrital grain-age distributions from 12 samples include the following grain-age populations and interpreted provenance: 2.5-2.7 Ga (late Archean southern Wyoming province); 1.6-1.8 Ga (Paleoproterozoic Yavapai province); 1.5-1.6 Ga (Early Meso proterozoic North American magmatic gap), 1.4-1.45 Ga (Colorado province A-type granite-rhyolite belt); 0.93-1.2 Ga (eastern Grenvillian orogen); and midNeoproterozoic volcanic grains (766 Ma). Sediment was transported by: (1) a major longitudinal west-fl owing river system tapping the Grenville orogen, (2) local southfl owing drainages off the southern Wyoming craton, and (3) northerly and westerly fl owing marine currents. The Uinta Mountain Group river system was one of several major transcontinental drainages that delivered Grenvillian zircon grains to the protoPacifi c Ocean. We propose that this river system ultimately supplied sediment to periGondwanan margins along the proto-Pacifi c to Antarctica, Australia, and South America, providing an alternative source for explaining the problematic provenance of Grenvillian grains in these areas.

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