MID-NEOPROTEROZOIC STRATA OF NORTHERN UTAH AND SOUTHERN IDAHO: DATING AND CORRELATION OF UINTA MOUNTAIN GROUP AND POCATELLO FORMATION

The middle Uinta Mountain Group in the eastern Uinta Mountains can be no older than 770 Ma as determined by the youngest detrital zircon ages in the formation of Outlaw Trail (OT). The OT is a mappable interval of green to gray sandstone, siltstone, and shale that is 50-70 m thick, thickens westward, and is exposed for about >50 km in the eastern Uinta Mountains near the Utah-Colorado border. The unit is located approximately 2.7 km from the extrapolated base and the eroded top of a locally 5.5-km-thick section of the UMG. The sandstone is a fine grained, subangular to subrounded, subarkosic to arkosic arenite, and exhibits symmetric and asymmetric ripplemarks, mudcracks, gypsum pseudomorphs, and mudcracks. This unit is overlain and underlain by coarser trough-crossbedded quartz arenite, and represents a shoreline deposit which transgressed across a braid plain.

The Red Pine Shale (RPS), in the western Uinta Mountains, may be as young as ~740 Ma based on C-isotope and paleontologic correlation with the Walcott Member of the Chuar Group. The RPS is ~<1200 m thick and is the uppermost unit in the 4-km-thick western UMG. It is dominated by organic-rich shales with lesser crossbedded sandstones (quartz and arkosic arenite). The utilitarian fossil assemblage includes acritarchs and vase-shaped microfossils, and associated C-isotope values range from -17 to -31‰ (PDB). This unit is interpreted as a marine prodelta to delta plain.

Probability density plots of detrital zircon ages from the RPS and OT in the Uinta Mountains and the Big Cottonwood Formation (BCF) in the Wasatch Range are very similar and show Grenville-age grains, grains derived from 1.45 Ga anorogenic granites, Meso- to Paleoproterozoic grains and an array of Archean grains. The BCF and UMG were part of the same <770 Ma to ~740 Ma rift basin. These strata, like the Chuar Group of the Grand Canyon, record rifting 10's of my prior to the volcanism, faulting and glacial sedimentation of the >709 to <667 Ma Pocatello Formation in southern Idaho.

Significantly, these new ages and correlations require a regional pre-Sturtian rifting episode in what is now Utah and Arizona, and suggest that the Pacific Laurentian margin experienced three phases of rifting during the interval 770 Ma to 550 Ma.