PALEOMAGNETIC RESULTS FROM THE NEOPROTEROZOIC UINTA MOUNTAIN GROUP, UTAH AND COLORADO

Sedimentary sequences deposited during Neoproterozoic breakup of the Rodinian supercontinent can be used to define the age and duration of the event. We have begun a regional paleomagnetic study of the Uinta Mountain Group (UMG), a Neoproterozoic sedimentary sequence exposed in the Uinta Mountains, Utah and Colorado, to better determine age, possibly duration of sedimentation, and better define paleogeography of Neoproterozoic North America. The UMG is exposed in the core of the east-west trending Uinta Mountains, a Late Cretaceous to early Tertiary Laramide-style high-angle reverse-fault bounded range. UMG strata are predominantly hematite-cemented sandstone with subordinate coarser and finer grained intervals, and range from about 7.3 km thick in the eastern Uintas to 4.0 km in the west. Bedding orientation varies from sub-horizontal in the south to overturned in the north. Biostratigraphic correlation between the UMG and other Neoproterozoic sedimentary sequences has been proposed based on presence of Chuaria and microfossils similar to Melanocyrillium. Paleomagnetic data from strata in Sheep Creek Canyon (SCC), 15 km west of Flaming Gorge show that early-acquired, possibly primary remanence is present. Samples from a 1 km stratigraphic section deformed into an overturned fold during slip along the Uinta fault in thermal demagnetization reveal magnetizations progressively unblocked between about 590 and 685° C, with no change in magnetic mineralogy or viscous behavior. From a collection of over 60 sites at different localities (1 site=2 to 4 beds, 10 to 20 samples collected per site), chemical demagnetization, fold tests and contact tests (elsewhere in the Uintas) were also performed. Complete demagnetization data from 31 SCC sites yield well-defined trajectories; mean of 17 sites (involving 123 samples not strongly contaminated by north-directed, positive inclination magnetizations) is Decl.=94° , Incl.=-12° , a₉₅=13.5° , k=71.5. This mean gives a 10° N / 160° E paleopole, which is comparable to previous results reported by Bressler (1981) and Chuar Group paleomagnetic poles. We therefore tentatively infer that much of the UMG has a mid-Neoproterozoic age.