ASHBY, James M., Department of Earth and Planetary Sciences, The Univ of New Mexico, Northrop Hall, Albuquerque, NM 87131-1116,, GEISSMAN, John W., Department of Earth and Planetary Sciences, The University of New Mexico, Northrop Hall, Albuquerque, NM 87131-1116, and WEIL, Arlo B., Department of Geological Sciences, The University of Michigan, 4534 C.C. Little Building, Ann Arbor, MI 48109-1063,

The Proterozoic Uinta Mountain Group (UMG) is exposed at the core of the east-west trending Uinta Mountains along late Cretaceous to early Tertiary Laramide-style high-angle reverse faults. The thickness of UMG strata ranges from 7.3 km in the eastern Uinta Mountains to 4.0 km in the west. Strata of the eastern and central regions are dominated by hematite-cemented medium- to fine-grained sandstone, interbedded shale, and subordinate coarser-grained siliciclastic intervals. Bedding orientations range from sub-horizontal in the south to overturned in the north, where UMG strata are structurally juxtaposed against Phanerozoic strata. Biostratigraphic correlation of the western-UMG (Red Pine Shale) with the Chuar (Grand Canyon) and Little Dal (Northwest Territories) groups has been proposed based on the presence of the fossil Chuaria and vase-shaped microfossils that allow correlation with the Chuar group (742 Ma), suggesting a mid-Neoproterozoic age for the upper western UMG.

To further define the age and duration of the UMG we have begun a regional paleomagnetic study. Here we report results from the Sheep Creek Canyon area, 15 km west of Flaming Gorge, Utah, where the eastern UMG is exposed. Samples were collected from a one-kilometer-thick stratigraphic section exposed within a locally overturned fold. Thermal demagnetization reveals apparently primary magnetization components that are progressively unblocked between 590 and 685C, with no change in magnetic mineralogy or viscous behavior during unblocking. Our preliminary collection includes over 60 sites from different localities (1 site=2 to 4 beds, with typically 10 to 20 samples collected per site). Completed demagnetization data from 6 sites yield well-defined trajectories; the mean of 34 samples not strongly contaminated by a north-directed and positive inclination magnetization is Decl.=96, Incl.=-9, a95=7.4, k=11.9. These data yield paleopoles of approximately 10N and 160E, comparable to Bressler's (1981) results and the paleomagnetic poles from the Chuar Group. We therefore tentatively infer that much of the UMG has a mid-Neoproterozoic age.

Rocky Mountain (53rd) and South-Central (35th) Sections, GSA, Joint Annual Meeting (April 29May 2, 2001)
Session No. 14--Booth# 15
Tectonics of the Rocky Mountain Region (Posters)
Sheraton Old Town Hotel: Alvarado DE
1:00 PM-5:00 PM, Tuesday, May 1, 2001