CONSTRAINING DEPOSITIONAL AGES FOR NEOPROTEROZOIC SILICICLASTIC SEQUENCES THROUGH DETRITAL ZIRCON AGES: A CA.770 MA MAXIMUM AGE FOR THE LOWER UINTA MOUNTAIN GROUP

It is difficult to obtain accurate and reliable time constrains on siliciclastic Neoproterozoic rocks where tuffaceous horizons are absent. Further, even where tuffaceous rocks are present they often contain recycled zircon. One mechanism to place time constraints is via the detrital zircons, specifically by examining the youngest coherent age grouping in the detrital age spectrum.

The Uinta Mountain Group (UMG) of northeastern Utah is one such siliciclastic Neoproterozoic succession that has limited age constraints. From correlations based on fossil, C isotope and lithological evidence with the Chuar Group of the Grand Canyon region, the top of the western UMG (Red Pine Shale) is placed at ca.740 Ma. Few if any age constraints are available for the eastern UMG, nor for the lower parts of the sequence

An initial 60-grain sampling of the formation of Outlaw Trail, ~ 1km from the base of the 4-7 km thick (UMG) in the Swallow Canyon quadrangle, eastern Uinta Mountains, found a single zircon with an age of ca.760 Ma. Other components of that detrital zircon spectrum include a significant Grenville age cluster, grains derived from 1.45 Ga anorogenic granites, Meso- to Paleoproterozoic grains and the usual smattering of Archean grains. In order to obtain a credible maximum depositional age constraint, a population, rather than a single young grain, is necessary and so further analyses were undertaken. At this stage, from the analysis of 128 single zircon grains from two separate samplings of the same outcrop and 4 analytical sessions we have found 4 grains that are within analytical uncertainty and give a concordia age of ca.770 Ma. These four grains are igneous in origin as determined from CL images, but show clear evidence for surface transport; ie they are not primary volcanic grains. The ca.770 Ma age therefore constrains the maximum age on deposition near the base of the eastern UMG.

These data show once again the power of detrital zircon age determinations for both provenance information and constraints on maximum depositional age.