THE USE OF URANIUM-LEAD DATING OF DETRITAL ZIRCONS TO DETERMINE RIVER DIRECTIONS BEFORE THE RISE OF THE WASATCH RANGE, UTAH

A rock unit known only as “Conglomerate No. 1” and believed to be Paleocene to Eocene in age is exposed throughout the Wasatch Range in Utah between Salt Lake City and the Uinta Mountains. The conglomerate includes clasts of Precambrian quartzite and Paleozoic limestone as well as clasts of the Archean - early Proterozoic Farmington Canyon Complex. The conglomerate probably predates the rise of the Wasatch Range as it does not include characteristic Wasatch Range rocks such as granite from the Cottonwood stock. The mystery of Conglomerate No. 1, besides the uncertainty in its age, is that the provenance of the clasts is completely unknown. It is not even known whether the conglomerate was deposited by streams flowing from the east or the west. The importance of the provenance of Conglomerate No. 1 is that it could help explain the origin of westward flowing rivers that cut the Wasatch Range such as Spanish Fork, Provo and Heber Rivers. One model for the origin of these rivers is that the ancestral rivers flowed eastward and that the rivers changed direction as the Wasatch Range rose and the Great Basin dropped. The objective of this study has been to obtain U-Pb ages for detrital zircons of Conglomerate No. 1 in order to determine the provenance of the conglomerate. Six samples were collected from the entire geographic range of Conglomerate No. 1. Detrital zircons were extracted and U-Pb dating was carried out on 100 detrital zircons from each sample at the University of Arizona. Ages ranged from 36 Ma to 3500 Ma. The minimum age suggests that Conglomerate No. 1 was deposited later than the late Eocene. The most prominent peak in U-Pb ages (28% of zircons) corresponds to the Grenville Orogeny (1000 – 1250 Ma) with the second most prominent peak (21% of zircons) corresponding to the Mazatzal Orogeny (1600 – 1800 Ma). The above prominent peaks suggest transport from the east. On the other hand, a small fraction of zircons (3%), all but one of which were extracted from one sample, gave U-Pb ages in the narrow range 152 – 163 Ma, which could correspond to a Jurassic island arc, suggesting the possibility of some transport from the west. Further analysis will be reported at the meeting.