U-PB DATING OF ZIRCONS FROM THE SALT WASH MEMBER OF THE MORRISON FORMATION, CENTRAL UTAH

Previous attempts to find primary volcanic ash layers in the Salt Wash Member of the Morrison Formation that could be radiometrically dated have been unsuccessful. In this study, we examined two mudstones from different stratigraphic levels within the Salt Wash Member from a section near Notom, Utah. These mudstones are not bentonites, but they do have some swelling clay and contain both euhedral zircons, which may be primary volcanic grains, and rounded detrital zircons. We also examined one bentonite from the top of the overlying Brushy Basin Member and one bentonite from the underlying Tidwell member. These two samples serve as both a constraint on the ages and on the dating technique. Earlier ⁴⁰Ar/³⁹Ar ages of sanidines from volcanic ashes in the Tidwell Member (156.7 ± 1.0 Ma) and the Brushy Basin Member (152.2 to 150.0 ± 1.0 Ma) have been previously published in Kowallis et al. (1998). The zircons were extracted from these samples using standard rock crushing and heavy liquid methods. For dating, we selected only the most euhedral zircons. These were dated using laser ablation ICP-MS at Washington State University with a spot size of 30 microns. Tuffzirc ages from the two ash layers used as control samples are: Brushy Basin Member, 151 ± 2 Ma and Tidwell Member, 157 ± 2 Ma. These ages are identical within error. The two ages from the Salt Wash Member are 151 ± 2 Ma near the middle of the member and 148 ± 2 Ma near the top of the member. The Salt Wash ages appear to be similar to the ages from the Brushy Basin Member and appear to be significantly younger than the Tidwell Member. This may indicate an unconformity between the Salt Wash and Tidwell members of the Morrison Formation.