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 have examined four mudstones from different stratigraphic levels in the Salt Wash Member from a section near Notom, Utah, east of Capitol Reef National Park. These mudstones are not bentonites, but they do have some swelling clay and contain both euhedral zircons, that may be primary volcanic grains, and rounded detrital zircons. 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 ICP-MS laser ablation at Washington State University and the results will be presented at the meeting. Earlier dating of volcanic ashes from the underlying Tidwell Member (155Ma) and the overlying Brushy Basin Member (150-148 Ma) have been previously published in Kowallis et al. (1998) and provide a constraint and means of testing whether our new ages are representative of the age of the Salt Wash Member.