U-PB DATING OF ZIRCONS FROM THE SALT WASH MEMBER OF THE MORRISON FORMATION FROM NEAR CAPITOL REEF NATIONAL PARK, UTAH

Zircons were extracted and analyzed by laser ablation ICPMS from samples of both detrital sediment and altered ash beds found in the Salt Wash Member of the Morrison Formation from near the eastern boundary of Capitol Reef National Park. A sample from an altered volcanic ash near the top of the Brushy Basin Member gave an age of 151.2 ± 1.8 (2σ) Ma, compared to 150.0 ± 1.0 Ma age on sanidine previously reported for this same ash in Kowallis et al. (1998, ages recalculated with new decay constants). A second sample of altered volcanic ash from the Tidwell Member gave an age of 157.2 ± 1.9 Ma, very similar to the 156.7 ± 1.0 Ma age reported by Kowallis et al. (1998) on this same bed. Two samples of detrital zircon from the Salt Wash Member gave youngest age peaks of 151.6 ± 1.1 Ma and 148.5 ± 2.5 Ma, overlapping with the age of the Brushy Basin Member, but not with the Tidwell Member. These data suggest the possibility that the J-5 unconformity is located between the Salt Wash Member and the Tidwell Member, not between the Tidwell Member and the Summerville Formation. The ages also suggest that the span of time represented by the dinosaur bearing Brushy Basin and Salt Wash members of the Morrison Formation is fairly short, only about 2-3 million years.