NEW U-PB ZIRCON AGES FROM AN ASH BED IN THE BRUSHY BASIN MEMBER OF THE MORRISON FORMATION NEAR HANKSVILLE, UTAH

A bentonitic ash bed 8.7 m below the top of the Upper Jurassic Morrison Formation from a section near Hanksville, Utah, has been dated by laser ablation multi-collector inductively coupled plasma mass spectrometry and gives a zircon U-Pb age of 149.0 +2.5/ -2.2 Ma. This age is statistically the same as the 149.3 ± 0.5 Ma age of an ash sample 0.5 m below the top of the Brushy Basin Member of the Morrison Formation collected near Notom, Utah, about 25 km to the west and reported in an earlier publication (Kowallis and others, 1998). The Hanksville section has a short section of possible Cedar Mountain Formation (between 0.4 and 7.3 m thick) on top of the Morrison section. Two young contaminant zircons with a peak age of 122.5 Ma in the Brushy Basin ash sample indicate that any Cedar Mountain sediments present here are from the lowermost part of the Cedar Mountain Formation.