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The AAPG/Datapages Combined Publications Database

Utah Geological Association


Central Utah: Diverse Geology of a Dynamic Landscape, 2007
Pages 75-80

New U-Pb Zircon Ages from an Ash Bed in the Brushy Basin Member of the Morrison Formation Near Hanksville, Utah

Bart J. Kowallis, Brooks B. Britt, Brent W. Greenhalgh, Douglas A. Sprinkel


A bentonitic ash bed 8.7 m below the top of the Upper Jurassic Morrison Formation near Hanksville, Utah, has been dated by laser ablation multi-collector inductively coupled plasma mass spectrometry and gives a single-crystal U-Pb zircon 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 bed located 0.5 m below the top of the Brushy Basin Member of the Morrison Formation near Notom, Utah, about 25 km to the west. 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 the Cedar Mountain sediments present here are from the lowermost part of the Cedar Mountain Formation.

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