NEW U-P ZIRCON AGES FROM AN ASH BED IN THE BRUSHY BASIN MEMBER OF THE MORRISON FORMATION NEAR HANKSVILLE, UTAH INTRODUCTION During the spring of 2005, the Utah Geological Survey organized a field trip to examine the contact rela- tionships between the Morrison and Cedar Mountain Formations in central Utah. This contact has long been problematic, particularly where the basal Buckhorn Conglomerate Member of the Cedar Mountain Formation is missing (Anderson and others, 2004; Demko and others, 2004; Turner and Peterson, 2004; Eberth and others, 2006; Greenhalgh, 2006; Greenhalgh and others, 2006). One field trip stop was near Hanksville, Utah (figure 1), at a section presented to the group by Utah State Paleontologist James I. Kirkland and Dinosaur National Monument geologist Scott K. Madsen. At this section, no obvious Buckhorn Conglom- erate is present and the section below the Dakota Sandstone is rich in swelling clays almost up to the con- tact (figure 2). Participants on the field trip discussed different possibilities for the boundary between the Morrison and Cedar Mountain Formations at this locali- ty. Is the Cedar Mountain Formation absent or very thin in this area? Is it simply more bentonitic and similar to the Morrison Formation? Regional isopach maps of the Cedar Mountain Formation indicate that it is thin in this area and absent just to the south (Currie, 1997; and fig- ure 3). To help clarify the Morrison/Cedar Mountain contact relationships, we examined the section and identified several possible volcanic ash beds that might yield new age data, and darker clay layers that might contain paly- nomorphs. One volcanic ash bed (now bentonitic) near the top of the section (see figure 2) contained abundant, large biotite crystals and was sampled for dating. A return trip to the section in the fall of 2006 by the authors allowed for more precise measuring of the section, the collection of several samples for palynomorph analysis, and resampling of the ash bed so that it could be submit- ted for additional radiometric dating. The age of the upper Brushy Basin Member of the Morrison Formation (148–150 Ma) is known from ash beds dated from several sections across eastern Utah ranging from Dinosaur National Monument in the north to Montezuma Creek in the south (Kowallis and others, 1991; Kowallis and others, 1998). The age of the lower part of the Cedar Mountain Formation is less well known. Willis and Kowallis (1988) obtained fission- track ages as old as 105 Ma from a section in Salina Canyon, but these ages have large errors of as much as 10 Ma, making them less than ideal for good stratigraphic control. One high-precision age of 98.7 Ma was obtained by Cifelli and others (1997) from near the top of the Cedar Mountain Formation. More recently, Greenhalgh (2006) and Greenhalgh and others (2006) obtained an age of 125 Ma from zircons extracted from tuffaceous mudstone near the base of the Cedar Mountain Form- ation. With this previous age information, we hoped that an age from the ash bed in the Hanksville section would help resolve the issue of where to place the Morri- son/Cedar Mountain contact and perhaps help to better identify this contact at other localities. SEDIMENTOLOGY AND STRATIGRAPHY The basal 38 m of the section consists of slope-form- ing, interbedded, smectitic silty claystone and clayey silt- stone/sandstone with occasional rooted horizons and numerous pale red horizons (figure 2). Tuffaceous mate- rials increase up-section, with the dated ash horizon 11.4 m below the top of this generally fine-grained package. The uppermost 0.4 m of the package bears chert clasts in by Bart J. Kowallis 1 , Brooks B. Britt 1 , Brent W. Greenhalgh 2 , and Douglas A. Sprinkel 3 1 Department of Geological Sciences, Brigham Young University, Provo, UT 84602 bart_kowallis@byu.edu 2 Questar Market Resources, 180 East 100 South, Salt Lake City, UT 84145 3 Utah Geological Survey, P.O. Box 146100, Salt Lake City, UT 84114-6100 ABSTRACT 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 pos- sible 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. b