A NEW APPROACH TO DATE PALEOSOLS IN TERRESTRIAL STRATA: A CASE STUDY USING U-PB ZIRCON AGES FOR THE YELLOW CAT MEMBER OF THE CEDAR MOUNTAIN FORMATION OF EASTERN UTAH

The Cretaceous Cedar Mountain Formation (CMF) of Eastern Utah is one of the most prolific Early Cretaceous dinosaur fossil archives in the world. Lack of age control in the CMF has complicated stratigraphic correlations of paleontological sites. The basal member of the CMF, the Yellow Cat Member (YCM), is a terrestrial unit that accumulated in a local salt tectonic basin and lacks easily datable horizons such as volcanic ash beds. U-Pb analysis of populations of 300 zircon grains from paleosols by laser ablation- inductively coupled mass spectrometry (LA-ICP-MS) was used to determine maximum depositional ages for the Yellow Cat Member and the duration of the unconformity between the Cretaceous Cedar Mountain Formation and the underlying Jurassic Morrison Formation. This unconformity was estimated to be ca. 25 Myr in duration throughout the western interior. ⁴⁰Ar/³⁹Ar data from the top of the Brushy Basin Member of the Jurassic Morrison Formation suggest a maximum depositional age of 150.0 ± 0.52 Ma for the latest Jurassic in eastern Utah (Trujillo and Kowallis, 2015). Our new U-Pb zircon data place the maximum depositional age of a paleosol at the base of the Yellow Cat Member at 139.7 ±2.2 Ma (n=2). A paleosol sample from the middle of the Yellow Cat Member has a maximum depositional age of 136.4±1.1 Ma (n=4), and a paleosol from the Upper Yellow Cat Member shows a maximum depositional age of 137.2±2. Ma (n=4). These findings from paleosols of the Yellow Cat Member suggest that there is a smaller time gap between the Jurassic Morrison Formation and the Cretaceous Cedar Mountain Formation than previously proposed. Zircons extracted from fine-grained paleosol samples are thus demonstrated to be useful in constraining the continental stratigraphic record, potentially better than coarser-grained, fluvial, sediments whose zircon populations may be dominated by detrital grains much older than the age of deposition. By developing a technique to provide precise age control on terrestrial units in the absence of discrete ash beds, we can better calibrate stable isotope records and better constrain the evolutionary rates of terrestrial vertebrates.