URANIUM-LEAD AGES OF CRETACEOUS (ALBIAN-CENOMANIAN) STRATA FROM THE ROSE CREEK ESCARPMENT OF THE CRATONIC MARGIN OF THE WESTERN INTERIOR BASIN

Albian-Cenomanian strata of the Rose Creek Escarpment (RCE) along the Nebraska-Kansas border have been the subject of published works on fossil leaf beds, terrestrial C-isotope chemostratigraphy of OAE1d, dinosaur track beds, mid-Cretaceous sequence stratigraphy and paleosols, and stable isotope paleohydrology of mid-Cretaceous North America. Zircon separates from key stratigraphic horizons collected along RCE illustrate potential for developing a high-precision U-Pb (CA-ID-TIMS) geochronologic framework for terrestrial deposits in the Western Interior Basin. Zircon separates are screened for U-Pb ages via LA-ICP-MS analyses with uncertainty of ± 2%. Near the top of RCE, the Cenomanian X-bentonite, a widely-traced ash bed throughout the Western Interior Basin, produces a unimodal zircon age peak with Concordia age of 95.53 ± 0.36 Ma (n = 140). Despite the range of uncertainty from individual analyses, the statistically large sample produced result closely similar to the high-precision U-Pb age of the X-bentonite of 95.87 ± 0.10 Ma reported by Barker et al. (2011, CJES 48:543-556). A sequence boundary paleosol at the Rose Creek Pit (RCP) was described by Gröcke et al. (2006, Geology 34:193-196) as a feature in the δ¹³C profile of the Albian-Cenomanian OAE1d. A population of zircons separated from the RCP paleosol (n = 207) produces a stand-alone Albian-Cenomanian age peak defined by five grains (2.4 % of total), much younger than older Paleozoic-Proterozoic detrital zircons (DZ) otherwise contained in the sample. Near the base of the Dakota Formation along RCE, the Camp Jefferson (CJ) paleosol was first described by Joeckel (1987, UWCG 25:95-102). This plinthic paleosol immediately overlies estuarine strata of the Albian Kiowa-Skull Creek marine cycle. A population of zircons separated from CJ paleosol (n = 197) produces a stand-alone Albian age peak defined by 17 grains (8.6 % of total). A DZ population from a sandstone overlying the CJ paleosol (n = 554) also produces a stand-alone Albian age peak defined by 2 grains (0.3 % of total), illustrative of the potential of paleosols as a target facies to capture improved estimates of maximum depositional ages. Albian grains from the RCP and CJ paleosols are the subject of a high precision study to determine the ages of the paleosols.