SCIENTIFIC INVESTIGATIONS ON CRETACEOUS (ALBIAN-TURONIAN) STRATA FROM THE ROSE CREEK ESCARPMENT (RCE) ALONG THE NEBRASKA-KANSAS BORDER, CRATONIC MARGIN OF THE WESTERN INTERIOR BASIN

The RCE on the NE-KS border was incised by glaciofluvial discharge along penultimate glacial margin of the Laurentide ice sheet, and exposes about 150 meters of Cretaceous (K) strata. Strata of the RCE are subject of published works on fossil leaf beds, terrestrial C-isotope chemostratigraphy of Albian-Cenomanian OAE1d, dinosaur track beds, sequence stratigraphy/paleosols, and K stable isotope paleohydrology. Zircon separates from key horizons from RCE illustrate potential for high-precision U-Pb geochronologic framework for K terrestrial deposits. Zircons 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 North America, produces unimodal age peak with Concordia age of 95.53 ± 0.36 Ma (n = 140). Despite ranges of uncertainty from individual analyses, the statistically large sample produces a result closely similar to 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 OAE1d. A population of detrital zircons (DZ) separated from the RCP paleosol (n = 207) produces a stand-alone Albian-Cenomanian age peak defined by five grains (2.7 % of total), much younger than older Paleozoic-Proterozoic DZ otherwise contained in the sample. Studies of stomatal frequency data from leaf beds spanning the OAE1d interval at RCP indicate a short-lived buildup of pCO₂ during the event. Near the base of the Dakota Fm 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. DZ separated from CJ paleosol (n = 197) produces a stand-alone Albian age peak defined by 17 grains (8.6 % of total). DZ separated 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.