RADIO-ISOTOPIC DATING OF VOLCANIC ASH LAYERS FROM THE WHITE RIVER GROUP, WYOMING AND NEBRASKA

High precision U-Pb and ⁴⁰Ar/³⁹Ar dating of U- and K-bearing minerals (primarily zircon and sanidine) from volcanic ash layers from the Eocene-Oligocene White River Group is being undertaken as part of the GTSnext project (http://www.gtsnext.eu) in an attempt to develop improved constraints for the Paleogene timescale. The work currently being carried out focuses on events at and near the Chadronian-Orellan boundary and their correlation to the Eocene-Oligocene boundary in Europe. 17 ash layers from the Flagstaff Rim (Wyoming) and Toadstool Park (Nebraska) sections have been sampled so far. At Flagstaff Rim, the Chadronian-Orellan boundary coincides with an erosional unconformity but sedimentation appears to have been continuous during accumulation of the Chadron Formation, which hosts over 15 ash layers labeled A-J in chronological order. At Toadstool Park, sedimentation was continuous across the Chadronian-Orellan boundary, which is bracketed by two ash layers, the Upper Purplish White layer (UPW), and the Serendipity ash. Previous radio-isotopic constraints constrain the interval between ashes B and J at Flagstaff Rim to 35.87 and 34.81 Ma according to single-crystal laser fusion ⁴⁰Ar/³⁹Ar sanidine dates obtained by Obradovich (1995), and 36.34 and 35.17 Ma according to Swisher and Prothero (1990) (both recalculated using an age of 28.201 Ma for the Fish Canyon sanidine (Hilgen and Kuiper, 2009)). According to Prothero and Swisher (1992), Ash J is situated just above the base of magnetic polarity zone C13r, ashes D-I are in magnetic polarity zone C15r, while Ash B is in normal polarity zone C16n. At Toadstool Park, the base of polarity zone C13n is 9 meters above the UPW, and the top of C13n is 28 m above the UPW (or 6 m above the Serendipity Ash). Our aim is to obtain high precision U-Pb (zircons) and ⁴⁰Ar/³⁹Ar (sanidine) data from the sampled ash layers in order to better constrain the age of these key terrestrial sections that span the Eocene-Oligocene transition.