COUPLING ORBITAL AND RADIOISOTOPIC CHRONOMETERS: LESSONS LEARNED FROM THE CRETACEOUS WESTERN INTERIOR BASIN (U.S.A.)

The Cretaceous Western Interior Basin (WIB) preserves an extraordinary record of orbitally-influenced sedimentation, and also contains volcanic ashes that provide most of the radioisotopic ages that underpin the upper Cretaceous time scale, rooted in the pioneering ⁴⁰Ar/³⁹Ar geochronology of Obradovich (1993). Following recent advances in U-Pb geochronology, ⁴⁰Ar/³⁹Ar geochronology and astrochronology, reevaluation of the WIB deposits provides an opportunity to dramatically refine the Cretaceous time scale, while also allowing a comparison – and ideally an intercalibration – of these high-resolution techniques. To accomplish this goal, we have generated U-Pb (CA ID-TIMS) and ⁴⁰Ar/³⁹Ar (laser fusion) data from zircons and sanidines in WIB bentonites spanning the Cenomanian-Campanian stages, and have evaluated them within the context of floating astrochronologies that encompass ~9 Ma of time. The radioisotopic analyses utilize newly collected material, as well as legacy sanidine samples of Obradovich (1993), from more than 20 individual volcanic ashes in the WIB. For some bentonites, we have analyzed material from several locations throughout the WIB, to assess the temporal correlation from the perspective of both sanidine and zircon. Our approach considers sources of uncertainty associated with the radioisotopic ages (analytical, tracer solution or age of neutron fluence standard, decay constants), including geologic uncertainties (e.g., magma residence, inheritance, stratigraphic correlation), and uncertainties in the astrochronology. Results highlight the strengths and weaknesses of each geochronometer, and illustrate strategies for coupling them to resolve shortcomings and ambiguities associated with the individual methods. Overall, the new data support the astronomically-calibrated age of 28.2 Ma for the Fish Canyon sanidine standard (Kuiper et al., 2008), and result in new stage boundary ages with 2σ total uncertainties (radioisotopic/geologic/astrochronologic) that range from ~2-5 per mil. This presentation will examine strategies for deploying the WIB “floating” orbital time scales to better constrain radioisotopic ages (for individual bentonites) and stage boundary ages, as well as approaches for optimally “anchoring” the astrochronologies in time.