HIGH-PRECISION 40AR/39AR GEOCHRONOLOGY IN THE LOWER FORT UNION FORMATION (MONTANA, USA): INSIGHTS INTO FLUVIAL RESPONSES TO ORBITAL CLIMATE FORCING AND GEOLOGIC TIME SCALE CALIBRATION OF THE EARLY PALEOCENE

High-precision ⁴⁰Ar/³⁹Ar geochronology provides a key method to date fluvial sedimentary successions and hence offer the opportunity to distinguish between timescales of autogenic and allogenic controls on sedimentation. Moreover, the development of detailed chronostratigraphic frameworks from these successions have also the potential to provide a robust calibration of the Geomagnetic Polarity Time Scale (GPTS), therefore allowing accurate correlations between the marine and terrestrial realms.

The Tullock and Lebo Members in the lower Fort Union Formation, exposed in the Western Interior Williston Basin of eastern Montana (USA), represent an ideal setting to collect detailed chronostratigraphic data for the early Paleocene. Generally the lower Fort Union Formation consists of sandstones and siltstones interbedded with several lignite coal seams. The sequence preserves a reliable paleomagnetic signal (e.g. Sprain et al., 2016). The coal seams, traceable over distances of kilometers, contain multiple volcanic ashfall layers (tephras), generally sanidine-rich, suitable for ⁴⁰Ar/³⁹Ar radio-isotopic dating.

Here, we present new high-precision ⁴⁰Ar/³⁹Ar radio-isotopic ages of single and multiple sanidine grains separated from tephra samples, collected at several locations in eastern Montana. Integration of the radio-isotopic ages with existing litho- and magnetostratigraphic records provides a high-resolution time frame for the early Paleocene. Magnetostratigraphic records reveal the presence of polarity reversals comprised between Chron C29r and C28n, consistent among different sections. The presented ⁴⁰Ar/³⁹Ar geochronological data provides new absolute age constraints for these polarity reversals and the associated Chron durations, allowing the re-calibration of the GPTS. Moreover, our high-precision geochronology provides additional age control to test the impact of orbitally-forced climate changes on fluvial sedimentation (Noorbergen et al., 2018).