STRATIGRAPHICALLY COMPREHENSIVE U/PB TIMS ZIRCON GEOCHRONOLOGY OF THE CRB

The Columbia River flood basalt province (CRB) is the youngest and best-exposed large igneous province (LIP) in the rock record. As such, the CRB is an accessible, relatively pristine analog to older, much larger LIPs for which exposure, preservation, and access are less ideal. Numerous studies have constrained CRB extent and volume, yielded a comprehensive chemo-and magneto-stratigraphy, produced models detailing the processes and mechanisms important in province evolution, and proposed a role for CRB emplacement in driving the mid-Miocene climate optimum (MMCO), a period of global climate perturbation. Critical to the accuracy of these models and assessing the likelihood of CRB emplacement in driving the MMCO is the timing and tempo of CRB emplacement. As a result, multiple iterations of radiometric geochronology have built a temporal framework for CRB evolution suggesting emplacement over ~ 2 Ma, with the potential for high-eruption flux periods over the interval from ~17-15 Ma. These age models are predominantly built via the ⁴⁰Ar/³⁹Ar geochronometer on whole-rock CRB samples, which although capable of directly dating the emplacement age of lava flows, lacks the precision necessary to resolve timing/tempo better than about ± 200 kyrs, which in some cases exceeds the emplacement duration of large portions of the province stratigraphy.

Recent work on the CRB and Cretaceous-Paleogene Deccan Traps has produced a protocol wherein zircon crystals isolated from volcanic/volcaniclastic material intercalated between flood basalt flows are dated via the U/Pb TIMS geochronometer. When applied to comprehensive stratigraphic intervals and coupled with Bayesian statistical models, this protocol permits projection of time onto the record of LIP emplacement with unprecedented precision (± 20 kyrs). Following this protocol, we have constructed an age model bracketing stratigraphic intervals from beneath the Steens Basalt Formation to the upper interval of the Grande Ronde Formation, including the Picture Gorge Basalt Formation. We integrate this new U/Pb dataset with existing U/Pb and ⁴⁰Ar/³⁹Ar datasets for the CRB to project time onto the entire CRB stratigraphy. This permits determination of CRB eruption duration, the emplacement tempo of various CRB formations, and comparison of CRB timing to the MMCO.