A comprehensive chronostratigraphic framework for Oligocene and Miocene fossil plant sites of the Pacific Northwest (USA) from U-Pb zircon geochronology and Bayesian age modeling

A wealth of Oligocene to Miocene fossil floras in the Pacific Northwest of the United States—including Washington, Oregon, and Idaho—offers critical proxy evidence of regional terrestrial environments and climate during times of dynamic regional volcanism, tectonics, and global climate. However, the lack of a well-resolved chronostratigraphic framework informed by high-precision and accurate geochronology inhibits reliable correlation of many fossil floras to other terrestrial and marine proxy records and, thus, limits their use in regional to global climate-model validation. Here, we present new high-precision U-Pb zircon ages of volcanic tephra paired with Bayesian stratigraphic age modeling within comprehensively documented lithostratigraphic sections to assess depositional dynamics and precisely calibrate several key Pacific Northwest fossil floras in numerical time, typically resolved within sub-orbital time scales. These reliably dated fossil floras are most concentrated during the Miocene Climatic Optimum (16.9−14.7 Ma) and across the Middle Miocene Climatic Transition (14.7−13.1 Ma), with fewer in the late Oligocene and Early Miocene. In addition, we precisely correlate fossil floras in numerical time to the development of Pacific Northwest mafic and silicic volcanic fields, including recent revisions to Columbia River Basalt Group geochronology. This new framework gives Pacific Northwest fossil floras greater utility for future studies reconstructing regional plant community and climate response across key intervals of global climatic change, pervasive regional volcanism, and dynamic regional tectonics.