Mid-Oligocene age of the Refugian Stage in western Washington: Evidence for diachronous provincial biozones along the northeast Pacific margin

Provincial benthic foraminiferal and molluscan stages have long anchored Paleogene chronostratigraphy along the northeast Pacific Ocean margin, yet their synchrony has not been rigorously tested with independent absolute age control. Our new detrital zircon U−Pb and strontium isotope data from the lower Lincoln Creek Formation in western Washington State (USA) constrain deposition to ca. 30−26 Ma, placing this local Refugian Stage foraminiferal fauna firmly in the mid-Oligocene and ∼7−8 m.y. younger than the Refugian type section in California. These results revise the magnetostratigraphic correlation of provincial Refugian assemblages in the Canyon River reference section from subchrons C15r−C12r to C11r−C8r, a reassignment made possible by independent absolute age constraints unavailable to earlier studies. Published detrital zircon data from the Makah Formation and rocks of Bulson Creek independently corroborate this younger calibration, demonstrating that Refugian-bearing strata across western Washington extend into at least mid-Oligocene time. Together, these results establish that some provincial benthic biozones in the northeast Pacific are diachronous eco-stratigraphic units that reflect persistent paleobathymetric and oceanographic conditions—not synchronous time horizons. Recalibrating the Washington Refugian Stage into the Oligocene shifts the temporal context for associated marine vertebrates, including early Pacific cetaceans and plotopterids, by millions of years. Until the geochronology of other Refugian strata is similarly tested, inferences about faunal origination and evolution near the Eocene−Oligocene transition remain provisional. More broadly, our study demonstrates that provincial biostratigraphic frameworks cannot be reliably linked to the global time scale without independent absolute age control.