Vegetation, climate, and age of the mid-Miocene Mascall flora reinvestigated: leeward environments of the Cascade Mountains

Abstract

The Mascall Formation of eastern Oregon preserves a rich fossil flora, offering crucial insight into Miocene vegetation and climate in the Pacific Northwest (PNW). However, a continuous age-depth model for the formation is lacking, limiting correlation with regional tectonic and global climate trends. Additionally, it is unclear whether the humid forests previously inferred from the Mascall flora reflect widespread regional vegetation or localized lakeside habitats, leaving the timing of forest-to-open habitat transitions in the PNW unresolved. To address these gaps, we present new U-Pb geochronology and Bayesian age-depth modeling, paired with a multiproxy analysis of plant macrofossils, phytoliths, and palynomorphs, and a review of taphonomic literature, to reconstruct regional vegetation and climate. A dated ignimbrite at White Hills (16.131 ± 0.011 Ma) helps constrain the base of the formation, and age modeling enables precise correlation with Miocene global climate events. Fossil evidence indicates deciduous angiosperm-dominated closed-canopy forests under warm temperate, humid conditions with minimal summer drought. Phytoliths additionally reveal that thermophilic palms and gingers and grassy patches existed in the overall forested landscape. These results contrast with more open contemporaneous habitats in the Great Plains and Rocky Mountains and suggest a unidirectional open habitat transition in the PNW was delayed, occurring after deposition of the Mascall flora. The persistence of a humid-adapted flora east of the Cascades also implies the range had not yet reached elevations sufficient to generate a prominent rain shadow. This study demonstrates the value of multiproxy approaches in resolving Miocene climate and vegetation structure in the PNW.