The early Oligocene Willamette flora of the Little Butte Volcanics in western Oregon provides a classic example of the mixed broadleaf evergreen and coniferous forests that developed in the lowlands of the Pacific Northwest immediately following the major temperature decline of the latest Eocene to earliest Oligocene. Foliar physiognomic analyses of this flora using J. A. Wolfe’s Climate-Leaf Analysis Multivariate Program (CLAMP) and Leaf Margin Analysis (LMA) tests Wolfe‘s earlier climatic interpretation of the Willamette flora. These earlier estimates indicate low cold season temperatures and high warm season temperatures which appear incompatible with the presence of diverse conifers and many thermophylic lineages seen in the Willamette flora. Our study of substantially large collections from the Willamette flora produce climate estimates more compatible with the presence of conifers and thermophyls.

In conjunction with recently gathered paleontological and magnetostratigraphic data (Retallack and Prothero, in prep), the correlation and dating, using high resolution 40Ar/39Ar geochronology, of three late Eocene/early Oligocene tuffs near Eugene, Oregon, Bond Creek (34.85±0.22 Ma), Spores Point (31.27±0.57), and an unnamed tuff stratigraphically above the Willamette flora (30.64±0.51 Ma) has resulted in the re-calibration of regional biostratigraphic correlation schemes. This study has resulted in a better understanding of the rate and magnitude of climate and vegetational change across the Eocene/Oligocene boundary.