CLIMATICALLY INDUCED FLORAL CHANGE ACROSS THE EOCENE-OLIGOCENE TRANSITION IN THE JOHN DAY AND CLARNO FORMATIONS, EASTERN OREGON

The Eocene-Oligocene transition marks a major cooling event in Earth history. Effects of this cooling event are well-documented in North American fossil floras where largely paratropical forests of the Eocene were replaced with mixed mesophytic forests by the early Oligocene. While ä18O values of marine benthic foraminiferans show a pronounced increase around 33.5 Ma, fossil floras from the Clarno and John Day Formations in eastern Oregon indicate gradually decreasing mean annual temperature (MAT) values beginning around 40 Ma. Through leaf margin analysis, it is estimated that MAT values decreased ~10°C from 44 to 33 Ma. Along with the decline in temperature, is a marked decrease in floral diversity, especially in the earliest Oligocene Bridge Creek Flora. Taken as a whole, the Bridge Creek Flora appears to be moderately diverse with the occurrence of least 125 species (Meyer and Manchester, 1997). This level of diversity represents the entire Bridge Creek assemblage which is a compilation of taxa found at seven spatially and temporally separated localities. When the Bridge Creek Flora is quantitatively sampled from discrete intervals from individual localities and compared to identically sampled middle Eocene Clarno Formation leaf assemblages, a drastic drop in floral richness from 44 Ma to 33 Ma is indicated.

Reference: Meyer, H. W. and Manchester, S. R., 1997, The Oligocene Bridge Creek Flora of the John Day Formation, Oregon: University of California Publications in Geological Sciences, v. 141, 195p.