Abstract

Thick ash-flow tuffs provide marker beds through fossiliferous Eocene and Oligocene marine and non-marine sedimentary rocks near Eugene, Oregon. New mapping, radiometric dating, and paleomagnetic stratigraphy of these tuffs and rocks now allow dating of local fossil floras. The Comstock, Goshen, Rujada, and Willamette floras have been widely used as evidence for Eocene-Oligocene climatic cooling and drying. Eocene leaves from Comstock and Hobart Butte included such thermophilic taxa as Liquidambar. The early Oligocene Goshen flora lacked Liquidambar but retained many thermophilic species with large leaves that have entire margins and acuminate apices (drip tips). In contrast, fossil leaves from later Oligocene Rujada and Willamette floras are small and serrate, and most lack drip tips. Marine faunas also indicate climatic cooling and local disappearance of thermophilic molluscs such as Anadara, Ficus, and Conus. Our dating and compilation of plant and molluscan fossil occurrences indicate a steady rise in species diversity from 46 Ma to maximal diversity of thermophilic taxa at 35–34 Ma, then extinctions of 60% of plant species after 33.4 Ma and 32% of marine invertebrates after 33.2 Ma, both significantly postdating the Eocene-Oligocene boundary at 33.7 Ma. Plant diversity rebounds during the early Oligocene, but marine invertebrates continue to decline into the Oligocene in part due to the retreat of fully marine environments from the Eugene area. Neither these data, nor evidence from coeval fossil plants and soils in central Oregon, support the notion of a “Terminal Eocene Event,” nor any other single, abrupt paleoclimatic shift or extinction. The Eocene-Oligocene biotic and climatic transition was drawn out over some 6 m.y. Abrupt forcings such as meteorite impacts or volcanic eruptions are less likely explanations for cooling and diversity decline than long-term processes such as mountain building, changing ocean currents, or reorganization of the carbon cycle by coevolution of grasses and grazers.

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