Cenozoic Paleobotany of the John Day Basin, Central Oregon
The Geological Society of America Field Guide 15 2009 Cenozoic paleobotany of the John Day Basin, central Oregon Richard M. Dillhoff Thomas A. Dillhoff Evolving Earth Foundation, P.O. Box 2090, Issaquah, Washington 98027, USA Regan E. Dunn Department of Biology, University of Washington, Box 351800, Seattle, Washington 98195, USA Jeffrey A. Myers Department of Earth Sciences, Western Oregon University, 345 N. Monmouth Ave, Monmouth, Oregon 97361, USA Caroline A. E. Strömberg Department of Biology & Burke Museum of Natural History and Culture, University of Washington, Box 353010, Seattle, Washington 98195, USA ABSTRACT The John Day Basin of central Oregon contains a remarkably detailed and well- dated Early Eocene–Late Miocene sedimentary sequence, known for its superb fos- sils. This fi eld trip examines plant fossil assemblages from throughout the sequence in the context of their geological and taphonomic setting and regional and global signifi cance. The Early to Late Eocene (>54–39.7 Ma) Clarno Formation contains fossil plants and animals that occupied an active volcanic landscape near sea level, interspersed with meandering rivers and lakes. Clarno assemblages, including the ca. 44 Ma Nut Beds fl ora, record near-tropical “Boreotropical” rainforest, which was replaced during late Clarno time by more open and seasonal subtropical forest. The overlying John Day Formation (39.7–18.2 Ma) was deposited in a backarc landscape of low hills dotted with lakes and showered by ashfalls from the Western Cascades. Fossils and paleosols record the advent of the “Icehouse” Earth during the earliest Oligocene, with decreasing winter temperature and more seasonal rainfall that sup- ported open deciduous and coniferous forest, much like that of the southern Chinese highlands today.
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