INDICATIONS OF LOCAL CLIMATE FROM A MIDDLE MIOCENE FOSSIL SITE IN CLE ELUM, WASHINGTON

Several rhyolitic ashfall deposits and lahars of varying sizes in the Yakima River Valley of Central Washington have been classified as part of the middle-late Miocene Ellensburg Formation. This formation, which underlies, intercalates and overlies various members of the Columbia River Basalt Group (CRBG), contains a small flora exposed in a roadcut located between Cle Elum and Ellensburg. This flora, which locally overlies the older Sentinal Bluffs unit of the Grande Ronde Basalt member of the CRBG, was studied to infer aspects of climate of the Middle Miocene in Central Washington. The studied specimens are housed at the Denver Museum of Nature and Science and at the Burke Museum at the University of Washington, Seattle. Morphotyping of the specimens was performed in preparation for climate analysis and to determine their systematic placement. Sixteen distinct angiosperm morphotypes have currently been distinguished, eleven out of which have been identified to the family level and five to the genus level. The flora appears to consist of plants whose modern relatives inhabit different continents, but most taxa identified so far have broad climatic preferences (e.g., Salix) and are therefore less helpful for inferring local climate. This can be compared to previous analyses of contemporaneous floras in Washington and Idaho, which suggest that climate in this region may have been warmer and wetter during the Middle Miocene than today. A clearer indication of local climate will be possible as the morphotypes are further analyzed.