Dillhoff, Thomas .
A Miocene forest assemblage from the Columbia River Basalts of Washington State, USA.
FOSSIL wood has been known from localities in the Miocene Columbia River Basalt Group since the early 20th century, however early collectors seldom recorded sufficient locality data to allow detailed studies of individual sites. A new locality near Yakima, Washington, with standing silicified tree trunks in situ has been excavated over a period of several years. The locality consists of a basalt flow up to seven meters thick underlain by a clay layer of unknown thickness. Over 150 silicified tree trunks have been found within the flow. The logs are nearly all preserved in a vertical orientation and extend from the basalt/clay layer contact to the erosional surface at the top of the basalt flow. There is no indication that tree roots penetrate into the clay layer. On the whole, the wood is well preserved and an initial survey has been conducted to identify the wood type. The results of the survey indicate that the assemblage consists of approximately 70% Carya, 20% Ulmus, plus minor amounts of Acer and Liquidambar. Geochemical analysis of the basalt places the deposit within the Frenchman Springs Member of the Wanapum Basalt Formation, with an approximate age of 15 ma. The presence of basalt pillows and palagonite in the flow indicates that the basalt was deposited in a water environment. The preliminary interpretation is that the locality represents a forest assemblage which was flooded by water, and then subsequently inundated by lava.[c.e.:srb]
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1 - Evolving Earth Foundation, PO Box 2090, Issaquah, Washington, 98027, USA
Presentation Type: Poster
Location: Auditorium/Bell Memorial Union
Date: Tuesday, August 1st, 2006
Time: 12:30 PM