PALEOENVIRONMENT OF A NEWLY DISCOVERED MIDDLE MIOCENE "BOG" LOCALITY, COLUMBIA RIVER BASALTS, NEAR ELLENSBURG, WA: A PRELIMINARY REPORT

A new locality of silicified “bog” material has been discovered in the Columbia River Basalts approximately 24 km southeast of Ellensburg, Washington near Interstate Highway I-82. The site is similar to the permineralized flora we have studied at the County Line Holes in nearby Yakima Canyon. Basalt geochemistry indicates that the I-82 site is slightly younger than those at Yakima Canyon. The deposit is intriguing because, unlike the Yakima Canyon flora which is known from small discontinuous pods, this flora is preserved as a solid block ca 1 meter thick of sideromelane and palagonite that shows the transition in preserved plant material from top to bottom as well as the expected alternation rind associated with basaltic glass deposits. Changes in floristic composition appear to correlate with vertical position within the matrix. At the top of the beds is a highly opalized translucent matrix that contains seeds and occasional twigs of the aquatic plant Decodon Gmelin.(Lythraceae), a taxon rare in the Yakima Canyon flora. This layer more closely resembles material we have studied previously from the Middle Miocene Cessily Anne locality in the Virgin Valley of northwestern Nevada, a flora which also contains abundant Decodon. Lower layers of the I-82 site are dominated by taxodiaceous conifer remains and Nyssa endocarps, but otherwise are floristically less diverse than Yakima Canyon. We suggest that the differences in floristic composition coupled with preservation may reflect in part differences in sampled paleoenvironment. The processes that led to plant material being preserved in basaltic quenched glass are not yet understood. Based strictly on the elements of the flora preserved, the various sites in Washington State appear to sample a small fragment of the wetland vegetation in the Miocene.