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Identification of a Kulshan caldera correlative tephra in the Palouse loess of Washington State, northwest USA

King, Georgina E., Pearce, Nicholas J.G., Roberts, Helen M., Smith, Victoria C., Westgate, John A., Gaylord, David R., Sweeney, Mark R.
Quaternary research 2016 v.86 no.2 pp. 232-241
crystallization, lakes, lava, loess, streams, tephra, trace elements, Washington (state)
The Kulshan caldera formed at ∼1.15 Ma on the present-day site of Mt. Baker, Washington State, northwest USA and erupted a compositionally zoned (dacite-rhyolite) magma and a correlative eruptive, the Lake Tapps tephra. This tephra has previously been described, but only from the Puget Lowland of NW Washington. Here an occurrence of a Kulshan caldera correlative tephra is described from the Quaternary Palouse loess at the Washtucna site (WA-3). Site WA-3 is located in east-central Washington, ∼340 km southeast of the Kulshan caldera and ∼300 km east-southeast of the Lake Tapps occurrence in the Puget Lowland. Major- and trace element chemistry and location of the deposit at Washtucna within reversed polarity sediments indicates that it is not correlative with the Mesa Falls, Rockland, Bishop Ash, Lava Creek B or Huckleberry Ridge tephras. Instead the Washtucna deposit is related to the Lake Tapps tephra by fractional crystallisation, but is chemically distinct, a consequence of its eruption from a compositionally zoned magma chamber. The correlation of the Washtucna occurrence to the Kulshan caldera-forming eruption indicates that it had an eruptive volume exceeding 100 km³, and that its tephra could provide a valuable early-Pleistocene chronostratigraphic marker in the Pacific Northwest.