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

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• 作者：Georgina E. King^a ; ^{georgina.king@gmail.com" class="auth_mail" title="E-mail the corresponding author} ; Nicholas J.G. Pearce^a ; Helen M. Roberts^a ; Victoria C. Smith^b ; John A. Westgate^c ; David R. Gaylord^d ; Mark R. Sweeney^e
• 关键词：Kulshan caldera ; Lake Tapps tephra ; Palouse loess ; Trace element geochemistry
• 刊名：Quaternary Research
• 出版年：2016
• 出版时间：September 2016
• 年：2016
• 卷：86
• 期：2
• 页码：232-241
• 全文大小：1333 K

文摘

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.