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

Journal article

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) in east-central Washington. Site WA-3 is located ~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 this tephra could provide a valuable early-Pleistocene chronostratigraphic marker horizon for the Pacific Northwest, which may have had an eruptive volume of ~124 km3, considerably larger than previously thought.

Authors: King, G; Pearce, N; Roberts, H; Smith, V; Westgate, J

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Elsevier
• Journal: Quaternary Research
• Volume: 86
• Issue: 2
• Pages: 232-241
• Publication date: 2016-08-01
• DOI: 10.1016/j.yqres.2016.06.004
• ISSN: 0033-5894
• Keywords: Kulshan caldera; trace element geochemistry; Palouse loess; Lake Tapps tephra