A new deposit of the 1.15 million-year-old ash that erupted during collapse of the Kulshan caldera has been discovered near Washtucna in eastern Washington, 350 km (220 miles) southeast of the source. The Kulshan caldera is on the east margin of the Mount Baker volcanic field. The ash is called the Lake Tapps tephra; the lake is near Sumner, the town east of Tacoma where the 20-30 cm ash deposit was first discovered; it was originally described in a 1980 paper by John Westgate and his colleagues, when the source eruption was yet unknown. A 3-cm-thickness of the tephra was also found in Frigid Creek, 90 km west of Sumner in Mason County
The discovery was made by Nick Pearce and his colleagues from Aberystwyth University (Wales) during research into Mount St. Helens ash deposits. They discovered a 15 cm thick ash layer in the Palouse Loess (thick, long-lived windblown deposits). The sample site was already known from earlier paleomagnetic work to be around 1.2 Ma, close to the age of the caldera eruption. Nick reports that the ash was sampled and chemically analyzed. It was found to be an excellent match to the Lake Tapps tephra, which was shown by Wes Hildreth to be the same as the intracaldera deposits. Nick was sent pumice and ash samples from within the caldera to further substantiate his work.
The new find suggests that the caldera tephra is distributed further and more thickly to the east or southeast, rather than to the south. Perhaps other deposits will be discovered by alert geologists in the future. We'll post more details of this research as it progresses, and references when they are published.
Hildreth, W., 1996, Kulshan Caldera: a Quaternary subglacial caldera in the North Cascades, Washington: Geological Society of America Bulletin, v. 108, p. 786-793.
J.A. Westgate, D.J. Easterbrook, N Naeser, R Carson, 1980, Lake Tapps tephra: An early Pleistocene stratigraphic marker in the Puget Lowland, Washington. Quaternary Research, v. 28, p. 340-355.