Age, distribution, and stratigraphy of Glacier Peak tephra in eastern Washington and western Montana, United States

Abstract

Tephra layers from Williams Lake Fen, Wildcat Lake, and East Wenatchee, Washington, and Kearns Basin, Lost Trail Pass, Sheep Mountain Bog, and Marys Frog Pond, Montana, were analyzed by electron microprobe (EMP), and associated lake deposits were radiocarbon dated. Though the tephra layers can be grouped by source (Glacier Peak, Mount Mazama, Mount Saint Helens, and unknown source), statistical analyses of both glass and mineral compositions show that finer distinctions within a group (for example, Glacier Peak B, M, and G) cannot be made on the basis of chemical data obtained using conventional EMP techniques. It appears that more-sensitive analytical techniques may be needed to discriminate among the Glacier Peak tephras. Tephra stratigraphy at the various sites reveals a potentially greater complexity in Glacier Peak tephra distributions and ages than was anticipated. All sites, except Sheep Mountain Bog and East Wenatchee, contained two Glacier Peak tephras. Taken as a whole the Glacier Peak tephra layers may record closely timed, multiple eruptions with restricted ash falls as well as widespread tephra from large eruptions. Radiocarbon dating generally confirms a 14C age of 11 200 years BP for a distal Glacier Peak couplet(s) that occurs, stratigraphically, both above and below Mount Saint Helens J tephra in east-central Washington.