MAPPING OF MAJOR LATEST PLEISTOCENE TO HOLOCENE ERUPTIVE EPISODES FROM THE GLACIER PEAK VOLCANO, WASHINGTON—A RECORD OF LAHARIC INUNDATION OF THE PUGET LOWLANDS FROM DARRINGTON TO THE PUGET SOUND

Volcanic hazards in the Pacific Northwest are typically associated with more visible stratovolcanoes such as Mt. Rainier. Glacier Peak, a dacitic stratovolcano near Darrington, Washington in the North Cascades, has produced at least three large eruptive episodes since the culmination of the last continental glaciation with each episode likely lasting a few hundred years. Data from recent detailed geologic mapping, new ¹⁴C ages, stratigraphic relations, flow and lahar clast petrographic and geochemical analyses, and laharic sand composition indicate that three large eruptive episodes occurred in the latest Pleistocene and Holocene—the information refines and expands on the pioneering work of Beget (1981). These voluminous events and traveled up to 135 km down valley of the edifice and reached the Puget Sound via the ancient Skagit and Stillaguamish deltas. Glacier Peak has erupted dacite of similar composition throughout its history. Hypersthene-hornblende-(augite)-phyric vesicular lahar clasts found from Glacier Peak to La Conner, Washington have similar geochemistry to dacite flows sample on the volcano. Radiocarbon dating of the White Chuck assemblage shows an eruptive episode of ~11,900 yrs B.P. This implies ice occupation during part of the White Chuck event and is further supported by mapped deposits of the White Chuck assemblage that directly overlie recessional outwash along high inset terraces. Charcoal obtained from the Kennedy Creek assemblage yielded ¹⁴C ages of ~5,000 yrs B.P. Near La Conner, Washington, the Kennedy Creek lahar run-out is locally interbedded with mid-Holocene estuarine and deltaic sediments of the Skagit delta. The Chocolate Creek assemblage yielded many ¹⁴C ages of ~1,800 yrs B.P.. This assemblage occurs as the lowest and most inset lahar terrace near Darrington. Log-bearing flood deposits near Padilla Bay are 120 km from the edifice and contain dacitic sands and silts and may be the most distal deposits of this complex. Many cities down valley of Glacier Peak are built on river terraces comprised of lahar, lahar run-out (hyperconcentrated flood deposits) and volcanic alluvium. Although Glacier Peak is generally not visible from urban areas, it is an active volcano that periodically erupts in an explosive manner and could affect several downstream communities.