DECIPHERING THE PYROCLASTIC ERUPTIVE HISTORY OF THE THREE SISTERS VOLCANIC CLUSTER, CASCADE ARC, OREGON, USA BY INTEGRATING PROXIMAL AND DISTAL TEPHRA RECORDS

The Three-Sisters–Tumalo volcanic region, located in central Oregon, USA, is associated with a relatively active segment of the Cascade volcanic arc. This region’s potential for future eruptions became especially evident to the public as a result of a period of ground deformation and earthquakes in the 1990s to early 2000s. This region is also associated with rhyolitic volcanism. Although several late Holocene (e.g. ~2.2 ka Rock Mesa tephra) to late Pleistocene (e.g. ~300 ka Bend Pumice/Tumalo tuff, and ~ 600ka Desert Springs tuff) pyroclastic deposits are well known, the overall pyroclastic eruptive history of the area remains poorly understood. Recent work in the Three-Sisters–Tumalo volcanic region has obtained stratigraphic and geochemical information on numerous proximal tephras and silicic lavas. To date, more than 50 field locations have been studied, yielding more than 180 samples analyzed by multiple undergraduate students. Utilizing glass geochemistry from the ARL-SEMQ electron microprobe at Concord University, statistical clustering suggests at least 30 and perhaps more than 40 geochemically and stratigraphically distinct populations. This adds substantially to a known catalog of eruptions from this area. At least six tephra beds (II1, JJ, JJ0.2, G, F, and E1) at Summer Lake, Oregon are correlated with their proximal equivalents near Three Sisters. Summer Lake bed F is the ~30-35 ka Wono tephra, a key regional time marker in Oregon, California, and Nevada, which forms a visible tephra bed as much as 500 km distant. The ~200 ka bed, JJ, has previously been correlated (Coleman to Bear Lake, Utah/Idaho) about 850 km distant. At least two Three Sisters tephras correlate to Carp Lake, Washington, about 200 km to the north-northeast.