DECIPHERING THE PYROCLASTIC ERUPTIVE HISTORY OF THE THREE SISTERS VOLCANIC CLUSTER, CASCADE ARC, OREGON, USA

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. New field studies and analyses of tephra glasses by EPMA are currently underway. To date, more than 50 field locations have been studied, yielding more than 180 samples. Due to erosion, the most proximal deposits available are preserved predominantly on unglaciated high ridges. Other samples come from scattered exposures at lower elevations as much as 30 km to the east. Utilizing the ARL-SEMQ electron microprobe at Concord University, the glass in most samples has been analyzed for SiO2, TiO2, Al2O3, FeO, MnO, MgO, CaO, BaO, Na2O, K2O, P2O5, and Cl to help identify correlations between tephra deposits. EPMA data reveals more than a dozen distinct geochemical populations, and each of these potentially represents a separate eruption. Geochemical correlations suggest that at least two of these eruptions deposited ash at Carp Lake, Washington, about 200 km to the north-northeast. Five tephra beds (II1, JJ, JJ0.2, G, and E1) at Summer Lake, Oregon, located 170 km to the southeast, also match the chemistries of Three Sisters area tephra deposits. Geochemical affinity and distal tephra distributions also a Three Sisters source for the widespread, ~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.