THREE SISTERS VOLCANOES, OREGON, A POTENTIALLY SIGNIFICANT TEPHRA SOURCE

Volcanic ash and pumice (tephra) deposits provide important records of past volcanism. By understanding the eruption history and processes of volcanoes, better estimates can be made about eruptions. Volcanic ash beds can also provide important time-markers or studies including archeology, earthquake hazards, and long-term environmental change. Lakes downwind of volcanic regions often preserve numerous tephra layers. At Summer Lake, Oregon, for example, as many as half of the distal tephra layers do not yet have source identification, implying that proximal eruption records at source volcanoes are incomplete. One of these distal beds is the widespread, ~ 25-35 ka Wono tephra, which is a key regional time marker in Oregon, California, and Nevada. Thickness and grain size distributions appear to point to the Three Sisters region as a potential source for the Wono tephra and perhaps also other distal tephra beds. The Three Sisters volcanoes, located in an active part of the Cascade Range in central Oregon, have a relatively unstudied pyroclastic history.

We obtained archival samples collected from 6 different locations by W. E. Scott (USGS CVO) in 1983 and 1997. Using the electron microprobe at Concord University, the glass in 19 samples was analyzed for SiO2, TiO2, Al2O3, FeO, MnO, MgO, CaO, Na2O, K2O, P2O5, Cl. The resulting geochemical data forms 11 populations, suggesting an equivalent number of eruptions. Five of these eruptions can be correlated between at least two proximal localities. Three of these match distal rhyolitic tephras at Summer Lake. For example, two samples collected from Little Brother ridge matched a sample from western Broken Top and bed G at Summer Lake. Two samples from Newberry Ridge at South Sister matched two samples from western Broken Top with the same stratigraphic order. Samples from Bottle Creek and the East Tumalo cinder pit match the rhyolitic component of three layers at Summer Lake, including the JJ tephra bed. A single sample from western Broken Top matches Summer Lake bed G. Multiple samples overlap with the broader compositional range of the Wono tephra, but none matched perfectly. This similarity suggests a Three Sisters area source but is not conclusive. If corresponding proximal tephra can be located, there is potential to obtain better materials for dating this important marker bed.