DESERT SPRING TUFF, CENTRAL OREGON CASCADE RANGE: NEW PERSPECTIVES ON SOURCE, GENESIS, AND HAZARDS

The circa 600 ka Desert Spring Tuff is the oldest of three voluminous Brunhes age ash-flow tuffs in central Oregon. The tuff is generally pink and vapor-phase altered, but some quarry exposures are fresh, and there the tuff is grayish-brown, with sparse dark rhyodacite pumice and two obvious flow units. Thickness is variable, from < 5 to > 30 m, mostly due to deposition on irregular, faulted terrane. Also, the base is usually unexposed, and thus the original volume is difficult to estimate, but must be on the order of 1-3 km³. The present outcrop area extends NNW from the vicinity of Bend, OR on both sides of Hwy 20 nearly to Hwy 126, a distance of approx. 25 km. The likely source of the tuff is a dome/lava complex poorly exposed in the south wall of Tumalo Creek, near Tumalo Lake, about 19 km WSW of Bend. The bulk, glass, and mineral chemistries of the dome and the tuff are indistinguishable. Both contain abundant phenocrysts of large plagioclase (An_30-46), two pyroxenes, two oxides, and apatite. The Tumalo Lake exposures seem to be near the base of the Brunhes section, so their relative age is likely similar. There is a small but significant chemical variation in the bulk pumice, from 67.8 to 69.2 wt% SiO₂, with concomitant changes in other elements (e.g. CaO, Sr, TiO₂, etc.), probably a reflection of minor magma mixing. The ⁸⁷Sr/⁸⁶Sr ratio of the tuff is 0.7038, consistent with a genesis chiefly involving crustal melting. Clearly a repeat of the Desert Spring eruption would devastate Bend and much of central Oregon, but our present knowledge is not complete enough to fully evaluate this potential hazard. Current mapping and dating suggest that volcanism has waned on the Bend Highland in the past 200-300 ka, but further work is needed to track eruption rates during Brunhes time. If magma supply has indeed declined with time, perhaps the present hazard is less, as exemplified by small Holocene, dome-fed, relatively cool high-silica rhyolitic pyroclastic eruptions around the South Sister. If, however, future dome emplacement (the growing bulge?) near the South or Middle Sister involves hotter dacitic or rhyodacitic magma similar to that of the Desert Spring, even a modest magma supply could lead to a very hazardous eruption.