HR: 1340h
AN: V43E-1452    [Abstracts]
TI: New Thoughts About Newberry Volcano, Central Oregon USA
AU: * Donnelly-Nolan, J M
EM: jdnolan@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025 United States
AU: Champion, D E
EM: dchamp@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025 United States
AU: Lanphere, M A
EM: alder@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025 United States
AU: Ramsey, D W
EM: dramsey@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025 United States
AB: New geologic mapping, geochemistry, argon dating, and paleomagnetic data suggest some significant new ideas about the history and evolution of Newberry Volcano. Previous mapping (MacLeod et al., 1995) made use of K-Ar dating done in the 1970's. A new $^{40}$Ar/$^{39Ar}$ age of 83$\pm$5 ka for the rhyolite of Paulina Peak, a thick lava flow cut by the southern caldera-bounding fault and previously thought to be hundreds of thousands of years old, indicates that the caldera formed since $\sim$83 ka. The ash-flow tuff (Qat) that forms the western rim of the caldera contains rhyodacitic pumices with unusually high Na2O (6.1 wt.%), identical in composition to a subset of pumices found in the underlying and more widespread basaltic lapilli tuff (Qbt), suggesting that these two ash-flow tuffs represent parts of the same eruptive event. Their paleomagnetic directions are very similar. Lapilli identical in composition and appearance to those in the Qbt are found in gravel underlying basalt that has been dated at about 80 ka. Taken together, the data suggest that the caldera formed about 80 ka when Qbt and Qat erupted. An earlier caldera likely formed about 300 ka, the approximate age of each of several mapped rhyolitic to dacitic ash-flow tuffs on the eastern side of the volcano, one of which was previously thought to be $>$500 ka in age. Preliminary paleomagnetic data suggest that these ash-flow tuffs have similar remnant directions. Previous identification (Kuehn, 2002) of some 40-50 Pleistocene tephras indicates either that Newberry is far more explosive than previously thought or that many of these tephras (of which no more than 6 occur in any single hand-dug pit) may represent different lobes of tephra generated by compositionally-zoned caldera-forming eruptions. The widespread Olema ash bed (a centimeter thick in the San Francisco Bay Area, $\sim$700 km SSW of Newberry Volcano) may represent one lobe of the $\sim$80-ka caldera-forming event. In the course of geologic mapping, 1- to 2-meter-sized boulders of a variety of lava flow types have been found on the tops of cinder cones at elevations as low as 5782 feet. Interpreted as glacial erratics, they suggest that ice extended far down the flanks of the volcano, rather than forming only very small cirques and limited moraines at the caldera rim (highest elevation 7984 feet), as previously thought. Large areas of mixed pumice and gravel at higher elevations may represent till that grades into outwash gravel at lower elevations. Periodic outburst floods from lakes formed in or beneath the ice cap (possibly linked to eruptive activity?) would explain the large channels that are so abundant on the eastern and, to a lesser extent, on the western (and younger) side of the volcano. As many as half a dozen postglacial ($<$$\sim$12 ka?), but pre-Mazama ($\sim$8 ka), basaltic lava flows occupy several of the eastern and southeastern dry channels. The dry channels on the NE side appear to drain into the canyon of Dry River, a large, dry box canyon several hundred feet deep, which may have been carved by multiple large floods originating primarily from Newberry Volcano.
DE: 8404 Ash deposits
DE: 8499 General or miscellaneous
DE: 3640 Igneous petrology
DE: 1821 Floods
DE: 1863 Snow and ice (1827)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting