DISTAL CORRELATION OF NEWBERRY VOLCANO TEPHRAS AND UPDATED TEPHRA STRATIGRAPHY FOR THE SUMMER LAKE SUB BASIN OF PLUVIAL LAKE CHEWAUCAN

Newberry volcano has a long history of silicic pyroclastic volcanism and has produced many extensive tephra deposits. During the last approximately 550,000 years, the volcano has erupted at least 60 rhyolitic and dacitic tephras that record a range of eruptive styles. Glass compositional variations in these tephra units have been characterized by electron microprobe and compiled into a database containing more than 8,600 individual point analyses. Potential distal ash correlations have been established by using the similarity coefficient of Borchardt et al. (1971 and 1972) in conjunction with discriminant function analysis. Similarity coefficients for correlated tephras range from 0.95 to 0.99 and correspond to probabilities of 90-100%.

The Holocene Newberry Pumice correlates to at least two distal locations in Oregon and three that span central Idaho. This distribution suggests that recognizable distal deposits are likely to exist in western Montana. The Pleistocene Paulina Creek tephra is the source of the Olema ash bed and correlates to at least two sites in Oregon and several locations in northern California. Tephra 9917C is highly similar to Pringle Falls tephras K and D. Lava Pass tephra and three related deposits are highly similar to a distal ash preserved at a depth of 142 m in the Knolls core in the Bonneville basin, Utah.

More than 75 individual tephra beds are intercalated with Pleistocene lacustrine sediments at Summer Lake, Oregon. More than 50 are exposed in the Ana River (AR) outcrop section. Others are known from the Wetland Levee (WL) and Bed and Breakfast (BB) cores. Similarities in glass compositions suggest that at least 14 of the tephra beds may originate from Newberry volcano. Glass in Newberry tephras Paulina Creek, Ice Quarry, 9912D, 9920C, 984F, 984G5, 978D, 9917C, and 9881C tephras are highly similar to ash layers at Summer Lake. Paulina Creek correlates to WL-7-2. Ice Quarry correlates to bed AR-2/WL-9-4. 9912D is similar to AR-N1 (equivalent to WL-37-I and WL-37-II), AR-N, and AR-M. 9920C is similar to AR-Q/WL-37-IV and AR-P/WL-37-III. 984F correlates to AR-S. 984G5 correlates to AR-T/WL-38-3. 978D is similar to AR-V. 9917C is similar to AR-DD, FF, and GG. 9881C is similar to AR-NN. In addition, two tephras of uncertain source found at Summer Lake, the Wono tephra and AR-G, are present at Newberry.