NEW DETAILS FROM THE TEPHRA STRATIGRAPHY AT SUMMER LAKE, OREGON, A SUB-BASIN OF PLUVIAL LAKE CHEWAUCAN

Sediments at Summer Lake, Oregon preserve an outstanding tephra sequence which has provided age control for several paleomagnetic, paleoclimate, paleoenvironmental, and tectonic studies in the basin and for the eruptive history of Newberry Volcano. Previous work (e.g. Davis, 1985; Negrini et. al, 2000; Sarna-Wojcicki et. al, 2001) has identified 54 tephra beds ranging from about 15 to 250 ka in age exposed in as much as 16 m of section in canyon walls along the Ana River and additional tephra beds from three sediment cores.

Recent re-excavations of outcrop locations E and F along the Ana River and examination of three additional cores (AR-C, SPG-A, and SPG-B) has revealed many previously undescribed tephra beds. The 31 m AR-C core was collected in 1992 adjacent to outcrop site C and parallels a much deeper (64 m) core collected nearby by the U.S.G.S. in 1986. The 32 m SPG-A and 36 m SPG-B cores were also taken in 1992 about 3 km to the southeast. At total of 68 tephra beds have been recognized during the re-excavation of the two outcrop locations. Published glass compositions are available for only 33 of these. The lower half of the AR-C core includes at least 30 tephra beds below tephra NN, the lowermost tephra in the outcrop sequence. The upper 2/3 of the SPG cores replicates much of the outcrop sequence (top missing) and adds several tephra beds. The lower portion includes at least 24 tephra beds below tephra NN. Major-element analyses of the tephra glasses by electron microprobe are in progress.