NEWLY DISCOVERED, LARGE SCALE, VOLCANIC DEBRIS AVALANCHE, DONNER PASS REGION, SIERRA NEVADA, AND ITS RELATION TO UNDERLYING TERTIARY VOLCANICLASTIC SUCCESSION

Recent geologic mapping by UCSB students disclosed widespread exposures of a massive debris avalanche in a 30 km² area in the Donner Pass region approximately 15km west of Truckee, California. The main foci of the mapping were the peaks and ridges around Mount Lincoln, Mount Disney, and Crow’s Nest. The avalanche fills an apparent paleochannel in the underlying, flat-lying, volcaniclastic succession of fluvial and debris flow deposited units. The debris-avalanche is poorly sorted, containing various volcanic clasts and clast sizes lacking coherent internal organization. Clasts include fractured andesite breccia, polylithic andesite breccia, stratified fluvial blocks, reworked tuff, granite, basaltic andesite flow blocks, and metasedimentary clasts. Clast size ranges from 1cm to 25m. Mixed within the bulk of the debris avalanche are large subrounded blocks of basaltic-andesite with a distinctive breadcrust texture. We regard these clasts as juvenile rocks and evidence of an eruption-related trigger to the avalanche. K-Ar dates from lava flows stratigraphically above and below the debris avalanche give an age range of 13.5/12.9 to 7.4 Ma. The exposed depth of the paleochannel, which trends approximately east to west, varies from 3m on Mt. Lincoln to 280m on Mt. Disney. Clast size increases from NW to SE, suggesting a source that may include Tinker Knob or Squaw Peak 6 and 12 km to the SE, respectively. The units underlying the debris avalanche are stratigraphically continuous across Mt. Lincoln, Mt. Disney, and Crow’s Nest, and comprise a stratified section of intercalated lapilli tuff and fluvially reworked volcaniclastic rocks. Lapilli tuff units consist of matrix-dominated, polylithic breccia. Fluvial units consist of pebble and boulder conglomerate and pebbly sandstone. The lithology of volcanically derived units includes clasts of hornblende andesite, pumice, cinders and metasedimentary rock and granite, the local basement rocks. Paleocurrent indicators lower in the section record a north-south channel direction, whereas flame structures upsection indicate a NW paleocurrent direction.