Preliminary Geologic Map of Newberry Volcano, Oregon

The late Pleistocene and Holocene rear-arc Newberry Volcano is located in central Oregon east of the Cascades arc axis. Total area covered by the broad, shield-shaped edifice and its accompanying lava field is about 3,200 square kilometers, encompassing all or part of 38 U.S.G.S. 1:24,000-scale quadrangles. Distance from the northernmost extent of lava flows to the southernmost is about 115 km; east-west maximum width is less than 50 km. A printed version of the preliminary map at its intended publication scale of 1:50,000 is 8 ft high by 4 ft wide. More than 200 units have been identified so far, each typically consisting of the lava flow(s) and accompanying vent(s) that represent single eruptive episodes, some of which extend 10’s of kilometers across the edifice. Vents are commonly aligned north-northwest to north-northeast, reflecting a strong regional tectonic influence. The largest individual units on the map are basaltic, some extending nearly 50 km to the north through the cities of Bend and Redmond from vents low on the northern flank of the volcano. The oldest and most distal of the basalts is dated at about 350 ka. Silicic lava flows and domes are confined to the main edifice of the volcano; the youngest rhyolite flows are found within Newberry Caldera, including the rhyolitic Big Obsidian Flow, the youngest flow at Newberry Volcano (~1,300 yr B.P.). The oldest known rhyolite dome is dated at about 400 ka. Andesite units (those with silica contents between 57% and 63%) are the least common, with only 13 recognized to date. The present 6.5 by 8 km caldera formed about 75 ka with the eruption of compositionally-zoned rhyolite to basaltic andesite ash-flow tuff. Older widespread silicic ash-flow tuffs imply previous caldera collapses. Approximately 20 eruptions have occurred at Newberry since ice melted off the volcano in latest Pleistocene time. The mapping is being digitally compiled as a spatial geodatabase in ArcGIS. Within the geodatabase, feature classes have been created representing geologic lines (contacts, faults, lava tubes, etc.), geologic unit polygons, and volcanic vent location points. The geodatabase can be queried to determine the spatial distributions of different rock types, geologic units, and other geologic and geomorphic features. Map colors are being used to indicate compositions. Some map patterns have been added to distinguish the youngest lavas and the ash-flow tuffs. Geodatabase information can be used to better understand the evolution, growth, and potential hazards of the volcano.