DISCOVERY OF ASH IN SEDIMENTS AROUND THE LAS VEGAS VALLEY: IMPLICATIONS FOR FUTURE ASHFALL HAZARDS FROM DISTAL VOLCANOES
The Las Vegas Valley in Nevada has no active volcanoes nearby but could potentially be affected by eruptions from distal volcanoes. This is highlighted by the recent discovery of several ca. 9-9.5 Ma ash layers preserved in a half-graben south of Las Vegas, in the northern McCullough Range. Dubbed the Pipeline Ashes, the three rhyolitic ashfall deposits comprise a section approximately 2.5-3 m thick. The lower white ash, undated, is ~0.3 m thick and correlates to the Southwest Nevada Volcanic Field (SNVF). The middle, silvery-gray, ash (9.31 Ma) is 1.5-2 m thick and correlates to a source in the Yellowstone-Snake River Plain (YS-SRP) hotspot. The upper white ash, undated, is 0.3-0.5 m thick and correlates to the SNVF. Ashes from YS-SRP and SNVF have been documented in sedimentary packages as young as 6 Ma in northwestern Arizona and southern Nevada. Additionally, cryptotephra layers tentatively correlated to the 0.76 Ma Bishop Tuff (Long Valley caldera, CA) and the 32 ka Wilson Creek 15 ash (Mono Craters, CA) were discovered in Pleistocene spring deposits at Whitney Mesa and Tule Springs.
The three pipeline ashes, as well as the Whitney Mesa cryptotephra layer, likely came from caldera-forming eruptions. However, the Wilson Creek 15 ash was produced by a much smaller eruption. That all of these ashes, especially Wilson Creek 15, fell on what is now a major metropolitan area should convince local governments to consider planning for the possibility of ashfall from distal volcanoes.