SOURCE ROCK FOR OBSIDIAN PROJECTILE POINTS, NEVADA TEST SITE

The bedrock source for obsidian used to make prehistoric tools found around Yucca Mountain has long been sought. Secondary deposits of unmodified obsidian nodules are found in Fortymile and Topopah Washes and dispersed across alluvial terraces. Analyses of these nodules and prehistoric tools have shown a clear geochemical similarity. Recent geologic mapping revealed a potential bedrock source for the obsidian within the Rhyolite of Shoshone Mountain, which lies adjacent to Fortymile and Topopah Washes. This formation comprises four lava flows and four pyroclastic flow deposits, a central intrusion and several dikes. Obsidian xenoliths entrained within tuff "C" macroscopically resemble the unmodified cobbles found around Yucca Mountain. None of the various vitrophyres associated with the lavas and intrusions bear a similar resemblance. New geochemical data derived from XRF show the obsidian xenoliths are compositionally identical to the Rhyolite of Shoshone Mountain, and were likely incorporated into tuff C. Comparison of these geochemical data with XRF analysis of obsidian projectile points recorded throughout the NTS shows that 29 percent of the points are composed of obsidian from tuff C of the Rhyolite of Shoshone Mountain. Differing projectile point forms have been stratigraphically correlated to broadly limited time frames, and the sample of projectile points spans the entire Holocene. Therefore, identifying the bedrock source for these nodules allows archaeologists to address toolstone availability or abundance when developing inferences regarding changing land use patterns during the Holocene.