LITHIC SOURCE MATERIAL AND BEDROCK GEOLOGY OF THE SOUTHERN GREAT BASIN

Stone was used to fabricate projectile points and other tools by the aboriginal peoples of the Great Basin, Nevada and California. These objects are found throughout the Great Basin and document Native American land use patterns. However, the original bedrock sources for many of these lithic artifacts have often proven illusive. Identifying the primary source area for stones most frequently found in secondary context is invaluable when reconstructing and refining migration and trade patterns of native groups. To this end geologists have been working with archeologists at the Nellis AF Range and the Nevada Test Site (NTS) to identify bedrock sources for lithic tools, and to develop search criteria, maps, and databases to assist other archeological workers to continue this type of research in the southern Great Basin. Geologic maps of potential source rock for chert, obsidian, chalcedony, and welded tuff, the principal toolstones found in archeological sites, were prepared to guide field studies that will identify and segregated those bedrock sources that provide high quality materials suitable for differing subsistence tasks from similar resources that manifest unacceptable material properties for tools. To date this approach has yielded useful results in differentiating the known tool-quality obsidian from vitrophyre associated with tuffs, lavas, and volcanic plugs. The obsidian source was isolated to Shoshone Mountain on the NTS. Obsidian xenoliths were discovered entrained within tuff of the Rhyolite of Shoshone Mountain, and geochemical data confirmed these xenoliths are the source for the Fortymile Wash obsidian nodules. These nodules were used to fashion 29 percent of the projectile points from the NTS that have been analyzed for geochemical data. Identifying this bedrock source allows archaeologists to address toolstone abundance and geographic distribution in order to develope inferences regarding changing land use patterns during the Holocene.