Geochemical Characterization of Rhyolite Sources:A Search for Prehistoric Toolstone

Coffman, Sam; Lawler, Brooks; Adams, Jacob

Lithic sourcing studies are an untapped and understudied resource in Alaska archaeology, and have focused largely on obsidian (Cook 1995; Reuther et al. 2011). They offer a way of addressing prehistoric mobility, landuse, and in some instances trade. Common in archaeological site assemblages are fine-grained volcanic rocks (e.g., obsidian, rhyolite, basalt, and andesite) which are amenable to a variety of geochemical characterization techniques. This study continued to focus and build on an ongoing rhyolite sourcing project (cf. Coffman and Rasic 2015), with the intent of documenting and geochemically characterizing rhyolite sources in Denali National Park and Preserve and Wrangell St. Elias National Park and Preserve and attempt to match bedrock sources to prehistoric artifacts. The problem with rhyolite, and most igneous rocks in general, is that it tends to be common and widespread geologically, making it challenging to locate the “high-quality” or “tool-quality” rock within a formation on the landscape. Portable XRF (pXRF) technology has been used to geochemically characterize 1000 rhyolite artifacts from over 100 archaeological sites in interior Alaska. The results of these analyses have assisted in the characterizing of 10 distinct geochemical groups (i.e. the geologic sources). While the sources have not been identified, it is believed that the majority of the material is originating in the Alaska Range, where rhyolite outcrops are plentiful. Given this information, the 2015 research focus was to collect rhyolite samples from selected outcrops as described in a 1963 report by Plafker et al.