The Glass Buttes complex in the High Lava Plains of Oregon produced obsidians during a series of rhyolitic eruptions circa 5.8 to 6.5. Ma. These obsidians have been used to craft stone tools for millennia, from Clovis peoples to modern knappers, and have been recovered at sites throughout the Pacific Northwest. Glass Buttes is also the origin of much obsidian used for lithic replication experiments and to test new techniques for sourcing. Paradoxically, the spatial distributions of chemically distinct obsidians at this complex have received comparatively little attention. The only published study to connect obsidian compositional differences to the Glass Buttes landscape is Ambroz et al. (2001), who reported seven discrete, spatially constrained obsidian "chemical groups" based on clustered data in elemental scatterplots. Since its publication, their paper has been cited as an example of successful obsidian "subsource" characterization. During the course of a wider research program, we collected 337 specimens at Glass Buttes from loci originally sampled by Ambroz et al. (2001). While we could replicate the "chemical groups" observed by Ambroz et al. (2001), we were unable to reproduce the reported spatial distribution of sources across the landscape. Almost half of the resampled loci exhibited intermingled populations of chemical types due to their locations on alluvial-colluvial deposits. We also identified five additional obsidian chemical types at the complex; however, geochemically significant elements suggest that only a subset of these eleven types correspond to different flows and domes at Glass Buttes. A few appear to reflect chemical zoning within flows, whereas two other types may be exogenous, moved from nearby obsidian sources via secondary transport. Thus, we demonstrate here that the discrete subsource zones demarcated by Ambroz et al. (2001) do not reflect reality on the landscape. This, in turn, alters how artifact sourcing results are interpreted with respect to how mobile foraging groups interacted with the landscape and made choices regarding resource acquisition, toolstone provisioning, and land use.
Bibliographical noteFunding Information:
Charissa Johnson provided field and laboratory assistance, which was supported by the University of Minnesota's Undergraduate Research Opportunity Program (UROP) and a Sigma Xi Grant . Michelle Muth also provided research assistance and was supported by the National Science Foundation's Research Experience for Undergraduates (REU) program ( EAR-1062775 ) and the University of Minnesota's Earth Sciences Summer Internship program . Additional field assistance was provided by LeRoy Frahm and Sarah Johnson, and Liev Frahm assisted with the pXRF analyses. The pXRF instrument utilized in this study is part of the research infrastructure of the University of Minnesota's Wilford Laboratory of North American Archaeology, which is directed by Katherine Hayes. Funding for the instrument was provided by the College of Liberal Arts and the Office of the Associate Dean for Research and Graduate Programs . This research was also supported by the Department of Earth Sciences , the Institute for Rock Magnetism , and the Department of Anthropology at the University of Minnesota-Twin Cities . The comments of two anonymous reviewers enabled us to improve and clarify the article. This is Institute for Rock Magnetism contribution #1408.
© 2015 Elsevier Ltd.
- Glass Buttes
- High Lava Plains
- Obsidian sourcing
- Portable XRF