SOURCING OF CHERT ARTIFACTS AT GILLETT GROVE (13CY2) USING GEOCHEMICAL ANALYSIS

The focus of this project is to correlate chert artifacts from Gillett Grove, an American Indian archaeological site in northwest Iowa, with potential source regions and outcrops from Nebraska, South Dakota, Minnesota, Illinois, and Iowa. The chert samples, which are primarily lithic debitage from stone tool production, were initially catalogued on the basis of color, luster, mottling and banding, and the presence of fossils. Representative samples were powdered, fused into glass disks, and analyzed using ICP-MS. Complete trace element abundances and ratios were then correlated with other properties to establish both a physical and geochemical signature for each set of artifacts. The same types of data will be collected from potential source regions and compared with the artifact data in order to establish provenance of the artifacts.

Gillett Grove is a late prehistoric/early-contact Oneota site occupied in the late 1600s by the ancestors of the historic Ioway, Omaha, or Oto American Indian tribes. Glacial tills make up the landscape at and around Gillett Grove, meaning that its occupants had to bring chert in from outcrops up to three hundred miles away.

Several patterns have been identified based on the geochemical data of the samples. A sample of Tongue River Silica (WD13) showed higher abundances and ratios of most incompatibles and REEs than all other samples. For example, the Zr abundance is twenty-one times higher and the Y abundance is seven times higher than that of the other samples. Another pattern found was based on color. Cream-colored samples had the highest abundances of incompatibles and REEs. White colored samples had the next highest, followed by bluish gray, and finally dark gray, which had the lowest abundances. One probable artifact provenance was made using trace element abundances. Artifact 3.118 had very similar incompatible and REE abundances to that of CA328, a sample from an outcrop in east central Nebraska. Also, 3.118 and CA328 had similar physical properties.

The sourcing of artifacts from Gillett Grove will help in determining the seasonal travel patterns and trade routes of its occupants.