TEPHRA AND OSL RESULTS FROM THE FRENCHMAN HILLS – TONNEMAKER MAMMOTH SITE, ROYAL CITY, GRANT COUNTY, WASHINGTON

Mammoth bones were discovered while plowing an alfalfa field on the Tonnemaker Hill Farm, north of Royal City, Washington. Emergency excavation of the site, conducted over the next month, revealed three distinct tephra layers in Ice Age (Missoula) flood deposits beneath the bone bed and plow zone. Initial tephra glass analyses suggested that the two lower-most tephra layers were likely Mount St. Helens set S tephras. The uppermost tephra layer contained two dominant compositional groups, one similar to that of the lower two tephras (set S), and the second (consisting of only two glass shards) weakly similar to Mount St. Helens set J. The presence of two glass compositions in the uppermost tephra layer suggests that it is likely a product of redeposition (or perhaps bioturbation). Samples of two tephra layers recently collected from a test pit located about 400 meters away (outside of the alfalfa field), have confirmed the presence of Mount St. Helens set S tephra (set So being the upper tephra and set Sg the lower), but failed to confirm the presence of set J tephra. A small number of glass shards from the uppermost tephra had different tephra compositions, again suggesting that this layer may have been re-mobilized by fluvial or eolian action. The most reliable age for layers Sg and So is about 16 ka, and that for set J is about 13.86 ka.

Optically stimulated luminescence (OSL) analysis of a sample collected from just beneath the lowermost tephra at the mammoth site excavation produced an age of 19.9 ± 5.3 ka (at 2σ), and a sample collected above the uppermost tephra layer produced an age of 16.4 ± 2.8 ka (at 2σ). These results are consistent with the 16 ka age associated with intervening Mount St. Helens Set S tephra. Radiocarbon dating of bone and molar dentin from the site has, thus far, been unsuccessful. However, the stratigraphic position of the mammoth remains within fine-grained flood deposits above these tephra layers indicates that the mammoth skeleton was buried in Ice Age flood deposits sometime after 16 ka. This is consistent with the presence of ice-rafted erratic boulders at the site and its location along one of the many Ice Age flood channels in eastern Washington. The OSL and tephra-chronostratigraphic results provide age constraints for both the timing of Ice Age floods and the extinction of mammoths in this region.