Investigation of Late and Post-Fremont Alluvial Stratigraphy of Range Creek, East-Central Utah: Use of OSL When Radiocarbon Fails
Range Creek in east-central Utah hosts a unique abundance of Fremont-aged archaeological sites. Although good age control is available for cultural artifacts and sites, radiocarbon ages from alluvial sequences along the valley floor have generally produced age overestimates. In order to provide an alternative dating technique and test the reliability of the radiocarbon results, samples for single-grain quartz optically stimulated luminescence (OSL) dating were collected from three alluvial profiles where 18 radiocarbon samples of charcoal and aggregated pollen had been analyzed. While AMS radiocarbon age results were more precise, the single-grain OSL ages were stratigraphically consistent and suggest radiocarbon age overestimation of up to 2–6 ka, with the greatest divergence observed in the results from aggregated pollen samples. OSL results and geochemical analyses suggest that hydrocarbon contamination may have affected the accuracy of the radiocarbon results; age overestimates are likely due to contribution from oil-shale deposits within the catchment. OSL and stratigraphic results suggest fluvial deposition began prior to 0.88 ka (AD 1130) at the site. Floodplain aggradation continued by conformable deposition of tabular flood packages, commonly capped with charcoal and ash-rich sediments, until interrupted by channel (arroyo?) entrenchment prior to 0.34 ka (AD 1670). Sandy flood packages filled this channel and re-aggradation continued until historic arroyo entrenchment of Range Creek in the late AD 1800's. OSL age constraints indicate that the alluvial sediments at the study sites were deposited during and immediately following Fremont occupation of Range Creek at AD 1000–1200 and may contain valuable archives of environmental conditions leading up to and following the abandonment of Fremont peoples from the region. Moreover, OSL results point to the importance of the use of multiple chronometers to date alluvial deposits, as the radiocarbon chronology alone would suggest that the study sites contained pre-Fremont-aged deposits and may lead to the misinterpretation of associated cultural and environmental records.
Rittenour, T.M., Coats, L.L., Metcalfe, D., 2015. Investigation of late and post-Fremont alluvial stratigraphy of Range Creek, east-central Utah: Use of OSL when Radiocarbon Fails. Quaternary International 362, 63-76 https://doi.org/10.1016/j.quaint.2014.11.065