ALLUVIAL RECORD OF ARROYO CUT-FILL ACTIVITY DURING FREMONT OCCUPATION OF RANGE CREEK, EAST-CENTRAL UTAH: USE OF OSL WHEN RADIOCARBON FAILS

Range Creek, a tributary to the Green River, occupies a bedrock canyon between the Book Cliffs and Roan Cliffs of the Tavaputs Plateau in east-central Utah. Unique preservation of numerous archaeological sites and artifacts indicates that Range Creek hosted a high-density, yet relatively short-duration Fremont occupation 0.8-1.0 ka. While dating of cliff-top ruins and granaries with radiocarbon and dendrochronology has been highly successful, obtaining age control for alluvial and peat sequences along the valley floor have proven difficult. For example radiocarbon dating of alluvial sequences containing abundant charcoal and burned horizons exposed in arroyo walls along Range Creek has produced age reversals and generally unreliable results, possibly due to contamination by hydrocarbons and coal from the surrounding bedrock. However age control for these alluvial and peat sequences is important because they contain valuable archives of environmental conditions leading up to and following Fremont occupation of Range Creek.

In order to test an alternative to radiocarbon dating, samples for optically stimulated luminescence (OSL) dating were collected from an alluvial sequence containing evidence of arroyo entrenchment interrupting >4m of vertical floodplain accretion that was previously dated with multiple radiocarbon ages from charcoal and extracted pollen. Single-grain OSL ages provide stratigraphically consistent results that suggest aggradation began prior to 1.3 ka, was interrupted by entrenchment by 0.6 ka and was followed by continual aggradation until historical entrenchment in the late 1800’s. OSL results confirm contamination of some radiocarbon results and suggest caution when selecting material for radiocarbon dating from alluvial deposits in drainages where coal and hydrocarbons are found within the local catchment.