USING OPTICALLY STIMULATED LUMINESCENCE OF ANTHROPOGENICALLY PLACED ROCKS TO DETERMINE THE TIMELINE OF DRIVELINE CONSTRUCTION AND GEOMORPHIC CHANGE AT WIGGINS FORK BISON JUMP COMPLEX, WYOMING

The Wiggins Fork River drains the Absaroka Mountains in northwestern Wyoming. Terraces along the drainage provide a record of fluvial response to the glaciation of the Yellowstone Plateau and post-glacial climate change. The terraces host 46 km² of cairn drivelines that constitute a bison jump complex. A riverbank stratigraphic profile displays beds of butchered bison bone linked to the primary jump. This site yields radiocarbon ages indicating use in the late 1500s AD and projectile points typologically associated with Late Prehistoric Shoshonean people. The stratigraphy and topography of the deposit suggests buried cultural material. Evidence of adjustments to the driveline alignments adds to the uncertainty of the age and duration of site utilization.

The first terrace of the Wiggins Fork, 25 m above the river, has four parallel rows of cairns that follow the edge of the terrace riser and extend for ~1km. These drivelines are bisected by gullies cut into the terrace surface. The relationship between the gullies and successively inward-placed drivelines suggest adaptation in response to headward gully migration. Research uses single-grain optically stimulated luminescence (OSL) dating to determine the age of the underlying terrace and timing of successive driveline emplacement through the dating of sediment under placed rocks. In addition to dating the drivelines associated with the bison jumps, a secondary goal is understanding the timing of gully formation and rates of headward migration.

Samples were collected for single-grain analysis from sediment under thirty-two cairns from five interpreted jumps on terrace surfaces and upland positions. These samples were analyzed to determine placement age. Five samples from the terrace alluvium were also collected to date terrace formation. Older terraces and other deposits in the study area will be dated using small aliquots of quartz sand.

Sub-cairn samples collected from four parallel drivelines on the first terrace of the Wiggins Fork are used to provide rock-placement ages during driveline construction to help determine adaptive response to gully-head migration. Results provide a timeframe of driveline construction and site occupation of a North American hunting complex.