OPTICALLY STIMULATED LUMINESCENCE DATING AT ALM SHELTER, BIGHORN BASIN, WYOMING

Alm Shelter is one of many rockshelters in northern Wyoming’s Bighorn Basin (USA) that provides key information about the late Quaternary environmental and population history of the region. However, unlike most regional rockshelters, which contain fragmentary stratigraphic records, Alm Shelter is a near-continuous depositional sequence that extends back to the Last Glacial Maximum. The excellent stratigraphic integrity of cultural and natural deposits can inform the formation processes, including the chronology of sequential eolian deposits reflecting periodic Holocene droughts. Despite its archaeological importance, and relatively robust radiocarbon chronology, the age of certain key stratigraphic layers remains unknown. Here, we review the existing radiocarbon data and report new ages obtained from optically stimulated luminescence (OSL) measurements on quartz sand from late Pleistocene and Holocene deposits associated with Alm Shelter. The four OSL ages presented provide improved constraints on the archaeological sequence at Alm Shelter, as well as late Pleistocene valley incision. Our work shows that the basal deposits at the site are associated with the terminal late Pleistocene and have preliminary OSL ages of 20 to 14 ka. We also dated the sediments under a 2m boulder, to more accurately determine when a roof fall event occurred, to just after 8.7 ka. Previous work has found that late Pleistocene fluvial gravels deposits associated with Level I at the base of the sequence date to 12.9 ± 5.7 ka calBP, and Level V sediments associated with the late Holocene archaeological deposits date to 1.1 ± 0.6 ka calBP. We compare the luminescence ages with the previous chronologies for Alm Shelter and briefly discuss how the revised chronology fits in the context of existing archaeological records and paleoclimatic reconstructions for the eastern Bighorn Basin with implications for the Central Rocky Mountains as a whole.