Sourcing sandstone cobble grinding tools in southern California using petrography, U-Pb geochronology, and Hf isotope geochemistry

Procurement strategies for grinding tool lithic material among mobile societies are thought to rely on opportunistic selection of resources locally available at habitation sites and along migratory routes. In San Diego County, California, non-local appearing quartzarenite cobble handstones were identified in the ground stone assemblages of some hunter-gatherer archaeological sites dating from ca. 7000 years ago. Due to the nature of the cobble material, both natural and cultural processes may have played a role in the spatial distribution of the artifacts recovered by archaeologists. In this study we employ three techniques to investigate the geological origins and source location(s) of the quartzarenite cobbles: thin section petrography, U-Pb geochronology, and Hf isotope geochemistry. Results confirm the Neoproterozoic-lower Paleozoic age of the cobbles, while metamorphism of southern California basement rocks of similar age indicates that the cobbles must have been transported into the area, probably during Eocene times. People collected the cobbles from source locations and carried them at least 4-10 km and possibly farther. We consider the diagnostic value of the three techniques for characterizing resource distributions of sedimentary cobble material and related procurement strategies, and more broadly, their global applicability for sourcing other archaeological materials made of sedimentary and metasedimentary rock.