New radiocarbon dating performed on materials from the Guerrero Member of the Luján Formation considerably enlarges the time period previously assigned to this unit. New data suggest that deposition began about the Last Glacial Maximum (ca. 21kaBP) and continued until at least ca. 10kaBP. During this period several climatic and biological events occurred, among the latter the last processes leading to the extinction of Pleistocene megafauna.