Stratigraphy and formation processes of the Upper Pleistocene succession at Gruta da Oliveira (Almonda karstic system, Torres Novas, Portugal)

This is a 'classical' geoarchaeological study dealing with the stratigraphy and chronology of a Middle Palaeolithic site in Portugal. The starting point is that a detailed study of the formation processes of the site must be carried out in order to infer any kind of interpretation on the meaning and chronology of cultural change. Gruta da Oliveira is a cave located in the Almonda karstic system in central Portugal. Its Upper Pleistocene fill is made up of gravitational sediments with varied biogenic and anthropogenic inputs. The archaeological sequence (35–70 ka) is fairly homogeneous, reflecting a predominantly moist climatic context modulated by millennial-scale variations. Human occupation was almost continuous throughout. In the richer archaeological layers (8–14), the anthropogenic components are in primary position, and post-depositional processes are restricted, although penecontemporaneous biological activity—such as hyena scavenging—overprinted the record. In layers 15–19, the excavated area corresponds to the footslope of a talus, where accumulation proceeded mainly through runoff and where the lithics and bones derive from occupations situated ca. 5m outward. The base of the deposit is >2m from the current base of the excavation, suggesting that Oxygen Isotope Stage (OIS)-4 and OIS-5 sediments underlay the deposit already exposed. This study confirms the archaeological integrity of the Mousterian assemblages found in the different layers, and, given the dates for layer 8, supports the hypothesis that Neanderthals survived in western Iberia until at least 35,000–38,000 calendar years ago.