Towards a revised stratigraphy for the Middle to Upper Palaeolithic boundary at La Guelga (Narciandi, Asturias, Spain). Soil micromorphology and new radiocarbon data

The archaeological sequence of the Palaeolithic site of La Guelga apparently shows an interstratification of Aurignacian between the Mousterian and Chatelperronian layers, a sequence which disagrees with the stratigraphic model for the Middle to Upper Palaeolithic transition in SW-Europe. We analyzed the witness section of the interior sector in archaeological zone D using micromorphology and collected new radiocarbon dates for the Aurignacian and presumably Chatelperronian levels in order to provide detailed sediment descriptions of the site and scrutinize the presumed interstratification. Thin sections from Mousterian levels 8 and 9 show microstratification, signs of trampling and preferential sub-horizontal orientation of elongated particles. These features are less strongly developed in the Aurignacian deposit of levels 5 and 6 and are lacking in the presumably Chatelperronian ones of levels 1 and 2. The deposits of the latter show other features of reworked slope deposits such as a randomized distribution of limestone clasts. Radiocarbon dating on bone samples from level 2 places this layer to between similar to 41.5 and 45 ka cal BP, hence older than Aurignacian level 5, dated to between similar to 35.5 and 41.5 ka cal BP. Comparative dating of bones with and without ultrafiltration pretreatment conducted in two different laboratories yielded results in excellent agreement. Overall, the micromorphological observations and new radiocarbon ages strongly suggest that the few finds of presumably Chatelperronian affinity and sediments of levels 1 and 2 were transported by slope processes into the shelter.