Geoarchaeology of the alluvial plain deposits at Entrecampos, Lisboa: first insights for a stratigraphic reconstruction

In the framework of a preventive archaeological excavation at the Unidade de Execução de Entrecampos - Parcela A (UEE-PA) in central Lisbon, Portugal, a sedimentary succession preserving evidence ranging from Prehistory to the 20^th century was unearthed. In order to better understand the site stratigraphy, two trenches (N-S/E-W) of about 40 m long and 5.50 m deep were opened in February 2022 by mechanical means allowing us to identify a sedimentary succession ranging from the Miocene bedrock to Holocene deposits.  Considering the importance of this stratification and its almost unique state of preservation for the city, we decided to follow a broad geoarchaeological approach, including geomorphological, sedimentological and lithic techno-typological analyses, as well as radiocarbon and OSL dating. Here we present some preliminary data, while further microstratigraphic analysis and physical-chemical and mineralogical characterisation of the deposits (grain size analyses, pH and organic matter content, XRD of clays) are ongoing. The alluvial plain deposits at the UEE-PA at Entrecampos are framed in the upper basin of the Ribeira de Alcântara valley and are related to a branch of the hydrographic drainage network of the Lumiar and Campo Grande plateau, on the right bank of the Tagus River close to its Atlantic mouth; their geomorphological expression is evident in the smooth base profile and the quite large deposits resulting from the erosion and transport of tertiary bedrock softest formations. The infill and incision are presumably in accordance with the Quaternary fluvial model of the Lower Tagus River sedimentary record. At the site, the depositional dynamics consist of aggradation related to a high dynamic discharge recorded at the base of the alluvial deposits, with a progressive reduction of the transport flux capacity to the top, resulting in turn in a silty loam floodplain deposit. The latter preserves a consistent number of lithic artefacts at its base (Palaeolithic and/or Mesolithic), pointing to a late Pleistocene or early Holocene chronology. This situation suggests the occurrence of  subsequent intermittent depositions that marked the initial stage of the suspended watercourse load, followed by the activation of soil formation processes which slightly affected the underlying sandy deposits as well. Further investigation will provide an absolute chronology for the main depositional events and will therefore help to reach a better understanding of the local paleoenvironmental and geoarchaeological contexts during the Quaternary in a central area in Lisbon city.