The subsoil of Colosseum and the detection of the ancient Tiber river Paleo-valley (MIS 12-11) in Rome

Core data from five boreholes drilled on 2017 in the Colosseum hypogea are of great interest for defining the Middle Pleistocene fluvial paleogeography and paleoenvironment in the Rome Basin. A detailed facies analysis and lithostratigraphic correlation with many other core and outcrop data from nearby areas, comprising the Palatine, Roman Forum and Capitoline hill, were performed in order to precisely reconstruct the subsoil of the monument and of the wider archaeological area. Two well defined paleo-valley infills have been recognised, which are correlated respectively to MIS 12-11 and MIS 2-1, and referred to the ancient Tiber River and the recent Fosso Labicano stream. The Middle Pleistocene paleo-valley stretches in the N-S direction, is up to 500 wide and was deeply carved during MIS 12 into older volcano-sedimentary interfluves (MIS 16-13) and the Pliocene substratum. The valley infill is 40 m thick and corresponds to the Fosso del Torrino Formation, which was deposited at the MIS 12-11 transition. It is composed of basal gravels grading upward to channel-belt sandstone, cross bedded and vertebrate-bearing (Palaeoloxodon antiquus, Bos primigenius), and to floodplain clays and silt with interbedded lignite seams and tephra. The western side of Colosseum is founded on pebbles and sands of an eastward convex bend of the paleo-valley, whereas the floodplain-marshy deposits are found further west beneath the Palatine hill, in the concave bank of the ancient river. The paleo-valley trunk of the historical centre of Rome well correlates to the downstream fluvial sandstones cropping out at the fossiliferous site of Via Ostiense-San Paolo, while the floodplain fine deposits, rich in plant remains and pollen, have the potential to better define the local past climate at the MIS 12-11 glacial-interglacial transition. This new paleogeographic reconstruction can help define the environmental context of the early human occupation in the Rome area.