New stratigraphic evidence for the mid-late Holocene fluvial evolution of the Arno coastal plain (Tuscany, Italy) Nouveaux éléments sur l’interprétation stratigraphique de l’évolution fluviale de la plaine côtière de l’Arno (Toscane, Italie) à l’Holocène moyen-supérieur

A large core dataset was used to reconstruct the mid-late Holocene palaeohydrographic evolution of the Arno plain around the city of Pisa. The fluvio-deltaic succession, overlying a laterally extensive lagoonal deposit, is composed of isolated to locally amalgamated channel sand bodies encased in swamp or alluvial clays and silts. Five fluvio-deltaic units (I-V) were identified beneath the city of Pisa and chronologically constrained using top-of-sand body depths, integrated with published radiocarbon datings. The oldest, pre-Roman units (Units I-II) record the development of a wide delta plain characterised by swampy deposits in correspondence of the interdistributary areas and distributary channels that locally cut down 4-5 m the underlying lagoonal clays. The subaqueous erosion and dismantling of the lagoonal deposits was favoured by the low lateral erodibility of highly cohesive lagoonal clays. A gradual transition from deltaic to alluvial plain is recorded by the Roman channels (Unit III), laterally bordering swamp and alluvial overbank fines at distal and proximal locations, respectively. During the Roman Age Pisa emerged at the confluence of palaeo-Arno and palaeo-Serchio rivers. About 1 km north an abandoned meander of probable palaeo-Serchio provenance has been reconstructed. After the Roman period only one active fluvial course (modern Arno River), is recorded, due to the embankment of the palaeo-Serchio. Our results corroborate the validity of a stratigraphy-based approach to reconstruct the recent evolution of modern deltaic and alluvial plains and confirm its potential for understanding past fluvial patterns and dynamics.