Alluvial megafans in the Venetian-Friulian Plain (north-eastern Italy): Evidence of sedimentary and erosive phases during Late Pleistocene and Holocene

The Venetian–Friulian Plain is the eastern part of the foreland basin of the Southern Alps and is characterized by the presence of alluvial megafans. Existence of these large landforms results from the tectonic setting, but their Late Pleistocene and Holocene evolution has been mainly controlled by climatic change and eustasy. Geomorphological, geological information and radiocarbon dating allow the recognition of phases of sedimentation and incision in the megafans of the Brenta, Piave and Tagliamento rivers. The main phase of aggradation took place during the Last Glacial Maximum (LGM) (24–15 ka BP), when the glaciers emanating from the Alpine valleys reached the plain and supplied large amounts of sediments. During the Late LGM the rate of aggradation lowered in the Brenta megafan and a wide incision of the fanhead developed in the Tagliamento megafan. During Late Glacial and Early Holocene an important phase of incision took place, and smaller telescopic lobes formed in the distal portion of Brenta and Tagliamento megafans. Sedimentation was absent or very low between 14 and 8 kaBP and only since the Middle Holocene a new phase of deposition affected the coastal areas, probably related to a marine highstand. Widespread aggradation started once more around 4–3 ka BP, with the formation of fluvial ridges along the terminal tract of Alpine rivers. Since the Roman period, human influence affected the alluvial sedimentation, especially in the last centuries, due to land reclamation and construction of river embankment.