Alluvial megafans along the Italian Southern Alps

Large fan-shaped landforms represent most of the northern Po Plain (40,000 km2) and its eastern continuation, the Venetian-Friulian Plain (10,000 km2). Major Alpine rivers which feed this Alpine-Apennine foreland basin drain a total catchment of about 100,000 km2 (70,000 km2 Po R. tributaries, 25,600 km2 Venetian-Friulian rivers). The main depositional phase occurred during LGM (27-19 ka cal BP), when the glaciers hosted in the Alpine valleys reached the plain and fed the related glaciofluvial and fluvial systems. These latter experienced large and widespread aggradation as fan-shaped distributary systems, that have been described as alluvial megafans when their longitudinal axis is >30 km (i.e., Isonzo, Tagliamento, Piave, Brenta, Adige, Mincio, Chiese, Oglio, Adda, Olona river systems). These large landforms have an extent of 500-3000 km2. Pede-Alpine megafans are characterized by steep (1-0.4%) piedmont sectors consisting of amalgamated gravels down to 10-30 km from apex, while the distal sector is fine-dominated and channels are sandy braided. The thickness of LGM sediments in the plain is 30-10 m, thinning to <5 m on the Adriatic Sea shelf, where depositional bodies dating to LGM or previous low-stand units (i.e. MIS 3 and 4) still largely crop out. Following ice decay at around 17 ka cal, an erosive phase occurred in the pede-Alpine sector during Lateglacial and early Holocene. This led to river downcutting for tens of meters. Alpine tributaries of the Po River still flow in entrenched valleys down to their junction. In the Venetian-Friulian sector, where distal tract of megafans are directly connected to the Adriatic Sea, incisions have depths of 15-30 m and widths up to 2 km. Post-LGM sedimentation in the valleys consisted of predominant gravels and sands down to the present coastal area. After 8.0 ka cal sea level rise triggered the formation of the coastal wedge, the infilling of incised valleys and the widespread aggradation in the interfluves.