LGM sedimentation in NE Italy: the continuity of alluvial systems from the Alpine glaciers to the Adriatic floor

In northern Italy the LGM played a major role in shaping the mountain catchments and the Alpine side of the alluvial plains. The glacial fronts reached the mouth of the main Alpine valleys and fed the related fluvioglacial and fluvial systems, which experienced a dramatic aggradation between 28-17 ka cal BP. In the Venetian-Friulian Plain this sedimentation led to the formation of some alluvial megafans and fans related to the main fluvial systems (i.e. Isonzo, Torre, Cormor, Tagliamento, Cellina, Meduna, Piave, Brenta and Astico rivers) which still largely crop out also in the seabed. Due to the LGM-marine lowstand the megafans extended for tens of km in the northern Adriatic basin. Geomorphological and geological surveys recently considered large sectors of the Venetian-Friulian Plain and the Italian portion of the Northern Adriatic sea floor was mapped. Several tens of cores investigated the first 15–50 m in the land and 2–8 m from the seabed, allowing to sample and to date several organic horizons. About 400 radiocarbon datings dealing with the alluvial stratigraphy are available in the area and the detailed analyses of some deep cores permitted to construct the depth/age models for the distal sectors of Tagliamento, Piave and Brenta megafans, estimating the sedimentary rates along the LGM. In the sea, through about 6000 km of CHIRP-Sonar profiles the 2D and 3D setting of LGM deposits was described. Topographic data of the plain and bathimetric, information of the seabed were processed and joined to obtain the DEM of the whole Northern Adriatic basin, from the piedmont sector down to 50 m below sea. The LGM megafans are characterized by a piedmont sector (10–25 km from the apex) of amalgamated gravels related to unconfined braided channels; the distal sector is fine-dominated and channels are sandy braided, whereas the meandering typology started from the present coastal zone. The thickness of LGM span between 30-20 m in the plain and thins to 15–10 m in the seabed.