EVIDENCE OF LARGE PROGLACIAL FLOODINGS IN THE VENETIAN-FRIULIAN OUTWASH PLAIN DURING THE LAST GLACIATION

The outwash rivers of the Venetian-Friulian plain promoted a high-rate aggradation during the Last Glacial Maximum (LGM); large sedimentary bodies were fed by major glaciers lobes, namely Astico, Brenta, Piave and Tagliamento. LGM glaciers along the south-eastern side of the Alps were temperate-type glaciers, hold up by high precipitation rates and characterized by large amount of liquid water trapped in subglacial lakes or funneled in speedy streams. The stratigraphic analysis of these sedimentary bodies is possible through open sections granted by gravel quarries and natural scarps of the river banks. Rapid changes from coarse gravels to well sorted sandy gravels few kilometers downstream have been identified in all sectors. Large boulders, from 1 up to 2 m of b size, are present in many sites, especially in the Cormor megafan of the Tagliamento outwash plain, from the end-moraine system to about 13 km far to the south (Zanferrari et al., 2008). These were previously interpreted as the evidence of dismembered frontal moraines (Venturini, 1988). However, the boulders are located only in this sector of the plain and lie stratigraphically above a paleosoil radiocarbon dated to 27 14C ka BP (Monegato et al., 2007), thus belonging to the LGM sandur of the Tagliamento glacier. In some dugs these boulders are associated to large bars, showing a rough cross-bedding. A similar sedimentary structure was observed 2 km downstream of the LGM frontal moraines in the Astico outwash system, a transfluence tongue of the Adige-Brenta glacial complex (Rossato et al., 2013). In this case a well preserved bar fills a deep cut in the horizontal bedding of the outwash gravels, marking the evidence of a high-energy cut and fill process. The occurrence of cross-bedded bars, embedding large boulders up to 2 m of diameter, in the piedmont plain of the south-eastern Alps is peculiar. The break of natural dams, such as the frontal moraines or the glacier itself, may have triggered catastrophic floods that determined the formation of such gravel bars with meter-size clasts as well as moving iceberg with loaded boulders far from the terminus.