Late-Quaternary Evolution of the Semi-Confined Alluvial Megafan of Isonzo River (Northern Adriatic): Where the Fluvial System of the Southern Alps Meets the Karst

Abstract: The evolution of alluvial megafans has mainly been investigated in unconfined settings;however, at the boundary of these large depositional systems, the development of fluvial channelscan be affected by structural constraints with regional extent. Here we present the study of the easternsector of the megafan of Isonzo River, in the Gulf of Trieste, where this system fed through thesouthern Alps is constrained by the Karst and Istria cliffs. Although this area is now submergedunder the northern Adriatic Sea, stratigraphy from seismo-acoustic profiles, drill cores and multibeambathymetry allows us to reconstruct the paleochannel system of the Isonzo River in detail,which was likely active within the period of 21-17.5 ka cal BP, at the end of LGM. This was reconstructedfor over 50 km and currently represents the longest abandoned fluvial channel in the Mediterraneanseabed. The occurrence of the mountain fringe and competition with nearby alluvial systemsforced the paleochannel to follow the present coastline and conditioned the slope of its thalwegto decrease almost to null, resulting the transformation from the megafan to the undifferentiatedalluvial plain.