The interplay of fluvial and marine processes as recorded in the post-LGM Incised Valleys of Tagliamento and Piave rivers (NE Italy)

The rivers fed by the southern Alps experienced strong entrenching when local deglaciation began, right after the Last Glacial Maximum (LGM). This erosive phase was induced by the general sediment starvation of the fluvial systems that followed the retreat of the Alpine glaciers, and it affected most of the southern Alpine rivers. The major rivers generated some Incised Valleys (IVs) even in the distal sector of the Venetian-Friulian Plain, but the depositional activity occurred during mid and late Holocene left almost no evidence of these features in the modern landscape. However, their infilling recorded the gradual shift from fluvial to tidal-influenced environments (limans/estuaries). We focused our analysis on the IVs in the distal portion of the Piave and Tagliamento megafans, in a study area of 800 km2 between these rivers’ present courses. A parallel study is also investigating the fluvial incisions of the inner Northern Adriatic shelf. The current dataset comprises 3500 shallow stratigraphic cores, DTMs from Lidar altimetry and a considerable number of radiocarbon dates. These data were used to investigate the infilling stratigraphy, planform geometries and networks of the IVs. Thanks to the high density and quality of the subsoil investigations, in some areas it was possible to recognize complex patterns of larger main channel incisions, joined by smaller tributary scours. The latter are interpreted as headward cutting gullies, fed from rainfall and groundwater of the dissected megafan surface. Five main IVs, with a length of up to 20 km upstream of the present coast, have been analysed. These features have a maximum width of 2 km and they can reach a depth of 30 m below the LGM top. The basal contact is formed by a gravelly fluvial unit, some 5-10 meters thick, that fines upward. It marks width-constrained braided channels fed by post-LGM Alpine melt waters, which continued to function in incised position until the early Holocene. The upper filling, formed in the Middle and Late Holocene during transgression and high stand, is more differentiated and spatially variable. It represents the legacy of shifting fluvial inputs (avulsing channels), differences in relative elevations associated to differences in timing and reach of marine ingression, and variable degrees of sheltering as the barrier-lagoon coastal system developed. As a result, two main evolutional trajectories can be reconstructed for the upper part of the IV infilling: i) a lagoon/tidal dominated; ii) a fluvial dominated (predominant only in one of the five analysed infillings). The first is characterized by fine and laminated deposits, crossed by coarser sandy tidal channel units and intercalated by peaty beds. The second is marked by a gentler shift from the gravelly bottom to a laminated sandy unit, which eventually shifts to the Holocene lagoon deposits. Detailed sedimentary study of the IVs network, dimensions and basal infill, provides a rare opportunity to study the river system functioning in the period from ca. 19 to 9 ka BP, which in the rest of the study area marks a depositional hiatus. Detailed palaeogeographical study of the upper infill of the IVs can contribute to improving Holocene relative sea-level rise reconstructions for the Northern Adriatic, besides understanding river mouth response to base level rise.