Incised Valleys of the Northern Adriatic Shelf: a Window on the Late-Quaternary Fluvial and Coastal Evolution

On a global perspective, marine regressions and low stands are associated to the widespread entrenching of fluvial networks through continental shelfs. The very low gradient that characterizes the Northern Adriatic shelf inhibited this scouring mechanism during the LGM. On the contrary, stratigraphic and chronological data highlight the occurrence of a set of LGM/post-LGM infilled incisions produced by different driving processes. The wave and tidal action of the transgressive sea reworked and erased most of the units characterizing the former alluvial plain. Thus, the fillings of the Adriatic incisions constitute one of the few preserved deposits for investigating the morphological and environmental evolution of the period between the LGM and the phase of maximum flooding (i.e. circa 5.5 ka BP). The study was carried out through the analyses of a series of VHR CHIRP profiles, stratigraphic cores and radiocarbon dates. Two different generations of infilled scours have been described. The older generation (Atl1) reach a depth of up to 30 meters below the modern sea floor and would have been formed during the peak of the LGM marine low stand (about 25-22 ka BP). The architecture of the infilling unit suggests that Atl1 incisions were abandoned by their formative river and therefore shifted into a fluvio-lacustrine environment, fed from distal fluvial systems. The scouring and infilling process appears to be stretch some 10,000 years. In contrast, the younger generation of infilled incisions (Atl2) is the legacy of the coastal-marine lagoon environment that existed in the study area for just a few millennia in the Early Holocene (10-8 ka BP). They are a network of tidal channels hinged on few deep tidal inlets, likely sheltered by sandy barriers. Atl2 incisions reach maximum depths of 20 m and they are completely infilled.