INCISED VALLEYS OF THE NORTHERN ADRIATIC SHELF: A MORPHOSTRATIGRAPHIC MARKER FOR THE LAST TRANSGRESSION

In a global perspective, fluvial incisions generally form on the continental shelfs during marine lowstand and they can be filled and partly eroded along the subsequent transgression by fluvial and estuarine deposition, as well as by sediment reworking (e.g. Maselli and Trincardi, 2013). A peculiar setting characterizes the Northern Adriatic shelf, that has a very low gradient and, apparently, it was not prone to significant incision during the LGM lowstand. On the contrary, stratigraphic and chronological data highlight the occurrence of fluvial erosive processes in the first part of the transgression. After the LGM, the Northern Adriatic shelf, experienced a rapid submersion with dramatic environmental changes. The current morphology of the seabed above -45 m msl is the result of the strong shaping activity induced by the widespread erosive processes related to waves and currents activity after the transgressive submersion, whilst sedimentation occurred in very limited portions (Moscon et al., in press). This peculiar evolution heavily restricts the available sites that can supply information for reconstructing times and modes of the transgression. Together with the relicts of the coastal barrier systems, formed during pauses along the marine transgression, the fillings of these incised valleys are the only deposits preserved in the northern shelf between the end of LGM and the phase of maximum flooding. The investigations carried out for the geological mapping of the sea floor recognized several completely filled incised valleys (Trincardi et al., 2011). The oceanographic cruises RISA2009, NAD2012 and ASCI2014 were partly devoted to the collection of high-resolution data (CHIRP seismo-acoustic profiles, multibeam bathymetries and stratigraphic cores) aiming at characterizing these sedimentary features. In order to seize all the possible information out of these features a geodatabase is under construction: this database will collect all the information available for the incised valleys recognizable in the northern Adriatic area, between the Po Delta and the Slovenian-Croatian border, including VHR seismic data, shallow cores and radiocarbon dating. The preliminary work evidences several channel-like morphologies, few of them characterized by draped fillings, depth ranging from few to 20 meters, maximum widths of few hundred meters and a longitudinal development that can reach 10 kilometres. These paleo drainage systems can be generally associated to fluvial processes, but some of them underwent a shift to estuarine environments and, in some cases, lagoon tidal inlets. The analysis of their morpho-stratigraphy, along with radiocarbon dating, should offer new insights into the evolution of the area during the post-LGM transgression, providing constrains on the relative dating of these structures. Given the lack of the geological record, this approach may be of particular relevance in order to link the continental and the marine domains and to investigate the low order climatic fluctuation that strongly influenced the evolution of Lateglacial rivers.