The post-LGM incised valley of Concordia: a fluvial archive of the alluvial plain of Tagliamento River (NE Italy)

The Tagliamento River is one of the major alluvial systems in the Venetian-Friulian Plain (NE Italy) and its megafan is characterized by several incised valleys formed since the Late Glacial and now almost completely filled. Among these features, the most investigated is the one identified in a first time near the city of Concordia Sagittaria. A dataset consisting of almost 2000 cores, integrated with several geotechnical tests and a lidar topography, was used in order to characterize this incised valley. In particular, more than 1500 cores were realized with hand augers since 2012 by the Physical Geography students of the Utrecht University during a series of fieldwork activities organized in the area. Our dataset allowed to reconstruct the buried morphology of this incised valley, which can be traced for a length of almost 25 km and was characterized by a width up to 1.2 km and a maximum depth of about 20 m below the top surface of the LGM alluvial plain. A detailed characterization of the infilling led to the recognition of three main depositional units. The lower one is the result of the direct activity of the paleo Tagliamento River within the valley. It consists of a 10 m thick gravelly unit, deposited after 19.5 ka cal BP and capped by an organic-rich layer with an age ranging from ca. 9.5 to 6.5 ka cal BP. The middle unit can be linked to the Holocene marine transgression, which led to the formation of a lagoon environment within the incised valley and the consequent deposition of a 15 m thick unit of muds between ca. 8.0 and 2.0 ka cal BP. The absence of an evident riverine input suggests an upstream diversion of the paleo Tagliamento River. The presence of some peat horizons within this unit indicates a pulsating freshening of the lagoon, which may be the consequence of phases of enhanced freshwater discharge. Nevertheless, whether the peat formation was fostered by fluvial (upstream) rather than marine (downstream) controls is still matter of debate. The upper unit is represented by alluvial sediments deposited by the Tagliamento River in a renewed phase of activity in the area. The formation of this unit started after ca. 2.0 ka cal BP, but some major floods occurred between 6th and 8th century AD and deposited up to 6 m of sediments that sealed the incised valley. This study provides a detailed description of a fluvial archive that encompasses almost the entire post-LGM history of the Tagliamento River, allowing the paleoenvironmental reconstruction of the area and providing new data on the fluvial evolution and on its interplay with the marine processes that occurred in the Venetian-Friulian Plain during the Holocene.