The MIS 5 highstands along the northern Adriatic Sea: stratigraphic data and paleogeographic reconstruction

The north-eastern Adriatic represents the foreland basin of the south-eastern Alps. But the short distance separating the mountains from the sea, in combination with the gentle slope characterizing the Adriatic shelf, created a particular setting which do not exist along the other sides of the Alps and that has very few analogues around the World. In fact, this condition allows to analyse in the same area both the fluvio-glacial deposits related to glacial advances and the sediments connected to marine high stands. The sedimentary sequence deposited during Late Quaternary was strongly influenced by the alternation of colder and warmer periods, but also the distal tectonic effect of both Alps and Apennines played a constraining role in the stratigraphic evolution of the area (Antonioli et al., 2009). It is important to remark that Pleistocene marine sediments do not crop out in whole coast of the Adriatic, neither in the western nor in the eastern side of the sea, pointing to a general long-term tectonic subsidence. In the last decade several new cores has been collected along the coastal plain between the Po River Delta and the Karst Highplain, strongly improving the available information about the transition between Middle and Late Pleistocene. The boreholes reached a depth between 50-120 m, but the core AZX arrived at 272 m of depth (Pini et al., 2009; Fontana et al., 2010). In the research also several cores already described in literature were considered, analysing the existing logs in the framework of the new information. For every core the following parameters where described: lithostratigraphy, sedimentary facies, macrofossils assemblages. For most part of the boreholes also a micropaleontological log was realized and, in some selected cores, pollen and paleovegetational analyses were conducted. In particular a new pollen sequence, recording the end of MIS 6 and the MIS 5, has been produced in core ULLOA near Mestre, close to the Lagoon of Venice. An important correlating sequence for vegetational variation occurred at the transition between MIS 6 and 5 is represented by the cores collected in the Fimon Lake, in the Berici Hills (Pini et al., 2010). In boreholes PNC and BLG, near Marano Lagoon and Grado Lagoon respectively, the MIS 5.5 age of the coastal deposits has been checked with ESR datings. In most of the cores the magnetic susceptibility was measured, obtaining an independent parameter, very helpful in correlating the different sequences, specially in the intervals of the cores where micropaleontological or pollen information is not available. Considering the general setting, a strong relation between alluvial aggrading phases and the glacial period occurred at the end of MIS 6 is evidenced, whereas during the MIS 5 the sedimentation mainly occurred in the coastal sectors. During the so-called Tyrrhenian transgression (MIS 5.5) the coast reached a more landward position than the present and the inner lagoon margin was about 10-25 km landward than the present one. The top of the MIS 6 alluvial deposits is marked by a well-developed soil, comparable to the soil capping the LGM deposits (the so-called “caranto” in the Venice Lagoon). The new information seems to confirm that the marine highstands occurred during MIS 5.3 and MIS 5.1 do not directly deposited coastal facies in the area northern than Po Delta. But the transition between the final part of MIS 5 and MIS 4 is still characterized by some chrono-stratigraphic uncertainties. Applying the method used to produce the compilation of the MIS 5.5 high stand (~125 ka) sites spanning the coastline of Italy (Ferranti et al., 2006), the new cores provide many new data about the downlift rates along NE Adriatic coast. From the Gulf of Trieste to the Po River Delta, the base of MIS 5.5 paralic deposits, which were sedimented around +6 m a.s.l. (Antonioli, 2012), now lie respectively from 40 to 130 m below sea level. This setting demonstrates a general subsiding trend, characterized by a westward increasing values from 0.3 to >1 mm/a; in the area of Venice the information provide a mean subsidence of 0.5-0.62 mm/a.