Geomorfologia e stratigrafia della bassa pianura veneta tra S. Donà di Piave e S. Stino di Livenza

GEOMORPHOLOGY AND STRATIGRAPHY OF THE DISTAL VENETIAN PLAIN BETWEEN S. DONA’ DI PIAVE AND S. STINO DI LIVENZA This thesis analyzes a portion of the distal sector of the Venetian-Friulian Plain, extending for about 100 km2, between the Piave and Livenza rivers. The area has been partly reclaimed in the 20th century and the present topography spans between +5 m and -2.5 m above mean sea level (msl). Alluvial, lagoon and swampy deposits are well represented and their age formed between Last Glacial Maximum (LGM) and the present. One of the main goals of the research is the comparison between surface geomorphological data and high-resolution subsoil information. The selected zone allow to test this thanks to the quality and quantity of data now available. Besides the production of detailed geomorphological, geological, pedological maps, this zone has been also investigated for the general framework of the first subsoil in the framework of the mapping projects carried out by the Provinces of Venice and Treviso. Moreover, since 2012 the area considered in this thesis has been part of the fieldwork zone analyzed by the student of the Department of Physical Geography of the University of Utrecht for the course “Low Land Genesis” (responsible Prof. Esther Stouthamer). The course is part of the Bachelor course in Physical Geography and is organized in collaboration with the Department of Geosciences of Padova University (Dr. A. Fontana). The field activity mainly consists of carrying out hand augering up to the depth of 15 m from the surface along lines with a length of 3-4 km and an average distance of 100 m between neighboring corings. This activity brought to the creation of a large database of detailed stratigraphic logs for the first 3-16 m, that has almost no analogues for depth and areal density in Italy. I attended the fieldwork activity in 2015, between May 22th and June 19th, joining 2 students of Utrecht and coring 33 boreholes along the line named 303. This experience allowed to learn the theory and the technical skills in coring and logging and the use of the software LLG2012. This has been designed by Utrecht Department to store and process the stratigraphic cores, helping in the creation of long profiles and their drawing. The Venetian-Friulian Plain is generally described as the eastern part of the Po Plain, but it can be differenciated for the presence of alluvial megafans. These can be recognized from the mouth of the major Alpine valleys up to the inner boundary of lagoons. The megafans formed during the LGM, supplied by the sediment generated by the fluvioglacial deposition. Since the withdrawal phase of the Alpine glaciers, in the Lateglacial and early Holocene, the lack of sedimentary input led the main river to incise in respect of the LGM surface and funneled the depositional flux along alluvial valleys. These erosive lanforms have been recognized in the distal sector of Tagliamento and Piave river systems. The fluvial valleys can be up to 2000 m wide and 25 m deep, but now they are almost completely filled by fluvial, lagoon and swampy deposits of mid and late VIII Holocene. Thus, for the recognition of these paleovalleys the use of stratigraphic cores is necessary. Few previous studies identified traces of incised-filled valleys in the studied area, but the new data collected through the fieldwork of Utrecht students allow to characterize in detail these buried landforms. In particular, this thesis aimed to describe the planform framework of the paleovalleys, the facies of the sedimentary infill and the age of the deposits. The research started from the interpretation of remote-sensed images (e.g. rectified aerial pictures and satellite images) and the comparison with high-resolution topographic data. This information was obtained through the analysis of Digital Terrain Models (DTMs) derived from airborne laser altimetry (LiDAR) and partly from heights points of the Regional Topographic Map (CTR) at scale 1:10,000. The new investigation dealing with the geomorphology and geology have been mainly devoted to increase and check the map recently produced by the Province of Venice (SINPAV and DOGE projects). The interpretation of the litho-stratigraphic and chronological information were strongly improved by the new data of the database produced by students of Utrecht. This thesis realized 6 stratigraphic profiles along the lines 301, 302, 303, 304 (cored in 2015) and the lines 223 and 224 (cored in 2014), corresponding to a total number of 245 boreholes and a total length of 20.5 km. Moreover, in some key sites, several single drills were carried out for detecting geological and morphological boundaries. The stratigraphic data have been also improved with the pedological profiles documented in the literature. The area is characterized by many traces related to the ancient anthropogenic activity and, in particular by the Roman and early Medieval site of Cittanova and the path of the Roman road Via Annia. Notwithstanding, the archaeological traces and remains were considered only for their chrono-stratigraphic meaning. The research allowed to recognize 2 main fluvial paleovalleys formed in the Lateglacial by the activity of Piave River, that merge together slightly north of the archaeological site of Cittanova. Here a maximum width of 1200 m is reached and the bottom is below 15 m from the present surface. The sedimentary infill is mainly characterized by fine dominated deposits and the occurrence of estuarine/lagoon sediments is documented since -14 m msl, with a radiocarbon are of 8256±70 years cal BP. This information documents that sea level was slightly above -14 m msl at that time, while it was around -9 m msl 7400 years cal BP, testifying the rapid rise of the marine level in that time interval. Moreover, the stratigraphic data demonstrate the existence of a geological unit belonging to Piave megafans, which formed at the end of the LGM and consists of coarse sand and fine gravels. This depositional body can be probably correlated to the so-called cataglacial unit, previously recognized only in the Tagliamento megafan and dated to the withdrawal period of the Alpine glacier. The new data and the reinterpretation of the previous available geomorphological and pedological information allow to build a robust stratigraphic framework that support a geological reconstruction with an unpreceded high resolution. This thesis can strongly support geotechnical and surface hydrogeological investigations and territorial planning.