Studio petrografico dei depositi tardo quaternari distali del Fiume Piave.

This thesis deals with the characterization of late-Quaternary distal deposits of the Piave River, one of the main fluvial system draining the south-eastern Alps. The work is based on the collection of numerous samples of sands from cores Torre di Mosto (TdM), Cessalto (SD4 and SD5bis) and Jesolo (JES2010), that reached the depth of 94, 40 and 60 m from ground surface, respectively. The boreholes were realized through a mechanical probe collecting cores of 101 mm in diameter, with a simple sampler and collected in plastic or polystyrene boxes. TdM survey was realized between September and October of 2003 in the framework of the carrying out of the new geological map of Italy (CARG Project, sheet 107 "Portogruaro"). The stratigraphic succession consists of alluvial deposits of MIS 6, followed by coastal sediments of the Tyrrhenian transgression (i.e. MIS 5.5) and after by alluvial deposits of the MIS 3 and 2. In the last 7.8 m Holocene lagoon deposits are present and their base is radiocarbon dated at about 7 ka cal BP. Cores SD4 and SD5bis were collected near the town of Cessalto between October and November 2008, for the construction of the third lane of the motorway A4 between San Donà di Piave and Alvisopoli. The core sampled only alluvial deposits that, at the base, related to floodplain facies radiocarbon-dated at about 40 ka cal BP. In SD4 alluvial sediments of Last Glacial Maximum (LGM, 30-20 ka cal BP) are present between 32.5-2.8 m and they are sealed by fluvial-ridge deposits of the Piave River, which was active in the area between 5.5 and 3 ka cal BP. The basal part of core SD5bis is well related to SD4, but from 28.8 m of depth it consists of an incised fluvial valley formed by Piave River during lateglacial and filled by distal sediments of this course since 9 ka cal BP. The last 5 m correspond to the top portion sampled in SD4. The core JES2010 was carried out in April 2010 and it passed through almost the same chronostratigraphic units and the same depositional facies recognized in TdM, except MIS 6 floodplain that was not reached. The stratigraphic reconstruction of the whole area analyzed in this research was obtained through the detailed stratigraphic logs of the cores and supported by sedimentological analysis. This study allowed to identify the depositional facies and their associations. Moreover, the stratigraphic data were interpreted at the light of the general stratigraphic framework of the late-Quaternary evolution of the distal sector of the Piave river system. Particle-size analyses were carried out on all the samples collected, but only on the most significant ones petrographic and mineralogical composition were investigated. In particular, the compositional study through the method of counting for points was carried out, using a mechanical stage with constant increments of 0.5 mm. At least 300 grains of the framework were determined. The study considered 6 samples from TdM, 1 from SD4, 2 from SD5bis and 5 from JES2010. The results obtained from the data of the calculations allowed the construction of triangular diagrams. The analyses were plotted using triangular diagrams showing as corners NCE (Non-Carbonate Extrabacinal), CE (Carbonate Extrabacinal), CI (Carbonate Intrabacinal), NCI (Non-Carbonate Intrabacinal), Q (Quartz), F (Feldspars), L (Lithic fragments). In particular, triangles NCE-CE-CI+NCI, Q-F-L+CE and Q+F-L-CE represent first, second and third level of classification respectively. The use of these analytical techniques has played a primary role to perform provenance studies. These were based on the detailed study of the main composition and recognition of diagnostic elements related to drainage systems which have influenced the studied area. The study of river sediments allowed to highlight that the composition of the sediments experiences an abrupt change from the Tagliamento to the Po delta. In particular, the percentage of carbonate grains (mostly dolomite) and fragments of volcanic rocks gradually decreasing toward south, while an enrichment in quartz and fragments of metamorphic rocks occurs. Deposits of continental facies of MIS 3 and MIS 2, both in TdM and SD4+SD5bis, show a typical composition of the Piave River catchment. Coastal deposits of MIS 5.5 in TdM have a mixed composition between Piave and Tagliamento with very little influence from Po; on the other hand, coeval sediments of JES2010 show a typical composition of the Po basin. These two petrofacies reflect the different source areas: 1) litharenitic sands with southern Alpine origin; 2) quartzolithic sands fed by western-central Alps and northern Apennines. The new information highlights that, probably, Po River had a stronger influence during MIS 5.5 in the Venetian Plain and its sediments were transported by a clockwise coastal current up to the area of Jesolo.