Bresolin, Mario (2013) Evoluzione della bassa pianura friulana nel Pleistocene Superiore = Late Pleistocene evolution of the Low Friulan plain (NE Italy). [Magistrali biennali]
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This thesis deals with the evolution of the distal portion of the Friulian Plain in the Late Pleistocene. The work begun with the description and the sampling of the cores Piancada (PNC1) and Beligna (BLG), which reached the depth of 64,5 and 60,0 m respectively. Both the boreholes were realized in June 2008; the first near the town of Piancada in the municipality of Palazzolo dello Stella (UD); the second in the locality of Beligna of Aquileia (UD), near the so-called dunes of Belvedere - San Marco. The cores were stored in the fridge room of the Department of Math and Geosciences of the University of Trieste, were between June and September 2012 they have been cut and documented through detail photography, high-resolution description and drawing of the lithostratigraphic logs. The detailed study of the sedimentary characteristics allowed to identify sedimentary facies and their association. Many samples were collected with the aim to process them in the future with geochronological, paleoenvironmental and biostratigraphic analyses. In particular macro- and micropaleontologic research will support paleobathimetric information, while pollen investigations will reconstruct the vegetational cover of the region and allow correlation with other deep cores available in the Venetian and Friulian Plain. Two peaty horizons from BLG1 (28.78 and 32.52 m) and two from PNC1 (26.71 and 31.88 m) were radiocarbon dated and gave the calibrated ages of 27.0-26.1, 31.4-31.0, 31.4-31.0 and 42.6-41.5 ka cal BP. These represent some of the very few geochronological constrains available for the Pleistocene subsoil of the Friuli Region. Moreover, in BLG1 the shell fragments from 51,1 m of depth were processed with Electron Spin Resonance (ESR) and this analysis established their deposition during MIS 5.5. In this research also the core CNC4 was considered in detail, re-interpreting the original data; this borehole was realized in 2003 near the center of Concordia Sagittaria (VE). Pollen and paleontologic investigations had been already carried out on CNC4 in the framework of the Geological Map of Italy, sheet 107 Portograuro and allow to consider this stratigraphy as a reference for Late Pleistocene evolution of the Friulian Plain in the coastal sector. Several other stratigraphic logs were selected after the analysis of the well log data based of the Friuli Venezia Giulia Region; the ones described in detail and within a distance of 15 km were compared to BLG1 and PNC1 trying to correlate some stratigraphic markers. Moreover, during this thesis was possible to join the ocenographic cruise NAD12, on board of the r/v “Urania”, organized by the CNR-ISMAR of Bologna and that surveyed the Italian and Slovenian shelf of Northern Adriatic. Chirp sonar geophysical profiles were acquired and some of them and others collected in previous cruises were analyzed looking for correlation with the information obtained from the stratigraphic logs available along the coast of Friuli. The paralic deposits referred to the MIS 5.5 are found at 35,77-50,23 m in PNC1, at 39,00-55,41 m in BLG1 and 39,30-51,50 m in CNC4. They sedimented during the Tyrrhenian highstand (132-116 ka cal BP), when sea level reached a maximum elevation of +6±3 m asl; their current lower elevation has to be related to the tectonic subsidence affecting the area, which is the sum of sedimentary compaction, isostasy and specially crostal deformation. Considering these paralic deposits in cores CNC4, PNC1 and BLG1, the difference between their original and present elevations and the time elapsed in between, the average values of tectonic subsidence were estimated. Results turned out to be 0.37 mm/yr (CNC4), 0.41 mm/yr (PNC1) and 0.41 mm/yr (BLG1) respectively. These are in accord with the raw estimations obtained by previous studies. Considering the geographycal distribution of the MIS 5.5 lagoon deposits and their vertical position, the position of the inner margin of the lagoon at the MIS 5.5 highstand was tentatively hypothesized. This reconstruction was based on the original topographic gradient (i.e. 0.1%) of the alluvial plain formed during the penultimate glaciation (MIS 6) and that was after onlapped by the Tyrrheninan transgression. The gradient rapidly increased upstream from the present coastal belt and, thus, the inner margin of the MIS 5.5 lagoon was estimated between 5-10 km northern than BLG1 and PNC1. To be noted that inside the coastal sequence recognized in CNC4 and PNC1 there is the evidence of a temporary progradation of the alluvial plain and the following re-advance of the lagoon environment over the emerged alluvial plain. After Tyrrheninan and until the beginning of LGM in the alluvial plains of Northern Adriatic the sedimentation was very limited and probably lacking for prolonged periods encompassing large part of MIS 4 and MIS 3. Sedimentary starvation can be probably explained with the limited sedimentary flux supplied from the mountain catchments and with incision of the fluvial channels. This depositional hiatus is represented by some important peaty layers that accumulated in a swampy environment up to 3 m of thickness. Sometimes, as in CNC4, these organic horizons rest directly on top of the lagoon deposits of MIS 5.5. These peaty layers can be also identified in some of the geophysical profiles acquired on the Adriatic shelf and allow to evidence their importance as stratigraphic marker of regional extent. The study of core BLG1 demonstrates that the sandy ridges of Belvedere and San Marco, that characterize the area between Aquileia and the rim of Grado Lagoon, formed at the end of the LGM and not during MIS 6, as postulated in the past by several authors. These relieves were probably formed by eolic rework of fluvial channels of Isonzo River, that was flowing in the area at that time.
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