Evoluzione geomorfologica della bassa pianura friulana e sue relazioni con dinamiche insediative antiche

This is a research of alluvial geomorphology and geoarchaeology that analyses the evolution of the lower sector of the Friulian plain (NE Italy) since the last 25 ka ca. The work recostructs the sequence of the natural and anthropic events that stratified in time and space and originated the base of the modern landscape; the environmental constraints that forced the settlement choices and the changes induced in the area by human activity are highlighted. A strong multidisciplinar approach was applied, based on the correlation and integration of information collected with geomorphological, geological and archaeological investigations. Researches were carried out at different scales, from remote sensing to microscopic thin sections. Analyses of aerial pictures and satellite images, covering the whole area, led to identify most of the natural landforms (e.g. abandoned river and lagoon channels) and the tracks of ancient human activity (roads and field system divisions). These data were crossed with a digital terrain model allowing to detect areas of interest on which field survey has been later carried out. Field activity consisted of the description of stratigraphic sections along the slope of canals and ditches, in road, pipe and building yard and in archaeological excavations; a number of bore-holes were performed, mainly with a hand gouge and partly with mechanical gouges. Many sediment samples were collected and some of them submitted to radiocarbon dating. The low Friulian plain is characterized by the presence of Tagliamento river and a number of groundwater- fed rivers. These are fed by many springs, arrangend in a belt that separates the “high” plain from the “low”; the first one is gravelly and is characterized by the lack of a surface drainage, whereas the second one mainly consists of clayey-silt sediments and is abundant in surface water. The Friulian plain is formed by alluvial megafans that are characterized by a telescopic geometry due to the existence of aggrading and erosive phases, which are mainly related to climate changes and eustasy. The main phase of sedimentation occurred during the acme of the Last Glacial Maximum (LGM), 25-18 ka BC, when the glacier hosted in the mountain catchment reached the plain and its melting water fed the megafans of Tagliamento, Corno, Cormor and Torre rivers. Deposition occurred over the whole surface of the plain with a thickness of 20-30 m. Gravel deposition stopped about 30 km far from megafan apex, leading to the formation of the gravelly high plain; only sands or fine sediments reached the distal sectors. During glacier decaying (18-15.5 ka BC) the rivers fed by glacier entrenched the apex of their megafan, but limited aggradation continued in the low plain, originating a telescopic lobe with narrow gravelly-sandy fluvial ridges. In the post-LGM, since 15.5 ka BC, sedimentation stopped abruptly leading Cormor and Corno megafans to deactivate and inducing the formation of several incisions also in the distal portion of the Tagliamento megafan; these “valleys” (15-25 m deep, 0.5-1.5 km wide) confined the sedimentary flux which by-passed the plain and could transport coarse gravel up to the present coast. Thus a period of non-deposition affected the whole Friulian plain until 6-5 ka BC, when the onset of the marine high stand triggered lagoon and delta formation. Anew aggradation phase affected the coastal portion, while an erosive trend is recognizable in the other sectors of the Tagliamento megafan up to 2-1.5 ka BC. At that time widespread aggradation started and the pre-existing incisions silted up completely. Fluvial ridges formation characterized the lower 30 km of megafan in the last 3 ka and gravel transport stopped about 20 km upstream from the coast. The cross correlation of the stratigraphic, archaeological and geochronological datings allowed to build a robust chronological framework of the post-LGM geomorphological phases. A reliable hypothesis about the different directions followed by Tagliamento river and their period of activity in the post-LGM is presented. Large surfaces of the Friulian Plain date to the LGM and allow a good visibility of the archaeological sites; whatever, the marine transgression submerged large sectors, previously available for settlement. In the area the first evidence of human presence corresponds to few artifacts dating to Final Epigravettian (11.500-9500 BC) and Mesolithic, but only in the ancient Neolithic (5500-4800 BC) a first period of widspread peopling occurred, coinciding with lagoon formation and warm climate. Many villages of this phase are known and they were totally found on top of cataglacial fluvial ridges, demonstrating that natural soil fertility and topography had strongly influenced the position of the settlements. Human peopling strongly decreased since the middle Neolithic and it was almost dissolved until the ancient Bronze Age, probably reflecting also a nomadic or semi-nomadic living style. Part of this period coincides with a colder climate and a very high river activity developed around 2800-2400 BC. An important peopling stage is represented by the recent Bronze Age (1350-1150 BC) in which a well developed territorial system existed; it was characterized by some major sites, sometimes defended with rampants (castellieri) or wooden fences, and controlled several minor settlements. Complex irrigation systems and connections between sites existed. At the transition between recent and final Bronze Age, around 1200-1150 BC, a dramatic crisis invested the whole low Friulian plain; as the main villages situated upstream than the spring belt maintained and expanded their control role, the sites set along lagoon fringe were completely abandoned; thus a possible influence of the sea-level rise in limiting the available land along the coast could be postulated. Since the final Bronze Age, people concentrated in some major sites, but Concordia Sagittaria is the only large village settled in the low plain, probably supported by its particular physiographic position: a terrace completely isolated by lagoon inlets. Besides Concordia, which reached a proto-urban size already in the lower Iron age, until the Roman epoch in the rest of the area only few villages existed, and they controlled the most important paths and fluvial junctions. Since the 2nd century BC Roman colonization transformed the Friulian plain in to an agricultural landscape with field division systems and good road connections; territorial management affected also part of the groundwater-fed rivers that underwent artificial calibration and diversions. The area experienced the most important human impact till the Modern epoch. In the last part of Roman period and especially in the upper Middle Age, a moister and colder climatic phase took place, triggering an important avulsion sequence in the Tagliamento river between 6-8th century AD. In the same period the lagoons rapidly expanded inland, over large sectors which were inhabited until the 4th century and artificially reclaimed only in the 18-20th century.