386 research outputs found
Recenti indagini su contesti archeologici sommersi di età romana nella laguna nord di Venezia
No full textA team coordinated by C. Beltrame and P. Mozzi is working on the investigation of the dynamics of settlement of the lagoon of Venice in the Roman period through an innovative approach which is going to review past discoveries made by E. Canal. The team, which comprehends specialists in the sciences applied to archaeology and which uses innovative techniques for the underwater investigation and documentation, has re-interpreted the so-called Roman submerged Tower of the Canale San Felice and is excavating underwater the so-called Roman Villa of Lio Piccolo. In this last site a tank for oysters has been found while the first site has been interpreted as a well-cistern
AIQUA WORKSHOP: UPDATES ON THE CHRONOLOGY, STRATIGRAPHY AND ENVIRONMENTS OF THE LAST GLACIAL MAXIMUM AND THE LATE GLACIAL ON THE SOUTHERN SIDE OF THE ALPS
No full textThe AIQUA workshop, sponsored by the TERPRO and SACCOM INQUA Commissions and by the IGG-CNR and the Department of Geosciences of the University of Padova, was held at the Department of Geosciences of Padova, on June 22nd-23nd, 2017 (scientific and organizing committee: P. Mozzi, G. Monegato, A. Fontana, S. Rossato). The workshop focused on the geomorphic, sedimentary and palaeoenvironmental events that occurred in the southern Alpine region and related Po Plain, Venetian-Friulian Plain and Adriatic basin during the Last Glacial Maximum and the Late Glacial. About 60 participants attended the workshop, with nine solicited talks, five short communications and posters, and ample time devoted to discussion. The second day consisted in a field trip in the Venetian Prealps
Alluvial megafans in the foreland of Southern Alps
No full text1. Introduction
Along the Southern Alps several large cone-shaped landforms are present. These relate to the alluvial systems fed by the main valleys and presently display an extent of 300-3000 km2, with a length of 30-70 km. We reviewed the available information related to the geomorphological, stratigraphical and geochronological aspects of these alluvial systems with the aim to identify the global forcing factors and the local constraints which influenced their late Quaternary evolution.
The study area extends in the Alpine foreland for about 25,000 km2 and corresponds to the northern sector of the Po Plain from Ticino River to Garda Lake (i.e. the Lombard Plain: LP), the Venetian-Friulian Plain (VFP) and part of the Adriatic shelf. The related Alpine rivers drain a total mountain catchment of about 50,000 km2, with maximum elevations ranging between 2800-4810 m.
2. Results
The largest cone-shaped depositional systems of the VFP have been described as alluvial megafans for their evident longitudinal differentiation (Fontana, et al., 2008; 2010). This characteristic is displayed also by several systems of the LP, but these were before considered only in their gravelly sector (Guzzetti et al., 1997; Marchetti, 2001). The steep piedmont sector consists of amalgamated gravels, while the distal portion has a gradient <2‰ and is dominated by fine sediments.
In all the megafans the major depositional phase occurred between 26-19 ka cal BP, during the LGM marine lowstand, when the Alpine glaciers reached the plain and fed the related glaciofluvial systems. The easternmost megafans partly extended also on the Adriatic shelf whereas, West from Garda Lake, their downstream development was limited by the existence of the Po river plain. The thickness of LGM alluvial sedimentation ranges between 30-15 m and pinches out about 25 km from the present coast.
Soon after the ice decay, since 19-17 ka cal BP, the sedimentary delivery from Alpine catchments to the plain dramatically decreased and in the central Alps large intramontane lakes formed, trapping the bedload almost completely. Thus, an erosive phase affected the whole pede-Alpine sector, leading the rivers to entrench for tens of meters in their LGM megafans. The funneling effect created by the incisions allowed the gravels to arrive tens of kilometers downstream than in LGM. In the Venetian-Friulian megafans a single valley formed in the piedmont sector, while 2-5 fluvial incisions developed in the distal sector. But these incised landforms have been almost completely filled by a depositional lobe formed in the last 8 ka, partly triggered by the marine highstand.
The Alpine tributaries of Po haven’t yet been affected by sea-level influence and they still flow along a single incision from their megafan apex to the junction with Po.
3. Conclusions
Alluvial megafans along the Southern Alps were mainly built by the activity of LGM Alpine glaciers and could be explained as a product of their erosive power in the mountain valleys and their efficiency in evacuate the sediment to the foreland.
Since the glacial withdrawal occurred at the end of LGM
the Alpine rivers incised in their megafans and were more sensitive to the local characteristics of their catchment. In particular the presence of large intramontane lakes, that trapped most part of the sediment.
The main aggrading phase occurred in the megafans during the marine lowstand, while the post-LGM transgression had no important effects until 8 ka cal BP and the marine highstand influenced only the distal sector of the VFP megafans, but not the LP. Thus, in the last glacial cycle the alluvial evolution along Southern Alps was in antiphase in respect to the typical model proposed by sequence stratigraphy.
The coupling between glacial aggradation and interglacial incision occurred also during MIS 6 and MIS 5 and it is likely also for the previous glacial cycles. Thus, along the Alps and in other similar temperate regions, the occurrence of megafans in the Quaternary stratigraphy could be a testimony of the colder phases.
References
Fontana, A., Mozzi, P. and Bondesan, A. (2008) Alluvial megafans in the Venetian–Friulian Plain (north-eastern Italy): Evidence of sedimentary and erosive phases during Late Pleistocene and Holocene. Quaternary International, 189, 71–90.
Fontana, A., Mozzi, P. and Bondesan, A. (2010) Late Pleistocene evolution of the Venetian-Friulian Plain. Rendiconti Lincei, 21 (Suppl.1), 181-196.
Guzzetti, F., Marchetti, M. and Reichenbach, P. (1997) Large alluvial fans in the north-central Po Plain (Northern Italy). Geomorphology 18, 119-136.
Marchetti, M. (2001) Fluvial, fluvioglacial and lacustrine forms and deposits. In: Castiglioni G.B., Pellegrini, G.B. (Eds.), 2001. Illustrative Notes of the Geomorphological Map of the Po Plain. Geografia Fisica Dinamica Quaternaria (Suppl. 7), 73-104
Alluvial Megafans of Europe: Morphologies, Architectures and Formation Phases
No full textAlluvial megafans are not very common in Europe and the use of this term is not diffuse in the continent because of the persistent habit of describing as "fans" event the huge divergent alluvial landforms (>1000 km2). Considering the megafans that are currently still recognizable in the topography, they can be found only in some of the alluvial areas facing the Alps and the Carpathian chain.
Along the southern Alps, megafans are present from Milan (central Po Plain) to the whole Venetian-Friulian Plain. The major landforms are the ones formed by Adda, Olona, Oglio, Adige, Brenta, Piave and Tagliamento rivers. All these systems experienced a strong depositional phase in the LGM (29-17.5 ka BP), when the Alpine glaciers stationed at the mouth of their valleys and the rivers played as glacial outwashes. Sedimentary starvation characterized all the megafans of northern Italy since Late Glacial. Thus, the Alpine megafans can be mainly considered as relict products of the last glaciation, mainly controlled by the climate forcings.
Megafans are documented also in the Little Hungarian Plain (Danube River near Bratislava and Rába River megafans; mainly fed by the Alps) and in the Great Hungarian Plain (e.g. megafans of Maros, Szamos and Timis rivers). The largest megafan is the one formed by the Maros River, which consists of two lobes covering an overall area of at least 7000 km2. The dated traces of braided and meandering channel belts testify a continuative activity until late Holocene. Compared to the Alpine ones, the megafans of the Great Hungarian Plain are fed by larger catchments, but these were not severely glaciated during LGM and sustained important depositional phases also in Late Glacial and after. In the Carpathian Basin the megafan evolution was strongly influenced by differential subsidence
Padova città tre volte murata
No full textLa pianura alluvionale su cui sorge Padova è costituita da diverse unità geomorfologiche di età comprese tra l'ultimo massimo glaciale (29 - 17,5 ka) e l'attuale, relative ai fiumi Brenta e Bacchiglione. Il centro storico della città corrisponde ad un ampio mound archeologico che si è andato formando a partire dall'età del Ferro, in un'area perifluviale già insediata nella tarda età del Bronzo. Lo sviluppo della città antica è avvenuto sulle sponde del F. Bacchiglione, che attraversa Padova dall'inizio del I millennio. La presenza di paleoalvei del Brenta ha condizionato la struttura urbana in età veneto antica, romana e medievale. Alcuni tratti delle mura trecentesche seguono dei paleoalvei
Investigating evidences of Quaternary glaciations in the Prealpine environment of the Astico valley
Get PDFIn the Astico valley (Venetian Prealps) some glacial, fluvioglacial and glaciolacustrine deposits were found. They are attributed to a pre-LGM glacial advance which reached further out in the valley. Geomorphological and stratigraphic data support this hypothesis. It is also pointed out the presence of a tectonic guidance in the developing of the Astico fluvial network
L’assetto geomorfologico della pianura veneta centro-orientale: stato delle conoscenze e nuovi dati
No full textDa: Scritti in memoria di G. Brunett
Sedimentary evidence of glacial lake outburst floods (GLOFs) during the last glacial maximum in the Venetian-Friulian plain (NE Italy)
No full textThe outwash rivers of the Venetian-Friulian plain promoted a high-rate aggradation during the Last Glacial
Maximum (LGM), with large sedimentary bodies fed by major glaciers along the south-eastern side of the Alps.
Glaciers were temperate-type, hold up by high precipitation rates and characterized by large amount of water
trapped in subglacial lakes or funneled in speedy streams. The stratigraphic analysis of these sedimentary bodies
showed sharp changes from coarse gravels to well-sorted sandy gravels few kilometers downstream. Large
boulders, 1–2m in size, are present in many sites, especially in the Tagliamento outwash plain, from the endmoraine
system to about 13 km far to the south. Most of the boulders are located on the eastern area of the
sandur and lie stratigraphically above a paleosol, radiocarbon dated to 31,114 ± 643 cal a BP, thus belonging to
the LGM sandur of the Tagliamento glacier. In some locations these boulders are associated to large bars,
showing a rough cross-bedding. Similar sedimentary structures are observed 2 km downstream of the LGM
frontal moraines in the Astico outwash system, a 25-km-long transfluence tongue of the Adige-Brenta glacial
complex. In this case the occurrence gravel dunes containing many boulders marks the evidence of a high-energy
deposition. The occurrence of cross-bedded bars, embedding large boulders up to 2m of diameter, in the
piedmont plain of the south-eastern Alps is peculiar. Breaks of natural dams, such as the frontal moraines or the
glacier itself, may have triggered glacial lake outburst floods (GLOFs) that determined the formation of such
gravel bars with meter-size clasts
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