1,721,194 research outputs found
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 plain formation during the Late Quaternary between the southern Alpine margin and the Lagoon of Venice (northern Italy)
No full textThe geomorphology of the central Veneto plain has been analysed using a high-resoluti on DEM, remote sensing and field survey. Th e study area consists of three alluvial megafans: the Montebelluna megafan (Piave River , pre-LGM); the Nervesa megafan (Piave River, LGM – Upper Holocene); the Bassano megafan (Brenta River, LGM). The gravelly, coneshape d, steep (0.8-004 %) Montebelluna megafan outcrops just in the piedmont sector, as its distal reaches are buried by the Bassano and Nervesa megafans . These latter extend from the Southern Alps piedmont to the lagoon of Venice. Within a distance of 10-15 km from the Prealpine foothills, their apical parts are cone-shaped and rather steep (0.6- OJ %), consisting mainly of gravels. Through a gradual transition , the distal parts become markedly less steep (reaching values 0.1%) and articulated in systems of fluvial ridges and elongated depressions. The ridges are generally 1 km wide and several km long. They are mainly composed of sandy deposits, while the depressions are silty-clay. The most significant erosive landforms are related to: i) the head trenching of the Bassano megafan , which probably took place at the end of the LGM because of a disequilibrium between solid and liquid discharges of the Brenta river, related to the de-glaciation of the mountain catchment; ii) the downcutting and lateral erosion of the eastern lobe of the Montebelluna megafan during the Upper Pleistocene and the Holocene, due to the tectonic uplift of the piedmont sector between the Aviano and Sacile faults. Active tectonics at the buried Southern Alpine thrust from led to the fault ing of the apical portions of the Momebelluna, Nervesa and Bassano megafans, with the formation of tectonic scarps. Geomorphological evidence of a south-west tectonic tilting of a relict alluvial surface is also detectable in the distal fringes of the Bassano megafan
“La territorialisation hydraulique dans la vallée du Sourou (Burkina Faso) – Lignes pour la recherche”
No full textViene presentato il progetto di ricerca sulla Valle del Sourou, con i quadri di riferimento teorico-metodologici e l'analisi della documentazione di bas
Holocene stratigraphy of the south Venetian Plain: new insight data and correlation
No full textThe late Holocene distal tract of the Adige sedimentary system is bounded to the north by the Brenta River megafan and to the south by the Po sedimentary system (MURST, 1997; FONTANA, et alii, 2008). This latter boundary is not well defined, as branches of the Po River have occasionally intersected the Adige alluvial plain and, vice versa, Adige channels have crossed the Po Plain.
The study area is located within the belt where the Adige and Po rivers have interfingered in the past. This alluvial plain is characterized by a complex network of alluvial ridges formed by the aggradation of sandy and silty channel deposits, natural levees and minor proximal crevasse splays (VEGGIANI, 1972; PERETTO, 1986; MARCOLONGO, 1987; CASTIGLIONI, 2001; PIOVAN et alii 2010). The interdistributary depressions are generally silty-clay, but frequently show evidence of accumulation of organic deposits in extensive swamps or are occupied by dense networks of distal crevasse channels and splays.
High-resolution sedimentological and geochronological data, here presented, allow the description of major alluvial ridges. As well, the correlation among stratigraphical elements is shown in a number of cross-sections.
A stratigraphic marker is provided by a calcic horizon which has been reached at average depth of 7 m in Cona and Conselve cross-sections. It can be correlated with the “caranto” palaeosoil of the Lagoon of Venice (GATTO & PREVIATELLO, 1974; MOZZI et alii 2003), which formed during the sedimentary hiatus and sub-aerial exposure of the alluvial plain between the end of the Last Glacial Maximum and the post-glacial transgression. This buried soil marks the top of the Late Pleistocene sequence in the whole Venetian-Friulian Plain (FONTANA et alii, 2008).
Another important element for stratigraphic reconstruction is a 1 m thick peat layer recognized in the whole study area at the average depth of 5 m. Chronostratigraphic evidence indicates that this major organic-rich sedimentary event in the fluvial series may correlate with the marine maximum flooding surface.
The radiocarbon dating of the bottom of this peat layer is 4435‒5306 cal BP at Cona and 3557‒3699 cal BP at Conselve, just ca. 1 m above the calcic palaeosoil; this points to condensed alluvial sedimentation during the early Holocene. Other ages from the bottom of this layer are 4570‒5330 cal BP in Saline and 5595‒5754 cal BP in Santa Margherita. The radiocarbon dating at the top of the peat provided the ages of 3455‒4094 cal BP in Cona, 3201‒3354 cal BP in Conselve, 4237‒4979 cal BP in Saline and 3158‒3383 cal BP in Santa Margherita cross sections.
Radiocarbon datings evidence that the base of the peat layer is progressively younger in more landward locations. Peat started to form around 5700 cal BP in Santa Margherita, the place nearest to the coastline, at 5300‒4400 cal BP in Saline-Cona and 3600 cal BP in Conselve.
After peat deposition, a phase of major aggradation occurred in the area, which resulted in the formation of alluvial ridges. Radiocarbon datings on organic clays in natural levee deposits in Conselve and Santa Margherita cross sections give the ages of 1950‒2119 cal BP at 3.53 m depth and 1995‒2157 cal BP at 2.5 m depth respectively. This evidences continuous aggradation until Roman times, mainly related to the Adige alluvial system as around 3000 BP the Po di Saline-Cona palaeochannel was not anymore active (PIOVAN et alii, 2010)
L'environnement physique et les problematiques environnementaux dans la gestion des projets d'irrigation
No full textInferring LGM sedimentary and climatic changes in the southern Eastern Alps foreland through the analysis of a 14C ages database (Brenta megafan, Italy)
No full textThe analysis of a database of radiocarbon ages is proposed as a tool for investigating major glaciofluvial systems of the Last Glacial Maximum (LGM) in the Alpine foreland, and their relations with glacier dynamics and climatic fluctuations. Our research concerns the Brenta megafan (NE Italy), where 110 radiocarbon dates integrate a robust regional stratigraphic and palaeoclimatic framework. Age-depth models allowed us to calculate sedimentation rates, while the time distribution of peat layers, which recurrently formed in this region during the LGM, were estimated through meta-analysis. The reliability of statistical results was carefully evaluated using Pearson and Spearman coefficients. Sedimentation rates in the Brenta megafan markedly fluctuated during LGM: ≈1.8 m/ka between 40 and 26.7 ka cal BP; ≈3 m/ka between 26.7 and 23.8 ka cal BP and ≈1.4 m/ka from 23.8 to 17.5 ka cal BP, when the distributary system deactivated due to fan-head trenching. This is evidence that sediment input and routing in the glaciofluvial distributary system was particularly efficient during the central part of LGM, when glaciers were stable at their outermost position. Meta-analysis indicates an increase in peat formation in correspondence with global (Heinrich Event 3 and/or the Greenland Interstadial 5.1 and 4 for the 30.5, 29.6 and 28.8 ka cal BP peaks) and regional (23.5 ka cal BP) wet events. Other peaks at 22.2, 21.8, 20.2 and 19 ka cal BP correlate with fluctuations of south-eastern Alpine glaciers. Significant peat formation continued until ≈18 ka cal BP, when the last peak occurred. A marked decrease in peat formation is recorded concomitantly with the onset of Heinrich Event 2 (i.e. the 26 ka cal BP trough). The good correspondence of sedimentary events in the Brenta glaciofluvial system with the dynamics of glaciers and glaciofluvial and lacustrine systems in the southern Eastern Alps suggests a common climatic forcing on the whole region during the LGM. Peat layer formation in the floodplain fens increased significantly in correspondence with glacier withdrawals and/or wetter climatic episodes, constituting a good proxy for climatic fluctuations during glacial periods. It also allows correlations across different continental environments and regions in the northern hemisphere
Recognizing avulsion events in the Adige River alluvial system
No full textPalaeohydrographical reconstructions of the Adige River alluvial system has recently begun to exploit chronostratigraphy and detailed geomorphological analysis focused on the role of major avulsive events. Following this research line, this work sums up the state of the art and discusses new data at a comprehensive scale
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
Integration of LiDAR and optical remote sensing for the study of fluvial and anthropogenic landforms in the Brenta-Bacchiglione alluvial plain (NE Italy)
No full textThe geomorphological study of alluvial plains takes great advantage from the integration of detailed altimetry with
high resolution images, especially in the lower relief sectors, like those in the distal plain of the Brenta and
Bacchiglione rivers near the city of Padua (mean slope 0.4‰).The LiDAR data which were specifically acquired
for this research (Riegl LMS-Q560; mean density 7 points/m
1101
2
, over 123 km
2
), were classified and interpolated in
order to map fluvial and anthropogenic landforms. The DEM (z accuracy <5-10 cm) was processed and analyzed
in integration with high resolution oblique and vertical (VIS+IR) aerial images, acquired in specific seasonal time
windows, to maximize the vegetation response (cropmarks) to soils, deposits, and palaeohydrographic features.
A detailed field survey (hand augerings, soil profile description, deep borehole stratigraphy) was conducted to
validate the remote sensing interpretations.
The detailed topography allows to individuate low rise interfluves and scarps (<1-2 m), paleochannels, scroll bars
and crevasse splays. The reconstruction of the precise “ground” surface has allowed a better definition of the
geometry of the multi-stratified archeological mound in the historical center of Padua.
Cropmarks result the most appropriate to complete the mapping of the numerous fluvial forms, that have no
topographic relevance being smaller (i.e. minor crevasse channels and splays) and/or flatted by anthropic activity
(mainly ploughing). LiDAR intensity permits a precise individuation of LGM deposits that have shown peculiar
reflectivity related to specific soil characteristics (i.e., presence of calcic and argillic horizons).
At this scale (cell size <0.5-1 m) the real complexity of this highly anthropized low plain environment fully
emerges. The results bring advances in the comprehension of the Holocene evolution of the Brenta-Bacchiglione
plain, also in close relation to ancient settlements
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