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Quaternary International 189 (2008) 71–90

Alluvial megafans in the Venetian–Friulian Plain (north-eastern Italy):Evidence of sedimentary and erosive phases during

Late Pleistocene and Holocene

Alessandro Fontana�, Paolo Mozzi, Aldino Bondesan

Department of Geography ‘‘G. Morandini’’, University of Padua, via del Santo, 26, 35123 Padova, Italy

Available online 14 September 2007

Abstract

The Venetian–Friulian Plain is the eastern part of the foreland basin of the Southern Alps and is characterized by the presence of

alluvial megafans. Existence of these large landforms results from the tectonic setting, but their Late Pleistocene and Holocene evolution

has been mainly controlled by climatic change and eustasy. Geomorphological, geological information and radiocarbon dating allow the

recognition of phases of sedimentation and incision in the megafans of the Brenta, Piave and Tagliamento rivers. The main phase of

aggradation took place during the Last Glacial Maximum (LGM) (24–15 kaBP), when the glaciers emanating from the Alpine valleys

reached the plain and supplied large amounts of sediments. During the Late LGM the rate of aggradation lowered in the Brenta megafan

and a wide incision of the fanhead developed in the Tagliamento megafan. During Late Glacial and Early Holocene an important phase

of incision took place, and smaller telescopic lobes formed in the distal portion of Brenta and Tagliamento megafans. Sedimentation was

absent or very low between 14 and 8 kaBP and only since the Middle Holocene a new phase of deposition affected the coastal areas,

probably related to a marine highstand. Widespread aggradation started once more around 4–3 kaBP, with the formation of fluvial

ridges along the terminal tract of Alpine rivers. Since the Roman period, human influence affected the alluvial sedimentation, especially

in the last centuries, due to land reclamation and construction of river embankment.

r 2007 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction

A number of recent studies have focused on theinvestigation of alluvial megafans in several areas of theworld, concentrating on foreland basins and especially onthe Indo-Gangetic Plain, where these depositional featuresare particularly evident and were first described by Geddes(1960). The earliest papers investigated the main character-istics which differentiate alluvial megafans from other fan-shaped features, i.e. their large size, low-relief morphologyand downstream facies partitioning (e.g. Wells and Dorr,1987; Gohain and Parkash, 1990; Rasanen et al., 1992;Singh et al., 1993; Singh and Ghosh, 1994; Singh, 1996).The petroleum geologists analysed several megafansrecorded in the pre-Quaternary sequences (e.g. Schwans,

e front matter r 2007 Elsevier Ltd and INQUA. All rights re

aint.2007.08.044

ing author. Tel.: +39049 8274085; fax: +39 049 8274099.

esses: alessandro.fontana@unipd.it (A. Fontana),

nipd.it (P. Mozzi), aldino.bondesan@unipd.it

1988; Mohindra et al., 1992; Kronberg et al., 1998;DeCelles and Cavazza, 1999; Horton and DeCelles,2001), while, more recently, the geomorphologists and theQuaternary geologists studied the evolutionary history ofsome active alluvial megafans. Phases of incision anddeposition have been recognized and, in some cases, relatedto climate, eustasy, tectonics or other external forcingfactors (e.g. Gupta, 1997; Shukla et al., 2001; Singh, 2001;Goodbred, 2003; Jain and Sinha, 2003; Shukla and Bora,2003). This provides information on alluvial depositionalprocesses, periods of activity and volumes of involvedsediments.The Venetian–Friulian Plain lies at the north-eastern

end of the Po Plain and constitutes part of the forelandbasin of the Southern Alps (Fig. 1). This stretch of plain,that includes the hinterland of the Lagoon of Venice,consists of several large, fan-like features. These elementshave been recognized since the first half of the 20thcentury (Feruglio, 1925; Comel, 1955) and the term alluvial

fan was used until recent years to describe them (Guzzetti

served.

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Fig. 1. Geological sketch of the Venetian–Friulian area, with cross-section (after Regione Veneto, 1990; Gasperi, 1997; Peruzza et al., 2002, modified).

A. Fontana et al. / Quaternary International 189 (2008) 71–9072

et al., 1997; MURST, 1997a). Fan-shaped morphologieswere considered to extend only from the valley outletas far as the limit of gravel sedimentation (the lattercoinciding with the spring belt in Fig. 2), whereas,downstream, an undifferentiated alluvial plain was de-scribed (e.g. Guzzetti et al., 1997). The completion of thegeomorphological map of the Po Plain, which includes alsothe Venetian–Friulian Plain (MURST, 1997a; Castiglioni,1999; Castiglioni and Pellegrini, 2001), and of the relatedmicrorelief map (MURST, 1997b), allowed the recognitionof the continuity of some alluvial fan-shaped features as faras the coastal area. Thus, these landforms appearedremarkably larger and the drastic diversity of texturebetween their apical and distal portions was highlighted(Mozzi, 1995; Castiglioni and Pellegrini, 2001). Thesedepositional features have been recently described asalluvial megafans because of their dimensions, longitudinal

continuity and internal downstream facies changes; actu-ally megafans are differentiated from the smaller, gravellyalluvial fans of the piedmont zone (Fontana et al., 2004;Mozzi, 2005).A number of papers concerning the Venetian–Friulian

Plain have been recently published (Avigliano et al., 2002;Bondesan et al., 2002; Michelutti et al., 2003; Mozzi et al.,2003; Bondesan and Meneghel, 2004; Bondesan et al.,2004a, b; Ragazzi et al., 2004; Mozzi, 2005; Fontana,2006), but they consider only limited sectors or specificaspects. Most of them are published in local reviews or inItalian monographs which are not easily available forinternational researchers. This paper is an attempt toassemble these data in a general synthesis of the area,considering the geochronology, the stratigraphy and thegeomorphology of the main alluvial megafans of theVenetian–Friulian Plain from the Last Glacial Maximum

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Fig. 2. Scheme of the Late Quaternary depositional systems of the Venetian–Friulian Plain. In the down-right corner, a simplified sketch of the alluvial

megafans and fans is shown. Symbols: (1) river, (2) fluvial scarp, (3) upper limit of the spring belt, (4) mountains and hills, (5) tectonic terraces, (6) end-

moraines systems, (7) interfan and intermontane deposits, (8) coastal-deltaic systems and (9) groundwater-fed river systems. Grey-tone areas: (A) Adige

Alluvial Plain, (B) Brenta megafan, (C) Astico fan, (D) Montebelluna megafan, (E) Piave megafan, (F) Monticano–Cervada–Meschio fan, (G) Cellina fan,

(H) Meduna fan, (I) Tagliamento megafan, (L) Corno fan, (M) Cormor megafan, (N) Torre megafan, (O) Isonzo megafan and (P) Natisone fan.

A. Fontana et al. / Quaternary International 189 (2008) 71–90 73

(LGM, corresponding to the Marine Isotopic Stage 2:MIS 2). The work highlights the similarities and thedifferences of the analysed megafans, compares theregional setting with the global glacio-eustatic cycle andtries to establish the driving forces in the development ofsedimentation and erosive phases.

2. The study area

2.1. Geographical and geomorphological setting

The Venetian–Friulian Plain is the eastern sector of thePo Plain sensu latu, even if it has been formed by

the deposits of Alpine rivers which are not tributaries ofthe Po River (Castiglioni, 1999). The investigated areaextends for about 10,000 km2 from the Karst MountainFringe, close to the Slovenian border, to the Berici andEuganei Hills. The Isonzo, Tagliamento, Piave and Brentaare the major Alpine rivers flowing in the area (Table 1).These rivers drain a total basin of about 12,000 km2 ofunglaciated mountain terrain. Large parts of the coastalplain, almost 4000 km2 between the Adige and IsonzoRivers, are lands situated below sea level, which have beenreclaimed. Mean annual rainfall is important if comparedboth to the Mediterranean and central Europe values, andit is characterized by a maximum during autumn and

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Table 1

Hydrologic and drainage basin characteristics of the main Alpine rivers of the Venetian–Friulian Plain (after Negrisin and Stefani, 1974; Surian and

Rinaldi, 2003)

River Drainage

basin area

(km2)

Length (km) Maximum

basin relief (m)

Mean annual

precipitation

(mm/yr)

Mean annual

discharge

(m3/s)

Flood peak

discharge

(m3/s)

River bed

elevation at

valley mouth (m)

Isonzo 3430 140 2760 1800 230 4400 65

Tagliamento 2580 172 2780 2150 109 4500 130

Piave 3899 222 3162 1330 132 4250 85

Brenta 1787 160 3079 1386 71 2810 130

Adige 11,954 410 3890 933 220 4000 100

A. Fontana et al. / Quaternary International 189 (2008) 71–9074

a secondary peak during spring (Fazzini, 2005); very highand concentrated precipitation may cause extreme floodevents in the plain.

The catchment basins of the Venetian–Friulian Plainconsist mainly of carbonate rocks (limestones and dolo-mites) (Fig. 1) which produced the supply of large amountsof clastic debris which consequently led to the deposition ofhundreds of metres of gravel in the piedmont plain. Thissector, called the ‘‘high plain’’, has a dry surface because ofthe high permeability of gravelly sediments and theconsequent depth of the groundwater table. Thus, eventhe main rivers are often dry from their valley outlet as faras the threshold where the gravels of the ‘‘high plain’’ passto the silty–clayey sediments of the ‘‘low plain’’. Thislithological transition forces the groundwater table to riseto the surface, supplying a number of springs which liealong a belt. This ‘‘spring belt’’ extends from the east to thewest across the study area, with a downstream width of3–15 km (Fig. 2). In the low plain the water table generallylies between 1.5 and 3m from the surface and manygroundwater-fed rivers are present. These are intrabasinalstreams, meaning that they only rework the deposits of thelarge Alpine rivers without bringing new sediments to thealluvial basin. All are characterized by a rather steady flowduring the year, reaching 20–50m3/s for the Stella,Lemene, Sile and Bacchiglione Rivers.

2.2. Tectonic framework

The Venetian–Friulian Plain represents the surface of theTertiary to Quaternary sedimentary infilling of the forelandbasin of the eastern sector of Southern Alps, a south-vergent thrust belt chain which developed since thePalaeogene (Massari, 1990; Doglioni, 1993). The presentAlpine thrust front is buried under the piedmont alluvialplain but, in the eastern sector, some of the more externalthrusts partly outcrop in the middle of the Friulian Plain,creating several narrow uplifted tectonic terraces nearUdine (Zanferrari et al., 1982) (Figs. 1 and 2). In the Friuliarea, the Alpine Neogene structures interfered with theDynaric tectonics, the latter characterized by west-vergentthrust tectonics of the Palaeogene Age (Peruzza et al.,2002). The southern sectors of the Venetian Plain havebeen influenced since the Upper Miocene by the activity of

the northward expanding Apennine foredeep, being thussubjected to southward tilting; the influence of Apenninetectonics is felt up to the Venice area (Doglioni, 1993).Subsidence, also driven by sediment compaction, was

continuous during the Quaternary and still strongly affectsthe coastal areas (Bortolami et al., 1977, 1984; Carbogninand Tosi, 2002).

2.3. The Last Glacial–Interglacial cycle

The uppermost pre-Holocene marine deposits arerepresented by the shallow-sea and lagoon sediments ofthe Last Interglacial (MIS 5e—Eemian). East of the PiaveRiver they are at a depth of about 35–50m below sea level(Feruglio, 1936), while in the Venice area they lie at ca.70–80m (Massari et al., 2004), and in the Po Delta at100–120m (Amorosi et al., 2004; Ferranti et al., 2006).The LGM glacio-eustatic sea lowstand brought the

whole northern Adriatic shelf under continental conditions(Correggiari et al., 1996). Between 21 and 18 kaBP, thealluvial plain extended to about 300 km south of thepresent Venetian coastal area. In that period, the glacier inthe Tagliamento catchment reached the plain and built alarge end-moraine system, feeding several fluvioglacialoutlets; the directions of the main ones correspond to thepresent Torre, Cormor, Corno and Tagliamento Rivers(Comel, 1955). In contrast, the Isonzo glacier ended withinthe valley, tens of kilometres upstream from Gorizia(Bavec et al., 2004) (Figs. 2 and 6). The lower portion ofthe Piave glacier was divided in two main branches: aneastern branch that followed the Val Lapisina and reachedthe plain near Vittorio Veneto and a western branch thatfollowed the present Piave valley and built end-moraineswithin the valley, 12 km from the plain (Casadoro et al.,1976; Venzo, 1977; Pellegrini et al., 2005). Though withsome uncertainties, all studies undertaken agree that theBrenta glacier most probably ended near Valstagna, about10 km upstream from the valley mouth near Bassano(Trevisan, 1939; Dal Piaz, 1946).After about 8kaBP the Adriatic coastline moved north-

ward, from the position now occupied by the Po Delta, as aconsequence of the post-Glacial sea-level rise (Correggiariet al., 1996). The Lagoons of Venice, Caorle and Maranostarted their formation during the Holocene marine highstand,

ARTICLE IN PRESSA. Fontana et al. / Quaternary International 189 (2008) 71–90 75

at about 6.0–4.5kaBP (Favero and Serandrei Barbero, 1980;Marocco, 1991; Galassi and Marocco, 1999; SerandreiBarbero et al., 2001; Fontana, 2006).

3. Methods

The research was carried out using the geomorphologi-cal, the geopedological and the geoarchaeological methodsof investigation. The use of microtopography was helpful,especially in the distal sectors of the plain, where thelandforms are characterized by very low elevation andslope that have often no evidence in the field. Large sectorsof the Venetian–Friulian Plain have been analysed using ahigh-resolution digital terrain model (pixel 30� 30m)derived from microrelief maps, with 0.5m (below 10masl) and 1m (above 10m asl) contour lines.

The recognition of palaeochannels and other landformsrelated to palaeohydrography (e.g. fluvial scarps, incisedvalleys and alluvial ridges) has been mainly based onremote sensing. Ancient coastal dune systems, lagoonchannels and archaeological tracks (e.g. ancient road,canals and field divisions) were recognized in the effort toreconstruct chronological relationships existing betweendifferent morphological elements. Remote sensing con-sisted of interpretation of 1:20,000 to 1:10,000 verticalaerial photographs taken in a series of flights dating from1954 to 2003. Some Landsat TM 5 and SPOT 3 imageswere processed to get a general view of the investigatedarea and to distinguish large morphological elements suchas sectors of megafans with homogeneous characteristics,meander belts and fluvial ridges. Thus, parts of palaeo-channels identified separately in the aerial photographscould be recognized as portions of the same meander belt.The results of microrelief analyses and remote sensing havealready been partly published in the geomorphological mapof the Province of Venice (Bondesan et al., 2004b) and ofthe Friulian low plain (Fontana, 2006).

The geological and soil surveys covered the whole distalsectors and some portions of the apical areas of themegafans (Mozzi, 1995; Avigliano et al., 2002; Micheluttiet al., 2003; Bondesan et al., 2004a, b; Ragazzi et al., 2004;Fontana, 2006). The field work consisted of describing andsampling stratigraphic sections and soil profiles from 1.5 to5m depth. Many of them have been described duringplanned surveys, but significant data were collected also insome temporary outcrops exposed during the constructionof gas pipelines, houses, roads and archaeological excava-tions. Sediment colour and texture, vertical and laterallithofacies relationships, primary sedimentary structuresand the type and concentration of accessory materials (e.g.roots, plant debris, organic matter and palaeosols char-acteristics) were used as basic tools for stratigraphiccorrelation.

The extensive use of hand augering with the Edelmangauge, down to 3–9m depth, led to the reconstruction ofdetailed stratigraphic sections, hundreds of metres long,crossing abandoned channels, fluvial terraces and other key

elements for the fluvial evolution, as already successfullyaccomplished in other alluvial and coastal plains such asthe Rhine–Meuse Delta (Berendsen and Stouthamer,2001). Some drill cores (down to 30–50m) have beenconsidered in the aim of understanding the regionalstratigraphic setting and measuring the depth of themeandering palaeochannels in the distal sectors, where itcan exceed 20m. New boreholes were funded by the VenetoRegion (CARG Project) and by the Province of Venice(LEO-DOGE Project) which also made available itsdata base of geological drills and dynamic and staticpenetrometries. Further information was obtainedfrom the wells and drills data base of Friuli VeneziaGiulia Region (Regione Autonoma Friuli Venezia Giulia,1990). The integration of remote sensing, microreliefanalysis and field surveys has allowed the recognition ofthe sedimentary facies associated with different palaeohy-drographic features.Investigations mainly concentrated on the distal portions

of the megafans, which represent the area characterized bysilt and clay sediments. In these sectors palaeosols andorganic layers are often recorded in the stratigraphy,allowing radiocarbon, pollen and micropalaeontologicalanalyses. In the apical portion of the megafans, character-ized by gravelly sediments, very rare organic layers areavailable and age estimates are mainly based on the relativeage of the landforms and on the degree of soil development(Avigliano et al., 2002; Ragazzi et al., 2004).In the framework of the geological and geomorphologi-

cal surveys, about 100 samples of peat, woods and organicsediments have been radiocarbon dated with the conven-tional and the AMS methods in order to assess the age ofthe activation and disactivation of river channels and/oralluvial phases. The results of these 100 dates, with other150 radiocarbon dates derived from literature dealing withalluvial evolution, are plotted in Fig. 3. Radiocarbon datesare expressed in uncalibrated 14C kaBP. The upperdiagram shows the frequency of 14C dates in intervals of500 yr while the lower diagram is a plot of age vs. depth.The average value of the uncertainties of the 250 radio-carbon dates is 780 yr and also the value of the statisticmode of the uncertainties is about 780 yr. In the lowerdiagram of Fig. 3 each cross represents a date and thelength of the cross corresponds to 500 yr and, therefore,due to the scale of the diagram, the uncertainty of eachdate is included in the cross. Only few dates have anuncertainty larger than 7250 yr and they generally have anage older than 25.0 kaBP. Only a few dates are presentedalong the text in some of the stratigraphic sections but, dueto the large number of radiocarbon dates considered andthe large extent of the area they represent, Fig. 3 could beconsidered as a good tool to highlight the different phasesof the alluvial evolution.The pollen and the palaeobotanical analyses provided

further chronological and palaeoenvironmental informa-tion, especially focused on LGM and the Middle and theUpper Holocene (Miola et al., 2006). Important proxies of

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Fig. 3. Diagram of 250 14C dates performed on the alluvial sequences in the Venetian–Friulian Plain. Above: frequency of dates grouped in 500 yr clusters;

below: depth vs. age. Dates are expressed in uncalibrated 14CkaBP.

A. Fontana et al. / Quaternary International 189 (2008) 71–9076

the age and stability of surfaces are provided bygeopedological studies (Mozzi et al., 2003) and by thestudy of archaeological sites and their stratigraphicposition (Bondesan and Meneghel, 2004; Bondesan et al.,2004b; Fontana, 2006). The original extent and character-istics of the portions of the megafans drowned by theHolocene transgression have been inferred using marinegeology information based on shallow gravity cores andhigh-resolution seismic profiles (Correggiari et al., 1996;Gordini et al., 2002, 2003).

4. Late Pleistocene and Holocene evolution of alluvial

megafans

4.1. General characteristics

In the study area, the heads of alluvial megafans andfans are located at the outlets of the major Alpine valleys,which are quite regularly spaced with intervals of20–40 km. The main axis of the megafans are more than50 km long. During the LGM sea lowstand they wereconsiderably longer, as they probably continued for10–30 km on the northern Adriatic shelf before joiningthe Po River system (Table 2).

Within about 10–25 km from the mountain front, themegafans are cone shaped and relatively steep (topographicgradient: 7–3%), consisting of gravel deposits hundreds ofmetres thick. Sections exposed in gravel pits and strati-graphic logs of water wells show that the uppermost 50mare organized in 1–2m thick, few to tens of metres widechannel bodies, stacked in continuous sequences withscattered sandy and silty lenses. The analysis of the surfacethrough remote sensing and field surveys indicated thepresence of palaeochannels over almost the whole surface ofthe upper part of the megafans. The traces are tens tohundreds of metres wide and they often cross each other.The information matches well with a braided-type fluvialsystem, similar to that which still characterizes the Alpinerivers for a distance of 30–50 km from the Alpine piedmont.Further downstream, south of the spring belt, the

general slope of the plain gradually decreases to less than1%. The main landforms are sandy fluvial ridges, separatedby silty-clay floodbasins. The palaeohydrography consistsof the abandoned river beds of the single-channel type,either elevated or less often incised, with varying sinuosity.They can be continuous for several kilometres. Theresulting sedimentary architecture of the lower sectorsconsists of lenses of sandy channel deposits, generallyscarcely interconnected, within overbank fines. The

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Table 2

Characteristics of the main alluvial megafans of the Venetian–Friulian Plain

Alluvial system Drainage

basin area

(km2)

Alluvial

megafan

(km2)

Maximum

length (km)

Maximum

width (km)

River bed

elevation at valley

mouth (m)

Maximum elevation,

uppermost terrace (m)

Topographic

gradient of apical

portion (%)

Tagliamento 2580 1200 65 40 130 155 7–4

Piave 3899 1050 55 35 85 100 5–3

Brenta 1787 2600 75 50 130 145 6–4

Fig. 4. Geomorphological sketch of central Venetian Plain (modified from Mozzi, 2005).

A. Fontana et al. / Quaternary International 189 (2008) 71–90 77

Venetian–Friulian megafans show a downstream change influvial style and sedimentary facies similar to thatrecognized in the megafans of the Indo-Gangetic Plain(Singh et al., 1993); but differ from most of the megafansdescribed in the literature because of their relatively smalldimensions; in fact, the extent of megafans recognized innorth-eastern Italy ranges between 1000 and 2500 km2.

Each of these megafans has a complex history. Theyconsist, both at the surface and in the stratigraphy, oflaterally and vertically juxtaposed sedimentary bodies ofdifferent ages, spanning the Upper Pleistocene to thepresent. The Brenta megafan shows such a large and well-outlined LGM portion, that it has been recognized as asingle-phase megafan: the ‘‘Bassano megafan’’ (Mozzi,2005) (Figs. 4 and 5). In this paper the Bassano megafan is

regarded as the main aggradation phase of the Brentasystem. In the Tagliamento megafan, the inset of the LGM,Late Glacial and Holocene alluvia in cut-and-fill sequences,or their deposition on the top of the older deposits, resultsin more complex sedimentary and geomorphic geometries.No single-phase megafans are recognizable. The Piavedepositional system is further complicated by the switch ofthe river’s entry onto the plain from the west to a positioneast of Montello Hill (Figs. 2 and 6). This occurred becauseof a fluvial capture by the Soligo River on the left side ofthe Piave induced also by the tectonic uplift of theMontello Hill (Zanferrari et al., 1982). The westerndrainage direction was active before ca. 30 kaBP, mostprobably still in the Upper Pleistocene, and formed the so-called Montebelluna megafan (Mozzi, 2005). Only the

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Fig. 5. The 3D display of the digital terrain model of the area represented in Fig. 4. In the left the Brenta megafan is represented, with the apical portion

characterized by the post-LGM incision still occupied by the river; the distal portion has a lower gradient and has a rolling topography which consists of

several fluvial ridges. The megafan of Brenta covers the distal portion of the older megafan of Montebelluna and the contact between them is evidenced by

the Sile River upstream from Treviso. Dowstream, the same river represents the limit between Brenta and Piave megafans.

A. Fontana et al. / Quaternary International 189 (2008) 71–9078

gravelly, cone-shaped, upper reach of the Montebellunasystem outcrops; both the surface and stratigraphicevidences indicate that the distal portions of this megafanare buried by the younger deposits of the Brenta and PiaveRivers (Bondesan et al., 2002). In this paper, the expression‘‘Piave megafan’’ refers only to the megafan with the apexlocated east of the Montello Hill, near Nervesa, andcorresponds to the ‘‘Nervesa megafan’’ described in Mozzi(2005) (Fig. 2).

In the following paragraphs, a concise overview of theevolution of the Brenta, Piave and Tagliamento megafansduring the last 30 kaBP is presented. These are the mainand best understood megafans of the area.

4.2. Pre-LGM (o24.0 ka BP)

The only pre-LGM fluvial depositional system out-cropping in the study area is the Montebelluna megafan,formed by the Piave River into the plain west ofthe Montello Hill. This age estimate is based on thestratigraphic correlation of the distal tracts of theMontebelluna megafan buried by the LGM deposits ofthe Piave and Brenta megafans. The corings in the Piavemegafan show that the top of the Montebelluna gravelslies few metres below a peat level radiocarbon dated31,63071100BP (Bondesan et al., 2002). The high degreeof soil development in the Montebelluna megafan (Alfisols

with 30–50 cm thick Bt argillic horizon) (Regione Veneto,2005) is consistent with the stratigraphic evidence of thisbeing the older depositional system in the Venetian Plain.Considering the whole study area, little information on

the pre-LGM alluvial evolution is available because of therelatively low number of deep cores. The scatter of theradiocarbon age estimates shows a fairly good linearrelationship between age and burial depth of the sediments(Fig. 3), suggesting that sedimentation was rather contin-uous during the pre-LGM period.During MIS 3 the Tagliamento River had an eastern

valley outlet; it shifted to the present position near Pinzano(Fig. 2) about 19.0 kaBP, during the peak of the LGM(Paiero and Monegato, 2003).

4.3. LGM (24.0–14.5 ka BP)

During the LGM the Glacial and the Periglacialenvironmental conditions allowed a substantial productionof debris in the Alpine area. The delivery of this debris tothe Venetian–Friulian alluvial basin resulted in an im-portant phase of vertical aggradation, well dated thanks tothe numerous radiocarbon dates of peat layers in the wholedistal plain (Fig. 3). Due to the sea lowstand, fluvialaggradation extended onto the North Adriatic shelf. LGMsediments are largely sub-outcropping in the Adriaticbottom below the 15–20m bathymetric contour, and wide

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Fig. 6. Age of the alluvial surfaces in the Venetian–Friulian Plain. (1) River, (2) bathymetric lines, (3) fluvial scarp, (4) upper limit of the spring belt,

(5) interpreted limit of maximum Holocene transgression at 6.0 kaBP ca., (6) cross-sections cited in the paper, (6a) location of the fluvial ridge of Mezzavia

cited in Section 4.6, (7) tectonic terraces, (8) pre-LGM, (9) LGM end-moraines systems, (10) LGM and (11) post-LGM.

A. Fontana et al. / Quaternary International 189 (2008) 71–90 79

thin layers of peat, dated between 20 and 18 kaBP, areoften present under some decimetres of marine sediments,separated by a ravinement surface (Correggiari et al., 1996;Gordini et al., 2002).

The heads of the Brenta and Piave megafans were locatedat the mouth of the Alpine valleys (the latter river was at theeastern outlet near Nervesa; as already described, theMontebelluna megafan was active just in pre-LGM times);distances from the end-moraines ranged from 10 to 20km.The Tagliamento megafan had its fanhead in a gorge locatedat the western end of the LGM end-moraines system, so thatduring the LGM it was directly connected to the glacier andworked as the main fluvioglacial outlet.

The active portions of the Brenta megafan during theLGM had an extent of about 2500 km2 down to the present

coastal margin (Fig. 2). This extent is similar to the areacovered by the overall fluvioglacial outwash of theTagliamento glacier (2300 km2), but it is double the areaof the Tagliamento megafan (1200 km2). This puzzlecan be easily understood considering that the Tagliamentomegafan was just one of the four major glacial streamsfed by the glacier, while the others correspond to thepresent Corno, Cormor and Torre streams. In contrast,the Brenta valley funnelled all the melting waters from theglacier to the plain. The Piave megafan as well was onlyone portion (1100 km2) of the whole fluvioglacial system ofthe Piave glacier, while the rest was the sandur of theVittorio Veneto end-moraines (250 km2). Thus, the PiaveFluvioglacial Plain was about half the size of the Brentaand Tagliamento Fluvioglacial Plains.

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The apical and distal portions of the megafans formedduring the LGM are well differentiated. In the apicalportion, both the palaeohydrographic pattern inferredfrom remote sensing and the stratigraphy of the graveldeposits indicate that the rivers were of the braided type.They could move gravels and blocks, but their transportcapacity rapidly decreased downstream, and the rivers lostlarge part of their sedimentary yield within 15–25 km fromthe fanhead. Further downstream no gravels are found inthe LGM alluvium and only sandy sediments characterizechannel fillings. In the lower sectors of the PleistoceneBrenta megafan there is evidence of downstream sorting ofboth the channel sands and the overbank fines. Within ca.15 km, the former passes from coarse sand with pebbles tofine sand, while the clay percentage in the latter increasesfrom o20% to 440% (Ragazzi et al., 2004).

In the distal sectors, in the aerial photographs the tracesof rivers dating to LGM are usually single channel with low

Fig. 7. Cross-section in the Ca’ Tron area, in the distal sector of Piave megaf

expressed in 14C yrBP uncal. and lab-codes are reported. (1) 17,5307120

(4) 16,190750 Beta-170848, (5) 17,9207130 Beta-173735, (6) 19,7707140 Be

(modified from Bondesan et al., 2004a).

sinuosity; sometimes traces of fluvial islands are present,suggesting a sort of wandering style river. Channel beltswere slightly elevated over the plain, which led to theproduction of fluvial ridges that are 1–3m higher than thesurrounding plain, tens to hundreds of metres wide andseveral kilometres long. Examples of the LGM alluvialarchitecture can be seen in Figs. 7 and 8, concerning thelower reaches, respectively, of the Brenta and Piavemegafans. At the surface, both areas consist of low, sandyalluvial ridges separated by extensive silty-clay floodbasins(Fig. 4). The channel bodies consist of medium and finesilty sands; they are usually o2–3m thick, scarcelyinterconnected and separated by clayey–silty sediments;this situation could be interpreted as an indication of anavulsion-dominated system, as an extensive meandermigration would had rather formed continuous tabularsand sheets than separated channels (Miall, 1996; Berend-sen and Stouthamer, 2001).

an (for location, see Figs. 4 and 6). Conventional radiocarbon dating are

Beta-173736, (2) 3650740 Beta-173729, (3) 20,3007220 Beta-173730,

ta-169479, (7) 21,1507190 Beta-169480 and (8) 20,9707140 Beta-169481

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Fig. 8. Cross-section in the Pleistocene distal reaches of the Brenta megafan (for location, see Figs. 4 and 6). (1) Channel deposits (sand, silty sand),

(2) overbank, crevasse and natural levee deposits (sandy silt, silt), (3) floodbasin deposits (silty clay), (4) peat, (5) stratigraphic correlation of the top of the

peat layers and (6) borehole; radiocarbon dates are expressed in 14C yrBP uncal. (Bondesan et al., 2002).

Fig. 9. Simplified scheme of the telescopic alluvial megafan of Tagliamento River. (1) LGM gravels and sands, (a) LGM clayey silt, (2) Late LGM gravels

and sands, (a) Late LGM clayey silt, (3) post-LGM gravels and sands, (a) post-LGM fine sediments, (4) peat and organic sediments, (5) lagoon and

shallow-marine deposits dating to MIS 5 and (6) hypothetical limit between LGM and pre-LGM deposits.

A. Fontana et al. / Quaternary International 189 (2008) 71–90 81

An interesting characteristic is the abundance of thin peatlayers and their significant lateral continuity. In the wholestudy area, the cold LGM climate and the high fluvioglacialdischarges of pensile fluvial channels probably triggered therising of the groundwater table which, in turn, kept soils ofdepressed areas waterlogged (Miola et al., 2006). Fensdeveloped in these badly drained depressions in the alluvialplain, leading to peat formation. Because of the high

aggradation rates, some fens were probably active only forfew centuries before being buried by alluvial sediments. Theresulting peat and/or organic layers, usually o20 cm thickand embedded within overbank fines, have been documen-ted to extend for 104–106m2 in different sites in theVenetian–Friulian Plain. Almost all the LGM radiocarbondates represented in the diagram (Fig. 3) have been carriedout on these ubiquitarious peat layers (e.g. Figs. 7–9).

ARTICLE IN PRESSA. Fontana et al. / Quaternary International 189 (2008) 71–9082

In the distal sector of the megafans the numerousradiocarbon dates available show that the LGM depositshave a thickness of 15–30m. A rough estimate of thesedimentation rates occurred during the LGM has beenobtained considering some long corings, carrying outsome radiocarbon dates on different levels of the samestratigraphic sequence. Even if correction for sedimentcompaction and subsidence was not considered, the roughaverage sedimentation rate is at least 2–3mm/a, with peaksof 10mm/a (Bondesan et al., 2002, 2004a; Bondesan andMeneghel, 2004).

The Piave glacier maximum advance in the area ofVittorio Veneto dates to 17.6 kaBP (Bondesan et al., 2002),while by 16–15 kaBP the glacier had likely disappearedfrom the main valley (Casadoro et al., 1976; Pellegriniet al., 2005). In the Nervesa megafan, the very intenseLGM sedimentary phase stopped at around 16 kaBP,showing a good connection between glacial activity andalluvial aggradation.

The Tagliamento glacier reached its maximum extentbetween 19 and 18 kaBP. Its regression phase startedshortly after (Paiero and Monegato, 2003), during the LateLGM. In this phase of deglaciation, the TagliamentoRiver, as well as all the other fluvioglacial outlets of thisglacier (Corno, Cormor and Torre Rivers), downcut apexesof their depositional systems. The entrenchment of themain rivers led to the deactivation of large sectors of thehigh plain. Apexes of the younger lobes of the megafanswere encased in the valley cuts and sedimentation shiftedtens of kilometres downstream (Fig. 9). Aggradation tookplace from the spring belt, where systems of fluvial ridgesburied the silty–clayey LGM surface and in some zonesthey buried also layer of peat or organic clay with aradiocarbon dating between 18.0 and 15.0 BP. In themegafans of Tagliamento and Cormor several Epigravet-tian and Early Mesolithic archaeological sites were foundon the top of the natural levees and channels (Ferrari and

Fig. 10. Simplified cross-section of a Late LGM fluvial ridge of Tagliamento

cross-bedding), (2) natural levee (silty sand), (3) overbank floodplain (clayey silt

(6) peat, organic clay and (7) carbonate calcium concretions (Fontana et al., 2

Pessina, 1992), showing that fluvial activity had alreadyceased at least by 12–10 kaBP (Fig. 10). In the deglaciationphase, gravels could be transported further downstreamthan during the LGM, reaching the present day lagoonarea. The fluvial ridges which formed in the distal plainwere characterized by a single channel with gravelly sand asfar as the present lagoon area, with a maximum grain sizeof 1–2 cm (Fig. 10). This may be due to the channelling ofthe river within the terraces for several kilometres from theAlpine piedmont, which led to the confinement of the waterflow and consequent higher velocity and higher transportcapacity. Longitudinal bars, scour-and-fills and reactiva-tion surfaces are the most common sedimentary structuresobserved in the channel deposits, which have a mean depthof 2–4m and a width of 40–250m. The natural levees aresandy loam, have a length of 30–100m, and sometimescross-bedding is present. The related alluvial plain is 1–2mthick and is characterized by tabular layers of clayey silt;on the natural levees and on the alluvial plain the soil ischaracterized by horizons with pluricentimetric carbonatecalcium concretions whose formation is related to the plantactivity and the existence of some less permeable layers.No data are available concerning the timing of the LGM

Brenta glacier dynamics, while more information isavailable on the megafan evolution in the same time span.The alluvial chronostratigraphy indicates that, between 22and 18 kaBP, the deposition of 15–20m of sediments tookplace in the distal portions of Brenta megafan. Between 18and 14.5 kaBP the vertical aggradation was less than 5m.This evidence points to a drastic decrease in the sedimenta-tion rates immediately after 18 kaBP. The cross-sectionof Fig. 8 indicates the presence of extensive peatlevel radiocarbon dated 17.0–16.0 kaBP, which can becorrelated on a distance of about 4 km (Bondesanet al., 2002). The formation of peat took place in fenswith low minerogenic sedimentation (Miola et al., 2006).Afterwards, fluvial activity started again, leading to the

megafan (for location, see Fig. 6). (1) Channel deposit (gravelly sand with

), (4) LGM floodplain (clayey silt), (5) channel deposit (fine–medium sand),

004).

ARTICLE IN PRESSA. Fontana et al. / Quaternary International 189 (2008) 71–90 83

deposition of three channel bodies and connected overbankfines. The 14C dating of occasional lenses of peat,embedded in the overbank deposits, shows that thislast sedimentary event took place between 16.0 and14.5 kaBP.

Even if in the central and western Po Plain loess coverdating to LGM is diffused (Cremaschi, 1987), in theVenetian–Friulian Plain there is no evidence of loessdeposition (MURST, 1997a; Castiglioni and Pellegrini,2001); in a very limited sector at the southern foot of theEuganean Hills some small Pleistocene continental duneshave been described, but their origin has not yet beenstated (Rizzetto et al., 1998).

4.4. Late Glacial and Early Holocene (14.5–8.0 ka BP)

The Brenta and Tagliamento megafans have been bothaffected by a major phase of incision since the Late Glacialand during the Early Holocene, but some differencesbetween the two alluvial systems exist. The last dateddepositional event in the LGM lobe of the Brenta megafan(‘‘Bassano megafan’’ in Mozzi, 2005) took place at about14.5 kaBP (Fig. 8); the following deactivation of themegafan was due to the fanhead trenching at the Brentavalley mouth near Bassano. The erosional scarp related tothe river downcutting is still more than 15m high at themegafan apex, and decreases gradually until it disappearsat about 25 km downstream. Where the scarp is present,the Late Glacial–Holocene Brenta floodplain lies at itsfoot. Further downstream, the Holocene deposits cover theLGM surface. Other 2–4m high scarps are located in theapical portion of the LGM megafan, creating three majorterraces with a rolling topography. Therefore, consideringthe LGM and the post-LGM deposits, a telescopicgeometry of the megafan is recognized.

In the Tagliamento megafan, the Late Glacial erosivephase enhanced the incision already developed during thefinal part of the LGM and further confined the active riverchannel into the apical portion. The elevation of the erosivescarps is 60–70m at the fanhead and is still ca. 30m as faras about 10 km downstream, then it progressively de-creases, until disappearing close to the spring belt. As aresult, the fanhead of the Late LGM lobe has beenentrenched as well and sedimentation shifted furtherdownstream. Thus, the Tagliamento alluvial megafanconsists of three different depositional bodies: theoldest dates to LGM, the one active during the deglaciationphase between the final part of the LGM and the beginningof the Late Glacial, and the last, active since theLate Glacial (Fig. 9). Apexes of younger units are inset inthe older ones and a telescopic geometry is clearlyrecognizable.

Since the Late Glacial, 5–10 km downstream from thespring belt the Alpine rivers have been characterized bymeandering channels. Until the Middle Holocene, riversshowed an erosive tendency also in their lower tract,incising beneath the LGM surface by 5–25m. Post-Glacial

erosion was particularly important in the distal sector ofthe Tagliamento megafan, where the formation of severaldeep and wide valleys took place (Figs. 9 and 11).Probably, these landforms developed mostly during theLate Glacial, but they have been in use or reactivatedduring later periods, causing an internal aggradation thatfilled up the incisions and often deleted their topographicalevidence. Upstream from Portogruaro two ancient valleysare still well recognizable thanks to the preservation oftheir surface morphology, owing to an early deactivationand a secondary use by groundwater-fed rivers. In thelower tract, these depressions silted out because of theHolocene lagoon deposition and the post-Roman fluvialsedimentation (Fig. 11). Five kilometres downstreamfrom Portogruaro, near Concordia Sagittaria, a seriesof mechanical and hand drills and archaeological excava-tions allowed to reconstruct some stratigraphic sectionsand one is presented in Fig. 11. The thickness of thechannel deposits is ca. 8m and the maximum diameterof pebbles is 3–5 cm and the incision is cut into theLGM deposits. In the deactivated valley, peat formationtook place since 7.8 BP until the Holocene lagooningression, dated to 5.7 kaBP. The lagoon environmentlasted until the Roman period causing the partial silting upof the incision which was completely filled between the 6thand the 9th century AD, during an alluvial phase ofTagliamento.Considering all the post-LGM valleys recognized in the

lower sector of Tagliamento megafan, also the incisionswhich do not have any topographic evidence and have beendetected with corings, they have an average width of500–2000m and a depth of 8–25m (Figs. 9 and 11);gravelly deposits are present at the base with a thickness of5–12m. In all the megafan considered, since the LateGlacial gravels have been generally deposited far beyondtheir LGM limit, as already described for Tagliamento andalso testified along the Brenta, Piave and Isonzo Rivers(Fassetta et al., 2003); in some cases gravels have beentransported several kilometres beyond their present thresh-old of sedimentation. This could be probably related to theincision of the apical portion of megafans and theconcentration of river flows.In the whole Venetian–Friulian Plain, chronostratigra-

phical data concerning the Late Glacial and EarlyHolocene are lacking. This gap in information is particu-larly evident for the interval 14.5–8.0 kaBP in thecumulative frequency of radiocarbon dates (Fig. 3) and isrelated to the absence of organic sediment formation.Nevertheless, in the distal sectors of the plain, no climaticor palaeoenvironmental reasons exist to explain such adrastic decrease in the production of wood, peat or organicsediment during the Late Glacial and especially in theEarly Holocene. One of the most reliable hypotheses couldbe a lack of alluvial deposition until 8.0 kaBP. This wasprobably induced by the confinement of fluvial activitywithin incised channels, which led to the transformation oflarge sectors of megafans into bypass areas (Fig. 6). Traces

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Fig. 11. Cross-section of an incised valley in the distal sector of Tagliamento River near Concordia Sagittaria (for location, see Fig. 6). (1) LGM clayey

silt, (2) LGM medium sand, (3) Late LGM clayey silt, (4) Late LGM coarse sand and gravel, (5) medium sand, (6) buried soil (inceptisol) over Late LGM

deposits with carbonate calcium concretions, (7) Late Glacial gravel with sand, (8) Late Glacial sand and silty sand, (9) peat and humic clay, (10) silty clay

with lagoon shells, (11) silty sand, (12) silt, (13) medium–fine sand, (14) sandy silt, (15) Roman archaeological remains, (a) Iron Age, (16) modern

buildings, (17) isochronal line 6th century AD and (18) contact between alluvial and lagoon sediments. (a) Borehole, (b) projection of borehole not along

the section, (c) archaeological excavation and (d) projection of archaeological excavation. Radiocarbon dates are expressed in 14C yrBP uncal. and lab-

codes are reported.

A. Fontana et al. / Quaternary International 189 (2008) 71–9084

of deep erosion are clearly evident eastward of the LivenzaRiver, but also in the westernmost sector of the plain riveractivity was partly confined and aggradation did not takeplace until the Middle Holocene. Well-developed soils(calcisols) formed on the large free-flood surfaces (Mozziet al., 2003).

In the central Po Plain a similar evolution has beendescribed for the alluvial fans and megafans that lie at theAlpine margin. After the major phase of the LGMaggradation, Alpine rivers deeply entrenched in theirdeposits at their valley mouths for tens of kilometresdownstream (Marchetti, 1996, 2001, 2002; Guzzetti et al.,1997).

One of the main causes for the lack of sedimentarysupply to alluvial megafans and consequent downcuttingwas the contraction of glaciers within the Alpine valleys.Recent researches indicate that by 15–14 kaBP thedeglaciation of the Alpine valleys was probably completed(Pellegrini et al., 2005). In the deglaciated valleys theformation of large valley lakes commonly occurred becauseof the presence of end-moraines and/or post-Glaciallandslide which temporarily dammed the valley (Castiglio-ni, 2001; Hinderer, 2001; Marchetti, 2001, 2002). Thesefeatures, described in the mountain catchments of Isonzo(Bavec et al., 2004), Tagliamento (Venturini, 2003) andPiave (Surian and Pellegrini, 2000), trapped large amounts

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of debris and caused drastic decreases in the sedimentaryflux towards the plain. In the terminal tract of the Alpinevalley of the Piave River, the so-called ‘‘Vallone Bellu-nese’’, an important phase of internal aggradation,characterized the post-Glacial period at least until8.3 kaBP (Surian and Pellegrini, 2000), involving thetemporary storage of fluvial sediments. Moreover, sincethe end of the LGM, climate amelioration triggered therecolonization of vegetation in the mountains, improvingslope stability and inducing the rate of erosion andsedimentary yield to decrease.

It must, nevertheless, be noted that some alluvial systemsdeviate from this general trend. Such is the case of thecoarse gravelly alluvial fans of the rivers Cellina andMeduna, close to the Tagliamento megafan. Here, aggra-dation took place until the Younger Dryas (10.0 kaBP) buta quick and deep incision of the fanheads developed soonafter (Avigliano et al., 2002). Another exception seems tobe the Piave megafan, where the Late Glacial erosive phaseaffected the tract upstream of the Montello Hill while noimportant incision has been up to now recognized in thepiedmont plain. This difference of the Piave with respect toBrenta and Tagliamento megafans may be explained by thelower topographic gradient of its LGM surface, due to thelow elevation of the valley mouth of Piave, ca. 50m lowerthan those of the other rivers (Table 2).

4.5. Middle Holocene (8.0–3.5 ka BP)

In the northern Adriatic, the maximum flooding surfaceof the Holocene transgression was reached at about5.7–4.0 kaBP (Correggiari et al., 1996). It was severalkilometres landward in the southern sector of the VenetianPlain and also along the lower tracts of the Piave andTagliamento Rivers (Fig. 6). In these areas, later fluvio-deltaic progradation took place. In the coastal zonesbetween the active sectors, not affected by fluvial deposi-tion, the LGM surfaces often outcrop and the presentposition of the sea is the most landward (Marocco, 1991;Fontana et al., 2004).

The oldest recognized delta of the Brenta–Adige–Posystem dates to about 5.0 kaBP (Favero and SerandreiBarbero, 1980) but the existence of a delta of the PiaveRiver about 5.4 kaBP is known (Bondesan et al., 2002); forthe Tagliamento River only the existence of a pre-Romandelta of undetermined age is known (Fontana, 2006).

The former alluvial topography strongly affected themarine transgression and the deep valleys of the Taglia-mento River, already abandoned in the Early Holocene,allowed the formation of tidal inlets; thus, narrow lagoonintrusions developed as far as Portogruaro (Fontana, 2006)(Figs. 6 and 11). Within the valleys, the bases of lagoondeposits have ages of about 5.9–5.7 kaBP.

During the Holocene, in the distal portions of themegafans, Alpine rivers had a meandering course withdifferent values of sinuosity, width, wave amplitude andwavelength of the channels. In a number of meander belts,

the organic infillings of the residual channels have beenradiocarbon dated. A large part of the Holocene radio-carbon dates represented in Fig. 3 has been carried out atthe base of such organic layers and may be regarded asreliable age indicators of the deactivation of the differentchannels (Berendsen and Stouthamer, 2001). Other datesconcern organic layers of lagoon environment (mostly salt-marsh deposits) that give information about the coastalplain aggradation.Until 5–4 kaBP, while the marine highstand allowed the

progradation of fluvial deltas, Alpine rivers showed ascarce depositional tendency also in their lower tracts. Inthe Tagliamento River, an important phase of valleyformation lasted until 3.0 kaBP. This was characterized bydeep channels incised by up to 25m, and by graveltransport several kilometres further downstream thantoday. In the Piave megafan, along the Sile River, at thelimit of the present lagoon, a 12m deep incision was cut bythe Piave River within LGM deposits between 7.0 and3.6 kaBP (Fig. 7) (Bondesan et al., 2004a).

4.6. Late Holocene (3.5–0 ka BP)

In the lower sectors of the megafans, the post-LGMphase of incision and/or no alluvial aggradation continueduntil the last part of the Subboreal period, when importantchanges in the fluvial sedimentation are recognized.Some chronological differences among the Brenta,Piave and Tagliamento systems are apparent. NearMezzavia (see Fig. 6), the radiocarbon dates of a treestump in living position on a palaeosol, buried by thenatural levees of a palaeochannel of the Brenta River, andof charcoals embedded in the alluvial deposits, show thatthe aggradation on the LGM surface started around5.0 kaBP. Nevertheless, the most widespread aggradingphase with numerous ridges took place after 3.0 kaBP(Mozzi, 2004).The oldest Holocene ridge of the Piave River developed

downstream from the city of S. Dona after 3.0–4.0 kaBP;moreover, this Late Holocene aggrading trend is alsoevidenced in the Piave megafan by the complete infilling at3.6 kaBP of the 12m deep incision along Sile River area byPiave deposits (Fig. 7). In the Tagliamento megafan, thefluvial ridges started to form only after 3.0 kaBP.This evidence suggests that the formation of fluvial

ridges started to influence large parts of the lower portionof the megafans after 4–3 kaBP. This trend is still active,even if during the last centuries human control limited thedepositional activity of rivers to artificial dikes.Holocene fluvial ridges are rather different from

Pleistocene ones, both in dimensions and internal strati-graphy. Ridges built in the last millennia are usually higher(2–5m compared to the surrounding plain) and wider(500–2000m wide) than the previous ones (Fig. 12). Riversform meanders in their lower tracts, with channel depositsusually 7–15m thick. Avulsion has been the main processtriggering new flow directions.

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Fig. 12. Cross-section of present Tagliamento fluvial ridge near Latisana (for location, see Fig. 6). (1) Sandy gravel, (2) medium sand, (3) fine sand,

(4) sandy silt and clay silt, (5) silty clay, (6) buried soil (entisol), (7) clay silt, (8) buried soil (inceptisol), (9) peat and organic sediment, (10) Roman

archaeological remains and (11) boreholes and projection of boreholes (light grey). Radiocarbon dates are expressed in 14C yrBP uncal. and lab-codes are

reported.

A. Fontana et al. / Quaternary International 189 (2008) 71–9086

In the Tagliamento megafan, Late Holocene sedimenta-tion affected also some of the incisions that had beenpreviously abandoned and caused them to silt upcompletely, as recognized in near Concordia (Fig. 11)and along the present day Tagliamento River nearLatisana (Fig. 12). In the section represented in Fig. 12the river was incised into the LGM deposits up to the endof the Subboreal period (ca. 3.0 kaBP), but the depth of thechannel was significatively shallower than those formedduring the Late Glacial and the Early Holocene and, alongthe meander belts, gravelly channel lags and bars stoppedupstream of the position reached during the Early and theMiddle Holocene. Both the shallowing of depth of channelbottom and the decrease in downstream transport capacityof the river may be related to a lowering in the hydraulicgradient, probably triggered by the relative rise of sea level,as demonstrated in some coastal plain (e.g. Blum andTornqvist, 2000; Berendsen and Stouthamer, 2001).

Late Holocene fluvial deposition affected considerablysmaller areas than during the LGM. The thickness ofHolocene sediments burying Pleistocene surfaces generallyreaches the maximum value of 4–6m along fluvial ridges.In the related floodplain only 1–3m of Holocene deposits ispresent.

In the Late Holocene stratigraphic sequences, wide-spread organic sediment or peat layers are present only inthe lagoon environment, while in the alluvial deposits peatis available only as a part of the infilling of the residualchannels. In contrast, buried soils are quite common andtheir degree of development was used to estimate theperiod of stability of the surface on which they formed(Mozzi et al., 2003; Fontana et al., 2004; Ragazzi et al.,2004). Buried soils with archaeological remains are

particularly important for the chronostratigraphic recon-struction; in fact they help the dating of surfaces, eventhose in which no organic material is available.In all the alluvial megafans of the Venetian–Friulian

Plain an important period of floods is evident during theEarly Middle Age (4–5th to 10th century AD). Thisinterval corresponds to a period of high rainfall recorded inthe historic chronicles, and coincides also with the collapseof the field and drainage systems settled during the Romanperiod (Dall’Aglio, 1994; Marchetti, 2002). Between the6th and 11th centuries, the Tagliamento had a highavulsive activity that is mentioned in the historicalchronicles and that led to the alternate activation of threedirections with consequent formation of new ridges(Fontana, 2006). Since the 11th century, only the presentday direction has been active and the ridge related to thisriver has a width of about 4 km and surrounds the alluvialplain for ca. 3m (Fig. 12). Many Roman sites and theRoman road Annia had been buried by the alluvialdeposits related to the present direction of the river andtheir stratigraphic and topographic positions testify thatthe formation of the ridge started after the Roman period.Different ridges have been formed during the Early

Middle Age also by the Piave River, which also had anaggradation phase in the eastern sector of its apical portion(Bondesan et al., 2002). Similarly, the Brenta Riverfollowed different directions in the post-Roman times,and has been prone to human interference since the 11thcentury. Human management strongly affected all therivers involved in the Venice Lagoon catchment to avoidsedimentation in it and to preserve large areas under tidalinfluence from fluvial progradation. Especially, since the15th century, numerous artificial deviations and river

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regulation have affected the Brenta, Piave, Bacchiglione,Sile and Livenza Rivers. Since the 18th century theVenetian–Friulian Plain could be considered an embank-ment plain where, except catastrophic floods, sedimenta-tion has been drastically reduced. Moreover, landreclamation in the 20th century has induced a rapid anddiffused subsidence on large coastal areas, as a conse-quence of the combined compaction of the formerlywaterlogged sediments and of oxidation of the abundantorganic matter contained in the marsh deposits.

5. Conclusions

The presence of alluvial megafans in the Venetian–Friulian Plain seems to be basely related to the role of thisarea as the foreland basin of the uplifting Alpine chain. Asnoticed by Castiglioni (2001), construction of large fan-shaped landforms in the Po Plain can be interpreted,without underestimating other factors, as an answer to thetectonic activity of the mountain belt. Considering timescales larger than 104–106 yr, the geodynamic setting hasbeen the main necessary constraint for megafan develop-ment in the study area, as already highlighted in otherareas of the world where these large-size landforms havebeen recognized (e.g. Rasanen et al., 1992; Gupta, 1997;Horton and DeCelles, 2001). Nevertheless, the evolution ofthe studied megafans during the last 24 kaBP shows thatclimate change and related eustasy are the main drivingfactors on medium time scales (103–104 yr). The majorenvironmental changes which took place since the LGMapparently triggered four main sedimentary phases:

1.

A phase of maximum aggradation took place during thepeak of the last glaciation, between 24 and 18 kaBP, andhad a primary role in shaping the Late Quaternarylandscape of the Venetian–Friulian Plain. In this period,the alluvial megafans achieved their maximum areal sizeand shape, with the deposition in the distal sector ofalluvial sequence with an average thickness of about20–30m. The LGM Brenta and Tagliamento watershedsdiffered quite extensively from the present fluvialcatchments, due to the existence of a network of valleyglaciers with important glacial transfluences from theinner Alps. The fact that the Brenta megafan used to bethe largest depositional system active in the region maybe regarded as the direct consequence of the extrasupply of sediment allowed by glacial transfluences fromthe Adige valley.

2.

During the Late LGM (18–15kaBP), when the Alpineglaciers started to contract and the more external terminalmoraines systems were abandoned, the Brenta megafandepositional rates decreased, but aggradation still tookplace. In the Tagliamento megafan and in the otheralluvial systems connected with the Tagliamento glacier(Corno, Cormor and Torre Rivers), the Late LGM wascharacterized by the downcutting of the fanhead anddeposition in the distal sector; thus, a first telescopic lobe

formed in the megafan, which was considerably smallerthan the area of LGM sedimentation.

3.

After 14.5 kaBP, through the Late Glacial and into theEarly Holocene, a widespread erosive phase affected themegafans. The eastern sector of the Brenta megafan wasdeactivated due to the entrenching of the river at thevalley outlet, while sedimentation shifted tens of kilo-metres downstream. In the Tagliamento megafan, theerosive phase affected the lobe built in the Late LGMand the formation of deep and wide fluvial incisionstook place also in the lower sector of the megafan.Megafan incision was probably triggered by the reduc-tion of the sedimentary supply to the plain. The decreaseof the river solid discharge led to exceeding streampower and river downcutting. This fall in the sedimen-tary input to the alluvial system could be related to:(i) the cessation of the glacial sedimentary input near theplain; (ii) a gap in the sediment transfer from mountainsto plain, due to the trapping of the sediment load in thepro-Glacial lakes and/or dam lakes that normally formduring valley glaciers retreat; (iii) a reduction in theproduction of debris, because of the stabilization of themountain slopes by the post-Glacial vegetation cover.

Between 14.5 and 8.0 kaBP there are no indications ofsedimentation, neither in the apical portions, nor in thedistal sectors of the megafans. This was probably due tothe confinement of fluvial activity inside erosivechannels, which bypassed the sediments to the Adriaticshelf. The largest and deepest incisions exist in theTagliamento megafan, where fluvial valleys have a widthof 1–2 km and a maximum depth of 25–30m. At thebase of the incisions, gravels were transported as far asthe present lagoon area. Also, in the other fluvialsystems gravels could be moved ca. 10–20 km moredownstream than during the LGM. The shift of thethreshold in gravel sedimentation may have been due tothe higher velocity, and the corresponding highertransport capacity, of the fluvial water channelled inthe post-Glacial incisions. Remarkable is the fact thatthis erosive phase took place during the Flandrian sea-level rise, showing that the eustatic base level rise hadminimal effect on these alluvial systems during thetransgressive phase.

4.

Since the Middle Holocene a new phase of sedimenta-tion is recorded in the lower sectors of the megafans,with some chronological differences among the analysedsystems. Deposition affected only limited portions alongthe lower tracts of rivers until 4–3 kaBP, when awidespread phase of aggradation took place. Never-theless, volumes and areas of sedimentation are con-siderably smaller than that during the LGM. Fluvialridge formation started in the Brenta megafan about5 kaBP, about one millennium later in the Piave system,and after 3.0 kaBP in the Tagliamento megafan.

Since the Middle Holocene the limit of sedimentation ofgravels moved upstream than during Late Glacial and

ARTICLE IN PRESSA. Fontana et al. / Quaternary International 189 (2008) 71–9088

Early Holocene, but it still reaches the distal sector ofmegafans and its position is considerably downstream thanLGM.

Marine highstand is considered one of the mostimportant factors in triggering and forcing the LateHolocene alluvial aggradation. The Holocene climaticfluctuations probably had some influence on the millen-nial-scale evolution of fluvial systems, but the evidencegathered up to now is scarce and does not permit to clearlydistinguish their role in alluvial phases.

Since the Roman period, human activity, as noticed inthe whole Po–Venetian Plain (Marchetti, 2002), has beenclearly affecting both the river directions and the produc-tion and dispersal of sediments with increasing intensity.During the last five millennia, man may be considered themost outstanding factor in controlling fluvial dynamicsand, therefore, the evolution of megafans.

Acknowledgements

The research was supported by the Italian Ministry ofResearch and Technology (MURST) and the University ofPadua within the Project ‘‘Geomorphological evolution ofthe Venetian–Friulian Plain’’ (coordinator A. Bondesan).Information was obtained also in the framework of otherprojects funded by the Veneto Region: the CARG Projectfor the new geological map of Italy at the scale 1:50,000(coordinator F. Toffoletto); the ‘‘Soil map of the watershedof the Lagoon of Venice at the scale 1:50,000’’ and the‘‘Soil map of Veneto at the scale 1:250,000’’, carried out bythe Soil Observatory of the Environmental ProtectionAgency of the Veneto Region (ARPAV) (coordinatorP. Giandon). The Geological Survey of the Province ofVenice provided geological and geomorphological datawithin the project LEO-DOGE ‘‘Geomorphological mapof the Province of Venice’’ (coordinator A. Vitturi).Investigations performed in Ca’ Tron area were sponsoredby Fondazione Cassamarca. Drills and analyses of the areaof Concordia Sagittaria were funded by the projectINTERREG IIIA-AAVEN 322065. Special thanks toN. Surian and M. Meneghel for stimulating discussionsand to F. Ferrarese for the production of the microreliefmaps and digital terrain models. We wish to express ourthanks to D. Maddy and M. Marchetti for their accuraterevision which improved the quality of the paper.

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