Land subsidence and Late Glacial environmental evolution of the Como urban area (Northern Italy

ARTICLE IN PRESS

1040-6182/$ - se

doi:10.1016/j.qu

�Correspond

Quaternary International 173–174 (2007) 67–86

Land subsidence and Late Glacial environmental evolution of the Comourban area (Northern Italy)

Valerio Comercia,�, Sabrina Capellettib, Alessandro M. Michettib, Sabina Rossib,Leonello Servaa, Eutizio Vittoria

aAPAT – Geological Survey of Italy, Via V. Brancati 48, 00144, Roma, ItalybUniversita dell’ Insubria, Dipartimento di Scienze Chimiche e Ambientali, Via Valleggio 11, 22100, Como, Italy

Available online 27 June 2007

Abstract

The Late Glacial and Holocene geological and environmental evolution of the Como lacustrine–palustrine basin has been broadly

reconstructed based on (a) the collection and analysis of more than 100 borehole stratigraphies, (b) the drilling of three new shallow

boreholes and the analysis of a new geological section and (c) several in situ and laboratory analyses (mineralogical, organic and pollen

content, 14C dating). The long-term, natural tendency of the ground surface to sink where the Como urban area has developed has been

verified and assessed. This tendency has resulted clearly controlled by the compaction of young unconsolidated sediments accumulated at

the hydrologically closed SW tip of Lake Como. The definition of a minimum age for the deglaciation of the basin and for a correlated

stratigraphic marker allowed broad quantification of the average velocity of land subsidence during the last ca. 17 000 cal BP, with a

gradient of values close to 2.5mm/yr along the axial zone of the basin toward the present-day lake. Burial depth of archaeological

remains, related to Roman Age, and levelling monitoring since 1928 are in agreement with this estimate. During 1950–1975, because of

deep water withdrawal, the Como area was affected by a human-induced, accelerated subsidence, with velocity (10–20mm/yr) one order

of magnitude higher than the natural one. After a period of deceleration (between the end of the 1970s and the end of the 1990s), during

which some benchmarks even reversed their trend, persistent scatterer interferometry (PSI) data provided by Tele-Rilevamento Europa

(TRE) for the period 1992–2003, and levelling data obtained by APAT in 2004, show that during the last decade the subsidence rate has

returned to its physiologic long-term trend.

r 2007 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction

Land subsidence phenomena are known to occur in avariety of geomorphic, stratigraphic and tectonic settings(some reviews of worldwide examples can be found inCarbognin, 1986; Prince et al., 2001, and referencestherein). They are caused by natural processes, such astectonics, isostatic adjustments, sediment load and oxida-tion of organic soils, or by human activities such as fluidextraction and mining, artificial loading, traffic vibrationsand explosions. Even if in general land subsidence is not anabrupt event, it generates environmental changes that maybe harmful, especially when it affects urban areas. Thetown of Como, located at the SW edge of Lake Como(Fig. 1), is affected by subsidence phenomena that haveposed and continue to pose significant hazards to the local

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

aint.2007.06.014

ing author.

community, in particular exposing the lakeshore area to anincreasing risk of inundation (Apuani et al., 2000; Comerci,2004). Remarkable rates of ground sinking (up to over20mm/yr) were recorded immediately after World War II,prompting concern and attention to this phenomenon(Ascheri, 1972; Comune di Como, 1980). A specificcommission (Comune di Como, 1980) determined thatsuch an accelerated subsidence was triggered by humanactivity, and principally by the intensive exploitation of thedeep aquifer. Moreover, the commission, on the basis ofthe 1975 and 1979 levelling data, concluded that thesubsidence was decelerating and such a trend wouldcontinue in the future.In this paper, the subsidence is analysed as part of the

geological and environmental evolution of the Como townbasin since Late Glacial. The role of the different natural(climate, geology, tectonics) and later human (e.g., hydro-logical works, water withdrawal from wells) components

served.

dx.doi.org/10.1016/j.quaint.2007.06.014

ARTICLE IN PRESS

Fig. 1. Lake Como; inset shows the study area centred on the town of

Como (orthophoto: Terraitaly—it2000TM—C.G.R. S.p.A.—Parma).

V. Comerci et al. / Quaternary International 173–174 (2007) 67–8668

affecting the physical evolution of the territory wasconsidered in order to discriminate between the naturaland the man-induced ground lowering in the investigatedarea. Understanding the role played by land subsidence inthe triggering of sudden variation in coastal areas isrelevant for the main scope of the Dark nature—rapid

natural change and human responses project, aimed atrefining the record of rapid (o100 yr) environmentalchanges affecting physical environments during the LateHolocene.

The investigations made it possible to describe thegeneral environmental evolution of the area for differenttime windows. Literature data provided information ongeologic evolution during the pre-Quaternary period.Geologic–geomorphologic surveys and stratigraphic data(some of them coming from new shallow boreholes)allowed (a) delineation of the general environmentalevolution since Late Glacial and (b) estimation of thenatural, long-term, average rates of land subsidence. Thearchaeological data permitted assessment of the averagesubsidence rate during the last ca. 2000 yr. Availablegeometric levelling data, re-levelling performed by APATin 2004 and the new radar persistent scatterer interfero-metry (PSI) analyses, performed by Tele-RilevamentoEuropa (TRE) for the years 1992–2003, allowed apprecia-tion of the subsidence trend during the last ca. 80 yr with

increasing precision. This paper focuses on the generalassessment of the natural tendency to subside of the Comotown area as determined by the geological setting, and onthe comparison of long-term and present-day landsubsidence rates, in order to predict the near-future trendof ground surface lowering and deformation.

2. Geologic and geomorphologic framework

At the foothills of the Southern Alps, Lake Como (alsocalled Lario) occupies a narrow glacial valley whichbifurcates to the south giving a typical l shape (Fig. 1).The town of Como is located at the southern end of thewestern, hydrologically closed, branch of the lake (Fig. 2).The town stands on a NW–SE elongated plain (locallyknown as ‘‘convalle’’), gradually rising from the lake level(198m a.s.l.) up to 220m a.s.l., bordered by steep bedrockslopes: to the NE by the Mt. Uccellera (1027m a.s.l.)–Mt.Boletto (1236m a.s.l.) massif, to the SW by the Mt. dellaCroce (536m a.s.l.) and Mt. Caprino (455m a.s.l.) ridgeand to the south by the Baradello Hill, the Camerlatathreshold (276m a.s.l.) and Mt. Tre Croci (469m a.s.l.).The plain is drained by the Cosia Stream (Fig. 2), which,after receiving only one modest tributary (the so-called‘‘Fiume Aperto’’), flows into the lake. Only one modestactive fan can be recognized, fed by the Valduce Creekcoming from the Mt. Uccellera slope.The regional geological setting and the Quaternary

evolution of the area have been described by severalauthors, such as Nangeroni (1940, 1954, 1970), Orombelli(1976), Gianotti and Perotti (1986), Bernoulli et al. (1989),Montrasio (1990), Bini (1993, 1996), Zanchi et al. (1997),Bini et al. (2001) and Giardina et al. (2004). From thelithological point of view, three main sectors can beidentified: the Mesozoic carbonatic relief to the NE, thearea carved in the Oligo-Miocene conglomerates of theSouthern-Alpine molasses to the SW and the Como townQuaternary basin. The Mesozoic carbonatic sequence isexposed at Mt. Uccellera (Fig. 2), where well-beddedLiassic (Sinemurian) siliceous limestones crop out (‘‘Mol-trasio Fm.’’; e.g., Bernoulli, 1964; Bertotti et al., 1993).Molasse sediments constitute the sector which includes thehilly ridge of the Spina Verde Mts., extending from theTicino (Switzerland) border to Mt. Tre Croci. This sector isconstituted by coarse polygenic conglomerates and sand-stones belonging to the Lombardian Gonfolite Group(‘‘Como Fm.’’, Upper Oligocene), up to ca. 1000m thick(e.g., Cita, 1957; Sciunnach and Tremolada, 2004), includ-ing well-rounded pebbles of crystalline and subordinatelysedimentary rocks. The size of pebbles is extremely variablewith a higher frequency between 8 and 12 cm (Cita, 1957).The Como Fm. is in the hanging wall of a south-dippingthrust separating the Lombardian Gonfolite Group fromthe underlying Mesozoic and Lower Tertiary sequence(Bernoulli et al., 1989). This structure is part of a majorregional north-verging backthrust, dated to the Tortonian(see Fantoni et al., 2004; Sciunnach and Tremolada, 2004,

ARTICLE IN PRESS

Fig. 2. The town of Como at the southern, hydrologically closed, tip of Lake Como (orthophoto: Terraitaly—it2000TM—C.G.R. S.p.A.—Parma), and

placed on a NW-trending sedimentary basin.

V. Comerci et al. / Quaternary International 173–174 (2007) 67–86 69

for a recent revision). Zanchi et al. (1997) and Bini et al.(2001) described the evidence for post-Miocene to Pleisto-cene uplift of the Gonfolite hanging wall, and Sileo et al.(2007) suggest that the vertical component of the displace-ment along the Gonfolite backthrust during the MiddlePleistocene was in the order of ca. 200m.

During the Quaternary, the morphology of the wholearea was modelled by the erosional/depositional activity ofthe glaciers that recurrently covered this territory. Since theMiddle Pleistocene, several major glacial expansionsaffected the Central Alps and generated ice tongues up to2 km thick, which reached the piedmont belt and theNorthern Po Plain (Nangeroni, 1954, 1970, 1974, 1976;Bini et al., 1998; Moscariello et al., 2000; Muttoni et al.,2003). During the Last Glacial Maximum (LGM,22–20 kyr BP; e.g., Bini and Zuccoli, 2004; Ivy-Ochset al., 2004, and references therein), in the Como area theAdda glacier reached at least the elevation of 825m a.s.l.,as documented by glacial deposits and landforms onMt. Brunate (Bini et al., 1996, 2001). The LGM frontalmoraines expanded more than 10 km south of Como, at thelatitude of the town of Cantu, from which they retired wellbefore 15 kyr BP (e.g., Bini and Zuccoli, 2004). During theEarly Late Glacial retreat from the Como valley, a lakeformed between the glacier front and the Camerlatathreshold (at ca. 276m a.s.l.), which acted as a spillwayfor the south-draining waters (Castelletti and Orombelli,1986; Fig. 2). The glacier deposited on the lake bottom awaterlain till, constituted by clay and silt containing clasts

and pebbles of varying lithology. The lake level progres-sively dropped and, according to Castelletti and Orombelli(1986), a lacustrine–palustrine environment developed inthe Como town basin during the Dryas II–Allerød. Duringthe Holocene, the shallow water basin occupying the Comoplain was progressively filled up by the alluvial deposits ofthe Cosia Stream. With the arrival of man, fires anddeforestation increased the sediment load of the localstreams and consequently their depositional capacity.According to Bini (1996), in the eastern zone of the Comolakeshore, the thickness of the alluvial sand and graveldeposited during the last 3.5 kyr is larger than thatdeposited during the previous 10 kyr. Also slope and fandeposits contributed to filling up the basin.

2.1. Como town stratigraphy

A commission on the study of the subsidence phenom-ena, instituted by the Como Municipality in 1974 (Comunedi Como, 1980), collected and analysed 115 boreholes thatallowed to reconstruct the general stratigraphy of theComo town basin. From top to bottom, the derived generalsuccession is composed of 1–10m of heterogeneous fillingmaterial, upper sands and gravels down to the depth of15–24m, palustrine silts with organic content down to23–50m, inorganic lacustrine clays down to 40–60m andlower sands and gravels down to 170m. Geotechnicalinvestigations showed that the palustrine silts and lacus-trine clays are normally consolidated.

ARTICLE IN PRESS

Fig. 3. Map of the Como urban area, based on the ‘‘Carta Tecnica Regionale, Regione Lombardia’’ at a 1:10 000 scale, showing the location of collected

stratigraphic boreholes and water wells, and the indication of San Abbondio (1), Ticosa (2) and Via Valleggio (3) sites, where new stratigraphic surveys

were carried out. The position of the 20 boreholes drilled in the San Abbondio area is given in the inset map, based on the ‘‘Carta aerofotogrammetrica,

Comune di Como’’ at 1:2000 scale; SA1, SA2 and SA3 show the locations of the new shallow boreholes drilled during the present project. Section AB is

shown in Fig. 3, section ABCD is shown in Fig. 8.


Some boreholes drilled in the area of San Abbondio’schurch (Fig. 3), on the SW border of the plain, were analysedand interpreted by Castelletti and Orombelli (1986). Theydescribed the local stratigraphic succession as follows: 2–3m ofreworked filling material including pebbles in coarse sandymatrix and archaeological remains, such as fragments of brick;coarse gravels in sandy matrix to the depth of 8–10m,interpreted as alluvial fan deposits coming from the localbedrock slope; a ca. 1m thick bed of silty sands rich in organicmaterial and wood remains, interpreted as a lakeshore deposit;the following ca. 30m (down to the bottom of the boreholes)are constituted by a succession of laminated silty clay,interpreted as glacio-lacustrine rhythmic bedding, and siltand thin silty sand (diamict, waterlain till), containing dispersedcoarse pebbles (ice rafted pebbles). At the depth of 8m withinthe lakeshore sandy deposits, Castelletti and Orombelli (1986)sampled a fragment of Larix/Picea that yielded a 14C age of117307180BP (Laboratoire de Hydrologie et de GeochimieIsotopique, Universite Paris-Sud); the calibrated age (usingCALIB 4.3, Stuiver et al., 1998) is 13810calBP (Table 1).

At that time, the 13 810 cal BP age represented the firstdating to constrain the minimum age for the completedeglaciation of Lake Como. More recently, new investiga-tions on low-altitude afforestation at nearby sites in theInsubria Southern Alps suggest that it had already startedat ca. 16 500 cal BP (e.g., Hofstetter et al., 2006). In order toprecisely define the time of such a crucial moment in thelocal landscape evolution, which is a key point for thestudy of the Como town subsidence (see Section 7.1), wecarried out further stratigraphical and environmentalanalyses, as described in the next sections.

3. Database

The general reconstruction of the Late Glacial tohistorical geological and environmental evolution of theComo area and the assessment of the mean subsidencerates were based on several methodologies. The knowledgeof the lithological and environmental setting of the Comotown subsurface deposits is the essential background to

ARTICLE IN PRESS

Table 1

Conventional and AMS radiocarbon dates from San Abbondio and Via Valleggio sites

Lab. code Core/sample Material Elevation (m

a.s.l.)

Years (14CBP) Age (cala BP) 2s range (cal BP)

Laboratoire de Hydrologie et de Geochimie Isotopique, Universite Paris-Sud (Castelletti and Orombelli, 1986)

– Core Sr1 Wood 201.00 11 7307180 13 810 15 191–13 184 S. Abbondio site

Centre for Isotope Research, University of Groningen

GrA-23357 Core SA2 Wood 204.00 13 2307120 15 900 16 433–14 823 S. Abbondio site

GrA-29158 Sample VC4 Organic sed. 209.50 5100750 5890 5932–5727 Via Valleggio site

GrA-29436 Sample VC1 Organic sed. 209.00 AMS

13 8807200

16 650 17 326–16 019 Via Valleggio site

See location and stratigraphic logs in Figs. 3, 5 and 7.aCalibration was carried out with CALIB rev. 4.3 using the IntCal98 database (Stuiver et al., 1998). Cal ages are rounded to the nearest 10 yr.


understand the sinking phenomena affecting the groundsurface of the area. The definition of the stratigraphicrelations among the main sedimentary bodies permits afirst approximate evaluation of the subsoil geotechnicaland hydrogeological properties, directly responsible for thetendency to subside. Moreover, the litho-stratigraphicanalysis allows reconstruction of the changes of thepalaeoenvironment through time, and to define the localnatural evolutionary trend of the landscape.

All the data described below have been located using the‘‘Carta aerofotogrammetrica, Comune di Como’’ at 1:2000scale, calibrated through the new APAT levelling surveyconducted in 2004. Consequently, the error on location andelevation of boreholes and other relevant sites is much less thanthe expected variability of the geomorphic (lacustrine terraces),stratigraphic (shoreline deposits) and archaeological (roadpavements, house foundations) features that we compare andcorrelate.

3.1. Historical and archaeological data

Published historical and archaeological investigationswithin the Como urban area were compiled and revised.The town of Como was mostly shaped under Julius Caesarin the first century B.C. after the reclamation of the plain,which before the Roman colonization was an uninhabited,unhealthy marshy area. Detailed historical and archae-ological analyses available at 13 buried Roman to MiddleAge sites (e.g., Uboldi, 1993; Luraschi, 1997; Nobile, 2005)were used to map the subsidence rates in the last ca. 2000 a.

3.2. Stratigraphic analyses

The search of private and public archives made availablemore than 100 stratigraphic logs of geotechnical boreholesand water wells spanning from 10 to 187m of depth. Geologicand geomorphologic surveys were carried out, with a specificfocus on the San Abbondio, Ticosa and Via Valleggio areas,located at key sites along the Late Glacial lakeshores (Fig. 3).At these sites, new stratigraphic analyses were conducted.

At Via Valleggio, excavations conducted for buildingfoundations allowed a direct observations of several, up to

7m high and 15m wide, stratigraphic sections. In theTicosa area, we revised in the field the stratigraphic logs offive boreholes, up to 25m deep (Fig. 3). At San Abbondio,three new shallow boreholes were drilled by means of acontinuous core drilling system specifically devised formicrostratigraphic studies (‘‘AF shallow core system’’,patented; see www.afgtc.com), allowing 100% of sampleretrieving with a diameter of 44mm. The cores SA1–SA3,T1–T5 and the trench walls exposed at the Valleggio siteswere accurately described and logged.

3.3. Mineralogical and loss-on-ignition (LOI) analyses

The mineralogical composition of 23 samples from thecore SA2 was determined by means of X-ray diffractometry(Bruker’s Mod. D8 Advance, Insubria University). Tensamples of about 1 g (dry weight) of sediment were takenfrom core SA2, between 407 and 990 cm depth and dried at80 1C for 24h. The samples previously weighted were heatedat 550 1C until no weight loss was observed, then heated foradditional 3 h. After this processing, the final weight wasdetermined. The weight loss represents the content of organicmatter and carbonate of the sediment. The LOI was thencalculated as the percentage dry-weight loss after ignition.

3.4. Palynology

Pollen analysis was carried out on eight samples of 4–5ml involume from the SA2 core. Pollen was concentrated by HCl,HF, NaOH treatment, followed by heavy liquid separation(ZnCl2, density ¼ 2) and acetolysis. Pollen counting wasprovided for 230 pollen grains as a minimum (mean pollensum: 1384). Pollen and spores identification was based onpollen atlas (e.g., Moore et al., 1991) and reference slides.Pollen diagram was plotted by GpalWin and expressed aspercentages, excluding unknown pollen and spores.

3.5. Radiocarbon dating

Three organic samples (one from San Abbondio and twofrom Via Valleggio) were dated by conventional and AMS14C methods at the Centre for Isotope Research, University

http://www.afgtc.com

ARTICLE IN PRESSV. Comerci et al. / Quaternary International 173–174 (2007) 67–8672

of Groningen. The obtained ages were calibrated with theprogram CALIB 4.3 (Stuiver et al., 1998).

3.6. Topographic surveys and remote sensing

Data of several levelling surveys from 1928 to 1997 werereviewed and compared with a new geometric levellingsurvey, carried out in 2004 by the Geologic Survey of Italy(APAT, Agency for Environmental Protection and Tech-nical Services) by means of a Leica NA3000 digitallevel. Satellite imagery data (PSI) processed by TREallowed recognition of the very recent and contemporary(years 1992–2003) average subsidence trend. Details onlevelling data and radar analyses are beyond the scopeof this research and will be provided in a companionpaper.

4. New stratigraphic surveys

Although the Adda River continues its course at thesouth end of the Lecco Branch, the Como Branch, since theLate Glacial period, has had no outlet (e.g., Castelletti andOrombelli, 1986). For this reason, the stratigraphy beneathComo town is essentially continuous and represents anexcellent archive to reconstruct the post-Glacial environ-mental evolution of the area, and the related landsubsidence history.

In order to define the first order geometric relationshipsamong the Late Pleistocene to Holocene sedimentaryformations in the Como town subsurface, the availablestratigraphic logs were revised and interpreted. Correlationof the boreholes along the A–B trace of Fig. 3 allowedderivation of the generalized geologic section in Fig. 4.Only one borehole, located near the mountain slope at thewestern border of the Como plain, reached at 180m the

Fig. 4. Geologic section along the trace A–B of Fig. 3, showing the generalize

urban area subsurface.

Gonfolite bedrock. All the other boreholes did not reachthe bottom of the recent, unconsolidated sedimentaryfilling, which according to Cancelli (1980) is presumablydeeper than 200m in the centre of the Como basin.The most relevant feature for the scope of the present

note is the geometry of the depositional unit composed bypalustrine–shallow lacustrine sediments rich in organicmatter and plant remains (Unit 2 in Fig. 4; Apuani et al.,2000; Comerci, 2004).Based on the careful revision of the available logs, Unit 2

does not show vertical changes of facies. If the palaeoen-vironmental setting illustrated in Fig. 4 is correct, theoriginal depositional surface of the bottom of Unit 2 wasnearly horizontal. Today, Unit 2 clearly thickens towardthe centre of the basin. Its base is deeper than 50m belowthe current coastal area in the central part of the basin,while it crops out along the Como plain borders, as shownby Castelletti and Orombelli (1986). Therefore, the present-day subsurface arrangement and thickness of the Unit 2 areinterpreted as long-term consequences of the local landsubsidence. Once the age of the Unit 2–Unit 3 transition iswell constrained, the average land subsidence rate can beestimated throughout the Como plain. New stratigraphicsurveys were conducted at three sites where the base ofUnit 2 crops out and can be sampled, due to recentexcavations and/or drilling projects specifically (i) at thesite already studied by Castelletti and Orombelli (1986)around San Abbondio’s church, where 17 stratigraphiclogs were already retrieved, (ii) in the Ticosa industrialarea, sited close to the San Abbondio area, where coresfrom five boreholes were available and (iii) at the ViaValleggio site, within the university campus located on theSE margin of the plain, where several trench walls wereopened for realizing new campus buildings (boxes 1, 2 and3 in Fig. 3).

d geometric relationships among the main sedimentary units of the Como

ARTICLE IN PRESSV. Comerci et al. / Quaternary International 173–174 (2007) 67–86 73

4.1. San Abbondio site

Along the SW border of the Como plain close to SanAbbondio’s church, at least four orders of lacustrineterraces are present, at elevations ranging from ca. 207and ca. 215m a.s.l. (e.g., Bini, 1993; Comerci, 2004). Threenew shallow boreholes were drilled between the elevationsof ca. 209 and ca. 214m (SA1–SA3 in Fig. 3) andcorrelated with the other available 17 stratigraphic logs.

4.1.1. Stratigraphic analyses of the three new shallow

boreholes

SA1 and SA2 boreholes are adjacent to each other(Fig. 3) and at the same elevation of ca. 209m a.s.l. SA1could only reach the depth of 6.55m. The stratigraphicsuccession (Fig. 5) shows about 3.70m of artificial fillingmaterial, followed by more than 10 cm of subroundedpebbles (diameter up to 5 cm) in sandy matrix, interpretedas debris flow and/or slope deposits (B in Fig. 5). Theremaining sediments down to the bottom of the core areconstituted by a massive washed sand, rich in muscovite,with biotite and quartz, interpreted as a lakeshore deposit(C in Fig. 5, with photo).

The borehole SA2 reached the depth of 10.50m. Thestratigraphy of the first 5m is very similar to that of SA1(see Fig. 5), followed by a layer of silty sand containingsubangular clasts, more than 20 cm thick (D in Fig. 5).Between the silty sands and the washed pure sands, awooden fragment was sampled at 5.07m below the groundsurface. Down to 8.90m (layers E–H) there is a successionof clayey silt and silt with few dispersed subangular andsubrounded clasts, followed by massive clay (I in Fig. 5)with a dispersed clast of 4 cm. This layer at the bottom ofthe core is interpreted as a distal glacio-lacustrine depositfed by a melting glacier front. Above, the E–H silty layers

Fig. 5. Sketch of the stratigraphic succession of the SA1–SA3 boreholes, with

environment; the oxidation levels reveal water table fluctuations while the clay

macroremain, found in SA2 at the depth of 5.07m, yielded a radiocarbon age

are interpreted as glacio-lacustrine deposits more proximal,in comparison with the underlying clays, to the glacierfront, which was likely re-advanced in relation to a minorclimatic variation. The layers D and C are, respectively,silty sands and washed, massive sands probably depositedin submerged shore and foreshore environment. In theupper part of layer C the presence of sand with abundantsubangular and subrounded pebbles up to 3 cm big(backshore sands) indicates the transition to a subaerialenvironment (layer B), characterized by the deposition ofgravel and sand with pebbles up to 6 cm in diameter (slope,debris flow and fan deposits).The wood remnant found at the depth of 5.07m yielded

a radiocarbon age of 13 2307120BP. The calibratedcorrected age is 15 900 cal BP (Table 1). According to theregional climatic–stratigraphic review proposed by Ravazzi(2003), this 14C date and the one of 13 810 cal BP(Castelletti and Orombelli, 1986) belong to Late Glacial,being the first in the Early Late Glacial, and the second inthe Late Glacial Interstadial.Borehole SA3 was drilled ca. 80m northwest of SA1 and

SA2 (Fig. 3), on the 214m a.s.l. terrace, reaching the depthof 10.30m. The litho-stratigraphic succession is verysimilar to those of SA1 and SA2 (see Fig. 5), but probablycorresponds to an older depositional cycle, as discussedbelow. The only difference is the lack of the glacio-lacustrine silt, which could indicate the presence of ahiatus, not unlikely in a palaeolake coastal setting.

4.1.2. Pollen diagram and LOI analysis

Pollen analysis of SA2 core was carried out over eightsamples throughout the glacio-lacustrine deposits betweenthe depth of 5.31 (clayey silts) and 10.30m (clays). Thepollen diagram (Fig. 6) gives a general overview of themain vegetation features. The pollen sequence has been

a view of the muscovite rich sand. It was deposited in a lacustrine shore

ey levels should be related to temporary high stands of the lake. A wood

of 15 900 cal BP.

ARTIC

LEIN

PRES

S

Fig. 6. Percentage pollen diagram at the S. Abbondio site. The stratigraphic log and the major sedimentological units are reported on the left.

V.

Co

merci

eta

l./

Qu

atern

ary

Intern

atio

na

l1

73

–1

74

(2

00

7)

67

–8

674

ARTICLE IN PRESS

Table 2

Loss-on-ignition (LOI) analysis of 10 horizons from SA2 core

Sampling depth

(cm)

Lithology Medium organic

content (%)

407 Shore sand 1.8

497 Shore sand 2.4

566 Glacio-lacustrine silt 2.4







990 Glacio-lacustrine clay 2.6

The average content (in %) of organic material is reported for each

horizon; see location and stratigraphic logs in Figs. 3 and 5, respectively.


subdivided in two local pollen zones, on the basis of themain variations both in pollen percentages and totalconcentration. Pollen zone SAB 1 is characterized by alower pollen concentration, with dominating Poaceae(30%), Artemisia (10–20%), Chenopodiaceae (1–5%) andPinus sylvestris t. (15–30%). Atypical pollen types in thiszone, as Abies, Picea, Carpinus and Fagus, are interpretedas reworked pollen grains due to their corroded surface. Inpollen zone SAB 2 the total pollen concentration alwaysexceeds 15 000 grains/ml and taxonomic diversity increases.This phase is dominated by Betula (10–20%), Juniperus

(5–10%), P. sylvestris t. (15–20%), Artemisia (22–29%)and Poaceae (19–22%).

LOI analysis was carried out on 10 horizons from SA2core, between the depth of 4.07 and 9.90m. The valuesrelated to the analysed interval (Table 2) are representativeof an oligo-mesotrophic environment. In the investigatedtime window, the amount of organic sediments seems tohave been almost constant with a slight increase at the topof the glacio-lacustrine unit.

4.1.3. Pollen data at San Abbondio in the regional

framework

In the pollen diagram of Fig. 6, pollen data of SAB 1zone refer to a scattered vegetation of pioneer plants(mainly herbaceous taxa) after the retreat of the LGMglacier, as shown by sample 827, which has an acceptableconcentration. Open steppe vegetation was probablypresent at that time around the lake. The expansion ofwoody vegetation with the extent of Betula–Pinus openforests, together with open steppe, is documented by pollenzone SAB 2. The limit of the two pollen zones,conventionally located in the middle point of two samples,should likely correspond to the change in the sedimento-logical pattern (from grey silt to dark grey clayey silt) nearthe top of the glacio-lacustrine unit. No erosional featurescan be distinguished because lithological transitions aregradational. The upper dark grey clayey silt probablyreflects a slight increase in organic content, as indicated by

an increase in pollen concentration, not clearly evidencedby the LOI analysis (value 2.4 at 5.66m depth; Table 2).Even though the duration of such a transition cannot beestimated, the change in environmental conditions, fromdetrital-dominated sedimentation with scarce vegetationcover (typical of a cold climate) towards an organic-dominated sedimentation and a spread of pioneer plantswith a well-developed shrub and woody vegetation,suggests a milder climate.The interpretation of the vegetation history at San

Abbondio is chronologically constrained only by theradiocarbon age of 15 900 cal BP. The pollen profile wastherefore compared with other well-dated series in theSouthern Alps, the Lago di Annone (Wick, 1996, 2000),Lago del Segrino (Wick, 1996, 2000), Lago di Alserio(Wick, 2000), Lago di Ganna (Schneider and Tobolski,1985), Lago di Biandronno (Schneider, 1978), Iseo peatbog (Bertoldi and Consolini, 1989), Lago di Origlio (Tinneret al., 2000), Lago di Muzzano (Gobet et al., 2000),Balladrum Mire (Hofstetter et al., 2006), Lago Piccolo diAvigliana (Fisinger et al., 2006) and Lago Lucone(Valsecchi et al., 2006) sequences. These sequences show,after a phase dominated by Juniperus, Pinus, Artemisia andPoaceae, the spread of Betula, which marks the beginningof afforestation around 16 000 cal BP. This phase can becorrelated to the SAB 2 pollen zone, between the end of theEarly Late Glacial and the beginning of the Late GlacialInterstadial. In this sense, the vegetation development inthe Como area is in good agreement with the dynamicsof the Insubric Region and the pre-Alpine sequences southof the Alps. The change in the depositional environmentfrom lacustrine deposits to shore sediments preventedrecording the expansion of well-developed boreal forestand the increase of organic-dominated sedimentationwhich normally appears in the other South-Alpine profiles.

4.2. Stratigraphic data from Ticosa and Via Valleggio sites

In the Ticosa area the stratigraphic sequence related tothe five analysed cores (Fig. 3) is the same as in the SanAbbondio area, but each corresponding change of lithol-ogy is some metres deeper. The lacustrine shore depositsare here approximately at the same elevation of thepresent-day lake level, while they are above it at the SanAbbondio site.The litho-stratigraphic sequence of the Via Valleggio

area, sited on the SE border of the Como plain (Fig. 3) atan elevation of ca. 214m a.s.l., was also reconstructed,based on surface geologic surveys, borehole stratigraphies,and detailed analyses on newly excavated trench walls(Fig. 7). The upper part of the succession is composed ofcoarse alluvial fan deposits, characterized by lateralcontinuity and the presence of carbonate boulders withdiameter up to several tens of centimetres. This alluvial fansequence unconformably overlies well-bedded lake margin-al and alluvial plain deposits including several horizonsrich in organic matter. Fig. 7 shows the stratigraphy from

ARTICLE IN PRESS

Fig. 7. Stratigraphic section, between 360 and 530 cm below the ground surface (ca. 214m a.s.l.), observed along an artificial cut in Via Valleggio area; the

organic material from sample VC1 yielded a radiocarbon date of 16 650 cal BP (Table 1).


3.60 cm down to 5.30m below the ground surface. Itconsists of conglomerates and microconglomerates withcarbonate clasts, alternating with silty sands, sandy siltsand dark organic silty sands.

Organic material from samples VC1 and VC4 (Fig. 7)yielded, respectively, the radiocarbon ages of 13 8807200and 5100750BP, corresponding, respectively, to 16 650 and5890 cal BP (Table 1). VC1 was sampled at the elevation ofca. 209m a.s.l., corresponding to lake level A of Fig. 8.

5. Dating and stratigraphic correlations

The cross-section in Fig. 8 shows the stratigraphicrelations through the San Abbondio and Ticosa sites alongthe ABCD trace of Fig. 3. Notably, moving from AB to Cand D, from the border towards the centre of the Como

sedimentary basin, the same lithologic marker can befound at increasing depths. The shore sands lie around209m a.s.l. in boreholes SA3, Sgc1, Sgc2 and Sgc3, around204m in Sr6, SA2 and Sgs2, around 202m in Sr7 and Sr1and around 198m in Sr5 and T1. The same happens forother lithologies, such as the wood remains-rich silts, foundgenerally deeper moving basinward.This stratigraphic setting has been interpreted as a

succession of sedimentary lake terraces, younger from westto east, formed as Late Glacial Lake Como stood atprogressively lower elevations. The depositional settingsketched in Fig. 9 was repeated several times at differentelevations until the lake level stabilized around the currentelevation (198m a.s.l.). Moreover, the presence of severalmorphological terraces between the elevation of 215 and207m a.s.l. corroborates this interpretation.

ARTICLE IN PRESS

Fig. 8. Geologic profile along the ABCD section indicated in Fig. 3. Generally, moving from AB to C and D (from west to east), same lithologies lie

progressively deeper. This stratigraphic setting has been interpreted as the succession of sedimentary lake terraces, lower and younger from west to east, as

testified also by the two radiocarbon dates.


The two radiocarbon dates available in the SanAbbondio area are in good agreement with this reconstruction(Table 1). They differ by more than 2000 yr, which is asignificant divergence, being larger than the dating methodapproximation. The older age (15 900 cal BP) refers to theelevation of ca. 204m a.s.l. and the younger (13 810 cal BP)refers to the elevation of ca. 201m a.s.l. Therefore, theyounger date is relative to a lower elevation (ca. 3m) withrespect to the older one.

While on the basin edges, like at San Abbondio site, ashore environment developed, the centre of the basin wasoccupied by a lacustrine–palustrine environment. Underthese conditions, the organic-rich silty Unit 2 deposited

(Fig. 4); the base of this unit is currently found down to thedepth of 50m near the current coastal area (e.g., PiazzaCavour).The Via Valleggio site is located on the SE margin of

the plain close to the point where the Cosia Stream entersinto the Como alluvial plain (Fig. 10). Since LatePleistocene, after the water drainage was directed towardsthe plain, the palaeo-channel of the Cosia Stream depositedalluvial sediments interbedded with coastal wetland or-ganic layers in this lake marginal area, as shown by thestratigraphic column in Fig. 7. The radiocarbon dateof 16 650 cal BP, related to an organic silty sand (VC1 inFig. 7; Table 1), represents the oldest post-LGM 14C

ARTICLE IN PRESS

Fig. 9. Model of the lakeshore setting along the SW border of the Como plain (modified, after Fairbridge, 1968). These geomorphic and stratigraphic

relations recurred several times during Late Glacial at progressively lower elevations, until the lake level reached the current altitude.


date known in the Como area and can be used as refe-rence dating for the beginning of organic sedimentationin the Como plain. Therefore, it is here considered as abenchmark for estimating the average natural subsidencetrend.

6. Landscape history and environmental evolution

On the basis of all collected data, a broad environmentalevolution of the Como town basin since the LGM wasreconstructed, as illustrated in Fig. 11. The oldest organicsample at Via Valleggio (Fig. 7) demonstrates that at leastsince 16 650 cal BP the Como basin (‘‘convalle’’) wascompletely deglaciated and vegetation could develop inthe basin and the surrounding slopes. In that period thelake level was ca. 11m higher than today, and the datedunit was deposited in a marsh connected to the lake. The15 900 and 13 810 cal BP ages, yielded by arboreal speciessampled at ca. 204 and 201m a.s.l. in sand deposits at theSan Abbondio site, indicate that at that time the lake level

was some metres higher than today (respectively, ca. +6and +3m). The beginning of forestation is confirmed bythe pollen diagram (Fig. 6). Therefore, it is possible to inferthat deposition of the organic material-rich sediments (siltsand silty sands), that overlap the glacio-lacustrine clays andsilts in the centre of the basin (Fig. 4), started ca.17 000 cal BP, in a palustrine–shallow lacustrine environ-ment (Fig. 11). At this stage the drainage on the Camerlatathreshold had already inverted its course, being directedtowards the lake. During the Holocene the Como plain wasprogressively filled up by the alluvial sediments essentiallyfed by the Cosia Stream, and, along the edges of the plain,by slope and fan deposits.

7. Como town subsidence

7.1. Natural subsidence

Based on the available stratigraphic data, Unit 2 (Fig. 4)was deposited in a lacustrine–palustrine environment hydrau-

ARTICLE IN PRESS

Fig. 10. Via Valleggio site is located close to the mouth of the palaeo-Cosia Stream into the Como plain, protected at SE by the Mt. Tre Croci reliefs.


lically connected to the lake. Therefore, this layer settled out atapproximately the same level of the lake 16650 calBP, nearly11m above the present-day level. This layer was found deep in

the Como town boreholes. In particular, it lies at its greatestdepth of ca. 50m below the ground surface (Fig. 4) along thecurrent lakeshore near Piazza Cavour.

ARTICLE IN PRESS

Fig. 11. Profile sequence representing the simplified environmental evolution of the Como plain since Last Glacial Maximum, when the Adda glacier tip

was likely beyond the Camerlata threshold. During the Early Late Glacial, a proglacial lake was between the glacier front and the Camerlata threshold, ca.

270m a.s.l.. Since ca. 17 000 cal BP, in the middle of the Como basin a lacustrine–palustrine environment developed, draining the water from the

surrounding reliefs. During the Holocene, the area hosting today the town of Como was progressively filled up by the Cosia floods and, along the edge of

the plain, by slope and fan deposits.


No overconsolidated deposits were found in the LateGlacial–Holocene lake sediments of the study area(Comune di Como, 1980; Apuani et al., 2000), suggestingthat the Lake Como level never dropped significantlybelow the present-day level during the last ca.17 000 cal BP. This is also supported by the lack ofgeomorphologic evidence indicating a lake level at a lowerelevation, and of literature data compatible with thishypothesis. According to several studies from lacustrinesequences in the Swiss Plateau, the Jura mountains and theNorthern Pre-Alps in France, it appears that during theHolocene the climatically driven lake level oscillationsnever exceeded 5m (Magny and Ruffaldi, 1995; Magnyand Richoz, 1998; Magny, 2001; Magny et al., 2003). InItaly, studies are still in progress in the northern area of theGarda Lake (Magny, personal communication). Baroni(1997) singled out two abrasion surfaces 8 and 12m underthe present-day level of Lake Garda, but their chronologyis unknown and their origin is likely due to tectonicactivity.

Therefore, the overall architecture of the Unit 2 layerswas controlled by long-term land subsidence phenomena.As discussed before, since ca. 17 000 cal BP the Como plainwas occupied by a marsh-shallow water lake until it wasfilled up by the Cosia Stream alluvial deposits. Inferring anapproximate age of 17 000 cal BP for the bottom of theorganic-rich silts found in the Como boreholes, that is, anacceptable approximation for the purpose of this study, itis possible to calculate the average subsidence rate duringthis time span. The velocity of subsidence is given byV ¼ (S�Dz)/T, where V is the velocity, S the sedimentsthickness, Dz the sum of depth variations due to water leveloscillations and T the sediment age. Assuming Dz ¼ 11m(the difference between the 16 650 cal BP and the present-day lake levels), the average subsidence rate during the last17 000 cal BP is bracketed between 1 and 2.5mm/a, with thehigher values in the present-day near lake zone (Comerciet al., 2005).

Fig. 12 shows the equal subsidence rate areas obtainedby means of the stratigraphic correlation among the keyboreholes indicated in the same map. The maximumsinking velocities (2.5mm/yr) are distributed along theaxial zone of the basin toward the lake. The subsidence

rates progressively decrease to the south and the borders ofthe basin.Subsidence is not a constant-rate phenomenon, being

characterized by acceleration and deceleration phases.Nevertheless, the average values mapped in Fig. 12, evenwith their approximation, highlight satisfactorily the orderof magnitude of the natural subsidence, on which, later on,human-induced subsidence was superimposed.Archaeological evidence observed between 1889 and

1931, before the period of post-World War II acceleratedsubsidence, can help to evaluate the order of magnitudeof the subsidence during the last two millennia. RomanAge remains at several sites, some of them reported inFig. 13 were found buried under 2.4–3m of sediments.Their detailed chronological attribution is discussed inUboldi (1993), Nobile (2005) and Riva (2006). Assumingthe same lake level of today, they provide a historicalaverage subsidence rate of a little more than 1mm/yr,hence comparable to the average velocity obtained fromgeological data. For the natural subsidence, this valuerepresents the order of magnitude of the phenomenon.During Roman times, human activities could have had alocalized effect on subsidence, mostly due to landreclamation and building load. However, the average landsubsidence since then and until the post-war accelera-tion appears to be comparable to the average rate over17 000 yr.

7.2. Human-induced subsidence

The analysis of the geometric levelling data availablesince the beginning of the 20th century has alloweddetermination of the subsidence rates during ca. the last80 yr (examples of two benchmarks are in Fig. 14). Thealtimetry of the Como territory was measured since 1877but levelling data are available since 1928 (Rete Col.Quaglia). Afterwards, other levelling surveys were con-ducted in 1955, 1975, 1979, 1981, 1983 and 1990 by IstitutoGeografico Militare (IGM), in 1997 by Politecnico diMilano and in 2004 by APAT.These data show that during part of this period a much

faster human-induced subsidence was added to the onedriven by the natural consolidation of the Late Pleistocene,

ARTICLE IN PRESS

Fig. 12. Natural average subsidence rates since Late Glacial in the Como urban area, estimated by means of stratigraphical data. Different velocities have

been estimated on the basis of the depth at which the lacustrine–palustrine sediments (Unit 2 in Fig. 4) were found in boreholes. Among all the boreholes,

only in the ones reported in the map it was possible to recognize with certainty the Unit 2. The approximate age of 17 000 yr was attributed to the bottom

of such a unit. Points 1–3 indicate the archaeological sites described in Fig. 13. Crosses 60IV, 60, 42 and 38 indicate the locations of levelling benchmarks

of Figs. 14 and 15.


Holocene and Recent sediments. The human activities thathave contributed to accelerate the ground lowering wereloading following building expansion, road traffic andchiefly water pumping for civil and industrial uses. Inparticular, in the coastal area the main factor is thepresence of a thickness of filling materials used since 1722to reclaim new territory.

According to the available measurements, during1950–1975 subsidence reached a velocity one order ofmagnitude higher than the natural one. In particular,geometric levelling in this period showed subsidence ratesof 10–20mm/yr, with peak values that exceeded 20mm/yr.The intensive exploitation of the deep aquifer during thoseyears was interpreted as the main factor responsible for thisacceleration. Between 1975 and 1980, a sharp slowing ofthe sinking velocity was observed, probably determined bythe modifications brought to the water supply system. Theactivation of the industrial aqueduct allowed a significantpiezometric recovery.

Between the early 1980s and 1997, a slight increase ofelevation of several benchmarks was measured, probablycaused by the elastic reaction of the cohesive lithologies,

following the rise of the water table. Similar occurrencesare well known all over the world, for instance, in Venezia,Ravenna and Tokyo (Carbognin, 1986).From the comparison of 1990, 1997 (Giussani, 1997) and

2004 geometric levelling data, it emerges that subsidenceoccurred along the breakwater (with faster velocitiestoward the lake end) and along the coastal area and PiazzaCavour. From 1990 to 2004, the benchmark on thebreakwater sank with the velocity of 5–6mm/yr, whilealong the lakeshore the velocities ranged between 1.4 and2.8mm/yr. In the historical centre the subsidence rate wasbetween 0 and 1mm/yr (Comerci, 2004).Ground movements during the years 1992–2003 were

measured also by TRE by means of the PSI technique(Strozzi et al., 1999; Ferretti et al., 2000). The velocitiesobtained for the same period by the two techniques are ingood agreement, supporting the reliability of the data(Comerci, 2004; Comerci et al., 2004, 2005). Essentially,only in a small number of cases the velocity of levellingbenchmarks and the velocity of the nearest permanentscatterer differ more than 1mm/yr, which is the precisionlimit of the PSI technique.

ARTICLE IN PRESS

Fig. 13. Examples of Roman Age archaeological remains discovered under the Como urban area, buried under 2.4–3m of sediments; these data allow to

estimate the historical long-term average subsidence rate. The position of sites 1–3 is shown also in Fig. 12.


Another interesting issue revealed by the comparison ofseveral geometric levelling data is the increase of sub-sidence in the period 1997–2004, compared to the previousyears. This trend is shown by many benchmarks all aroundthe town: a benchmark on the breakwater moved at therate of �4mm/yr between 1983 and 1997, and at �8mm/yrbetween 1997 and 2004 (benchmark 60IV in Fig. 15), withthe other benchmarks on the breakwater having ananalogous behaviour. One of the benchmarks sited inPiazza Cavour showed a strong lowering between 1975 and1981 (�9.8mm/yr), followed by an attenuation up to 1997(�1.2mm/yr), and afterwards by a new increase of thesubsidence rate up to 2004 (�2.8mm/yr), even if at lessermagnitude than in the 1970s (benchmark 42 in Fig. 15); abenchmark in the Duomo perimeter even reversed its trend(benchmark 38 in Fig. 15) during the 1997–2004 period(�1.2mm/yr) as compared to the 1981–1997 time interval(+0.75mm/yr), as for several other downtown bench-marks (Comerci, 2004). According to the analysed data,

the Como urban area has been sinking again during recentyears (up to 2004), with a rate generally comparable to thatof the long-term average natural subsidence, and with evenhigher rates in some places along the coast.Moreover, the analysis of all the geometric levelling

surveys carried out in the Como area shows that thebenchmarks along the eastern border of the plain, sited onthe stiff bedrock, have a positive trend. The PSI datadisplay positive movements, with rates higher than 2mm/yrat Mt. Brunate (Comerci, 2004; Comerci et al., 2004). Thistrend is in agreement with the uplift and ongoing tectonicdeformation of the Como pre-Alpine and Alpine chain(e.g., Arca and Beretta, 1985; Sileo et al., 2007). Thisevidence suggests that the Como subsidence is a local, non-tectonic, phenomenon due to soil compaction, affectingonly the geomorphologic depression on which the townstands, filled by soft, under to normally consolidated, LateGlacial and Holocene sediments (Cancelli, 1980; StudioCancelli Associato, 1997; Apuani et al., 2000).

ARTIC

LEIN

PRES

S

. Vertical displacement time history of benchmarks 60 (Piazza Cavour) omo) indicated in Fig. 12. The 1928 measurements are afflicted by a ca. 50 cm uncertainty due to changes in the

al Geodetic Network which occurred after World War II. The highes nsidered to be more reliable (Comune di Como, 1980).

. The 1975–2004 vertical displacements of the benchmarks indicated in nchmarks 60IV (breakwater) and 42 (Piazza Cavour) are illustrative of the subsidence rate along the shoreline.

ark 38 (Duomo) presents a negative trend from 1997 to 2004, after a 81–1997) of uplift.

V.

Co

merci

eta

l./

Qu

atern

ary

Intern

atio

na

l1

73

–1

74

(2

00

7)

67

–8

683

Fig. 14

Nation

Fig. 15

Benchm

and 38 (Du

t value is co

Fig. 12. Be

period (19


8. Conclusions

The thorough examination of more than 100 boreholescovering the entire Como town plain, the drilling of threenew shallow boreholes in the south-western border of theplain, supported by several in situ and laboratory analyses(organic and pollen content, 14C dating), and the analysisof newly excavated geologic sections on the SE border ofthe plain allowed reconstruction of the broad geologic andenvironmental evolution of this territory since LateGlacial. The natural tendency of the Como urban area,in particular in the near lake zone, to subside has beenassessed. The order of magnitude of the natural subsidence,inferred by means of geological data, has resulted to bebetween 1 and 2.5mm/yr, with rates progressively reducingtowards the edges of the plain. Archaeological data, relatedto Roman Age remains, and instrumental monitoring arein agreement with this estimate.

During the 1950–1975 period, because of the indiscrimi-nate exploitation of water resources from the deep mainaquifer, the same area was affected by a human-inducedaccelerated subsidence. In this period the subsidence ratewas about 10 times larger than the natural subsidence rate,exceeding in some cases 20mm/yr. The modificationsbrought to the industrial water supply system at the endof the 1970s induced a general deceleration of thephenomenon and, between the 1980s and 1997, even aslight increase of elevation on several downtown bench-marks, probably due to the piezometric recovery of themain aquifer.

The APAT geometric levelling in 2004, also supportedby the PSI data (1992–2003), has shown that in recent yearssubsidence has been active again with rates comparablewith the long-term natural ones. This is the most likelyscenario to be reasonably expected for the near future, ifthe boundary conditions (e.g., lake level, exploitation ofthe aquifer, ground loading) remain unchanged.

Finally, the satellite geodetic data (1992–2003) show apositive regional movement along the eastern edge of theComo plain and on the neighbouring mountains, which isin agreement with the known Alpine and pre-AlpineQuaternary deformational history. Therefore, the subsi-dence tendency is a local phenomenon, strictly related tothe Como urban area, basically depending on the settlingof the thick, recent, unconsolidated sediment sequencefilling the basin.

Acknowledgements

The authors wish to thank Giuseppe Orombelli for hisrevision that contributed to sensibly improve the paper.Thanks are also due to Alessandro Ferretti and MarcoBasilico (TRE) for providing the PSI data; Luciano Bonci,Francesco Vullo and Domenico Matarazzo (APAT) for thegeometric levelling data; Francesca Giardina for drawingFig. 11; Kervin Chunga for the stratigraphy of ViaValleggio; Norberto Masciocchi, Simona Galli and Simone

Tumiati for the mineralogical XRD analysis and inter-pretation; Roberta Bettinetti for the LOI analysis; Gian-franco Peruzzo and Luca Demicheli for their help; Dr.G. Attardo and Dr. G. Landi, Geocipo, Geostudi andRodio for providing their stratigraphic data and the ComoMunicipality for logistic and practical support.

References

Apuani, T., Cancelli, A., Cancelli, P., 2000. Hydrogeological and

geotechnical investigations along the shoreline of the town of Como,

Italy. In: Moore, D.P., Hungr, O. (Eds.), Engineering Geology and

Environment, Proceedings, Eighth Congress of the International

Association of Engineering Geology, IAEG, 21–25 September 1998,

Vancouver, BC, Canada, vol. VI. Balkema, Rotterdam, pp. 4685–4692.

Arca, S., Beretta, G.P, 1985. Prima sintesi geodetico-geologica sui

movimenti verticali del suolo nell’ Italia Settentrionale (1897–1957).

Bollettino di Geodesia e Scienze Affini XLIV (2), 125–156.

Ascheri, A., 1972. Studio geologico sui fenomeni di subsidenza nell’area

urbana della citta di Como. Rapporto 1, Comitato per lo studio dei

fenomeni di subsidenza, Comune di Como, p. 28.

Baroni, C., 1997. The paleolevels of Lake Garda. In: Geomorphology of

the Central and Southern Alps. Fourth International Conference on

Geomorphology, Italy, 1997. Supplement, Geografia Fisica e Dina-

mica Quaternaria III(T.2), 13–47.

Bernoulli, D., 1964. Zur Geologie des Monte Generoso (Lombardische

Alpen). Ein Beitragzur Kenntnis der sudalpinen Sedimente. Beitrage

zur Geologie Karte der Schweiz, Neue Folge, p. 118.

Bernoulli, D., Bertotti, G., Zingg, A., 1989. Northward thrusting of the

Gonfolite Lombarda (‘‘South-Alpine Molasse’’) onto the Mesozoic

sequence of the Lombardian Alps; implications for the deformation

history of the Southern Alps. Eclogae Geologicae Helvetiae 82,

841–856.

Bertoldi, R., Consolini, M., 1989. Deglaciazione e sviluppo vegetazionale

tardi-postglaciale nella regione del Lago d’Iseo attraverso analisi

polliniche e datazione 14C. Memorie della Societa Geologica Italiana

42, 139–145.

Bertotti, G., Picotti, V., Bernoulli, D., Castellarin, A., 1993. From rifting

to drifting; tectonic evolution of the South-Alpine upper crust from

the Triassic to the Early Cretaceous. Sedimentary Geology 86,

291–301.

Bini, A., 1993. Geologia del Quaternario e geomorfologia della Piana di

Como. In: Uboldi, A. (Ed.), Carta archeologica della Lombardia, vol.

III. Museo Civico Archeologico P. Giovio, Como.

Bini, A., 1996. Intervento alla tavola rotonda. La regolazione del Lago di

Como: problemi e proposte. Kiwanis International Europe. Atti del

convegno. Villa Olmo, Como, 19 April.

Bini, A., Zuccoli, L., 2004. Glacial history of the southern side

of the central Alps, Italy. In: Ehlers, J., Gibbard, P.L. (Eds.),

Quaternary Glaciations—Extent and chronology. Elsevier, Amsterdam,

pp. 195–200.

Bini, A., Felber, M., Pomicino, N., Zuccoli, L., 1996. Maximum extension

of the glaciers (MEG) in the area comprised between Lago di Como,

Lago Maggiore and their respective end-moraine system. Geologia

Insubrica 1 (1–2), 65–78.

Bini, A., Tognini, P., Zuccoli, L., 1998. Rapport entre karst et glaciers

durant les glaciations dans les vallees prealpines du Sud des Alpes.

Karstologia 32, 7–26.

Bini, A., Felber, M., Pomicino, N., Zuccoli, L., 2001. Geologia del

Mendrisiotto (Canton Ticino, Svizzera): Messiniano, Pliocene e

Quaternario. Rapporti del UFAG, Serie Geologia, vol. 1, Berna.

Cancelli, A., 1980. Geotecnica del sottosuolo cittadino. In: Relazione di

sintesi della Commissione per lo Studio dei fenomeni di Subsidenza.

Comune di Como. Documenti e Ricerche, vol. 34, pp. 43–46.

ARTICLE IN PRESSV. Comerci et al. / Quaternary International 173–174 (2007) 67–86 85

Carbognin, L., 1986. La subsidenza indotta dall’uomo nel mondo. I casi

piu significativi. Bollettino Associazione Mineraria Subalpina XXIII

(4), 433–468.

Castelletti, L., Orombelli, G., 1986. Una nuova data 14C per la storia della

deglaciazione del bacino del Lago di Como. Geografia Fisica e

Dinamica Quaternaria 9, 56–58.

Cita, M.B., 1957. Studi stratigrafici sul Terziario Subalpino Lombardo.

Rivista Italiana di Paleontologia e Stratigrafia 63 (2), 79–122.

Comerci, V., 2004. Evoluzione geologica e ambientale recente in aree

subsidenti. Esempio di studio nella citta di Como. Ph.D. Thesis,

Universita degli Studi dell’Insubria, Como, 136pp.

Comerci, V., Bonci, L., Michetti, A.M., Vittori, E., Vullo, F., 2004.

Natural and man-induced subsidence in the Como urban area

(Northern Italy). In: Program with Abstracts, Italia 2004, 32nd

International Geological Congress, 20–28 August 2004, Florence,

Italy.

Comerci, V., Bonci, L., Michetti, A.M., Vittori, E., 2005. The subsidence

of Como: human impact or natural tendency? In: Dark Nature, Rapid

Natural Change and Human Responses—Final Meeting, 6–10

September, Como. Program and Abstracts, pp. 11–12.

Comune di Como, 1980. Relazione di sintesi della Commissione per lo

Studio dei fenomeni di Subsidenza. Comune di Como. Documenti e

Ricerche, vol. 34.

Fairbridge, R.W., 1968. The Encyclopedia of Geomorphology. Dowden,

Hutchinson and Ross, Stroudsburg, PA, 1295pp.

Fantoni, R., Bersezio, R., Forcella, F., 2004. Alpine structure and

deformation chronology at the Southern Alps–Po Plain border in

Lombardy. Bollettino della Societa Geologica Italiana 123 (3), 463–476.

Ferretti, A., Prati, C., Rocca, F., 2000. Measuring subsidence with SAR

interpherometry: applications of the permanent scatterer technique. In:

Land Subsidence—vol. II. Proceedings of the Sixth International

Symposium on Land Subsidence, 24–29 September 2000, Ravenna, Italy.

Fisinger, W., Tinner, W., van der Knaap, W.O., Ammann, B., 2006. The

expansion of hazel (Corylus avellana L.) in the southern Alps: a key

for understanding its early Holocene history in Europe? Quaternary

Science Reviews 25, 612–631.

Gianotti, R., Perotti, C.R., 1986. Introduzione alla tettonica e all’evolu-

zione strutturale delle Alpi Lariane. In: Proceedings of the Symposium

on the Geology of Como. Memorie della Societa Geologica Italiana

32, 67–99.

Giardina, F., Michetti, A.M., Serva, L., Doglioni, C., 2004. The seismic

potential of the Insubria Region (Southern Alps): insights from

topographic and rheological modelling. Bollettino di Geofisica Teorica

ed Applicata 45, 86–91.

Giussani, A., 1997. Studio dei fenomeni di subsidenza del territorio del

Comune di Como. Campagna di misure 1997. Politecnico di Milano,

Milano.

Gobet, E., Tinner, W., Hubschmid, P., Jansen, I., Wehrli, M., Ammann, B.,

Wick, L., 2000. Influence of human impact and bedrock differences on

the vegetational history of the Insubrian Southern Alps. Vegetation

History and Archaeobotany 9, 175–187.

Hofstetter, S., Tinner, W., Valsecchi, V., Carraro, G., Conedera, M., 2006.

Lateglacial and Holocene vegetation history in the Insubrian Southern

Alps—new indications from a small-scale site. Vegetation History and

Archaeobotany 15, 87–98.

Ivy-Ochs, S., Schafer, J., Kubik, P.W., Synal, H.A., Schluchter, C., 2004.

Timing of deglaciation on the northern Alpine foreland (Switzerland).

Eclogae Geologicae Helvetiae 97, 47–55.

Luraschi, G., 1997. Storia di Como antica. Como, 120pp.

Magny, M., 2001. Palaeohydrological changes as reflected by lake-level

fluctuations in the Swiss Plateau, the Jura Mountains and the northern

French Pre-Alps during the Last Glacial–Holocene transition: a

regional synthesis. Global and Planetary Change 30, 85–101.

Magny, M., Richoz, I., 1998. Holocene lake-level fluctuations in Lake

Seedorf, southern Swiss Plateau. Eclogae Geologicae Helvetiae 91,

345–357.

Magny, M., Ruffaldi, P., 1995. Younger Dryas and early Holocene lake-

level fluctuations in the Jura mountains, France. Boreas 24, 155–172.

Magny, M., Begeot, C., Guiot, J., Marguet, A., Billaud, Y., 2003.

Reconstruction and palaeoclimatic interpretation of mid-Holocene

vegetation and lake-level changes at saint-Jorioz, lake Annecy,

northwestern French Pre-Alps. The Holocene 13, 265–275.

Montrasio, A. (Ed.), 1990. Carta geologica della Lombardia. Scala

1:200.000. Servizio Geologico Nazionale, Roma.

Moore, P.D., Webb, J.A., Collison, M.E., 1991. Pollen Analysis. 2nd ed.

Blackwell Science, Oxford, 216pp.

Moscariello, A., Ravazzi, C., Brauer, A., Mangili, C., Chiesa, S., Rossi, S.,

Beaulieu, J., Reille, M., 2000. A long lacustrine record from the

Pianico Sellere basin (Middle–Late Pleistocene, Northern Italy).

Quaternary International 73/74, 47–68.

Muttoni, G., Carcano, C., Garzanti, E., Ghielmi, M., Piccin, A., Pini, R.A.,

Rogledi, S., Sciunnach, D., 2003. Onset on major Pleistocene glaciations

in the Alps. Geology 31, 989–992.

Nangeroni, G., 1940. Considerazioni sul Quaternario dell’alta pianura

lombarda occidentale. Bollettino del Comitato Glaciologico Italiano e

della Commissione Glaciologica del Centro Alpinistico Italiano 20,

63–72.

Nangeroni, G., 1954. Il glaciale Quaternario nell’anfiteatro del Verbano e

lungo i rami del Lario, vol. LXXXVII, Fasc. III. Rendiconti Istituto

Lombardo, Milano, pp. 653–664.

Nangeroni, G., 1970. Appunti sulla Geomorfologia del Triangolo

Lariano. Atti della Societa Italiana di Scienze Naturali e

del Museo Civico di Storia Naturale di Milano 110 (2), 69–149

(15-VI).

Nangeroni, G., 1974. La geomorfologia delle montagne lariane. Atti della

Societa Italiana di Scienze Naturali e del Museo Civico di Storia

Naturale in Milano 115, 5–116.

Nangeroni, G., 1976. A proposito di una sintesi geomorfologica sulle

Prealpi Lombarde. Rivista Geografica Italiana 83, 203–209.

Nobile, I., 2005. Indagini archeologiche a Como. Lo scavo nei pressi della

Porta Pretoria, Como, pp. 10–40.

Orombelli, G., 1976. Indizi di deformazioni tettoniche Quaternarie al

margine meridionale delle Prealpi Comasche. Quaderno Gruppo di

Studio Quaternario Padano 3, 25–37.

Prince, K.R., Sonenshein, R., Karavitis, G., 2001. International land

subsidence data base In: US Geological Survey Subsidence Interest

Group Conference, Proceedings of the Technical Meeting,

27–29 November 2001, Galveston, TX. Open-File Report 03–308,

pp. 147–162.

Ravazzi, C, 2003. An overview of the Quaternary continental stratigraphic

units based on biological and climatic events in Italy. Il Quaternario 16

(1 bis), 11–18.

Riva, S., 2006. Geologia e Archeologia a Como. Tesi di Laurea in

Valutazione e Controllo Ambientale. Unpublished Thesis, Environ-

mental Assessment and Monitorino Degree, Universita degli Studi

dell’Insubria, Como, 87pp.

Schneider, R., 1978. Pollenanalytische Untersuchungen zur Vegetations-

geschichte am Sudrand der Alpen zwischen Turin und Varese.

Botanische Jahrbuch System Pflanzengesch 100, 26–109 (in Italian).

Schneider, R., Tobolski, K., 1985. Lago die Ganna. Late-glacial and

Holocene environments of a lake in the Southern Alps. Dissertationes

Botanicae 87, 229–271.

Sciunnach, D., Tremolada, F., 2004. The Lombardian Gonfolite Group in

central Brianza (Como and Milano Provinces, Italy): calcareous

nannofossils biostratigraphy and sedimentary record of neo-Alpine

tectonics. Eclogae Geologicae Helvetiae 97, 119–131.

Sileo, G., Giardina, F., Livio, F., Michetti, A.M., Mueller, K., Vittori, E.,

2007. Remarks on the Quaternary tectonics of the Insubria (Lombar-

dia, North-Western Italy) and Ticino (South-Eastern Switzerland)

regions. Bollettino della Societa Geologica Italiana 126 (2),

127–142.

Strozzi, T., Wegmuller, U., Bitelli, G., Tosi, L., Carbognin, L., Galgaro, A.,

1999. Monitoraggio della subsidenza con l’interferometria SAR

differenziale: metodologia ed esempi. In: Atti Conv. Le Pianure, 8–11

November 1999, Ferrara, pp. 245–248.


Studio Cancelli Associato, 1997. Opere per la difesa dalle esondazioni

lungo la passeggiata a lago. Comune di Como.

Stuiver, M., Reimer, P.J., Braziunas, T.F., 1998. High-precision radio-

carbon age calibration for terrestrial and marine samples. Radio-

carbon 40, 1127–1151.

Tinner, W., Hubschmid, P., Wehrli, M., Ammann, B., Conedera, M.,

2000. Long-term forest ecology and dynamics in Southern Switzerland.

Journal of Ecology 87, 273–289.

Uboldi, M. (Ed.), 1993. Carta Archeologica della Lombardia, III. Como.

La citta murata e la convalle. Franco Cosimo Panini Editore, Modena,

155pp, ISBN 88-7686-211-0.

Valsecchi, V., Tinner, W., Finsinger, W., Ammann, B., 2006. Human

impact during the Bronze Age on the vegetation at Lago Lucone

(northern Italy). Vegetation History and Archaeobotany 15, 99–113.

Wick, L., 1996. Late-Glacial and Early-Holocene palaeoenvironments in

Brianza, N Italy. Il Quaternario 9 (2), 653–660.

Wick, L., 2000. Lago di Annone AN-2 Holocene. In: Wick, L. (Ed.),

XXIV Moorexkursion—Excursion guide, Southern Alps. Institute of

Plant Sciences University of Bern, pp. 33–37.

Zanchi, A., Bini, A., Felber, M., Rigamonti, I., Ugeri, A., 1997.

Neotectonic evidences along the Lombardian foothills of the Southern

Alps. Geologia Insubrica 2, 99–112.

Land subsidence and Late Glacial environmental evolution of the Como urban area (Northern Italy

Documents

Transcript of Land subsidence and Late Glacial environmental evolution of the Como urban area (Northern Italy