A multidisciplinary approach to the understanding of hydromedusan populations inhabiting...

A multidisciplinary approach to understanding carbonatedeposition under tectonically controlled hydrothermalcirculation: A case study from a recent travertine mound in theEuganean hydrothermal system, northern Italy

MARCO POLA*, ANNA GANDIN† , PAOLA TUCCIMEI‡ , MICHELE SOLIGO‡ ,RITA DEIANA§ , PAOLO FABBRI* and DARIO ZAMPIERI**Dipartimento di Geoscienze, Universit�a degli Studi di Padova, Via Gradenigo, 6, 35131 Padova, Italy(E-mail: [email protected])†Dipartimento di Scienze fisiche della Terra e Ambientali, Universita degli Studi di Siena, ViaLaterina, 8, 53100 Siena, Italy‡Dipartimento di Scienze, Sezione di Scienze Geologiche, Universita degli Studi ‘Roma Tre’, LargoSan Leonardo Murialdo, 1, 00146, Roma, Italy§Dipartimento dei Beni Culturali: Archeologia, Storia dell’arte, del cinema e della musica, Universitadegli Studi di Padova, Piazza Capitaniato, 7, 35139 Padova, Italy

ABSTRACT

A multidisciplinary study was carried out on an Upper Pleistocene traver-

tine mound (Montirone Hill; Abano Terme, south of Padova, Veneto, Italy)

with the aim of developing a comprehensive understanding of the processes

interacting in structurally controlled thermal springs and the consequent

deposition of thermal limestones. The sedimentology, geochemistry, struc-

tural characteristics and hydrogeology of the mound, which is associated

with the Euganean geothermal field and the related complex structural

framework of the Schio-Vicenza fault system, were investigated. A palaeo-

environmental model of the deposit has been reconstructed clarifying the

connection between the travertine deposition and the regional structural

setting. Calcium-rich thermal waters rose from spring orifices (estimated tem-

perature from 54°C to 61°C based on geochemical calculations) and produced

a mound made of coalescent shield-like bodies. The dominant lime-mudstone

facies resulted from the accumulation of lime mud at the bottom of shallow

crater-like basins located on top of stacked cones. Soft sediment deformations

affecting the lime mud sediment were most probably produced by the recur-

rent uprising of gas bubbles. The overlying crystalline crusts were deposited

on the flanks of the mound by waters overflowing from the vents with laminar

flux. A deep hydrothermal circuit with a long period of water–rock interaction

is supported by geochemical analyses. Furthermore, a local extensional

regime, enhancing the migration of thermal fluids to the surface, is inferred

based on the results of the structural analysis carried out on the fault/fracture

mesh that affects the mound. These results corroborate the well-established

relation among travertine deposits, hydrothermal systems and fault systems,

and substantiate the structurally controlled conceptual model of the Euganean

hydrothermal system, suggesting that both the thermal and the fault systems

were active at least since 34 ka.

Keywords Dilational stepover, Euganean hydrothermal system, isotopegeochemistry, resistivity measurements, travertine depositional facies,travitonics.

172 © 2013 The Authors. Journal compilation © 2013 International Association of Sedimentologists

Sedimentology (2014) 61, 172–199 doi: 10.1111/sed.12069

INTRODUCTION

Studies of the relations between hydrothermalcirculation, travertine deposition and tectonicactivity have been valuable elements of neotec-tonic investigations in the last few decades (Alt-unel & Hancock, 1993; Hancock et al., 1999;Brogi, 2004; Uysal et al., 2007; Brogi & Capezzu-oli, 2009; Temiz et al., 2009; Brogi et al., 2010,2012). Faults and their related fractures are rec-ognized as the most efficient path for fluidmigration in the Earth’s crust (Caine et al., 1996;Curewitz & Karson, 1997; Gudmundsson, 2000;Faulkner et al., 2010), allowing the developmentof structurally controlled, hydrothermal systemsmanifest at the surface by hot water uprising(thermal springs). If the hot waters are characte-rized by high calcium contents, then thermalcarbonate (travertine) may be deposited aroundthe thermal springs as well as in the fault/frac-ture meshes, sealing the path against fluid circu-lation. Because the thermal flow of fluids isstrictly related to the activity of faults, the distri-bution of active flow sites and/or preservedtravertine masses can provide a tool for studyingthe evolution of fault zones and their relatedhydrothermal systems (Altunel & Hancock, 1993;Curewitz & Karson, 1997; Hancock et al., 1999;Fouke et al., 2000; Brogi, 2004; Uysal et al., 2007;Faccenna et al., 2008; Brogi & Capezzuoli, 2009;Nelson et al., 2009; Temiz et al., 2009; Uysalet al., 2009; Brogi et al., 2010; Guido & Campbell,2011; Kele et al., 2011; Brogi et al., 2012; DeFilippis et al., 2012; Guido & Campbell, 2012).The tectonic control of thermal fluid migration,hot springs and travertine deposits is well-demonstrated (Curewitz & Karson, 1997; and ref-erences therein) and generally is related to activeextensional structures, such as normal or trans-tensional faults (Altunel & Hancock, 1993;Hancock et al., 1999; Brogi, 2004; Brogi & Cape-zzuoli, 2009; Nelson et al., 2009). However, inaddition to the active regional tectonics, traver-tine deposition and lithofacies distribution couldbe influenced by other factors, such as climate(precipitation and temperature), hydrodynamics(fluid artesian pressure, pH and composition),topography, inherited fault/fracture meshes, highheat flow and CO2 supply (Rihs et al., 2000;Pentecost, 2005; Faccenna et al., 2008; Uysalet al., 2009; Guido & Campbell, 2011; De Filippis& Billi, 2012; De Filippis et al., 2012). Therefore,detailed (palaeo) geographical, (palaeo) environ-mental and (palaeo) hydrological reconstructionsare needed to achieve a broader view of the fac-

tors that influence travertine depositional pro-cesses. Present knowledge of travertineenvironments results from sedimentological andstratigraphical investigations (Chafetz & Folk,1984; Folk et al., 1985; Pedley, 1990; Guo &Riding, 1992; Koban & Schweigert, 1993; Folk,1994; Guo & Riding, 1998; Chafetz & Guidry,1999; Guo & Riding, 1999; Riding, 2000; Pedley,2009) which are usually combined with: (i) mor-phological and structural analyses at the localand regional scales (Altunel & Hancock, 1993;Hancock et al., 1999; Brogi & Capezzuoli, 2009;Brogi et al., 2010; De Filippis & Billi, 2012; Guido& Campbell, 2011; Brogi et al., 2012); (ii) geo-chemical analyses of the stable isotope signature(Fouke et al., 2000; Rihs et al., 2000; Faccennaet al., 2008; Gandin & Capezzuoli, 2008; Keleet al., 2011; De Filippis et al., 2012; Rodr�ıguez-Berriguete et al., 2012); (iii) geochronologicalanalyses (Bargar, 1978; Uysal et al., 2007; Temizet al., 2009; Uysal et al., 2009; Brogi et al., 2010,2012); and (iv) geochemical investigations of thewaters in present-day thermal systems (Crosseyet al., 2006; Nelson et al., 2009; Banerjee et al.,2011). Integration of the results of these differentapproaches has the potential to provide a compre-hensive view of travertine depositional processesin terms of: (i) the sedimentological characteris-tics of the deposit; (ii) regional and local tectonicregimes, providing useful information regardingthe neotectonic history and palaeoseismology ofthe area; (iii) climatic control on travertine depo-sition; and (iv) features of the hydrothermal sys-tems controlling the chemical signature of thethermal fluids/travertines.This study reports a multidisciplinary study

carried out on a Late Pleistocene travertinedeposit (Montirone Hill) located within theEuganean geothermal field (EGF). The EGF is oneof the most important thermal fields of northernItaly and extends in a band of ca 36 km2 to thesouth-west of Padova (Veneto Region, Italy;Fig. 1A) comprising the well-known spa townsof Abano Terme, Montegrotto Terme and Batta-glia Terme. The temperature of the Euganeanthermal waters ranges from 60°C to 86°C, andtheir TDS (total dissolved solids) ranges from 3�5to 6 g l�1 with the principal constituents beingprimarily Cl� and Na+ (70 wt.%) and secondarilySO4

2�, Ca2+, Mg2+, HCO3� and SiO2 (Gherardi

et al., 2000; Fabbri, 2001; Fabbri & Trevisani,2005). At present, ca 250 wells are active (with adepth from a few hundred up to 1000 m), andthe average total flow rate of exploited thermalfluids is 17 Mm3 yr�1. Although studies of the

© 2013 The Authors. Journal compilation © 2013 International Association of Sedimentologists, Sedimentology, 61, 172–199

A multidisciplinary approach to hydrothermal tectonically controlled carbonate deposition 173

chemistry and temperature of the Euganean ther-mal waters (Piccoli et al., 1973; Sartori et al.,1997; Gherardi et al., 2000; Boaretto et al., 2003),and attempts to reconstruct the subsurface geol-ogy based on the thermal well stratigraphy havebeen carried out (Antonelli et al., 1993), the

structural geology of the EGF remains poorlyunderstood. The multidisciplinary approach usedin this study comprises: (i) sedimentological(lithofacies description and interpretation) andgeochemical (U/Th dating, oxygen and carbonisotope ratio) analyses of the travertine deposit;

A

C D

B

Fig. 1. (A) Location of the Euganean geothermal field (EGF) within the central part of the Veneto region (NE Italy)[the dashed white area corresponds to (B)]. (B) A structural sketch of the NW–SE trending Schio–Vicenza faultsystem (SVFS) bordering to the west the Veneto-Friuli foreland; the EGF is located in coincidence with a rhom-boid, left stepover that is bounded by two distinct, sub-parallel, NNW–SSE trending fault segments of the SVFS[the dashed white area corresponds to (C)]. (C) A schematic geological map of the subsurface of Montirone traver-tine mound in the Abano Terme area. (D) An aerial view of Montirone Hill (modified from Google Earth©): theblack contour lines represent the mound elevation (values given in metres). The red dots mark the sites of the twodrillings (ca 1 m in depth) made to sample the travertine lithofacies.


174 M. Pola et al.

(ii) structural analysis of the fracture mesh affect-ing the carbonate body; and (iii) geophysical mea-surements of the area (electromagnetic frequencydomain mapping and electrical resistivity tomo-graphy). The results of this integrated approachhave provided a clearer understanding of: (i) thepalaeo-environmental model of deposition, suit-able for the interpretation of similar, worldwidetravertine deposits; (ii) the faults, kinematics andactivity that have enhanced thermal water flux;and (iii) the distinctive features of the relatedhydrothermal system (Piccoli et al., 1973; Sartoriet al., 1997; Gherardi et al., 2000; Boaretto et al.,2003; Zampieri et al., 2009; Pola et al., 2010,2013).

GEOLOGICAL AND HYDROTHERMALSETTING

The Euganean geothermal field (EGF) is locatedalong a regional fault, the Schio-Vicenza fault(main trend: NW–SE), which extends for ca100 km in the Veneto alluvial plain from theSchio area (foot of the pre-Alps) to the Conselvearea (south–east of the Euganei Hills) (De Pretto,1931; De Boer, 1963; Semenza, 1974; Pellegrini,1988; Massironi et al., 2006). This fault repre-sents the western border of the north-easternAdria plate indenter (Viti et al., 2006; Burratoet al., 2008) and, together with the forelandstructural high of the Euganei Hills–Berici Hills-Lessini Mountains, separates the western fromthe eastern Southern Alps thrust belt fronts(Semenza, 1974; Zampieri et al., 2003; Massironiet al., 2006). Recent investigations (Zampieriet al., 2009) evidence a complex system of NW–SE and NNW–SSE trending, north-east dipping,high angle faults buried beneath the Veneto allu-vial plain and synthetic with the Schio-Vicenzafault (Schio-Vicenza fault system – SVFS;Fig. 1B). The SVFS is most probably an inheritedextensional structure that originated during thePalaeogene, when the Veneto plain constitutedthe forebulge of the south-west-vergent Dinaricthrust belt (Doglioni & Bosellini, 1987; Zampieri,1995). Subsequently, during the Neogene, thefault system was reactivated with a sinistralstrike-slip or transtensional kinematics acting asa kinematic transfer of the Southern Alps short-ening between the ENE trending eastern SouthAlpine front and the north-east trending Giudica-rie front (Massironi et al., 2006). A closer inspec-tion of the SVFS shows a rhomboid structurebounded by two distinct, sub-parallel, NNW–SSE

trending fault segments in coincidence with theEGF (Fig. 1B). Given the present sinistral strike-slip/transtensional kinematics superimposed onthe faults, the structure accommodates along-strike local extension that enhances the bedrockfracturing and allows the development of perme-ability. Consequently, this structure is inter-preted as a left stepover zone (Cunningham &Mann, 2007) and, in more detail, as a releasingor dilational stepover (De Paola et al., 2005,2007). Because of the high bedrock fracturing/permeability, dilational stepovers are potentialsites of high fluid/heat flow such as the Coso geo-thermal area of California (Lees, 2002) and theCerro Prieto geothermal area of Mexico (Glowa-cka et al., 1999).Recently, a new conceptual model of the

Euganean hydrothermal system has been pro-posed (Zampieri et al., 2009; Pola et al., 2010,2013) taking into account the well-established,structurally controlled, hydrothermal conceptualmodels (Curewitz & Karson, 1997; Faulkner et al.,2010) and the renewed SVFS architecture(Zampieri et al., 2009). The waters of meteoricorigin [infiltration area up to 1500 m above sea-level (a.s.l.); Piccoli et al., 1973; Gherardi et al.,2000] infiltrate 80 km to the north of the EGF inthe Veneto pre-Alps. The waters then flow to thesouth within a Mesozoic carbonate reservoir,represented by the Dolomia Principale (UpperTriassic), Calcari Grigi (Lower Jurassic–MiddleJurassic) and Maiolica formations (Upper Jura-ssic–Upper Cretaceous), reach a depth close to3000 m and warm up to ca 100°C by a normalgeothermal gradient (Gherardi et al., 2000). Theactive SVFS and the related highly permeabledamage zone (Caine et al., 1996; Gudmundsson,2000; Faulkner et al., 2010) enhance the migra-tion of the thermal waters in the middle part ofthe circuit. In the EGF area, the hot fluids inter-cept the dilational stepover and rise quickly tothe surface through the fault/fracture mesh devel-oped within the stepover. A water residence timegreater than 60 years (although probably on theorder of a few thousand years) is suggested bytritium and 14C isotope analyses (Piccoli et al.,1973; Sartori et al., 1997; Gherardi et al., 2000;Boaretto et al., 2003).

MONTIRONE HILL: MORPHOLOGICALAND GEOLOGICAL SETTING

The Montirone Hill represents the only naturaloccurrence of the Euganean thermal waters in the



Abano Terme area. The hill and its thermal poolshave been well-known since the Roman period(Vandelli, 1761; Mandruzzato, 1789; Da Rio,1836; Catullo, 1838) and were active until the1960s. Hot waters rose in natural ponds locatedon the top and at the foot of the hill and were par-tially channelized to feed anthropic pools and tomove a mill-wheel (Vandelli, 1761). In the 1960s,the increasing number of spa hotels in the EGFand the consequent intense exploitation of thehot fluids lowered the potentiometric level of thethermal aquifer from the surface (probably morethan 16 m a.s.l.) to about �15 m a.s.l. (year 1982),drying up the hot springs. At present, no activevents occur on the hill, but traces of bowl-likedepressions in its upper part can be related tothermal springs active in the recent past (Vandel-li, 1761; Mandruzzato, 1789; Da Rio, 1836; Cat-ullo, 1838). The temperature of the Montirone

Hill thermal waters ranged from 53�5°C to 86�5°C(Mameli & Carretta, 1954).The Montirone Hill is characterized by a flat

top connected to the alluvial plain by a lateralgently sloping surface, exhibiting a sub-circularshape in plan view and a mound shape in cross-section (Fig. 1D). It rises 6 to 8 m above thesurrounding alluvial plain (maximumheight = 15�75 m a.s.l.) and has a perimeter of ca360 m covering an area of 10 000 m2. The presentshape of the mound has been slightly modifiedover the last two centuries by the intense urbani-zation affecting the EGF area, as suggested by thesimilarity between present and historical mea-sures. In fact, Da Rio (1836) reported that in the19th Century, the hill rose 4�28 m above the allu-vial plain and had a perimeter of 321�3 m. TheMontirone Hill is made of poorly exposed traver-tine (Fig. 2) represented by lime-mudstone and

A B

C D

Fig. 2. Montirone Hill travertine lithofacies: (A) and (B) poorly stratified lime-mudstone with karst dissolutionpits (A, outcrop; B, core sample); (C) and (D) laminated crystalline crusts (C, outcrop; D, core sample).


176 M. Pola et al.

crystalline crusts etched by diffuse, small karstdissolution cavities. A network of high angle frac-tures, historically used to channel the hot waters,affects the calcareous body (Figs 3 and 9). Thefractures are characterized by varying widths (upto a few decimetres) and directions.A reconstruction of the geological framework

of the subsurface in the Montirone Hill area canbe obtained from the stratigraphy of the thermalwells. A NNW–SSE trending, WSW dippingfault buried beneath the Quaternary alluvialcover occurs to the east of the Montirone Hill,as shown in the EGF subsurface geological map(Fig. 1C). This fault juxtaposes the Scaglia Rossaformation (Upper Cretaceous–Lower Eocene) ofthe hangingwall against the Maiolica Formation(Upper Jurassic–Upper Cretaceous) of the foot-wall. A buried morphological high of the bed-

rock (Fig. 12B) can be inferred from thethickness of the alluvial cover, which reachesonly ca 140 m under the Montirone Hill versusca 200 m in the Abano Terme area.

METHODS

The Montirone travertine deposit has beenstudied using a multidisciplinary approach thatcombines specialist knowledge in sedimentology,geochemistry, structural geology and geophysics.Similar combined approaches have been appliedworldwide to study travertine and the relatedhydrothermal and fault systems (e.g. Guo &Riding, 1998; Hancock et al., 1999; Fouke et al.,2000; Uysal et al., 2007; Faccenna et al., 2008;Temiz et al., 2009; Brogi et al., 2010; Guido &

A B

C D

Fig. 3. The fissures affecting theMontirone travertine mound. TheESE–WNW set has a mean width ofca 50 cm (A, top of the hill, viewtowards NW). The NW–SE and theNNE–SSW sets exhibit widthsvarying from a few centimetres [(C)NE corner, view towards SW; (D),NW corner, view from top] up to20 to 30 cm [(B) southern part ofthe hill, view towards the north].The location of the pictures isgiven in Fig. 9.



Campbell, 2011; Kele et al., 2011; Brogi et al.,2012; De Filippis et al., 2012; Rodr�ıguez-Berrigu-ete et al., 2012). Such integrated studies detailthe travertine depositional processes both at thelocal (travertine body) and regional scales (tec-tonic and hydrothermal systems).Because of the absence of active thermal

springs on the Montirone Hill and its presentuse as a public park, the travertine is often cov-ered with a lawn and consequently is poorlyexposed. Therefore, two drillings were madeto collect samples for sedimentological andgeochemical analyses. Core 1 is located in thesouthern part of the Montirone carbonate bodyat ca 14 m a.s.l. (Fig. 1D) and was collected onthe flat bottom of a sub-circular depression,probably an old thermal pool, with the aim ofsampling the mudstone lithofacies (Fig. 2B).Core 2 is located on the western flank of the hillat ca 12�8 m a.s.l. (Fig. 1D) and was drilled per-pendicular to the ground surface that corre-sponds to travertine layers made of thincrystalline crusts dipping 12° WSW (Fig. 2D).Sedimentological and petrological studies

were carried out with optical microscopy on 23thin sections (dimensions: 7 cm 9 5 cm and2�5 cm 9 5 cm) obtained from different segmentsof the cores. For the purpose of reconstructing thepalaeo-depositional environments of the Monti-rone Hill, the observed depositional facies wereinterpreted and compared with those describedin old and recent travertine deposits.Using the information regarding the deposi-

tional conditions and the diagenetic history ofthe travertine, samples for U-series and stableisotope analyses were taken from the morehomogeneous portions of the cores, i.e. thoseless affected by post-depositional events. Datingof the upper part of the Montirone calcareousbody and estimation of the water palaeo-temper-atures (Friedman & O’Neil, 1977; Zhou & Zheng,2003, 2006; Kele et al., 2011) were performed,respectively, using the U-series and oxygen iso-tope ratio analyses. A detailed description ofthe analytical procedures for both U-series andstable isotope analyses is reported in theAppendix.Structural analyses of the Montirone Hill and

its fissures were carried out to define the tec-tonic regime that enhances fluid migration inthe thermal area and to improve the conceptualmodel of the Euganean hydrothermal system(Zampieri et al., 2009; Pola et al., 2010, 2013).Dip measurements of the travertine beds,detailed mapping of the fractures using ortho-

photos of the EGF and field measurements ofthe direction (measured each metre), the width(measured each 50 cm) and the depth (measuredapproximately each metre) of the fissures wereperformed.Geophysical measurements, particularly elec-

trical resistivity tomography (ERT) and electro-magnetic frequency domain surveys (FDEM)were used to define the subsurface geometry ofthe Montirone Hill. The high level of urbaniza-tion and the alteration caused by human use ofthis area produced strong limitations in terms ofthe choice of both methods and investigationdepth. According to these limitations, the entirearea was first covered with the FDEM surveys(depth of investigation 1�5 m), collected with aCMD 1 probe (GF Instruments s.r.o., Brno, CzechRepublic). Based on the FDEM results, five ERTlines were collected with a Syscal Pro 72 (IrisInstruments, Orleans Cedex 2, France) in thesame area using 48 electrodes spaced 2 m apart,with the dipole–dipole skip zero scheme (a totalinvestigation depth of about 20 m). The use of afull reciprocal acquisition ensured that the dataerrors were estimated and used in the Occam’sinversion scheme implemented in ProfileR ver-sion 2�5 (Binley, 2003). In particular, the dataexceeding 5% difference between the direct andreciprocal measurements were rejected. Conse-quently, the inversion of all ERT lines was per-formed to fit the observations at a 5% errorlevel.

RESULTS

Sedimentological analysis

Two main travertine lithofacies were recognizedon the surface and in the drilled cores: (i) apoorly stratified, massive lime-mudstone facies(Fig. 2A and B) with irregularly distributed tan-gles or rafts of microbial laminae, which cropsout in some depressions on the top of themound; and (ii) a centimetre-scale laminatedfacies made of crystalline crusts (Fig. 2C and D),which crops out on the lateral slopes.

Core descriptionsCore 1 (ca 0�95 m) is composed of homogeneous,cream coloured lime-mudstone that includesintercalations of laminar microbialites and tan-gles of microbial boundstone (Figs 4G and 7D).Core 2 (ca 0�8 m) is made of two different litho-logies. The bottom is represented by a homoge-


178 M. Pola et al.

A B

D E F

G H

C

Fig. 4. Microfacies of the microsparitic lime-mudstone: (A) faintly laminated with dispersed quartz and phyllosi-licate silt (white speckles), crossed by irregular cracks that may be related to dewatering shrinkage processes; (B)locally containing well-preserved remains of microbial filaments (Microbial mudstone); (C) reticulate boundstone(Microbialite) associated with the lime-mudstone lithofacies; (D) paper-thin raft accumulation (Floatstone/Wacke-stone) in a lenticular layer floored by bacterial shrubs. (E) Detail of a cluster of bacterial shrubs. (F) An indentedraft resulting from the intensive microbial colonization of both surfaces of a paper-thin raft. (G) Lime-mudstonelithofacies (segments from core 1): homogeneous (right sample) or finely porous because of the presence of micro-bialites (left sample); large dissolution vugs are evident in both samples. (H) A shield-shaped spring mound atYellowstone Park: lime mud is deposited within the shallow crater-like basin surrounding the vent of the thermalspring (red arrow); this could be a likely model for the reconstruction of the palaeo-depositional environment ofthe Montirone lime-mudstone lithofacies.



A B C

D E F

G

H

Fig. 5. Microfacies of the crystalline crusts: (A) and (B) superposed thin laminae of irregular thickness made offeather-like calcite crystals (C), (D) and (E); the interruption of the crystal growth is marked by thin rusty lines (A)and (B) resulting from microbial colonization/sparmicritization (E) and (F) during periods of non-deposition. (G)A gas bubble encrusted by calcite crystals and microbial communities (evidenced within the red circle), entrappedwithin a rusty-red interval of the crystal laminite. (H) The slope environment at San Giovanni Ridge (Terme SanGiovanni, Rapolano, Italy): feather-like laminae are deposited by thin sheets of water flowing down the slopeunder a laminar flux regime; this could be a likely model for the reconstruction of the palaeo-depositional envi-ronment of the Montirone crystalline crust lithofacies.


180 M. Pola et al.

A

B

D

E

C

Fig. 6. Soft sediment deformations in the mudstone facies: (A) recumbent asymmetrical folds in alternating lime-mudstone and bindstone layers resulting in microsparitic pseudo-nodules coated by boudinated microbialites;(B) cusps produced by diapiric uplifts of the underlying less dense sediment; (C) load structure in the laminatedlime-mudstone; (D) a sedimentary dyke induced most probably by escaping gas that in its upwards motionintruded with liquefied lime mud the semi-consolidated fenestrate microbial mudstone. (E) A mud pot vent atYellowstone Park: the dense lime mud is stirred and blended by small gas eruptions; this could be a likely modelfor the reconstruction of the physical conditions causing the Montirone soft sediment deformations.



A B

C

E F

D

Fig. 7. Post-depositional brecciation and karstic processes in the Montirone travertine: (A) intraformational intra-clast breccia (Rudstone) alternated with crystalline crusts (segments from core 2). (B) A thin section of a breccia(Floatstone) with clasts of crystalline crusts supported by lime-mudstone matrix. (C) Karstic solution pits in adisplaced block of travertine on the top of the Montirone Hill. (D) A dissolution cavity in the lime-mudstone(segments from core 1) filled with different generations of internal silt [(E) and (F) in thin section].


182 M. Pola et al.

neous, cream coloured lime-mudstone (35 cm)similar to that of Core 1. At the top, crystallinecrusts (45 cm) with thin intercalations of grain-supported and/or mud-supported intraforma-tional breccias (Figs 2D and 7A) occur.The lime-mudstone facies appears to be the

dominant lithology occurring in the MontironeHill cores. All the lime-mudstone facies fromthe cores (Figs 2B, 4G and 7D), as well as thoseobserved in the Montirone Hill outcrops (Figs 2Aand 7C), display a pervasive porosity representedby small open karstic pits and cavities (Figs 2A,2B and 4G) superposed to the finer primaryporosity of the microbialites (Figs 4G and 7D).The vugs are partially filled with carbonate inter-nal sediment (Fig. 7D to F) or are lined with apoorly developed microcrystalline cement. Con-versely, the exposed crystalline facies are lessaffected by pervasive karstic dissolution, althoughthere are traces of deeper karstic activity in somefloating blocks (Fig. 7C).

Depositional faciesLime-mudstone. Description: The homogenouslime-mudstone facies contains thin intercalationsof laminated microbialites and scattered tanglesof microbial boundstone, as revealed by the dis-tinctive fine porosity of the structural framework(Figs 4G and 7D). The matrix shows a mottledfabric made of patchy mosaics of inequigranularabiotic microsparite, mostly consisting of calcite(Fig. 4A). Less frequently, it corresponds tocoarse, microbial micrite containing the dis-persed remains of bacterial filaments (Fig. 4B)and/or clots of bacterial aggregates that locallyevolve into spongy/peloidal bindstone. The asso-ciated mud-supported facies (Floatstone/Wacke-stone) contain scattered allochems representedby microbial and/or paper-thin rafts (Fig. 4D),oncolite-like microbial tangles, encrusted bub-bles, shrub fragments and intraclasts. Fine, silt-size quartz and phyllosilicate grains are rathercommon exotic components that were foundeither diffused into the matrix or concentratedalong pressure solution seams (Fig. 4A). Abioticlaminites consisting of alternated dense micriticand lighter microsparitic thin laminae are locallypresent as intercalated layers, but more com-monly occur as contorted shreds indicative ofintense soft sediment deformation (Fig. 6).Interpretation: Accumulations of microcrystal-

line lime mud are found at present in shallowcrater-like basins surrounding the orifices ofactive thermal springs (Fig. 4H), where calciumcarbonate nucleates as aragonite and/or calcite

micrite by whiting and then settles from the sus-pension as a loose sediment (Riding, 2000; Pente-cost, 2005). The lime mud mixes with and trapsa variety of small, loose microbial filaments,coccoids or clumps forming ‘grumuleuse’ struc-tures (Pedley, 1990; Riding, 2000; Pedley, 2009).Over time, mudstone with scarce microbialaggregates (Fig. 4B) grades to fenestrate microbialmudstone and to micrite with dense clots, upto loosely connected frameworks (microbialboundstone/bindstone) with interspaces filled bymicritic or microsparitic calcite cement (Figs 4Cand 6A). Laminites are interpreted as the sedi-ment deposited on the bottom of the basin at theorifices of the thermal springs.The texture of the actual calcite matrix, which

is made of irregular, patchy microsparite mosaics,suggests neomorphic processes on a primaryaragonite lime mud (Lasemi & Sandberg, 1984,1993). Aragonite is known to precipitate as theonly mineral component, or partially mixed withcalcite, in the present thermal environments char-acterized by water temperatures higher than 40°C(Folk, 1994). Its neomorphic transformation insome localities has been estimated to be surpris-ingly rapid, i.e. in the range of 10 to 15 days(Malesani & Vannucchi, 1975; Cipriani et al.,1977). Consequently, the original aragonite com-position of the Montirone lime-mudstone is con-sistent with the historical temperature of theMontirone Hill thermal waters (Mameli &Carretta, 1954).

Microbialites. Description: The microbialites arecommonly associated with the lime-mudstonefacies. They are represented by: (i) small, dis-crete, ball-like, boudinated boundstone bodies(Fig. 4C); (ii) layers of microbial boundstonewith reticulate (Figs 4C and 6A) or thrombolitefabric; and (iii) poorly developed laminae of bac-terial shrubs (Fig. 4D and E). Bundles of stackedmicrobial sheets commonly contorted (Fig. 6A)and oddly curled are locally well-developed.The microbial core of the laminae is embeddedin two thin dense micrite coatings and rein-forced by more or less thick calcite cement(Fig. 4C). Encrusted bubbles and/or accumula-tions of paper-thin rafts (Floatstone/Wackestone)are often found in small shelter-like lenses(Fig. 4D) resulting from folds in the microbialsheets.Interpretation: Reticulate organosedimentary

structures with varied fabrics, which can beincluded into microbial bindstones, can be com-pared with those illustrated in several papers



concerning thermal spring deposits (Koban &Schweigert, 1993; Guo & Riding, 1998; Chafetz &Guidry, 1999; Guo & Riding, 1999; Fouke et al.,2000; Fl€ugel, 2010; Guido & Campbell, 2012;Rodr�ıguez-Berriguete et al., 2012). However, therafts (Paper-thin rafts; Folk et al., 1985; Guo &Riding, 1998) in the Montirone lime-mudstoneshow uncommon thickness and indented fabric,suggesting the unusual development of micro-bial communities on both raft surfaces (Fig. 4F).Similar microbial deposits can be observed indams or streams of active thermal fields, as wellas on the water surfaces of the thermal poolsforming floating rafts (Fig. 4H).

Crystalline crusts. Description: The crystallinecrusts are made of composite calcite crystalswith a feather-like arrangement (Feather-like cal-cite, Fig. 5C and D; Chafetz & Folk, 1984; Guo &Riding, 1992, 1998) set up in superposed rowswith syntaxial growth (Fig. 5A and B). Locally,their development is interrupted by dense,rusty-red, thin lines made of concentrations ofiron oxides with microsparite clots and traces ofmicrobial filaments (Fig. 5).Interpretation: The deposition of crystalline

calcite layers is characteristic of slope environ-ments (Fig. 5H) in which thin, millimetre-thicksheets of thermal water run with a regular andlaminar regime forming feather-like laminae(Guo & Riding, 1992, 1998; Gandin & Capez-zuoli, 2008). The crystals appear to be unaf-fected by biotic involvement, but traces ofmicrobial activity can be found in the dark redzones. The presence of irregular concentrationsof microsparite and the remains of microbial fil-aments suggest that a bacterial colonizationdeveloped on the exposed surface of the crustduring breaks in the water flux.

Post-depositional processesSoft sediment deformations. Description: Earlypost-depositional deformations (soft sedimentdeformations) are evident at the microscopicscale in both the abiotic, finely laminated lime-mudstone and the microbial facies. They consistof recumbent asymmetrical folds and associatedpseudo-nodules (Fig. 6A and C), cusps evidencedby the diapiric uplifts of the underlying sediment(Fig. 6B), fractures/faults with mud-filled verticalconduits (Fig. 6D) and boudinage of microbialboundstone layers (Figs 4C and 6A).Interpretation: Similar structures can be

ascribed to the process of liquefaction of muddy,laminated deposits in an inverted density regime

(Owen, 1996, and references therein) and suggestthe instability of the semi-consolidated sedimentsduring or shortly after deposition (Motenat et al.,2007; Owen et al., 2011). The more common driv-ing forces triggering such deformational struc-tures are shaking events. At the large scale, thedeformations may originate from earthquakes aswell as meteoric storms (Owen et al., 2011). Atthe small scale, gas escapes associated with boil-ing-like and/or geyser-like eruptions would trig-ger the deformation. In the context of the shapeand size of the structures occurring in the Monti-rone Hill travertine, the observed deformationscan be ascribed to the tremors induced by gasescaping in a dense muddy sediment depositedwithin crater-like vents (Fig. 6E).

Microkarstic processes and brecciation. Descrip-tion: Pervasive porosity generally consisting ofmillimetre to centimetre-scale vugs (solutionpans, solution pits; Ford & Williams, 2007) andintraclast breccias affect both lithofacies of Mon-tirone Hill (Figs 2A, 2B, 4G and 7). Most of theopen cavities are more or less infilled with inter-nal sediment and/or cement (Fig. 7D to F). Theyare sometimes lined by crystalline crusts made ofbladed/palisade calcite and filled with micriticinternal sediment and/or oxidized carbonate siltcontaining scattered sand-size grains of quartzand phyllosilicates. The internal lime mud sedi-ment frequently bears articulated or disarticula-ted valves of Ostracod carapaces and crystallinecalcite dust from the crumbling karstic calcitecrusts that line previous cavities. Multiple (atleast three) events of karstic circulation, dissolu-tion and deposition are documented by super-posed cross-cutting small bodies of geopetalinfilling (Fig. 7E and F).Intraclast breccias form thin and lenticular

intercalations at the boundary between the mud-stone facies and the overlying crystalline lami-nites. The breccias consist of unsorted fragments(Rudstone; Fig. 7A) of the including crystallineand muddy/microbial facies (feather to shrubfabrics) supported by a microsparite matrix(Floatstone; Fig. 7B), which shows the samecomposition and texture of the underlying lime-mudstone.Interpretation: Short-lived karstic circulation is

recorded by the pervasive, small-scale, dissolu-tion/infilling processes. Another karstic dissolu-tion, early post-depositional event is documentedby the occurrence of intraclast breccias derivedfrom the collapse of the brittle crystalline layersinterlaid with the still plastic mudstone. Both


184 M. Pola et al.

karstic activities can be related to periods of non-deposition, probably caused by diversion and/orlowering of the thermal water flux.

Uranium-series isotope analysis

Uranium concentration and activity ratios(Table 1) were measured on travertine samplescollected from the lime-mudstone facies (Ab. 10:core 1, from 0�64 to 0�78 m below ground level)and from the crystalline crust facies (Ab. 22:core 2, from 0�03 to 0�08 m below ground level)with the aim of dating the travertine.The 230Th/232Th activity ratios, ranging

between 1�8 and 2�3 (Table 1), require a correc-tion because of the presence of a detrital compo-nent in the samples. The scarce amount ofsamples available for the analysis does not allowapplication of the standard isochron correctionmodels (Bischoff & Fitzpatrick, 1991). Therefore,a crustal correction was applied (Richards & Do-rale, 2003) by assuming that all detrital thoriumhas an average crustal 230Th/232Th activity ratioof 0�85 (Wedepohl, 1995). The ages were calcu-lated by means of Isoplot/Ex (version 3�0), aplotting and regression program designed byLudwig (2003) for radiogenic-isotope data. Theerrors are quoted as 1r.The corrected 230Th/234U ratios and obtained

ages are reported in Table 1. The ages of the

travertine (30 � 4 ka and 25 � 5 ka) overlapwithin the error range dating the first metre ofthe deposit between 20 ka and 34 ka (UpperPleistocene), an age that can be related to isoto-pic stage 3 (Pentecost, 1995).Moreover, the U-series isotope analysis pro-

vides evidence of some features of the hydro-thermal system. The travertine is characterizedby: (i) extremely low uranium content (from 3 to12 ppb; Table 1) that is typical of a deep circuit,where reducing conditions change the oxidationstate of uranium from VI to IV favouring its pre-cipitation (Bourdon et al., 2003); and (ii)234U/238U activity ratio (ca 2; Table 1) higherthan unity (equilibrium conditions), which canbe related to a long-term water–rock interactionand is typical of hydrothermal systems charac-terized by long circulation paths at depth (Brogiet al., 2010, 2012). These data confirm that thetravertine parent solutions were thermal waterscharacterized by a long residence time at highdepth, in agreement with the Euganean hydro-thermal conceptual model (Zampieri et al.,2009; Pola et al., 2010, 2013).

Carbon and Oxygen isotope analysis

Oxygen and carbon isotope ratios were mea-sured on travertine samples collected from lime-mudstone facies (Ab. 10: core 1 a: from 0�64 to

Table 1. The Uranium abundance, uranium and thorium activity ratios and ages of the Montirone travertine coresamples. *The ratio corrected using the value of 0�85 as representative of the crustal thorium isotopic composition(Wedepohl, 1995).

Sample U (ppb) 230Th/232Th 234U/238U 230Th/234U measured 230Th/234U corrected* Age (Ka)

Ab. 10 11�7 � 0�5 1�79 � 0�12 2�15 � 0�09 0�35 � 0�02 0�24 � 0�03 30 � 4Ab. 22 3 � 0�3 2�34 � 0�46 2�01 � 0�2 0�28 � 0�04 0�21 � 0�04 25 � 5

Table 2. The stable oxygen and carbon isotope compositions of the travertine from Montirone Hill and the calcu-lated values of palaeotemperature using the calcite–water equilibrium fractionation equation (f; Friedman &O’Neil, 1977) and the aragonite–water equilibrium fractionation equation (z; Zhou & Zheng, 2003, 2006) correctedusing the non-equilibrium empirical ‘travertine curve’ (k; Kele et al., 2011). The d18O value of the thermal waterused in the calculation is �39�98& V-PDB.

Sample d18O (& V-PDB) d13C (& V-PDB) T (°C) calculated (f) T (°C) calculated (z) T (°C) corrected(k)

Ab. 10a �17�40 1�45 52�94 42�60 61�0Ab. 10b �17�40 1�43 52�94 42�60 61�0Ab. 22 �16�55 1�60 47�68 38�44 56�5Ab. 23 �16�45 1�63 47�07 37�96 56�0Ab. 32a �16�15 0�45 45�29 36�52 54�0Ab. 32b �16�40 1�17 46�78 37�72 56�0



0�72 m below ground level, and b: from 0�72 to0�78 m below ground level; Ab. 32: core 2 a:from 0�58 to 0�64 m below ground level, and b:from 0�51 to 0�58 m below ground level: Table2) and crystalline crusts facies (Ab. 22: core 2,from 0�03 to 0�08 m below ground level; Ab. 23:core 2, from 0�08 to 0�11 m below groundlevel).

The d18O values range from �16�15 to�17�40& V-PDB and d13C values range from0�45 to 1�63& V-PDB (Table 2). These data arein accordance with the typical isotopic signatureof thermogene travertine (Pentecost, 2005) andwith the values resulting from carbonates depo-sited in present-day thermal springs (Fig. 8;Gandin & Capezzuoli, 2008). These data suggest

Fig. 8. Combined plot of stable oxygen (d18O& V-SMOW/& V-PDB) and carbon (d13C& V-PDB) isotope valuesderived from carbonates presently forming in terrestrial (travertine, calcareous tufa and speleothems) and marine(skeletal-ooid sediments and pelagic muds) environments and from lithified marine limestones (slightly modifiedafter Gandin & Capezzuoli, 2008). The d18O and d13C values of the Montirone Hill carbonates (yellow area;Table 2) fall in the upper part of the diagram within the range of present-day travertine confirming the geothermalprovenance of the depositing parental water.


186 M. Pola et al.

that the original meteoric fluids were heated inthe subsurface and acquired a positive d13Csignature by contamination with the bedrockmarine limestones resulting from a long-termwater–rock interaction. These features are inaccordance with the Euganean hydrothermalconceptual model (Zampieri et al., 2009; Polaet al., 2010, 2013), as suggested by the U-seriesisotope analysis.The obtained d18O values of the travertine and

the d18O value of the thermal waters (�39�98&V-PDB; unpublished geochemical datum from athermal well near Montirone Hill) were used tocalculate the palaeo-temperature of deposition ofthe travertine waters by applying both thecalcite–water (Friedman & O’Neil, 1977) and thearagonite–water equilibrium fractionation equa-tions (Zhou & Zheng, 2003, 2006). At present, thesediment is made of calcite, but the primary limemud probably was made of aragonite. However,the concomitant precipitation of calcite andaragonite cannot be totally neglected; additio-nally, a secondary origin of calcite by dissolution-reprecipitation from the primary aragonite may beconsidered. An oxygen isotope fractionationbetween calcite–aragonite–water occurs duringthe precipitation–dissolution–reprecipitation pro-cesses depending on the presence/absence ofwater, its temperature or its chemical composition(Zhou & Zheng, 2006). Therefore, the present d18Ovalue measured in the Montirone Hill travertinemay not be the original d18O value of the primarylime mud. Hence, a conservative approach forcalculating the palaeo-temperatures is to use boththe calcite–water (Friedman & O’Neil, 1977) andthe aragonite–water (Zhou & Zheng, 2003, 2006)equilibrium fractionation equations, and thencorrect the calculated values using the non-equi-librium empirical ‘travertine curve’ proposed byKele et al. (2011). The corrected palaeo-tempera-tures, ranging from 54 to 61°C (Table 2), are inaccordance with the historical measurements ofthe Montirone thermal waters temperature (53�5to 86�5°C; Mameli & Carretta, 1954).

Structural analysis

The analysis of the dip direction measurementsof the travertine beds corroborates and improvesthe preliminary morphological study. The radialsymmetry of the dip direction and the typicalmound profile, ranging from an almost flatmorphology in the central and on the northernand eastern sides to a steeper slope (up to 25°)on the western flank (Fig. 9A), confirm the

sub-circular mound shape of the MontironeHill.A statistical analysis of the fracture widths

and directions was performed, showing threemain sets of fissures trending: NNE–SSW,WNW–ESE and NW–SE (Fig. 9C). The NNE–SSW fracture (mean direction: N15°E) affects thenorth-eastern side of the hill and has a length of20 m and a width varying from 59 to 3 cm(Fig. 3C) in the north-easternmost region (mean:43 cm; median: 42 cm). The WNW–ESE fracture(mean direction: N115°E) affects the central(Fig. 3A) and the south-eastern areas of thedeposit and has a length of 40 m with a widthvarying from 75 to 28 cm (mean: 42 cm; median:41 cm). The NW–SE fracture (mean direction:N135°–145°E) affects the southern area of thehill (Fig. 3B) and has a length of 35 m and awidth varying from 30 to 5 cm in the southern-most segment (mean: 17 cm, median: 12 cm).The last set was also observed in the north-wes-tern part of the deposit and has a length of ca50 cm and a width of 5 cm (Fig. 3D). The seg-ments with the minimum widths, measured inthe southernmost and north-westernmost regionof the body, are interpreted as the tip of aNW–SE or WNW–ESE trending ‘main fracture’that is only partially detectable in the fieldbecause of the anthropic activity. Moreover, itshould be noted that the width increases whenmoving from the tip to the middle part of thefracture, as shown by the graph plotting thewidth of the ‘main fracture’ (given by the sum ofthe width of NW–SE and WNW–ESE sets)against position along the fracture (Fig. 9B). Thegraph demonstrates that this fracture is affectedby an extensional regime producing a wideropening in its middle part.A good parallelism between the fracture mesh

and the regional structural framework resultsfrom plotting the trend of structural elements atthe regional [main direction of the Schio-Vice-nza fault system (SVFS): NW–SE; red arrows]and the Euganean geothermal field (EGF) scales(the direction of anisotropy depicted in the vari-ogram analysis of transmissivity calculated inthe EGF thermal aquifer: N110°E; blue arrows;Fabbri, 1997) on the Montirone fractures rosediagram (Fig. 9C). In particular, the anisotropydepicted in the variogram analysis of transmis-sivity (Fabbri, 1997) indicates a direction ofhomogeneous transmissivity values and conse-quently a preferential flow direction of the hotwaters within the EGF thermal aquifer. Consider-ing that open fractures represent paths for fluid



A

B C

Fig. 9. (A) An aerial view of the Montirone Hill (modified from Google Earth©) and map of the fissure meshaffecting the deposit. The bold letters (‘a’ to ‘d’) represent the locations where pictures of the fissure in Fig. 3 weretaken. (B) A diagram showing the width of the WNW–ESE and NW–SE fissure sets versus the position along thefissure: the width is largest in the middle part of the fissure and decreases towards the tips. (C) A rose diagram ofthe direction of the fissures measured each metre; a good parallelism between the fracture mesh and regionalstructural elements (main direction of the SVFS, red arrows; direction of anisotropy depicted in the variogramanalysis of transmissivity calculated in the EGF thermal aquifer, blue arrows, Fabbri, 1997) is observed.


188 M. Pola et al.

flow in the subsurface (Caine et al., 1996; Cure-witz & Karson, 1997; Gudmundsson, 2000; Faulk-ner et al., 2010), the parallelism between the

direction of anisotropy of the variogram of trans-missivity and the wider WNW–ESE fracture setdemonstrates that this set is the morphological

A

B

Fig. 10. (A) A map of the apparent resistivity distribution (ca 1.5 m depth) in the Montirone Hill obtained usingthe CMD 1 FDEM probe (GF Instruments). The base map is modified from Google Earth©. (B) A representativeERT line collected on the Montirone Hill with 48 electrodes, 2 m spacing, using dipole–dipole skip zero scheme.The dashed black area indicates the probably more fractured zone.



expression of open fractures in the EGF thermalaquifer/subsurface.

Geophysical measurements

Figure 10A shows the electromagnetic frequencydomain survey (FDEM) map obtained with theCMD 1 probe in high position mode (depth ofinvestigation ca 1�5 m). Two areas of high resis-tivity, respectively, on the western and on theeastern flanks of the hill (from 250 to500 Ohm*m), are separated by an elongated, cen-tral, low resistivity zone (from 9 to 100 Ohm*m).This trend of resistivity is also observed in thedeepest part of the deposit by the ERT lines (arepresentative example is shown in Fig. 10B;depth of investigation ca 20 m).Although it might appear that a clear discrep-

ancy exists between the FDEM resistivity valuesand the ERT values (from 0 to 5000 Ohm*m),the results are totally consistent. In fact, withthe FDEM methods, the first upper part of thedeposit (up to 1�5 m depth) was investigatedonly qualitatively, and the effective value of theapparent resistivity could be reduced by theorganic soil that locally covers the hill. On theother hand, using 2 m spacing for the ERT mea-surements, the depth of investigation is beyondthe first 2 m depth. It is clear that the target ofthese two methods differs both in terms of theresults (qualitative or quantitative) and thedepth of investigation. In summary, the anoma-lies in the field were isolated qualitatively withthe FDEM method, and the ERT acquisitionsaided quantitative reconstruction of the struc-ture of the deepest part of the hill in some repre-sentative sections.After these preliminary considerations, the

central low resistivity anomaly (dashed blackarea in Fig. 10A and B) is interpreted as a morefractured zone characterized by a higher watercontent (and consequently low resistivity), whichis not clearly detectable on the surface because ofthe human impact on the upper central part ofthe deposit. Furthermore, the sharp lateral transi-tion between the two zones of different resistiv-ity, as depicted by the ERT section, supports theinterpretation described above.The high resistivity anomalies show a vertical

extent of about 10 m and match well with thetravertine outcrops. In addition, a low resistivityanomaly can be observed below the high resisti-vity anomalies and its mound geometry confirmsthe morphology of the deposit in its deepestpart.

DISCUSSION

According to the results of recent investigationson present and palaeo-travertine deposits (e.g.Hancock et al., 1999; Fouke et al., 2000; Uysalet al., 2007; Temiz et al., 2009; Brogi et al., 2010;Guido & Campbell, 2011; Kele et al., 2011; Brogiet al., 2012; De Filippis et al., 2012), travertinedeposition is strictly connected to hydrothermalsystems and active, normal/transtensional faults.Furthermore, other factors such as the climate,hydrodynamics, inherited fault/fracture meshes,high heat flow and CO2 supply can influence thedeposition of travertines. A multidisciplinaryapproach (sedimentology, geochemistry, struc-tural geology, geophysics and hydrogeology) hasbeen applied to reconstruct the palaeo-deposi-tional setting of the Montirone Hill travertinebody. In addition, the results substantiate somefeatures of the related Euganean hydrothermalsystem and elucidate the factors controlling thetravertine deposition.The Montirone Hill exhibits sub-circular,

domical shape with a dip variable from a fewdegrees on the top to a maximum of 25° on itswestern slope (Figs 1D and 9), which is typicalof travertine mounds (Pentecost, 2005). Thismorphology is confirmed by the radial symmetryof the dip directions of the travertine beds(Fig. 9) and by Electrical Resistivity Tomography(ERT; Fig. 10B). Historical documents (Vandelli,1761; Mandruzzato, 1789; Da Rio, 1836; Catullo,1838) and poorly preserved potholes attest tothe past activity of the thermal springs locatedon top of the shield-shaped mound.Two travertine depositional lithofacies can be

observed on the Montirone Hill, including amassive, poorly bedded lime-mudstone (Fig. 2Aand B) and centimetre-scale laminated crystal-line crusts (Fig. 2C and D). The massive litho-facies, consisting of abiotic, locally finelylaminated or microbially mediated microspariticlime-mudstone and locally containing microbiallayers of aggregates (Fig. 4), can be related to themuddy dense sediment that is found at presentwithin the shallow basins surrounding the cen-tral orifice of active thermal springs (Pedley,1990; Riding, 2000; Pentecost, 2005; Pedley,2009; Fig. 4H). The associated microbialites,which are made of reticulated/spongy mats andtangles with shrubs and paper-thin rafts (Folket al.,1985; Koban & Schweigert, 1993; Guo &Riding, 1998; Chafetz & Guidry, 1999; Guo &Riding, 1999; Fouke et al., 2000; Fl€ugel, 2010;Guido & Campbell, 2012; Rodr�ıguez-Berriguete


190 M. Pola et al.

et al., 2012), probably developed on the marginsof the pools as floating rafts. The sediment wasdeformed by tremors that produced soft sedi-ment deformations (Motenat et al., 2007; Owenet al., 2011), such as asymmetrical folds,pseudo-nodules, diapiric uplifts of underlyingless dense sediment, boudinage of microbialboundstone layers and sedimentary dykes(Fig. 6). Such deformations were triggered mostprobably by the persistent bubbling of escapinggases within the dense muddy sediment(Fig. 6E). The dominant occurrence of neomor-phic microsparitic mosaics suggests that arago-nite rather than calcite precipitated, as observedin several existing hydrothermal environments(Folk, 1994; Fouke et al., 2000). The crystallinelithofacies with the typical elongated, feather-like calcite crystals (Fig. 5) is characteristic of aslope environment, where crystalline calcite hasbeen deposited by thin sheets of water flowing ina laminar regime (Fig. 5H; Chafetz & Folk, 1984;Guo & Riding, 1992, 1998; Gandin & Capezzuoli,2008). Therefore, the Montirone Hill was builtby lime-mudstone deposited by hot waters as adense, aragonite mud in shallow basins at theorifice of the thermal springs. Over time, thecoalescence of several cones with the associ-ated thermal pools gave rise to a composite,single, shield-shaped mound. The thermalwaters overflowed from the hot springs anddeposited crystalline crusts on the flanks of thehill. The U-series isotope analysis suggests anUpper Pleistocene age (20 to 34 ka; Table 1) forthe upper metre of the travertine. Furthermore,the U-series and stable isotope analyses supportthe thermal origin of the travertine parentwaters and their deep circulation. The palaeo-temperature of the thermal waters was calcu-lated from the oxygen isotopic ratio, using thecalcium carbonate–water equilibrium fraction-ation equations (Friedman & O’Neil, 1977;Zhou & Zheng, 2003, 2006; Kele et al., 2011).The obtained temperature ranges between 54°Cand 61°C (Table 2), in accordance withhistorical measurements of the Montirone ther-mal pools temperatures (from 53�5°C to 86�5°C;Mameli & Carretta, 1954). Therefore, the palae-o-environmental model, resulting from petrolog-ical data and palaeo-temperature calculation, isconsistent with the morphology/depositionalenvironments of present spring mounds (Pente-cost, 2005) and the historical description of theMontirone Hill and its hot springs (Vandelli,1761; Mandruzzato, 1789; Da Rio, 1836;Catullo, 1838).

In the Montirone Hill area, the relationbetween travertine deposition and brittle defor-mation is elucidated by the network of highangle fractures developed in three main sets(mean direction: N15°E, N115°E and N135°–145°E; Fig. 9C) on the travertine deposit (Figs 3and 9A). In particular, the WNW–ESE set is thewidest and reflects open fissures in the subsur-face, as suggested by the parallelism with thedirection of anisotropy depicted in thevariogram analysis of the EGF thermal aquifertransmissivity (Fabbri, 1997) (Fig. 9C). The Mon-tirone fracture mesh is interpreted as havingdeveloped within a dilational stepover (De Paolaet al., 2005, 2007) bounded by NNW–SSE trend-ing sinistral transtensional faults, such as thefault reconstructed in the bedrock to the east ofthe Montirone Hill area (Fig. 1C). This structureis analogous to the dilational stepover of theSVFS, which was recognized in coincidencewith the EGF (Fig. 1B), in accordance with thepower-law scaling relation (from centimetres tohundreds of kilometres in scale) between theinteraction zones of faults (Peacock, 2003; Long& Imber, 2011). Therefore, a calculation of thetheoretical fracture mesh that develops withinthe dilational stepover is performed to confirmthis interpretation. This method (De Paola et al.,2005, 2007) assumes that a transtensional defor-mation, which results from a regional directionof extension obliquely oriented to the deforma-tion zone boundaries (Sanderson & Marchini,1984), can be modelled as the simultaneousaction of a wrench simple shear and an exten-sional pure shear. Within the stepover, the defor-mations produce: (i) a rotation of the principalstress/infinitesimal strain field relative to theregional stress field (clockwise for sinistral faultsas the SVSF); and (ii) a typical ‘Hill-type’ fracturemesh (Hill, 1977) made up of interlinked ten-sional fractures and conjugate strike-slip shearfractures, which are symmetrical relative to thelocal stress directions (Sibson, 1996). Predictablegeometric relations exist between the direction ofthe boundary faults and: (i) the axis of maximuminfinitesimal strain within the stepover(expressed by the angle b); (ii) the regional direc-tion of extension (angle a); and (iii) the syntheticR-fracture planes (angle h) (Fig. 11A; De Paolaet al., 2007). The angle a was calculated using thedirection of the fault in the subsurface of theMontirone area (N150°E) and the regional direc-tion of extension (N12°W, rounded to N10°W;Fig. 11B), which was obtained by GPS data of thearea to the south of Padova and in accordance



with the regional N/NNW movement of the Adriaplate (D’Agostino et al., 2005; Grenerczy et al.,2005; Serpelloni et al., 2005). The angular valuesobtained are a = 20°, b = 55° and h = 5°(Fig. 11A). Furthermore, the principal stress/infinitesimal strain field within the dilationalstepover (maximum infinitesimal strain axisdirection: N25°E) is rotated clockwise relative tothe regional stress field (Fig. 11B) because ofthe sinistral transtensional kinematics affectingthe boundary faults. The calculated pattern offractures consists of a N115°E tensional fractureand two conjugate fractures trending N155°E(R) and N95°E (R’) (Fig. 11C and D). If the theo-retical fracture mesh is compared with both theMontirone Hill fracture mesh and the EGFfault/fracture mesh (Fig. 11D), it is evident that:(i) the theoretical tensional fracture (N115°E)parallels the widest (tensional) fracture mea-sured on the Montirone Hill (N115°E; Fig. 9C),a fracture set affecting the EGF subsurface(N100°E) and the direction of anisotropydepicted in the variogram analysis of transmis-sivity calculated in the EGF thermal aquifer(N110°E; Fabbri, 1997); (ii) the theoretical R

fracture (N155°E) parallels a fracture set recog-nized on the Montirone Hill (N135°–145°E;Fig. 9C) and reconstructed in the EGFsubsurface (N150°–160°E); and (iii) the theoreti-cal R’ fracture (N95°E) fits an E–W trendingsmall fracture depicted in the central part and onthe western slope of the Montirone Hill. Further-more, a NNE–SSW trending fracture set is observ-ed in addition to the described fracture network(N15°E on the Montirone Hill, Fig. 9C; N20°E inthe EGF subsurface; N20°E anisotropy evidencedby the variogram analysis of transmissivity).The good analogy between the fracture meshes

(Fig. 11D) demonstrates that the network of frac-tures affecting the Montirone Hill is consistentwith a network that develops within a dilationalstepover bounded by NNW–SSE trending sinis-tral transtensional faults. The transtensionaldeformation produces an extensional regimewithin the stepover and, in accordance with thewell-known power-law scaling relation thatexists between relay zones (Peacock, 2003; Long& Imber, 2011), a similar regime also affects theEGF dilational stepover. This regime inducesfracturing of the bedrock and, consequently, the

A B

C

D

Fig. 11. (A) A diagram for thecalculation of the angles b and hstarting from the divergence angle awithin a dilational stepover (DePaola et al., 2007); in the case ofthe Montirone Hill dilationalstepover, the angle a is 20° (thedashed red line) and consequentlyb is 55° and h is 5° (see text forexplanation). (B) The principalstress/infinitesimal strain field(grey ellipse) developed within thedilational stepover. (C) Theoreticalpattern of the fractures developedwithin the dilational stepover;tensional fractures (direction:N115°E) are interconnected byconjugate R fractures (R direction:N155°E; R′ direction: N95°E). (D) Acomparison between the fracturesmeasured on the Montirone Hill(top, red line; Fig. 9), the faults inthe subsurface of the EGF (top,blue line) and the theoreticalfracture mesh (bottom); a goodcorrelation between the ESE–WNWand NNW–SSE fissure sets isshown.


192 M. Pola et al.

development of permeability enhances theuprising of the hot waters in the EGF area, inaccordance with the Euganean hydrothermal

conceptual model (Zampieri et al., 2009; Polaet al., 2010, 2013).The results of the multidisciplinary study con-

tribute to the reconstruction of a complete circu-lation/depositional model of the Montirone Hilltravertine deposit (Fig. 12) and provide a betterunderstanding of the mechanisms driving thecirculation of thermal fluids, which in springmound environments are still poorly understood(Pentecost, 2005). The thermal waters rise upquickly from the deeper part of the Euganeanthermal aquifer through the fault/fracture meshaffecting the EGF bedrock (Fig. 12B), as demon-strated by the correspondence between the tem-perature of the thermal waters measured in thethermal wells (65 to 86°C) and the temperatureof the thermal aquifer estimated using geo-ther-mometers (80 to 100°C; Gherardi et al., 2000).The thermal fluids subsequently migrate to thesurface within the Quaternary alluvial cover(sands and clays) probably forming a spreadzone of uprising made of several pipe-like upris-ing paths. The hot waters (54°C to 61°C) reachthe surface through these pipes (Fig. 12A), buildsmall coalescing mounds with shallow pools atvent orifices (Fig. 4H), where lime mud accumu-lates, and may overflow from the pools runningdown the slope with laminar flow depositing cal-cite crystalline crusts (Fig. 5H). The rise of ther-mal fluids to the surface appears to be enhancedby: (i) thin Quaternary cover associated with themorphological high of the bedrock in the Monti-rone Hill area (Fig. 12B); (ii) a vertical extent ofthe travertine deposit beneath ground level, prob-ably up to the bedrock; and (iii) the fault/fracturemesh affecting the carbonate body. This interpre-tation is supported by the results of the geophysi-cal surveys that show a low resistivity anomalyin the central area of the mound (Fig. 10), inter-preted as a highly fractured zone of high fluidcontent. The depositional model proposed forthe Montirone Hill corroborates the well-knowncontrol of active, normal/transtensional fault onthe travertine deposition/thermal fluid flow(Altunel & Hancock, 1993; Curewitz & Karson,1997; Hancock et al., 1999; Brogi, 2004; Uysalet al., 2007; Brogi & Capezzuoli, 2009; Nelsonet al., 2009; Temiz et al., 2009; Brogi et al., 2010;Guido & Campbell, 2011; Brogi et al., 2012;Guido & Campbell, 2012). Although other factorscould influence the thermal depositional pro-cesses (for example, climate and hydrodynamic;Rihs et al., 2000; Pentecost, 2005; Faccennaet al., 2008; Uysal et al., 2009; Guido & Camp-bell, 2011; De Filippis & Billi, 2012; De Filippis

A

B

Fig. 12. A simplified circulation/depositional modelof the Montirone travertine mound. (A) A 3D view ofthe Montirone Hill and its palaeo-depositional envi-ronment: the mound was composed of several thermalpools, where lime mud accumulated, and from whichwaters, overflowing in thin sheets and running downthe slope, deposited the calcite crystalline crusts. (B)The E–W cross-section of the Montirone Hill area: thethermal fluids rise up quickly from the deeper part ofthe thermal aquifer through the fracture mesh thataffects the EGF bedrock, and reaching the surfacethrough the Quaternary alluvial cover deposit thetravertine.



et al., 2012), the available data do not allow eval-uation of their impact.The results, and in particular the results of

the geochemical analyses (both U-series and sta-ble isotope analyses), provide better knowledgeof the Euganean hydrothermal system at a regio-nal scale. The studied travertine is characterizedby: (i) a low uranium content (from 3 to 12 ppb;Table 1) and a 234U/238U activity ratio higherthan unity (ca 2; Table 1); and (ii) d18O valueslower than 0 (from �16�15 to �17�40& V-PDB;Table 2; Fig. 8) and d13C values higher than 0(from 0�45 to 1�6& V-PDB; Table 2; Fig. 8), inagreement with the present-day travertinedeposit environments (Pentecost, 2005; Gandin& Capezzuoli, 2008). These data are typical of adeep hydrothermal circuit that is characterizedby a long circulation path at depth and a long-term water–rock interaction resulting in the 13Cenrichment of the original meteoric water bycontamination with the marine limestones bed-rock, as well as the change in oxidation state ofuranium from VI to IV, which favours its precip-itation (Bourdon et al., 2003). Furthermore, noinput of CO2 produced by decarbonation phe-nomena of limestone buried at higher depthappears to be active in the Montirone Hill depo-sitional/hydrothermal environment, which is incontrast with other travertine systems (for exam-ple, Denizli Basin, De Filippis et al., 2012; LapisTiburtinus, De Filippis et al., 2013). In fact, thestable isotope ratios of the Montirone Hill traver-tine are relatively lower than the Denizli (�14 to�7& d18O V-PDB and 3 to 8& d13C V-PDB; DeFilippis et al., 2012) and Lapis Tiburtinus (�7to �4& d18O V-PDB and 8 to 11& d13C V-PDB;De Filippis et al., 2013) basins. This differencecan be explained by a higher depositional tem-perature and a slightly heavier oxygen composi-tion of the thermal fluids. Therefore, accordingto Gherardi et al. (2000), the Montirone d18Oand d13C values can be regarded as a result ofthe water–rock interaction (carbonate dissolu-tion) at 3000 m depth with a normal geothermalgradient. These features are consistent with therecently proposed Euganean hydrothermal sys-tem conceptual model (Zampieri et al., 2009; Polaet al., 2010, 2013). The model shows that originalmeteoric waters flow within a Mesozoic carbonatereservoir (Dolomia Principale, Calcari Grigi andMaiolica formations) reaching a depth of ca3000 m and warming to ca 100°C by a normalgeothermal gradient (residence time of the watersof a few thousand years). Furthermore, the age ofthe first metre of the travertine deposit (20 to

34 ka; Upper Pleistocene; Table 1) confirms boththe long activity of the Euganean hydrothermalsystem and the recent activity of the Schio-Vice-nza fault system (SVFS). Both systems had to beactive at least since 34 ka to allow the thermalfluid flow and the deposition of the travertine.This datum is relevant, especially for the study ofthe activity and seismic hazards related to theSVFS that is not included presently in the Data-base of Individual Seismogenic Sources (DISS3�0�2; Burrato et al., 2008) and consequently isconsidered as inactive or quiescent.

CONCLUSIONS

In this study, the Montirone Hill travertinemound was studied using a scientific multidisci-plinary approach comprising analysis of: (i) thedepositional facies of the travertine (sedimento-logy); (ii) the age and geochemical features of thecarbonates (geochemistry); (iii) the fault/fracturemesh affecting the deposit (structural geology);and (iv) the static features and the nature of thedeepest part of the deposit obtained throughextensive, non-invasive procedures (geophysics).The results of the sedimentological and petro-

logical analyses of the dominant lithofacies(lime-mudstone and crystalline crusts), as wellas the stable isotope ratio, revealed the thermalnature of the carbonate, allowing the reconstruc-tion of a likely palaeo-environmental model ofthe Montirone Hill travertines. The lithofacieswere deposited around thermal spring ventslocated on the flat top of a shield-like mound.The recurrence of gas bubbles produced softsediment deformation in the still plastic limemud. The coalescence of several thermal poolsgave rise to a composite, single, shield-shapedmound that extends below ground level. Thepalaeo-temperature of the thermal waters wasfound to range from 54°C to 61°C improving theproposed palaeo-environmental model. The dis-charge/upwelling of hot waters appears to becontrolled mainly by the local extensionalregime affecting the Euganean geothermal field(EGF). The network of fissures is consistent withthe extensional stress field developed within adilational stepover bounded by NNW–SSEtrending sinistral transtensional faults, which issimilar to the releasing structure of the Schio-Vicenza fault system (SVFS) located in coinci-dence with the EGF. Based on the power-lawscaling relation between the interaction zones offaults, it is likely that an analogous extensional


194 M. Pola et al.

regime affects the EGF, allowing the develop-ment of permeability and the uprising of hotfluid. The thermal system had to be active atleast since 34 ka, as demonstrated by the U-ser-ies isotope analyses, which dated the uppermetre of the deposit at the Upper Pleistocene(20 to 34 ka). At the regional scale, the thermalfluid flow is controlled by the SVFS as recentlyproposed in the Euganean hydrothermal systemconceptual model. Therefore, dating of the trav-ertine demonstrates that the SVFS, which isnow considered inactive or quiescent, had to beactive 34 ka ago, providing evidence of therecent and likely present activity of the faultsystem. Confirmation regarding some features ofthe hydrothermal system was obtained throughboth the U-series and stable isotope analyses,which evidence a long and deep hydrothermalcircuit with a long water–carbonate bedrockinteraction in accordance with the Euganeanhydrothermal system conceptual model.In conclusion, the results of this multidisci-

plinary study have produced an improvedunderstanding of the Euganean hydrothermalsystem from local (Montirone Hill), to medium(geothermal field), to regional (hydrothermal sys-tem) scales. Furthermore, this study corroboratesthe well-known relation that exists among trav-ertine deposition, thermal fluid flow and exten-sional faulting, thereby confirming that thestudy of travertine bodies is a useful tool forhydrothermal and tectonic studies. This casestudy demonstrates that an integrated approachcan provide a broader view of the hydrothermaland structural mechanisms that occur concur-rently in travertine depositional systems.

ACKNOWLEDGEMENTS

This study was partially funded by the CariPaRoFoundation Ph.D. thesis, grant to P. Fabbri, andthe University of Padova (Athenaeum project“A new conceptual model of the Euganean Geo-thermal System performed using a multidisci-plinary approach”; No.: CPDA102312; year:2010), grant to D. Zampieri. The authors aregrateful to D. Liotta, A. Brogi and S. Kele for thefruitful discussions on the relations betweentravertines and tectonics and on travertine iso-tope signature, and to A. Ricciato and R. Fantonifor the discussions on the regional structuralsetting. E. Capezzuoli, L. De Filippis and W.Wr�oblewski are thanked for their very construc-tive criticism and suggestions.

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APPENDIX

U-series isotope analysis

The U-series isotope method, which is based onthe extreme fractionation of the parent isotopes238U and 234U from their long-lived daughter230Th in the hydrosphere, is the most widely usedradiometric age determination technique for trav-ertines. Uranium, which is markedly more solu-ble than thorium in surface and near-surfaceenvironments, is readily mobilized as the highlysoluble uranyl ion (UO2

2+) and its complexes; incontrast, thorium is easily hydrolysed and precip-

itated or adsorbed on detrital particles. Uraniumis co-precipitated with CaCO3 on exsolution ofCO2, whereas thorium is generally negligible. Inthe absence of detrital thorium, 230Th forms onlyin situ by the radioactive decay of co-precipitateduranium. In a closed system, the extent to whichthe 230Th/234U activity ratio has returned towardsunity is a function of time taking into accountalso the state of disequilibrium between 234U and238U (Kaufman & Broecker, 1965). Unfortunately,travertines are often characterized by the occur-rence of a detrital component containing thorium,as in the case of the Montirone travertine, leadingto an unrealistic estimation of the age. Therefore,suitable correction models must be applied toestimate the amount of non-radiogenic 230Th andsubsequently the real travertine age. Approxi-mately 30 g of the sampled travertine was ultra-sonically washed in deionized water, spiked witha mixed 232U/228Th tracer and dissolved in nitricacid. A few millilitres of hydrogen peroxide wasadded and heated at 100 °C to destroy the organicmatter. The isotopic complexes of uranium andthorium were extracted according to the proce-dure described in Soligo et al. (2002) and werealpha-counted using high-resolution ionimplanted Ortec silicon surface barrier detectors.The analyses were performed at Laboratorio diGeochimica Ambientale e Isotopica, Dipartimen-to di Scienze Geologiche, Universit�a degli Studi“Roma Tre”, Rome.

Stable isotope analysis

The oxygen and carbon isotopes were deter-mined on the sampled travertine by isotopesratio mass spectrometry. A Finnigan MAT Kiel IIautomatic preparation device was used for phos-phoric acid digestion at 72 °C and CO2 purifica-tion, and subsequently a Finnigan MAT 252mass spectrometer was used to measure thecarbon and oxygen isotope ratios of the carbondioxide. The ratio is expressed in the traditionald notation, which represents the relative devia-tion in part per thousand (&) of the heavy iso-tope/light isotope ratio (d13C = 13C/12C,d18O = 18O/16O) relative to V-PDB. The reproduc-ibility is � 0�05& for both the carbon and oxygenisotope analyses. The d18O values were used tocalculate the deposition temperature of the trav-ertine by applying the calcium carbonate–waterequilibrium fractionation equation (Friedman &O’Neil, 1977; Zhou & Zheng, 2003, 2006) andassuming equilibrium isotope fractionation dur-ing carbonate precipitation from the thermal


198 M. Pola et al.

water. Unfortunately, isotopic equilibrium israrely maintained under natural conditions dur-ing calcium carbonate precipitation (non-equilib-rium calcium carbonate–water fractionation) inthe temperature range between 13 °C and 67 °Cas a result of elevated CO2 outgassing and extre-mely rapid carbonate precipitation (Kele et al.,2011; and references therein). Moreover, Keleet al. (2011) observed that the d18O of travertineequals the d18O (HCO3

�) in thermal waters at theorifices of the thermal springs and suggested twodifferent approaches to obtain a more reliable

palaeo-temperature: (i) use the water–bicarbonateoxygen isotope equilibrium fractionation equa-tion (Halas & Wolacewicz, 1982); and (ii) use thecalcite–water equilibrium fractionation equation(Friedman & O’Neil, 1977) and add 8 °C to 9 °Cto the calculated palaeo-temperature. The analy-ses were performed at Laboratorio Isotopi Stabili,Istituto di Geologia Ambientale e Geoingegneria,Rome, Italy (stable isotope analysis).

Manuscript received 31 May 2012; revision accepted19 July 2013



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