Travertine: Distinctive depositional fabrics of carbonates from thermal spring systems

Travertine: Distinctive depositional fabrics of carbonates fromthermal spring systems

ANNA GANDIN and ENRICO CAPEZZUOLIDipartimento di Scienze Fisiche, della Terra e dell’Ambiente, Universit�a di Siena, Via Laterina,8 - 53100 Siena, Italy (E-mail: [email protected])

Associate Editor – Martyn Pedley

ABSTRACT

The terrestrial limestones forming at the emergence of thermal springs show

a variety of unusual depositional facies. The specific lithological and petro-

logical features of these deposits have few counterparts in the marine, and

continental, karst-related carbonates, but they are typical of the epigean lime-

stone that has been quarried since antiquity in the surroundings of Tivoli

(Rome) under the name of travertine, where it is still forming in hydro-

geothermal fields linked to extensional tectonics. The physicochemical,

hydrodynamic and geological conditions specific to the Tivoli thermal spring

system imply hypersaturated alkaline–sulphate, warm to hot waters, upwelling

from springs fed through open fractures/faults in extensional and/or volcanic

regimes. These features, together with the hydrodynamic behaviour of the

water flows running from the vents, control the petrogenetic features of the

travertine, a well-bedded, mostly finely laminated, porous but quite compact

limestone. The results of a detailed comparative petrological analysis carried

out on the lithofacies of travertine limestones, and of those observed during

formation within numerous active thermal spring fields, provide the elements

required for an exhaustive textural classification of the travertine lithofacies,

which has not yet been described systematically. According to the genetic

processes and fabrics, the thermal deposits that originate from such hyper-

saturated alkaline–sulphate, warm to hot waters, can be subdivided into:

abiotic crystalline crusts, microbially mediated crusts (microbialites) and

granular deposits mostly represented by small accumulations of lime-

mudstone. Some of the granular deposits and the microbialites are only partially

comparable with analogous sediments forming on tidal flats/sabkhas or other

continental sites; however, the facies association of crystalline crusts and lami-

nar curled microbialites has no counterpart in the marine realm. The widespread

presence of thermophile bacteria and sulphobacteria, and the general absence of

autochthonous eukaryote organisms, unable to live in poisonous sulphate

waters, are also undeniable evidence of their thermal origin.

Keywords Calcareous crusts, continental carbonates, genetic classification,microbialites, thermal spring systems, travertine petrology.

INTRODUCTION

Travertine is one of the most diffuse continentalcarbonate rocks, well-known for its commercial/ornamental qualities but it is not yet well-definedin terms of lithological/petrogenetic features. The

carbonate rock named ‘lapis tiburtinum’ (stone ofTivoli, near Rome) by early Roman architects,subsequently corrupted into the word ‘travertine’,is still quarried and is produced by Ca-richsulphurous warm waters (23˚C) rising fromnumerous thermal springs known by the Romans

264 © 2013 The Authors Sedimentology © 2013 International Association of Sedimentologists

Sedimentology (2014) 61, 264–290 doi: 10.1111/sed.12087

as ‘acquae albulae’, i.e. whitish/milky waters(Faccenna et al., 2008), an attribute that describesthe continuous suspended material (whitings)forming in the hypersaturated spring waters dueto the precipitation of microcrystalline calcite.Continental, Ca-rich spring waters flowing in epi-gean conditions, here derive from groundwatersmostly of meteoric origin, which acquire specificgeochemical and physical proprieties, dependingon the depth of circulation/recharge paths andconnections with older carbonate and evaporiticbedrock of marine origin, geothermal fluids andmagmatic rocks (Pentecost, 2005, and referencestherein; Gandin & Capezzuoli, 2008). The shallowunderground circulation, promoted by regionalfracturing of the brittle carbonate rocks, gives riseto a karstic cave system and cool hypersaturatedalkaline waters; whereas the deep hydrothermalcircuit driven by tensional deep faulting leads tothe geothermal or magmatic warming of the origi-nal meteoric waters and their eventual mixingwith endogenous fluids and rocks (Altunel &Hancock, 1996; Brogi, 2004; Brogi & Capezzuoli,2009). The resulting karstic and hydrothermalspring systems, fed by waters with quite differentand exclusive physicochemical proprieties, origi-nate contrasting landscapes and non-comparabledeposits (Gandin & Capezzuoli, 2008; Pedley,2009). Evidence of the different genetic derivationof the two groups, the ‘meteogene’ and ‘thermo-gene’ carbonates (Pentecost & Viles, 1994; Pente-cost, 1995, 2005), can be proved by the comparedanalysis of their isotopic imprints, their differentlithological characteristics (Gandin & Capezzuoli,2008), and also by the mechanical characteristicsand permeability proprieties inherent in the tradename of travertine (Barazzuoli et al., 1988; Pente-cost, 2005). Because the significance of the spe-cific petrological features of these two distinctgroups of continental carbonates was not well-known, the term travertine has been and is stillused as a general term to designate terrestrial car-bonates of different origin, genetic connectionsand geological meaning (Gandin & Capezzuoli,2008, and references therein). The term travertineis employed regardless of the lithological charac-teristics, already known for both calcareous tufa(Ford & Pedley, 1996; Pedley, 1990, 2009, and ref-erences therein; Arenas-Abad et al., 2010) andtravertine (Chafetz & Folk, 1984; Folk et al., 1985;Koban & Schweigert, 1993; Guo & Riding, 1998,1999; Fouke et al., 2000, 2003; Jones & Renaut,2010, and references therein), and regardless ofthe potential information that their contrastingdepositional settings imply (Gandin & Capezzu-

oli, 2008, and references therein). The aim of thepresent study was to define the depositional pro-cesses, and related macro-facies and micro-facies,of the continental carbonates that can be pres-ently observed forming in the numerous microen-vironments of active thermal spring systems. Theidentification and comparison of the specific pet-rofacies resulting from a complex interaction ofphysical, chemical and biological factors, willprovide the criteria for the genetic attribution andunambiguous lithological identification of olddeposits of travertine no longer connected withtheir ‘parent spring’ waters.

PREVIOUS KNOWLEDGE

The petrological features and genesis of traver-tine limestone surprisingly have attracted com-paratively little attention (Pentecost, 2005; Jones& Renaut, 2010; Capezzuoli et al., 2013). Thefirst studies concerning travertine were geo-chemical and mineralogical, and performed forcommercial evaluation (Gonfiantini et al., 1968;Cipriani et al., 1972, 1977; Chafetz & Folk, 1984,and references therein). Later, the particular fab-rics of the travertine were described (Chafetz &Folk, 1984; Folk et al., 1985; Guo & Riding,1992, 1994, 1998; Fouke et al., 2000; Pentecost,2005; Jones & Renaut, 2010, and referencestherein) but petrological features have not yetbeen structured into a comprehensive classifica-tion, and the genetic relations with the deposi-tional environment have not been investigatedfully. Many of the more unusual/bizarre deposi-tional facies, and a schematic categorization ofdepositional sites, were described in the 1980sand partly interpreted from Italian travertines, atTivoli and Rapolano, and other localities inNorth America (Folk & Chafetz, 1983; Chafetz &Folk, 1984; Folk et al., 1985; Pentecost & Tor-tora, 1989). Research was also carried out on themuch debated problem of the organic contribu-tion to carbonate precipitation (Pentecost, 1990,1994, 2003; Folk, 1994). Facies analyses carriedout on Rapolano travertines and on the activefissure ridge complex at San Giovanni Terme(Guo & Riding, 1992, 1994, 1998, 1999; Guoet al., 1996) illustrated, for the first time, theintimate association of lithofacies and micro-environments in a hydrothermal system. Thisanalysis resulted in a schematic depositionalmodel consisting of three environmentalsystems: ‘Slope Depositional System’, ‘Depres-sion Depositional System’ and ‘Reed Mound

© 2013 The Authors Sedimentology © 2013 International Association of Sedimentologists, Sedimentology, 61, 264–290

Travertine depositional fabrics 265

Depositional System’ and eight types of relatedcarbonate deposits (crystalline crust, shrub tra-vertine, pisoid travertine, paper-thin rafts, litho-clast, reed travertine, palaeosol and coatedbubble travertine). An analysis of Angel Terracein Mammoth Hot Springs, Wyoming, by Foukeet al. (2000, 2003), defined five facies on thebasis of their fabrics (vent, apron and channel,pond, proximal slope and distal slope facies),each characterized by distinct depositional fea-tures. The main depositional structures recog-nized in the wide and complex system ofPamukkale (Turkey) have been described by Alt-unel & Hancock (1996) and €Ozkul et al. (2002).Investigations of numerous active thermal

fields in Central Italy (Rapolano, Bagni SanFilippo, Bagno Vignoni, Saturnia, Bagni di Petri-olo, San Casciani dei Bagni, Tivoli and Viterbo:Pentecost, 2005, and references therein; Cape-zzuoli et al., 2009, and references therein), inTurkey (Pamukkale: €Ozkul et al., 2002), in theUSA (Yellowstone: Fouke et al., 2000, 2003), inCanada (Grasby et al., 2003; Rainey & Jones,2009; Jones & Renaut, 2008) and in China (Bai-shuitai and Yunnan: Liu et al., 2003, 2010) sug-gest that the common architecture of thermalspring systems is related to analogous bedrocksources and hydro-geothermal regimes (Chafetz& Folk, 1984), and results in limestone bodieswith depositional fabrics and facies associationswhich are essentially similar. This view is alsosupported by the depositional features recentlyobserved in well-preserved thermal fields of theMesozoic Deseado Massif in Argentina (Guidoet al., 2010; Guido & Campbell, 2011), the Neo-gene in Southern Germany (Koban & Schweigert,1993) and the Messinian in Italy (Gandin et al.,2002) all of which display geological setting,depositional patterns and fabrics/facies associa-tions that are definitely comparable with thoseoccurring in active thermal spring systems.

DEPOSITIONAL FEATURES OFTHERMAL CARBONATES

Travertine is comprised of a variety of depositsarising from two different genetic processes: (i)carbonates deposited from flowing waters,lithified during deposition as hard crusts orflowstones (abiotic crystalline and microbiallymediated facies), either in epigean (subaerialthermal systems) or in hypogean (deep geother-mal conduits) conditions; and (ii) deposits that,like marine carbonates, settle in a subaqueoussetting (lakes, marshes, streams and ephemeralponds/pans), by suspension or transport as loosegrains (lime-mudstone to grainstone/rudstone)and are lithified after deposition, mostly afterburial (granular sediments). In order to unravelthe intricacy of the current state of understand-ing of the continental carbonates, an integratedclassification of the thermal deposits is proposedhere with the aim of defining the relationsbetween depositional processes and features/fab-rics of the related carbonate by-products.

CLASSIFICATION AND TERMINOLOGY

In active thermal deposits Ca-carbonate precipi-tates either as calcite and/or aragonite or both(Pentecost, 2005; Jones & Renaut, 2010, and ref-erences therein). Nevertheless, the mineralogydoes not influence the depositional processesand fabrics of the precipitates and, in this arti-cle, they are referred to simply as carbonates.Most of the carbonates produced in the thermalspring system are laid down as crusts (Table 1)that range from primary crystalline facies (abi-otic crystalline crusts) to microbially mediatedlaminites (microbialites), including minor accu-mulations of granular sediments (Table 2) rep-resented by lime-mud and carbonate sands to

Table 1. Textural classification of thermal-derived carbonates: travertine limestone.

Crusts facies--Lithified during deposition

Abiotic crystalline crusts Microbially mediated crusts/Microbialites

Feather-like/dendritic crystals Bindstone Dendrolite Cruststone

Fan/ray crystals Microbial mats Microbial shrubs Microbial laminites

Banded palisade crystals Thrombolitic fabric Flat to curled laminites

Foam rock Agglutinated fabric Puff pastry-like fabric

Calcite rafts Stromatolitic fabric Microbial rafts and flakes


266 A. Gandin and E. Capezzuoli

rudites. Not all of the fabrics, micro-facies andcomponents of these carbonates can be describedusing the standard textural classification devel-oped for marine carbonates because the sedimen-tary processes are somewhat different from thoseof the marine realm. Consequently, while the ter-minology for the granular facies group is based onthe textural categories of Folk (1962) and Dunham(1962) (Table 2), and that of the microbially medi-ated crusts utilized terms relative to some of theDunham (1962), Embry & Klovan (1971) and Cuff-ey (1985) biofabrics, a new category has beenformulated for the abiotic crystalline crusts(Table 1). Microbially mediated crusts or micro-bialites, an overall term comprising benthicorgano-sedimentary deposits (Burne & Moore,1987), include either non-laminated spongy/thrombolitic mats and rafts or laminated faciescharacterized by unusual structures and geneticprocesses (for example, microbial shrubs andcurled laminites).Flat, ephemeral basins prevail in the thermal

spring system, here they are described as terrace

pans; pan being a more appropriate denomina-tion for the lobate very shallow evaporativebasins (Fig. 1D). Natural, deep basins are veryrare, such as the Tivoli ‘lakes’, that are up to55 m deep and commonly agitated by robust gasescape bursts (Pentecost & Tortora, 1989), orsome crater-like basins at the mouths of somespring vents (at Yellowstone, USA, in Italy atBullicame-Viterbo and a fossil example at AbanoTerme – Pola et al., 2013). The depositionalfacies recognized in modern and ancient thermalcarbonate bodies (Fig. 1) record a variety of sedi-mentary niches, most of them resulting from thecomplex and continuously shifting self built/auto built, depositional architecture.

Abiotic crystalline crusts

Abiotic crystalline facies (Figs 1 and 2) consistsof aggregates of well-packed primary calcite/aragonite crystals showing varied uncommonmorphologies (e.g. Chafetz & Folk, 1984; Folket al., 1985; Chafetz et al., 1991; Jones &

Table 2. Textural classification of thermal derived carbonates: travertine limestone

Granular facies and components – Loose sediments lithified after deposition (during diagenesis)

Matrix-supportedLime-mudstone Wackestone/FloatstoneGrains <10% Grains >10% to >2 mm

Grain-supportedGrainstone Rudstone/FlakestoneGrains <2 mm Grains >2 mm

MatrixMicrobial lime-mudstone – Automicrite (whitings) to Organomicrite (clotted micrite – peloidal micrite)Silty lime-mudstone – Allomicrite (residual/skeletal debris) to Automicrite

Granular componentsAutochthonous/‘intrabasinal’

Peloids Intraclasts Oncoids/ooids Coated bubbles Skeletals

Bacterial aggregatesFaecal pellets

Rafts: crystalline andmicrobial flakesFragments ofcrystalline ormicrobial crusts

Tangles of microbialfilamentsVadose pisoidsUnstructured grains

Crystalline andmicrobialcoatings

Chara andostracodremains

Allochthonous/‘extrabasinal’

Bioclasts

Windblown/floated materials:Bones of vertebratesMolluscs and ostracod shellsLeaves, branches and stumpsPollens



Renaut, 1995, 1996, 2010; Guo & Riding, 1998,1999; Renaut et al., 2013). The dominant crys-talline lithofacies are represented by fan/raycrystals (Folk et al., 1985; Guo & Riding, 1992,1998; Fouke et al., 2000; Jones & Renaut, 2010and references therein) and feather-like crystals(Chafetz & Folk, 1984; Folk et al., 1985; Guo &Riding,1992; Koban & Schweigert, 1993) precip-itated in epigean conditions, and banded pali-sade crystals (or ‘banded travertine’ of Altunel& Hancock, 1993a,b, 1996; €Ozkul et al., 2002)exclusive of hypogean phreatic settings withinvent conduits. Less frequent crystalline depositsconsist of aggregates of microcrystalline coatedbubbles (Folk et al., 1985, or ‘bubble limestone’and ‘lithified bubbles’: Chafetz et al., 1991;‘honeycomb rock’ and ‘foam rock’: Chafetz &Folk, 1984; Guo & Riding, 1992, 1998; Jones &Renaut, 2010), or of thin flakes, chips, calciterafts or paper-thin rafts (paper-thin sheets: Folket al., 1985; or ‘hot water ice’: Allen & Day,1935; ‘calcite ice’: Bargar, 1978) which developat the surface of stagnant waters and, after sink-ing, accumulate at the bottom of the basin assmall lenticular bodies, or at the bottom ofdesiccated pans. The abiotic character of thecrystalline crusts, increasingly corroborated bydata (Jones & Renaut, 1998, 2010; Pentecost,2005), can be ascribed either to the fast precipi-tation of carbonate, that restrains the establish-ment and development of microbial colonies(Folk et al., 1985; Koban & Schweigert, 1993;Gandin & Capezzuoli, 2008; Rainey & Jones,2009), and/or to the high temperature of thewater (Folk et al., 1985; Jones & Renaut, 1998).Observations of active systems show that theprecipitation of subaerial crystalline crustsderives from fast running flows inducing highlevels of CO2 degassing and related supersatura-tion of the water, and is closely related to theirspecific level of energy and dynamic pattern(Jacobson & Usdowski, 1975; Folk et al., 1985;Guo & Riding, 1992, 1998; Gandin & Capez-zuoli, 2008; Rainey & Jones, 2009; Jones &Renaut, 2010).

Fan/ray crystalsFan/ray crystals form radiating bundles of clo-sely packed acicular, platy to dendritic crystalsgrowing in syntaxial continuity (Fig. 2D); theydisplay a very thin, sub-millimetric regularbanding (Fig. 2C and D) which reflects a fast rateof deposition that has been associated with sea-sonal or annual cadence, but is likely to bedaily, if not faster (Folk et al., 1985; Guo &Riding, 1992; Liu et al., 2006; Okumura et al.,2012). In present thermal systems, the crystalassemblages are arranged in diverse morpholo-gies varying from gently undulated juxtaposedand/or superposed, thick crusts (Fig. 2C to F)possibly associated with microbial mats(Fig. 2F), to botryoids/mamelons (Fig. 2B and C)or glove-shaped mamelons (Folk et al., 1985;Guo & Riding, 1992; Gandin & Capezzuoli,2008).Depositional conditions: The crystals grow in

fan-like bundles normal to the substrate andparallel to the direction of flow (Folk et al.,1985; Fouke et al., 2000, 2003), forming laminarbands on the subvertical slopes of a waterfall(Fig. 2E), whereas botryoids and mamelonsdevelop on less inclined but still steep sur-faces, in channels (Fig. 2A and B), or form therimstone barrages of lobate pans on terracedslopes (Fig. 1D). Precipitation of this fibrouscarbonate appears to be related to hypersatura-tion of turbulent waters as a consequence ofvaporization and rapid CO2 degassing inducedby the fast running, high-flux regime. The sprayproduced by the turbulent waters (Fig. 2A) rap-idly encrusts and coats any surrounding objectwith radial needles.

Feather-like crystalsFeather-like/dendrolitic crystals (Fig. 3A to E),also described as non-crystallographic dendritesor calcite dendrites/dendrite crystals (Jones &Renaut, 1995), crystal shrubs (Chafetz & Guidry,1999), feather dendrites (Rainey & Jones, 2009)and pseudodendrites made of branching bundlesof platy crystals (Jones & Renaut, 1998), display

Fig. 1. Travertine bodies. (A) The Pleistocene, well-bedded travertine lithofacies, exposed on the walls of OlivieraQuarry (Rapolano, Siena; Italy. Maximum thickness of the Quarry walls from the water surface is ca 20 m). (B) to(D) The carbonate body of the active thermal system of Pamukkale (B) seen from the Roman town of Laodicea(Turkey; distance from Pamukkale is ca 10 km). (C) The apron front: a white desert comprising two of the mainelements of deposition of a thermal edifice: a mildly inclined slope built by very shallow terraced pans boundedby rimstone barrages (D) where microbialites dominate (see also Fig. 10H) and sedimentation is affected by occa-sional desiccation, and a steep ramp inducing a waterfall and the growth of crystalline botryoids. Person for scalein (C) is ca 1�8 m tall.



A

B C

D



variable and complex morphologies that can begrouped into two main assemblages: those madeof aggregates of rhombohedral crystallites toform feather-like/plume-like units (Fig. 3D andE) or branching dendritic units (Jones & Renaut,2010). Plume-like aggregates (Fig. 3E) arereported to grow oriented normal to the sub-strate to form the rim of microterraces that coverthe outer surface of the crusts, up to 50 mmthick, which develop juxtaposed in steep slopes(Fig. 3A and B) (Guo & Riding, 1992; Foukeet al., 2000; Gandin & Capezzuoli, 2008). Thegrowth of these dendritic forms, as well as thecrystal shrubs, was initially attributed to exclu-sively abiotic processes (Pentecost, 1990; Guo &Riding, 1992; Rainey & Jones, 2009). However,minor microbial clumps are reported in associa-tion with crystal shrubs (or radiating dendritefacies), feather crystals and non-crystallographicdendrites (Jones & Renaut, 1995; Rainey & Jones,2009), suggesting the presence of organisms dur-ing the growth of these crystalline aggregates(Chafetz & Guidry, 1999).Depositional conditions: In present thermal

systems, the feather-like crystalline crusts preci-pitate directly from very thin sheets of water run-ning on variably steep surfaces, with smooth,laminar flow (Guo & Riding, 1998, 1999; Gandin& Capezzuoli, 2008). The plume-like aggregatescan be observed disposed normal to the substrate(Fig. 3A, B and E) and parallel to the direction offlux, progressively shaping the microterraces.Dendritic splays appear to result from the precipi-tation along the frontal crestline of interferencefans of laminar flows (Fig. 3C). The dendriticforms are observed in a great variety of morpho-logies in different thermal spring systems (Jones& Renaut, 2010, and references therein), and arereported from sloping surfaces, rimstone damsand rollovers (Rainey & Jones, 2009).

Banded palisade crystalsThe onyx-like banded palisade carbonatedescribed as ‘banded travertine’ (Altunel &Hancock, 1993a,b, 1996; €Ozkul et al., 2002) con-

sists of juxtaposed vertical sheets of closelypacked radiating acicular, or palisade columnarcrystals growing inward and in syntaxial conti-nuity, normal to the fracture walls (Fig. 3F toH). Calcite crystals commonly form alternatingbands of limpid and rusty-red stained crystals(Fig. 3G and H) that bear traces of tentativemicrobial colonization. The different morpholo-gies of the crystalline assemblages suggest ahypogean phreatic environment (Kendall &Broughton, 1978), similar to that of the calcitefilling the Neptunian dykes connected with thetensional tectonics of the marine carbonate plat-forms of the past (Fl€ugel, 2004).Depositional conditions: This lithofacies occurs

as vertically accreted bands that fill and plug frac-tures cut in the underlying substrate connectedwith the vent network (Fig. 3F) and represent thehypogean conduits that fed earlier linear, open fis-sure springs (Altunel & Hancock, 1993a,b, 1996).

Calcite raftsCalcite rafts (Folk et al., 1985; paper-thin rafts,pars: Guo & Riding, 1998; also ice calcite:Bargar, 1978) consist of platy crystalline flakes(Fig. 4A and B) sometimes with a dark micritefilm in the centre, lined on both surfaces by lim-pid microcrystalline calcite (Fig. 4C and D).Depositional conditions: Calcite rafts form at

the water–air interface of actively evaporating/cooling, carbonate-rich, still-water bodies, grow-ing in large glassy plates frequently shelteringgas bubbles (Fig. 4A and B). These rafts are com-monly broken by agitation of the water surfaceby wind, currents or by the pressure that the gasbubbles exert on the solid crystalline plate ontheir way upward. The irregular floating frag-ments may be enveloped by microbial coatingsand eventually sink and become embeddedwithin the sediment at the bottom of the basin.

Foam rockFoam rock consists of aggregates of lithified gasbubbles (Chafetz & Folk, 1984; Chafetz et al.,1991; Jones & Renaut, 2010) arranged in subver-

Fig. 2. Abiotic crystalline crusts – fan/ray crystals. (A) Turbulent flow in a drainage channel surrounded by a cloudof spray has deposited on the channel bed mamelons/botryoids of fan/ray crystalline calcite (B). In (A) the width ofthe channel (ca 50 m long) varies from ca 2 m upstream to 1 m downstream. Hammer for scale in (B) is ca 40 cm long.(C) Cross-section of old mamelons/botryoids (ca 20 cm thick) made of ray clusters crossed by thin growth laminae(San Giovanni Terme, Rapolano. Italy). (D) In thin section, the syntaxial continuity of the ray fans appears to be inter-rupted by dark bands resulting from microbial colonization (sparmicritization) during temporary depositional inac-tivity (Messinian travertine, Pignano, Volterra, Italy). (E) and (F) Juxtaposed fan/ray crystalline crusts deposited onthe steep front (E) of an active waterfall (Bagno Vignoni, Siena. Italy) and on a low-angle slope (F) where it is associ-ated with spongy microbial facies (see also Fig. 10E; Oliviera Quarry, Rapolano, Siena. Italy).



A B

C

E

F

D



tical, closely fitted pipes constricted at regularintervals like bead strings (Fig. 4E and F).Depositional conditions: In the thermal system,

bubbles are produced by gas discharge, mainlyoriginated by organic decay (Chafetz et al., 1991),and their thin-film, liquid envelope is promptlycoated by microcrystalline calcite (Fig. 4E). Inparticularly sheltered conditions, the steadyupward escape of gas may produce foam that israpidly lithified by carbonate precipitation, result-ing in an abiotic buildup (Fig. 4F) of sub-vertical,elongated chains of bead string-like pipes.

Microbialites

Bacterial communities that thrive in hot springwaters actively contribute to the formation ofmicrobially mediated deposits or microbialites.These deposits are the dominant components ofthe typical laminated lithofacies of travertinelimestone representing sediments formed eitherat the bottom or at the surface of very shallowbasins and pans or on the margins of shallowstreams or rivulets in rather flat areas (Figs 1Dand 5 to 8). In the thermal depositional environ-ment where warm to hot waters contain dissolvedsulphide that generally is poisonous for eukary-otes, the prokaryotes represented by thermophylebacteria and sulphobacteria, are the only livingorganisms that can contribute to carbonate depo-sition (Fouke et al., 2000, 2003; Pentecost, 2003).However, it is now evident that these microbialcommunities are involved in the formation ofmicrobialites, with a passive role of support andentrapment (biologically influenced mineraliza-tion; Dupraz et al., 2009) rather than contributingvia photosynthetic activity (Pentecost, 1990; Guo& Riding, 1998; Fouke et al., 2000; Rainey &Jones, 2009). The physicochemical conditions ofthe water and evaporation rates appear to be criti-cal factors in the development of the bacterial col-onies and the amount of carbonate precipitation.The preservation of the microbial communitiesdeveloping at the bottom of basins covered by avery thin sheet of warm, evaporating water

(Figs 1D, 6A, 9C and 9D), is consequently subor-dinate to the microcrystalline carbonate/lime-mud fallout (whitings), that is entrapped by fila-ments or cohesively fixed by the extracellularpolymeric substance (EPS) mucilage (Fig. 5E) andcan bury the organic mats and rapidly hardenresulting in laminar, leathery crusts (Figs 7G, 7Hand 8F). As long as the lime-mud at the bottom ofa basin is still covered or merely soaked by a filmof warm water, bacteria may continue growingwithin any available open spaces, such as in thecrevices of shrinkage cracks (Fig. 9B), or colonizefilament surfaces or framework spaces of curledlaminae (Fig. 7C to F) derived from syneresis pro-cesses.Microbialites correspond to organically bound

structures (Table 1) made of bacterial filamentsor coccoids, the cell walls of which may beembedded in, but never encrusted by, microcrys-talline calcite (Fig. 5G). These structures buildfabrics corresponding to: bindstone (Embry &Klovan, 1971) comprising microbial mats (Rid-ing, 1991; Fl€ugel, 2004) with reticulated/throm-bolitic (Fig. 5C), agglutinated (Fig. 5D), orcellular and spongy (Fig. 5F) fabrics; dendrolitesmade of bacterial shrubs (Fig. 6); and cruststone(Cuffey, 1985) which includes different types ofmicrobial laminites (Chafetz & Folk, 1984; Koban& Schweigert, 1993; Guo & Riding, 1994; Rainey& Jones, 2009) represented by peculiar facies,such as tightly packed flat to curled laminites(Fig. 7), puff pastry-like fabric made of wavysheets or flakes with large interlaminar openspaces (Fig. 8) and microbial rafts (Fig. 10). Someof these facies, also reported in the literature asstromatolites (Fl€ugel, 2004; Pentecost, 2005, andreferences therein), even if they retain a similar-ity with marine organically bound tidalites andreefal/‘algal’ buildups, differ however in fabric,depositional conditions and genetic processes.

Bindstone – microbial matsMicrobial mats (bacterial mats: Folk et al., 1985)exhibit thrombolitic or agglutinated fabrics(Fig. 5) made of microbial peloids (coccoid clus-

Fig. 3. Abiotic crystalline crusts – feather-like/dendritic crystals. (A) to (E) Thin, subvertical layers of feather-likecalcite growing normal to a smooth steep slope. (A) San Giovanni Terme, Rapolano, Italy. Each layer is ca 2 cmthick. (B) Sets of microterraced layers made of feather-like crystals grown on a rather steep slope (AcquaBorra,Siena, Italy). (C) Dendritic splays grown at the interference crestline of laminar flows (Pamukkale apron, Turkey).(D) and (E) Microscopic view of plume-like crystals (D) (Montirone, Abano Terme, Padova, Italy) and feather-like,dendritic aggregates (E) (Messinian travertine, Pignano, Volterra, Italy). Banded palisade crystals. (F) Vertical crys-talline bands fill the open fracture (ca 5 m wide) of an old fissure ridge complex (C�ukurbag, Pamukkale, Turkey.(G) and (H) Onyx-like calcite/aragonite in alternate bands of coloured and light rows of crystals with fibrous (G)or columnar (H) fabric (C�ukurbag, Pamukkale, Turkey. Hammer in the circle for scale is ca 40 cm long).



A

C D E

F

G

H

B



ters) and filamentous assemblages arranged inspongy cellular, reticular or vacuolar networks(Fig. 5C to F) that locally form small incipient

domal structures (Fig. 5A and B). The small-scaleframework porosity of these intertwined loosefabrics may be enclosed in a microbial lime-mud-

A B

CD

E F

Fig. 4. Calcite rafts and foam rock. (A) to (D) Crystalline rafts supported by gas bubbles and associated microbial,pancake-like rafts [pink flakes in (B)], float like an ice pack on the surface of a thermal pool (Acqua Borra, Siena,Italy). (C) and (D) Paper-thin rafts lined by crystalline calcite, with a dark, apparently biological central axis(Oliviera Quarry, Rapolano. Italy). (E) and (F) Aggregation of actively ‘growing’ bubbles (E) (Yenice, MeanderValley,Turkey) results in a massive foam rock (F) (San Giovanni Terme, Rapolano. Italy).



stone matrix, or totally filled by an irregularmicrosparitic, syngenetic cement, while the largervoids are commonly lined and sometimes par-tially filled with cement (Fig. 5C to F). Thesefabrics form lenticular layers irregularly alter-nated with the other microbial facies (Fig. 7Aand B), as well as associated with crystalline orgranular/mudstone facies (Figs 2F and 10E).Depositional conditions: This ubiquitous, por-

ous facies forms soft, mucilaginous bodies oroncoids (see components of the granular facies)at the bottom of more or less shallow water orlarge pancake-like rafts at the water surface(Figs 4B and 10H).

Dendrolite – bacterial shrubs fabricBacterial shrubs consist of composite colonies ofdensely packed bacterial clumps (shrub ‘buildingblocks’ of Chafetz & Folk, 1984) with an evidentvertical to radial pattern that forms dense clustersof dendrolitic arrays (Folk & Chafetz, 1983; Cha-fetz & Folk, 1984; Folk et al., 1985; Guo & Riding,1994; Chafetz & Guidry, 1999, 2003). The clumps(Fig. 6B) made of micrite, spar rhombs or needlecrystals (Pentecost, 1990) develop in arborescentstructures (Fig. 6C and E), in which small inter-skeletal spaces are commonly filled with unequal,fine-grained (microsparitic) cements (Fig. 6E andF). Seen from above the shrubs look like small,subspherical to mushroom-shaped clusters (Fou-ke et al., 2003; Fig. 4E) with low synoptic relief(Fig. 6D and F). Locally, fan-shaped shrub arraysare arranged in laterally continuous to lenticularbands closely alternated with thin intervalsof finely laminated micrite/bacterial silt (Fig. 6C).Depositional conditions: Bacterial shrubs are

reported from very shallow pools and extensivesheets of still water in waterlogged flats (Chafetz& Folk, 1984; Guo & Riding, 1994; Chafetz &Guidry, 1999; Rainey & Jones, 2009). In activethermal systems, small clusters of shrubs can beobserved to develop in several depositional set-tings, characterized by the rapid evaporation ofvery thin films of hypersaturated waters flowingon the subvertical walls of semi-dried channels,on the fringe of very shallow slow-runningwatercourses (Fig. 6A) or within macroterracesor microterraces (Fig. 6B). The origin of themicritic intervals alternated with the bands offan-shaped shrubs (Fig. 6C) has been interpretedas reflecting the reduced carbonate precipitationin the cold season (Chafetz & Folk, 1984). How-ever, in agreement with the observations madein active/present conditions, the bacterial shrubsappear to develop when the water evaporates and

the level of the flow drops while the deposition oflime-mud is a consequence of whitings actingduring flooding periods.

Cruststone – microbial, flat to curled laminitesand puff pastry-like fabricMicrobial, flat to curled laminites (laminarmicrobial crusts: Koban & Schweigert, 1993) con-sist of composite sheets made of microbial filmslined by crystalline cement (Fig. 7C to F). Thelaminae are assembled in laterally extended,tightly packed bundles locally split up by large,flat interlaminar cavities and folded in extraordi-nary curls and tepee-like/diapiric structures(Fig. 7A to D). Straw-like pipes (Fig. 7B), com-monly constricted at regular intervals like stringsof beads, appear to contribute to the deformationof the laminae, lifting and drawing them upwardinto tepee-like structures (Fig. 7C and D). Theresulting fabric-selective cavities, represented byinterlaminar spaces and loops, are commonlyopen and lined by thin ribbons of isopachous,more rarely geodic, fibrous early diagenetic/syn-genetic cements (Fig. 7C and D). The single cal-cite sheets, separated by thin interlaminar spaces(Fig. 7A, B and E), consist of very thin, arcuateor flat dark organic films commonly reinforcedby single or multiple linings of cement (Figs 7Cand 8B) that, in some cases, bear pendant den-drolitic clots (Fig. 7F). The microcrystallineflakes that locally are arranged to form rosettes(Fig. 8E and F) are made of bundles of very thindense micrite laminae containing brown matterprobably of organic origin (Fig. 8B and F). Thepeculiar arrangement of laminae and flakes issimilar to the fabric of the puff pastry which con-sists of wavy, thin sheets or large flakes looselysuperposed with intervening large lenticular orthin open spaces (Fig. 8C and D) forming unu-sual lamellar (Fig. 7A to F) or vesicular (Fig. 8A,B, E and F) structures. A similar laminar facies,although the characteristic curls and diapiric-like features were not mentioned, has beenreferred to as ‘bacterial stromatolites’ (Chafetz &Folk, 1984; Folk et al., 1985; Rainey & Jones, 2009)or ‘laminar microbial crusts’ (Koban & Schweigert,1993), but never described in detail.Depositional conditions: Flat to curled lami-

nae can be observed in very shallow pans wheremicritic lime-mud accumulates at the bottom asa blanket over thin films of bacterial coloniesand their mucilaginous EPS, forming sheets thatrapidly acquire a leathery consistency and, atthe same time, tend to shrink and form tepee-like structures evolving into curls (Fig. 7G and



H) and eventually breaking into intraclasts(Figs 7H and 8F). This process appears to betriggered by steady evaporation that at first givesrise to whitings and deposition of small quanti-ties of micrite mud (Fig. 9C and D) and then tosyneresis processes driven by variations of thefluid density and concentration, that lead todehydration of the microbial component of thesediment and deformation (Fig. 7G and H), and/or cracking of the carbonate sheets (Fig. 8D).The upward rise of the sheets (Fig. 7), beforethey crack, may also be driven by the accumula-tion of entrapped natural or biological gases.The subsequent, but still concomitant, precipita-tion of cement on the walls of interlaminarspaces, probably developed from residual con-nate waters in a vadose and/or phreatic regime,would eventually harden the laminar package.The dense microcrystalline laminae that form

the puff pastry-like fabric appear to be producedon the bottom of pans and puddles as leatherysheets and plates of desiccated organic-richlime-mudstone (Fig. 8F). Alternating cycles offlooding and desiccation of the bottom sediment,connected with the variable discharge regime,may account for the formation of the flakes aftermud-cracking, as well as for their accumulationin similar packages of laminae.

Microbial rafts and flakesMicrobial rafts and flakes (paper-thin rafts, pars:Folk et al., 1985; Guo & Riding, 1998) are repre-sented by a variety of forms that differ in thedevelopment of the microbial overgrowths; theserange from the typical paper-thin rafts with apoorly developed microbial lining on one orboth surfaces of the flake, to thick jagged rafts,with tooth-like protuberances on the downwardface (Fig. 10F) or biscuit-like flakes completelyenveloped in a thick microbial coating(Fig. 10G). Pancake-like rafts correspond to moredeveloped spongy microbial masses (Fig. 10Hand I). Coated bubbles are often associated withall types of rafts (Fig. 10A to C).Depositional conditions: Paper-thin and pan-

cake-like rafts develop on glassy calcite frag-

ments (Figs 4B and 10I) floating on the surfaceof actively evaporating and cooling still water,regardless of water depth. The thickness of themicrobial overgrowths appears to be related tothe time of residence of the rafts on the watersurface.

Granular facies

Granular deposits, the components of whichrange in size from micrite to rudite, are notwell-represented among the depositional faciesof a travertine body. Lime-mud or grain-sup-ported facies may occur as small lenses or thinlayers intermixed with the other more typicaltravertine crustose facies. The genetic origin ofthe components of the granular sediments inpart corresponds to that of the marine allochemsand, consequently, the terminology used todescribe them and their fabric (Table 2) isinspired by the classifications of Dunham (1962)and Folk (1962).Lime-mudstones exhibit different fabrics

depending on their mineralogical and organiccomponents, which reflect different genetic con-ditions (Table 2). As a primary sediment lime-mudstone occurs in lenticular bodies of greymassive limestone laterally interfingering withsets of microbial and/or crystalline crusts(microbial lime-mudstone) or as dark grey, pinkto rusty-red massive silty limestone (silty lime-mudstone), associated with sedimentary uncon-formities (Figs 9F and 11A). Otherwise, as apost-depositional sediment, it commonly fillsfabric-selective or karstic cavities with multipleinputs (Fig. 11A to C).

Microbial lime-mudstoneFine-grained, grey homogeneous limestone thatlocally can be finely laminated, mottled(Fig. 9A) or fenestrate (Fig. 9B) may sporadi-cally contain dispersed grains of silt quartzand phyllosilicates. It consists of micrite/finemicrosparite or coarse inequigranular microspa-rite mosaics with different amounts of organicparticles (automicrite to organomicrite: Fl€ugel,

Fig. 5. Microbialites – bindstone. (A) Irregularly distributed spongy microbial mats and laminites. Beady pipescross the mat apparently inducing an incipient small domal structure (Oliviera Quarry, Rapolano. Siena, Italy).(B) Irregularly alternated spongy microbial mats and microbial laminites. Small domes (arrow) appear to be builtby the combined activity of the microbial communities and gas escape (Acqua Borra. Siena, Italy). (C) to (F) Fab-rics of microbial mats: reticulated/thrombolitic (C); agglutinated (D); cellular and spongy (F) associated with lime-mud (Oliviera Quarry, Rapolano, Italy). (E) Soft aggregate of microcrystalline lime-mud cohesively fixed by theEPS mucilage (see Fig. 10H), harbouring encrusted gas bubbles (arrow) (Karahayit spring, Denizli, Turkey). (G)Microbial weeds not yet incorporated in carbonate mud entrap coated gas bubbles.



2004). Bacterial filaments and microbial aggre-gates (peloids sensu Chafetz, 1986) are com-monly included in the matrix as sparse clots

(clotted micrite) or as denser inequigranularclumps (peloidal micrite: sensu Pedley, 1992,1994). Layers and subrounded masses/tangles

A B

CD

EF

G



of reticulated microbialites and/or pancake-likespecimens may be found floating in the homo-geneous mud matrix (Fig. 10I). Locally in thelarger lenticular bodies, apparent syneresiscracks (Fig. 3B) and/or early soft sedimentdeformations were observed (see Pola et al.,

2013). This facies is also found in thin layersassociated with crystalline crusts (Fig. 2F) oralternated with microbial (Figs 5F, 6C, 7A and7B) and granular facies.Depositional conditions: Microbial lime-mud-

stone can be observed in crater-like vent

A B

C D

E F

Fig. 6. Dendrolite – microbial shrubs. (A) and (B) Microbial shrubs growing at the edges of a discharge stream(Bullicame spring, Viterbo. Italy; the channel is ca 25 cm wide) or within spoon-like small terraces with micriticmargins (B) (San Giovanni Terme, Rapolano. Italy). (C) Rows of well-developed shrubs separated by thin layers ofmicrite lime-mudstone (San Giovanni Terme, Rapolano. Italy). (D) and (F) Shrub dendrolite ‘heads’ seen on a slabcut parallel to the depositional surface (D) and in thin section (F). (E) Shrubs growing on the surface of a gas bub-ble entrapped in the microbial frame (Oliviera Quarry, Rapolano, Siena, Italy).



mouths. In shallower basins (stepped pans,mires and water puddles), the loose sedimentcomposed of microcrystalline calcite/aragonite,nucleates during whitings in the water column(Riding, 2000), accumulates by suspension aslime-mud at the bottom (Fig. 9C and D) and israpidly incorporated into loose microbial aggre-gates and their EPS (Figs 5E, 7H and 10H), toform a microbial micrite that may evolve intoregular microbial mats.

Silty lime-mudstoneCoarse, dark to rusty limestone commonly formslenticular beds characterized by irregular shapeand size (Figs 3F and 11A), and by the inclusionof large crust intraclasts (Fig. 11A). It consists ofmicrosparite/coarse micrite, likely to be partlyresidual or erosional in origin (allomicrite toautomicrite; Fl€ugel, 2004), with diffusedquartz grains, frequent pedogenetic concretions(Barazzuoli et al., 1988), such as rhizoliths andencrusted calcretic pisoids (Fig. 3G), nests ofapparent faecal pellets and articulated or disar-ticulated valves of ostracods, scattered stems ofcharophytes (Fig. 9H), shells of gastropods and,locally, bioturbation.Depositional conditions: Described by Guo &

Riding (1998) as ‘marsh-pool’ facies, its compo-sition suggests distal, palustrine conditions incool, shallow ephemeral ponds established onan unconformable substrate, clearly adapted to apreviously eroded surface (Figs 9F and 11A),probably during a pause or diversion of the ther-mal input.

Granular componentsThe most frequent elements found in continen-tal carbonates have been broadly described(see an extensive overview in Jones & Renaut,2010, and references therein). However, asystematic report and depositional definitionof the particles characteristic of carbonate ther-mal systems is still lacking. The granulesoccurring in mud-supported or grain-supportedaccumulations are in part autochthonous/‘intra-basinal’ comprising specific, environment-related particles (i.e. coated bubbles and rafts)and normal allochems, while particles of allo-chthonous/‘extrabasinal’ provenance are repre-sented by windblown or flooded skeletalremains of organisms (macrophytes, vertebratesand invertebrates) accidentally trapped in thecarbonate deposits but alien to the thermalecosystem.

Coated bubblesCoated bubbles are preserved as hollow sphe-roids with a microcrystalline thin coating(Schreiber et al., 1981; Chafetz & Folk, 1984;Chafetz et al., 1991) commonly followed bywell-developed microbial overgrowths. Coatedgas bubbles, strictly are not grains but maybehave as such and are found as isolated parti-cles (Fig. 10A) in lime-mud matrix (Fig. 10B),entrapped in microbialites (Figs 4E, 5G, 6E and10C) or associated with rafts (Figs 2F and 10E)and other grains. The hollow interior of thesebubbles may later be infilled with allochthonousinternal sediment (Fig. 10D).Depositional conditions: Gas bubbles grow in

low energy sites: on side-embayments of pans,along the edges of drainage streams or in associa-tion with floating rafts. These bubbles nucleateon water or microbial surfaces or within waterdroplets (Schreiber et al., 1981), are rapidly coatedby microcrystalline calcite (Chafetz et al., 1991),may be transported by flood currents (Fig. 4A) andcan be entrapped in microbial frameworks wherethey commonly develop thick microbial over-growths (Figs 4A, 5G, 6E, 7F, 10B and 10C).

Microbial raftsMicrobial rafts are commonly found in smalllenses, often associated with coated bubbles(Fig. 10G), entrapped within the microbialframework or scattered in the lime-mudstonematrix (Figs 2F and 10E).Depositional conditions: In active thermal sys-

tems, the floating rafts formed at the surface ofthe water body, may sink and be embeddedwithin the bottom sediment or be transported byoccasional currents to accumulate downstream.

IntraclastsAngular unsorted clasts of the underlying traver-tine lithofacies are commonly embedded at thebase of silty lime-mudstone beds (Fig. 9F). Thecrust fragments resulting from fractures alongthe bedding planes are locally associated withtepee-like structures. Evidence of transport,selection or accumulation is uncommon, withmost of the clasts found incorporated into thepalustrine lime-mudstone or slightly displaced.Depositional conditions: In active thermal

systems, intraclasts are commonly found as in-place residual material at the bottom of dry pud-dles, pans, streams or on inactive flow surfaces.The mechanical processes of fragmentation mayderive from desiccation (Koban & Schweigert,



1993) or syn-sedimentary tectonic fracturation,or advanced karstic dissolution during ratherlong periods of non-deposition.

Coated grainsOncoids/ooids locally occur either as unstruc-tured, snowball-like grains (bacterial pisoliths:Guo & Riding, 1998), as tangles of microbial fila-ments or with regular concentric laminae (Cha-fetz & Meredith, 1983; Folk & Chafetz, 1983).Concentric oncoids/ooids appear to be formed bywhirlpool currents flowing on terraced slopesand shallow pans or streams (Schreiber et al.,1981).

Organic/skeletal componentsOrganic/skeletal components are rarely found inthe typical depositional facies of the thermalcarbonates. Most of them are allochthonous ele-ments, evidently windblown or flooded in by epi-sodic storms or rainfall and enclosed within thelime-mud of the basins or in the primary laminarcrusts. Some of the organic particles found in thetravertine facies can be considered autochtho-nous but related to either distal or interthermalplustrine episodes (silty lime-mudstone beds) orto later karstic processes developed within arestricted area around open fractures of the trav-ertine main body. The occurrence of these parti-cles suggests a close relation of life withatmospheric and karstic waters rather than hydro-thermal conditions.

Faecal pelletsFaecal pellets are locally found accumulated atthe bottom of geopetal cavities, probably repre-senting the residence of some cave dwellingorganism. Ostracod carapaces are found as arti-culated or disarticulated valves scattered inthe matrix of the pink internal sediment, orconcentrated in nests of moults in open frac-tures.

Gastropod shellsGastropod shells of terrestrial/pulmonate forms(aquatic species have not yet been recorded intravertine/thermal water environments) evi-dently floated within active vent pools.

VertebratesVertebrates are found locally as disarticulatedskeletal parts scattered in the lime-mudstonethat infills rather large pools; they probably dieddue to inhalation of CO2 or their carcasses weretransported into the basin during floods.

Vegetal remainsVegetal remains represented by casts of macro-phyte parts, such as leaves and seeds or mouldsof encrusted stems (calcite straws) or woodfragments (branches/stumps), may be found pre-served in laminar crusts while very poor dwarfcharophyte (Fig. 9H) and pollen assemblages(Capezzuoli et al., 2011) occur within the siltylime-mudstone layers associated with thecruststones.

Terrigenous detrital quartzTerrigenous detrital quartz and phyllosilicategrains, likely to have been introduced by thewind or episodic floodings, are frequentlypresent in the palustrine silty lime-mudstone.

POST-DEPOSITIONAL PROCESSES

The post-depositional history of thermal carbo-nates is recorded mainly by syngenetic or earlydiagenetic processes, comprising: (i) sparmicriti-zation in the crystalline facies; (ii) cementationcommonly well-developed in the granular facies(intergranular porosity) and, more incompletewithin the large interlaminar spaces of themicrobialites; (iii) shrinkage and soft sediment

Fig. 7. Microbialites, cruststones, lamellar puff pastry-like fabric, flat and curled microbial laminites. (A) and (B)Bundles of flat and curled microbial crusts associated with thin lenticular beds of spongy microbial mat and ofdark lime-mudstone (B). The latter appears to be deformed in proximity of the folds of the underlying sheets. (C)and (D) Microbial laminites made of single or multiple organic films curled/folded to form large, open interlami-nar cavities lined/reinforced by calcite cement. The folds apparently derive from diapiric deformation/movements.(E) and (F) Irregularly superposed, flat laminae (E) made of thin, cement-supported microbial filaments with pen-dant dendrolites locally including coated gas bubbles (arrow) (Oliviera Quarry, Rapolano. Siena, Italy). (G) and(H) A possible model for the formation of curled laminites: lime-mudstone mixed with microbial mucilage on thefloor of an actively evaporating terrace pan, rapidly assumes a leathery consistence and, probably as a result ofsyneresis processes, forms sheets that detach in flakes of variable size [in (H) ca 15 cm large] from the underlyingsediment as a result of swelling, cracking and curling (Karahayit spring, Denizli, Turkey).



A B

C D

E F

G

H



deformations in lime-mudstone facies; and (iv)pedogenetic elaboration of silty mudstones.Post-depositional processes comprise aragonite/calcite neomorphism; fracturing of the brittletravertine crusts by desiccation (tepee-like struc-tures) or extensional tectonics and relatedkarstic processes.

Early diagenetic processes

SparmicritizationSparmicritization produces more or less thickbrown/rusty organic-rich microcrystalline calcitebetween consecutive rows of crystals in crystal-line crusts (Jones & Renaut, 2008; Pola et al.,2013). This feature involves short periods offlow discontinuity, promoting the start of micro-bial colonization and overgrowths on the crystalgrowth surfaces.

CementationCementation is poorly developed and mostlyrevealed by scanning electron microscope (SEM)analysis (Jones & Renaut, 2008) because primaryporosity in the compact crystalline facies isgreatly reduced. Conversely, the microbial crustsexhibit a very high porosity represented by a vari-ety of framework and structural cavities of anyshape and size (Figs 5 to 8). The small frameworkpores of spongy bindstones and dendrolites arealmost completely occluded by mosaics of fine-grained sparite, whereas the large fenestral/shel-ter-like structural cavities are commonly emptyand only lined with poorly developed geodiccements (Figs 7B, 8A and 8B) and/or partiallyfilled with geopetal speleothems. Geodic cementsformed by superposed rims of gothic arch, trigo-nal or euhedral rhombohedral calcite (Folk et al.,1985) contribute in the transformation from softorganic sheets in to hard microbial laminae.Therefore, the crusts appear to result from theultimate evaporation of the residual, hypersatu-rated connate fluids entrapped within the openspaces of the sediment.

ShrinkageShrinkage structures in the mudstone facies arerepresented by cracks generally of trilete shape,the arms and crevices of which are lined bymicrobial overgrowths (Fig. 9B). This type ofstructure, that can be related to syneresis-induced shrinkage processes on a still supplemicrobial/lime-mud blanket, attests to the acti-vity of microbes where warm water persisted onthe bottom of the pan.

Soft sediment deformationSoft sediment deformation, represented by loadstructures (pseudo-nodules, diapiric uplifts andboudinage), is locally well-developed (Fig. 11E)in the lime-mudstone facies, suggesting fluidifi-cation/liquefaction processes in unconsolidatedlime-muds. Whatever triggers the syngeneticfluid migration in the thermal restricted basins,these structures cannot be ascribed to load-induced compaction as is normal for the marinedomain. Other factors, more inherent to themechanics of the thermal system and capable ofstarting convective movements in dense lime-mud, can be detected in gas escape pulses, asobserved in some active spring mouths (Polaet al., 2013). This interpretation is supported bythe frequent occurrence of fenestral cavities withextended beady ‘necks’ suggesting the develop-ment, at smaller scale, of gas escape conduitswithin semi-consolidated sediments (Fig. 11F).

Karstic processes

In many old travertine bodies an irregularlydeveloped karst network is commonly estab-lished after open fractures (Fig. 11A and B) asso-ciated with unconformable surfaces (Fig. 11A).Evidence of circulation, dissolution, cement pre-cipitation and geopetal infilling of internal sedi-ment is restricted to the immediate proximity ofthe fractures (Fig. 11A and B). Some of the frac-tures show a very immature karstic elaborationand are filled with the dark, silty lime-mud-

Fig. 8. Microbialites – cruststones. Flaky puff pastry-like fabric. (A) and (B) loosely superposed thin sheets orlarge flakes made of bundles of very thin dense laminae (B). The intervening, large lenticular shelter-like spacesare still open and lined with thin coatings of cement that occlude only the small framework porosity (whitearrow) and the narrow connections among the large cavities (black arrow) (Oliviera Quarry, Rapolano, Siena,Italy). (C) and (D) Different types of puff pastry: the flaky Turkish B€orek (C) and the lamellar Neapolitan Sfogliatel-la (D). (E) and (F) contorted micritic sheets/flakes form rosettes (E – San Giovanni Terme, Rapolano, Italy) of densemicrite (F – Messinian travertine, Pignano, Volterra, Italy). (G) and (H) Accumulation of hard micritic sheets andbroken flakes. (G – San Giovanni Terme, Rapolano, Italy. Coin for scale is ca 2 cm wide) similar to those formedby desiccation at the bottom of a terrace pan (H – Pamukkale apron, Turkey).



A B

C D

E

G H

F



A B

CD

E F

G H

Fig. 9. Lime-mudstones. (A) and (B) Compact fine-grained limestone with scattered small patches of porousmicrobialite and black seams probably produced by compaction. (B) Fenestral micritic lime-mudstone with trilete-like cracks that are mostly open cavities (arrow) lined with microbial overgrowths (Oliviera Quarry, Rapolano, Sie-na, Italy). (C) and (D) Not much loose lime-mud sediment on the hard floor of the pan is available. Person forscale in (C) is ca 1�6 m tall. The micrite mudstone is pitted by gas escape pores heralding a fenestral fabric (D –Karahayit spring, Denizli, Turkey). (E) Microsparitic mudstone with microbial clots. (F) to (H) Dark, silty, paludallime-mudstone rests on an unconformable surface resulting from desiccation and fracturation of the exposedunderlying crystalline crust. The intraclasts enclosed in the matrix show no significant translation. (G) and (H)Microsparitic, peloidal mudstone with soil concretions that destroy the depositional fabric (G) and remains ofostracods with closed valves and Chara stems (H) (Oliviera Quarry, Rapolano, Siena, Italy).



A B

C D E

F G

H I

Fig. 10. Coated bubbles and microbial rafts as granular components. (A) Partially broken, encrusted bubblestransported and stranded as grains on the shores of an ephemeral discharge stream (C�ukurbag, Pamukkale, Tur-key. Hammer for scale is ca 40 cm long). (B) to (E) Bubbles coated by a microbial frame, within a small mud-stone lens (B), a microbial mat (C) and partially infilled by siliciclastic fine sand (D). (E) Small lenses and thinlayers of bubbles and paper-thin rafts associated with fan/ray crystalline crusts and microbial mats. (F) Jaggedraft lined only on one surface by teeth-like microbial protuberances. (G) Raft with a microbial overgrowth onboth sides of a very thin crystalline axial core (Oliviera Quarry, Rapolano. Siena, Italy). Pancake rafts: (H) Largeirregular rafts made of microbial colonies and large encrusted bubbles hold together by EPS mucilage (seeFig. 5E), floating on the surface of a terrace pan (Karahayit spring, Denizli, Turkey). (I) Spongy microbial struc-tures enclosing bubbles that can be interpreted as pancake rafts, floating in lime-mudstone (Oliviera Quarry,Rapolano, Siena, Italy).



stone, evidently percolated from the overlyingunconformable paludal sediment (Fig. 11A andB); otherwise they are filled with multiple gener-ations of internal sediments (Fig. 11C) or locallyby faecal pellets, bladed prismatic palisadecement and micro-stalagmites made of brownand lighter layers of calcite, lining the floor ofthe cavities (Fig. 11C and D). These fractures arecommonly followed by polyphase infillings oflime-mudstone of different provenances that canbe discriminated by the colour, texture, compo-sition and, locally, by siliciclastic or fossil con-tent (Fig. 9H). The lime-mudstone internalsediments found within dissolution cavitiesand/or subvertical fractures include micrite, mi-crosparite, carbonate silt (allomicrite) commonlystained red by iron oxides and fine siliciclasticsand. These sediments may contain nests ofpeloids (faecal pellets) or of articulated valves,or moults of ostracods, wood frustules, wholeshells or fragments of gastropods.

DISCUSSION

Carbonate deposition in a thermal spring systemreflects the characteristics of the geothermal/tectonic regime that provides warm to hothypersaturated alkaline–sulphate waters result-ing from a deep hydrothermal circulation into avolcanic and/or carbonate/sulphate-rich bedrock.The thermal spring system is a complex environ-ment governed by physicochemical and hydrody-namic proprieties that, when combined, make thethermal depositional conditions different fromthose of other marine and continental carbonates.Depositional processes in the thermal system

depend not only on the temperature of the waterbut are also constrained by its Ca-hypersatura-tion, the volume and regularity of discharge andthe mechanics of the fluxes, as well as the pres-ence of sulphur compounds that, on the path ofthe thermal floods, restrain the development ofmacrophyte vegetation and life of animal orga-

nisms. These factors come together to producetravertine: characteristically well-bedded andfinely laminated limestone that cannot be con-fused with karst-derived speleothems or calca-reous tufa. Unlike most marine and somecontinental carbonates, the thermal deposits areprecipitated mainly as crusts, while the loosegranular sediments (lime-muds/lime-sands) pre-vailing in marine, lacustrine and palustrineenvironments are definitely under-represented.The distinctive, depositional facies of thermalcarbonates are related to different genetic pro-cesses, mostly leading to nearly instantaneouslithification. Three depositional groups, eachrelated to specific depositional niches, havebeen detected: (i) abiotic crystalline crusts com-prising banded palisade crystals grown in hypo-gean phreatic environments underlying springvents; fan or ray crystal crusts and feather crys-tal crusts, precipitated from subaerial, fast run-ning, turbulent (waterfalls) or laminar (slopes)flows in evaporative/cooling regimes; (ii) micro-bialites or microbially mediated crusts compris-ing bindstones (thrombolitic and agglutinatedmicrobial mats), dendrolitic fabrics (shrubfacies) and cruststones (flat to curled and puffpastry-like laminites) developed in shallowslack to still waters in terraced pans or ephem-eral ponds; and (iii) granular sediments, mainlyrepresented by microbial lime-mudstone com-monly with clotted fabric and affected by softsediment deformations and silty mudstone withdiffused quartz grains, pedogenic concretions,remains of in situ organisms (ostracods, gastro-pods and Chara) and intraclasts, that reflectrain-diluted waters in palustrine settings com-monly located on distal unconformable surfaces.The rate of precipitation induced by the turbu-

lent or laminar motion of water flux and con-trolled by vaporization/degassing, appears toregulate the morphology and size of the crystalsforming the abiotic crystalline crusts: the turbu-lent flow in channels and waterfalls deposits fanor ray botryoidal bodies, while microterraces

Fig. 11. Karst features. (A) A karstic dissolution surface marked by residual terra rossa and pink, silty, paludallime-mudstone, records an interval of non-deposition of the thermal carbonates (San Casciano dei Bagni, Siena,Italy). (B) Poor karstic elaboration of a tensional fracture filled by silty lime-mud percolating from an overlyingunconformable palustrine basin (Oliviera Quarry, Rapolano, Siena, Italy); note in (A) and (B) that the travertineadjacent to the permeable fractured zones appears to be unaffected by karstic circulation/staining. (C) and (D) Dis-solution cavities carved in slightly altered microbial travertine, floored by palisade cements that evolve in smallstalagmites, and locally infilled (C) with multiple phases of geopetal internal sediments. (E) Soft sediment defor-mation of the silty lime-mudstone infilling a dissolution cavity. (F) Gas escape structures in the internal sedimentof a cavity cut in the primary microbial frame (Oliviera Quarry, Rapolano, Siena, Italy).



A B

C

D

E

F



associated with feather-like crystals develop fromlaminar flows running on smooth slopes. On thecontrary, the rate of carbonate precipitation fromslack to still waters in very shallow terraced pansor ephemeral ponds appears to be controlledmainly by evaporation and desiccation promotinghypersaline conditions that enhance the develop-ment of microbial communities on the bottom ofthe basin and, simultaneously, the precipitationof lime-mud on the microbial sheets or mats. Con-tinuous replacement of the evaporating fluid bywater sheets slowly flowing in the pans results inthe superposition of microbial sheets and theirrapid hardening, due to the growth of crystallinecement on the surfaces of the open spaces. Thedevelopment of microbial sheets contorted bysyneresis processes gives rise to the irregularsuperposition of laminites with a peculiar puffpastry fabric marked by a fenestral porosity withlarge-scale and small-scale cavities that are partlylined, but never occluded, by cements most prob-ably derived from residual connate fluids. There-fore, the limited syn/diagenetic circulation offluids in the very porous microbialites results inpartial cementation, mostly concentrated in thethin connections and passages between cavitiesthat makes the travertine impervious to furtherwater circulation. Consequently, the later, poorlydeveloped karstic water circulation in the other-wise impermeable travertine advances only in theimmediate proximity of the fractured zones.

CONCLUSIONS

The results of a detailed investigation supportedby a comparative petrological analysis of thepresent/active depositional processes in thermalspring systems and of the lithofacies of traver-tine bodies exposed in adjoining sites providecriteria for the univocal genetic identification oftravertine the ‘thermogene’ continental lime-stone, associated worldwide with hydrothermalspring systems in extensional tectonic regimes.The geothermal affinity of the waters depositingthe travertine limestone is substantiated by thecharacteristic positive values of d13C supportingthe recycled, marine/evaporitic provenance ofcarbon, unrelated to the meteoric/biochemicalactivity of vegetation and soils (Gandin & Cape-zzuoli, 2008).The distinctive depositional facies of these car-

bonates, presently deposited in numerous nichesaround thermal springs, were recognized in lime-stone bodies as the components of facies associa-

tions dominated by irregularly alternated,lenticular units of abiotic crystalline laminites,microbialites and the less represented mudstone/granular facies. Most of the typical travertinelithofacies result from fast depositional processesand immediate lithification, related to flows ofhypersaturated alkaline, sulphurous geothermalwaters. Both the compact crystalline crusts andthe largely porous microbialites, despite theirmanifestly different porosity, are characterizedby a generally low permeability. The crystallinefacies appear to result from dominantly abioticprecipitation, while the primitive/prokaryoteorganisms which are able to live in the warmsulphurous waters are involved mainly as pas-sive support in the formation of microbial lami-nites. All of these features, associated with therarity of remains of organisms that are mostlyalien to the depositional system, reflect the pecu-liar depositional setting of the thermal springsystem.

ACKNOWLEDGEMENTS

B. Charlotte Schreiber, Elaine Richardson,Stephen Rice, Peter Swart, Tracy Frank andAgustin Martin Algarra, made pertinent sugges-tions and language ameliorations that improvedthe final version of this paper. E. C. is pleased toacknowledge a P.O.R.-F.S.E. 2007–2013 (Regio-nal Competitiveness and Employment) grantfrom the Tuscan Regional Administration. Thesubstantial contribution of Barbara Terrosi inthe assemblage of figures is greatly appreciated.

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Manuscript received 6 August 2012; revision 17 June2013; revision accepted 29 October 2013



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