Comparison of the Quaternary travertine sites in the Denizli extensional basin based on their depositional and geochemical data

which formed from the hotter water, is more widespread than the tufa that formed in the cooler spring waters. Deposition of the travertine, which formed largely on slopes, in depressions, and along ﬁ ssure ridges (mostly on northern basin margins), was controlled by the interplay between various intrinsic and extrinsic parameters. The travertines are formed largely of calcite with only minor amounts of aragonite in some of the vertically banded, crystalline crust, raft and pisoid travertines found in some of the northern sites. The aragonitic samples, rich in Sr, are typically found around the spring ori ﬁ ces and along the central axis of the ﬁ ssure ridges. The stable isotope values of the travertine found in the northwest and southeast parts of the basin are different. The δ 13 C values of the northern travertine deposits are more positive (3.7 to 11.7 ‰ VPBD) than those found in the south – southeast areas ( − 4 to 5.8 ‰ VPDB). In contrast, the travertine and tufa in the southeastern areas have higher δ 18 O values ( − 15.2 to − 7.8 ‰ VPDB) than those of the northern areas ( − 16.6 to − 4.8 ‰ VPDB). Available evidence indicates that spring activity and associated travertine precipitation in the Denizli Basin were controlled largely by tectonic activity rather than by climatic conditions. © 2013 The Authors. Published by Elsevier B.V.

Many studies on travertine have been based on a single locality (e.g., Fouke et al., 2000;Kele et al., 2008Kele et al., , 2011) ) and little attempt has been made to integrate data from numerous springs into one model.Notable exceptions to this general assessment include the studies by Chafetz and Lawrence (1994), Minissale et al. (2002), andSant'Anna et al. (2004).Chafetz and Lawrence (1994), for example, integrated information from six calcareous spring systems in U.S.A. and Italy to determine the relationships between the stable isotopic composition of the calcareous spring deposits (i.e., travertine and tufa) and waters from which they were deposited.Similarly, Minissale et al. (2002) demonstrated that there were consistent differences in the geochemical and isotopic signatures of modern and late Quaternary travertines, thermal spring waters, and gas vents on the east and west sides of the Tiber Valley in central Italy.
Travertine deposits are commonly associated with normal or transtensional faults (e.g., Hancock et al., 1999;Brogi and Capezzuoli, 2009).The depositional architecture and geochemical characteristics of travertine deposits are controlled by the balance between extrinsic factors (e.g., climate, spring water composition) and intrinsic factors (e.g., spring flow patterns, biota).Separation of the extrinsic and intrinsic factors can be achieved by comparing different spring deposits found in one basin.The Denizli extensional basin in western Anatolia, Turkey (Fig. 1A) is ideal for this purpose because it contains numerous carbonate spring deposits (mostly travertine), particularly along its northern boundary (Altunel and Hancock, 1993a;Çakır, 1999;Özkul et al., 2002;Uysal et al., 2009;Özkul et al., 2010;Kele et al., 2011).Accordingly, this paper describes and compares the depositional, mineralogical, and geochemical features of six travertine deposits found in the Denizli Basin with the view of establishing the intrinsic and extrinsic factors that controlled their development.Conclusions reached from these comparisons can provide the basis for evaluating similar deposits throughout the world.
The Neogene basin fill and old bedrock that underlie the travertine deposits are exposed on the graben shoulders and in the mountainous horst areas (Fig. 1).The bedrock in the main catchment areas, exposed on Honaz (2571 m asl) and Babadağ (2308 m asl) in the south and Çökelez (1840 m asl) in the north, is composed of schists and marbles that cover the Menderes Massif (Bozkurt and Oberhänsli, 2001;Erdoğan and Güngör, 2004) and the allochthonous Mesozoic limestone, dolomite, and gypsum of the Lycian Nappes, which tectonically overlie the Menderes Massif (Okay, 1989).
The Neogene fill in the Denizli Basin is formed from alluvial, fluvial and lacustrine deposits.The basin was initiated as a half graben in the late Early Miocene when deposition gradually evolved from alluvial fan settings into fluvial deposits and finally into lacustrine environments (Alçiçek et al., 2007).Deposition continued until the Late Pliocene.By the early Quaternary, there was a change in the regional tectonics and the Neogene Denizli half graben became a full graben as the Pamukkale Fault on the north margin became active (Fig. 1B, C).Quaternary deposits are evident on local fluvial terraces that become progressively younger towards the basin centre, and in the form of large travertine deposits that are widespread along the northern and southern margins of the graben.The Quaternary faults and fissures that are common in carbonate bedrock along the margins of the graben are natural pathways that allow meteoric waters to descend into the subsurface and hydrothermal fluids to come to the surface (Minissale et al., 2002;Dilsiz, 2006).These faults and associated fissures cut through the travertine bodies in some localities (Altunel and Hancock, 1993a,b;Van Noten et al., 2013).
The Denizli Basin is important in terms of its seismic activity as well as its geothermal potential (Aydan et al., 2005;Tan et al., 2008;Utku, 2009).The ancient city of Hierapolis at Pamukkale, for example, was damaged several times by earthquakes (Altunel and Barka, 1996;Piccardi, 2007) with magnitudes up to 6.0 (Hancock et al., 2000) that were triggered by normal faulting and extension of the basin (Kaypak Fig. 1 (continued).and Gökkaya, 2012).Most of these earthquakes had focal depths of 5 to 15 km.Local seismicity of the Denizli Basin strongly depends on the deep and shallow geothermal systems in the region (Kaypak and Gökkaya, 2012).

Sampling and analytical procedures
The travertine exposed at Gölemezli, Pamukkale-Karahayıt (including Çukurbağ and Akköy), Ballık, Kelkaya, Kocabaş and Honaz in the Denizli Basin was examined and sampled between 2006 and 2009.These sites are located in different parts of the basin.The quarry faces provided excellent opportunities for establishing the three dimensional depositional architectures of the deposits.Approximately 65 travertine samples were collected from these sites for mineralogical, petrographical, and geochemical analyses (including stable isotopes), and radiometric dating.
In situ measurements of temperature (T), pH, Eh, and electric conductivity (EC) were carried out using a Hach-Lange HQ40d instrument.The free CO 2 and alkalinity analyses were performed in the field using titrimetric methods.The ions (SO 4 , Cl, Ca, Mg, Na, K) and SiO 2 were analyzed with ion chromatography (Dionex ICS-100) and spectrophotometric (Hach DR4000 UV/Vis) methods, respectively.Sr analyses were performed with Optima 2100 DV ICP-OES instrument at the Water Chemistry Laboratory at Pamukkale University, Denizli.Water samples were collected in 100 ml glass bottles for stable isotope analyses of δ 18 O and δD.Stable isotope compositions of the waters were determined at the Institute for Geochemical Research of the Hungarian Academy of Sciences (Budapest).
The travertine samples were examined by optical microscope using polished thin sections.Thirteen fracture samples, selected from different sites and various lithotypes, were mounted on stubs and sputter coated with a thin layer of gold before being examined on a JEOL JSM 6490 LV scanning electron microscope (SEM) at the Turkish Petroleum Corporation (TPAO) in Ankara, Turkey.The mineralogical composition of 48 samples was determined by the X-ray powder diffraction (XRD) technique using a Philips PW 1730 diffractometer located at the Institute for Geological and Geochemical Research (Budapest, Hungary) with CuK α radiation at 45 kV and 35 mA.For this purpose, randomly oriented powders obtained from fresh unweathered samples were used, and semi-quantitative mineral composition was determined according to the modified method of Bárdossy et al. (1980).For some samples, layers formed from different materials were analyzed separately.If a sample was too small for normal preparation, it was attached directly to a steel slide for analysis.Estimates of the amount of aragonite and calcite in each sample produced by these analyses have a 5-10% error margin.
Cathodoluminescence examination was performed at the Institute for Geological and Geochemical Research, Hungarian Academy of Sciences (Budapest, Hungary) using a Reliotron type cold-cathode equipment that is attached to a Nikon Eclipse E600 optical microscope.The equipment was operated at 8 to 10 kV accelerating voltage with a 0.5 to 1.0 mA current.Photographs were taken with a Nikon Coolpix 4500 digital camera using automatic exposure and a defocused electron beam.
Elemental determinations were completed for 56 samples of recent (i.e., actively forming) and old travertines that came from different sites in the Denizli Basin.These analyses, done by the Acme Analytical Laboratory (AcmeLabs, Vancouver, Canada), utilized inductively coupled plasma mass spectrometer (ICP-MS) techniques.The total abundances of the major oxides and several minor elements are reported on a 0.1 g sample analyzed by ICP-emission spectrometry following a lithium metaborate/tetraborate fusion and diluted nitric digestion.Loss on ignition (LOI) is by weigh difference after ignition at 1000 °C.The TOT/C and TOT/S were determined by Leco.The determination of element composition was performed also by ICP-MS technique, following a lithium metaborate/tetraborate fusion and diluted nitric digestion.
Stable carbon and oxygen isotope values were obtained from 65 samples at the Institute for Geological and Geochemical Research, Hungarian Academy of Sciences, Budapest, Hungary.Carbon and oxygen isotope analyses of bulk carbonate samples were carried out using the conventional phosphoric acid method (H 3 PO 4 digestion method at 25 °C) of McCrea (1950) and the continuous flow technique of Spötl and Vennemann (2003).Standardization was conducted using laboratory calcite standards calibrated against the NBS-18, NBS-19 standards.All samples were measured at least in duplicate and the mean values are given in the standard delta notation in parts per thousand (‰) relative to VPDB (δ 13 C) and VSMOW (δ 18 O) according to δ[‰] = (R sample / R reference − 1) × 1000.Reproducibility is better than ± 0.1‰.

Results
In this section, the travertine sites are described with age determination (Table 1) and the results of water chemistry (Table 2), mineralogical composition (Table 3), and geochemical signatures (Table 4).

Gölemezli (site 1)
This fossil site, located ~4 km northwest of Gölemezli village, is found at elevations between 350 and 450 m on a southwest facing hillside (Fig. 1A).The deposit, which is ~35 m thick over an area of b1 km2 , is located at the west end of the NW-SE trending Akköy fault (Çakır, 1999).This fault segment forms the boundary between the Neogene deposits and metamorphic bedrocks.The volume of travertine is estimated to be ~0.035km 3 .
Spaces that developed along the vertical to subvertical fault planes and extensional fissures in the metamorphic bedrocks have been filled by vein travertines that are up to a few metres thick (Fig. 2A).Multiple generations of emplacement are evident from different veins that commonly cross-cut each other (Fig. 2B, C).Downslope, the light green and honey coloured banded travertines pass into the bedded travertines.Vein travertines from this site yield U-Th ages of 344 ± 18 ka, and 613 ka (Table 1).
The Karahayıt subsite, located northwest of Pamukkale (Fig. 1A,  B), is characterized by thermal water with a temperature of ~53 °C (Table 2) that emerges from a fissure and precipitates red to brown travertine that is covered with microbial mats (Fig. 3A, B).Consequently, the local people call it 'Kırmızı Su' (meaning red water in Turkish).
The Pamukkale travertine, designated as a UNESCO world heritage site (Şimşek, 2003), is characterized by its dazzling white colour.The ancient Roman city of Hierapolis, built on this site, is at 365 m asl.The warm waters (~35 °C) that flow from four springs which are located along the Pamukkale Fault and connected fissures (Fig. 1A, B), are now directed to the slope aprons by a closed concrete channel system (Gökgöz, 1994;Şimşek et al., 2000;Kele et al., 2011).Precipitation The saturation index with respect to calcite and partial pressure of CO from these spring waters (Table 2) has produced the vast white calcite deposits with their spectacular arrays of terraces and rimstone pools (Ekmekçi et al., 1995) (Fig. 3C, D).The white slopes are covered mostly by crystalline calcite, whereas lime mud (Fig. 3E), shrubs, pisoids, rafts, and coated gas bubbles are common in the terrace pools (Özkul et al., 2002;Jones and Renaut, 2008;Kele et al., 2011).
The first U-Th ages from the fossil travertine at Pamukkale, obtained by Altunel (1994Altunel ( , 1996)), ranged from 24 to 400 ka.More recently, a U-Th age of 55.4 ka was obtained from the vertically banded travertines along the scarp of the Pamukkale fault (Uysal et al., 2007).
The Çukurbağ subsite is located near the slope apron, just in front of Jandarma spring, Pamukkale, which lies at an altitude between 250 and 300 m asl, and encompasses a relatively flat to gently concave area that is characterized by several fissure ridges that are variably oriented.The E-W trending Çukurbağ fissure ridge (Fig. 4A), is ~400 m long, 10 m high, and 40 m wide (Altunel, 1994).Past quarrying has exposed the central part of the ridge (Fig. 4A-C).Samples from the vertical bands that fill the central fissure space (Fig. 4B, C) yielded U-series ages that range from ~24.7 to 152 ka (Uysal et al., 2007(Uysal et al., , 2009)).One sample from the banded travertines (Fig. 4B, C), collected during this study, yielded a U-Th age of 25.3 ka (Table 1).
Today, a mesothermal spring with a temperature of 57.1 °C (Table 2) is located at the east end of the Çukurbağ ridge.Travertine is presently being precipitated in small, shallow pools (a few metres wide) and narrow channels that are located around the spring orifice.Calcite rafts and coated gas bubbles around the pool margins are associated with thermophilic microbial mats (Fig. 4D, E).
The Akköy subarea, situated ~1 km southeast of Akköy town at ~250 m asl and southwest of the Karahayıt-Pamukkale travertine plateau, is dominated by a large fissure ridge (Figs. 1, 5A).The northwest trending ridge, 1400 m long, 40 m high, and up to 800 m wide at its base, is also known as 'Karakaya Hill' or the 'Akköy fissure ridge' (Altunel, 1994).A depression lies between the ridge and the Karahayıt subarea to the north.Recent quarrying provides a three-dimensional view of the structure and depositional architecture of the Akköy ridge (Fig. 5A-C).Secondary fissures are associated with the main fissure (Altunel, 1994).The fissure spaces in the ridges are filled with vertically banded/vein travertines (Fig. 5B), which become younger towards the central part of the fissure space (Altunel et al., 1993a,b).Some veins bifurcate towards the surface and cross cut the older bedded travertines (Uysal et al., 2009).Evidence of repeated fracturing and filling events are apparent in some of the fissures (Fig. 5C, D).Bundles of raft flakes, earlier considered as bedded travertine (Uysal et al., 2009, their Fig.2E), red mudstones rich in Fe-oxide, and angular clasts of older travertine and metamorphic bedrocks are found in some of the fissures (Fig. 5C-E).The white, horizontal ledges (Fig. 5E) formed when the fissure spaces were filled with thermal waters.Some rafts were thickened due to subsequent encrustation (cf., Guo and Riding, 1998) whereas other rafts are now in vertical to subvertical positions.Bedded travertine, which dips at 10-20°away from the ridge crest, is present on both sides of the fissure axis (Fig. 5A, B).These beds are formed from alternating layers of crystalline shrubs, rafts, micrite, and coated gas bubbles (Fig. 5F-H).Palaeosols, up to 60 cm thick, and lithoclast interbeds are found in some parts of the succession.A sample from the uppermost part of the fissure ridge gave a U-Th age of 18.4 ± 0.3 ka (Table 2).
The Pamukkale travertine plateau, which includes the Karahayıt, Çukurbağ and Akköy subsites, has an average thickness of 50 m and covers an area of 11.8 km 2 .Thus, the volume of travertine is estimated to be about 0.6 km 3 .

Ballık (site 3)
Ballık, located ~5 km northwest of Kaklık, has travertine deposits that are exposed on a stepped southwest facing slope (Fig. 1A, C) that is 500 to 1000 m asl.It is the largest travertine site in the basin with travertine up to 120 m (average 75 m) thick that covers an area of 12.5 km 2 .The volume of travertine at this site is approximately 0.94 km 3 .The travertine deposit is dissected by northwest-trended normal faults and extensional fractures.
Today, there are about 50 quarries operating in this area.The cut surfaces in the quarries allow investigation of the spatial architecture of the depositional settings that developed as the travertines were being deposited.In the Ballık area, the travertine sequences are formed mostly of horizontally to subhorizontally bedded travertine, particularly in the lower and middle parts (Fig. 6A, B), that extends laterally for a few hundred metres.These successions are formed mainly of thin shrub layers, micritic laminae, coated bubbles, and locally included reed casts.Throughout the successions exposed in the quarries there are intercalations of fluvial conglomerate, sandstone, and red and green mudstone (Fig. 6A, B), palaeosol horizons, and erosional surfaces (Özkul et al., 2002).At Killik Tepe, south of this area, the horizontally bedded travertines at the base pass gradually upward into the low angle slope travertine that, in turn, passes upward into the tufa facies at the top (Fig. 6C, D).The slope facies is composed mainly of crystalline calcite, shrub, and micritic layers that are locally cut by erosional surfaces.
The uppermost tufa horizon, which can be followed laterally for several hundred metres, is formed from numerous mound and waterfall build-ups, each of them are convex upward and dominated by reed and bryophyte tufa.The mound facies, up to 25 m thick, typically have high porosity due to the presence of big cavities of a few metres in diameter and reed cast moulds (Fig. 6C, D).

Kelkaya (site 4)
This travertine at this site, located near the village of Aşağıdağdere, is at the foot of the north facing slope of the Kelkaya Hill at elevations between 520 and 750 m asl (Fig. 1A).Kelkaya Hill is formed from Mesozoic limestone bedrock.The travertine is ~50 m thick and covers an area of 0.75 km 2 .The estimated volume of travertine is 0.04 km 3 .The spring waters arise along a normal fault plane that dips to the north.Waterfall tufas with porous textures dominate the upper part of the site (Fig. 7A).In contrast, the horizontally bedded and low-angle slope travertines that dip to the north cover the lower part of the area (Fig. 7A).The terrace pools and their rims are clearly evident on the vertical wire-cut surfaces (Fig. 7B).In the quarry, the uppermost bench (~10 m high) is separated from the lower benches by a palaeosol and green claystone horizons.

Kocabaş (site 5)
Travertines around the villages of Kocabaş and Gürlek, herein referred to as the Kocabaş site (Fig. 1A, C), are located on a relatively flat area that lies between 370 and 495 m asl.In this area, the travertine is ~40 m thick and covers an area of ~33.7 km 2 .The estimated volume of travertine is about 1.0 km 3 .Highway 320 (from Denizli to Afyon) passes through the site.The area, bounded to the north by the Ballık site, is formed mainly of several NW-trending inactive fissure ridges that are flanked by bedded travertines (Özkul et al., 2002;Altunel and Karacabak, 2005).The main fissures in the ridges are filled by vertically banded travertines that are up to 16 m thick.Based on the observation from the quarry faces that are perpendicular to the main fissure axis, the bedded travertines (Fig. 8A), with thickness up to 50 cm, dip away at angles of 5-10°.Within the bedded travertine, bedding-parallel crystalline layers, up to 60 cm thick (Fig. 8A, B), are formed almost entirely of calcite and/or aragonite (Table 3).The bedding-parallel crystalline layers (or veins) are commonly brecciated (Fig. 8B, C) and similar to the 'jigsaw puzzle' or 'crackle textures' found in the Akköy fissure ridge near Pamukkale (Uysal et al., 2009).In the Kuşgölü fissure ridge, immediately north of Highway 320, vertical and horizontal veins cut the bedded travertines and each other many times (Fig. 8E, F).U-Th dates obtained from these vertical and horizontal veins yielded ages of 90.5, 95.1, and 144.9 ka (Table 1).Thermoluminescence dates of 330 ± 30 and 390 ± 40 ka (Özkul et al., 2004) and U-Th dates of 105 ± 9.8 to >400 ka (Altunel and Karacabak, 2005;De Filippis et al., 2012) have been obtained from samples collected from this travertine.
Near Gürlek (site 5a), ~7 km west of Kocabaş (Fig. 1A), the horizontally bedded travertines are exposed at ~370 m asl.The travertine benches are composed of light, medium and dark coloured horizons (Fig. 9A) that are locally interbedded with palaeosols, claystone, and mudstones.Some of the medium and dark horizons contain numerous gastropods.Travertine samples from the quarries yielded U-Th ages of 231.5 ka, 128.8 ka, 114.3 ka and 108.7 ka (Table 1).

Honaz (site 6)
This area is (site 6) located at the west end of the east-west trending Honaz Fault, which forms the southern margin of the basin (Fig. 1A, C) and is still seismically active today (Bozkuş et al., 2000).Sites at Değirmenler, Kayaaltı and Karateke are included in the Honaz site.Both travertine and tufa are found at these localities (Fig. 9A-D).At Honaz, the travertine is 30 m thick and covers an area of 2.7 km 2 .The volume of travertine is estimated at 0.08 km 3 .At Kayaaltı (subsite 6a), fossil tufa of Holocene age is exposed on a northwest facing cliff face (Fig. 9B) that is ~20 m high (Horvatinčić et al., 2005).The upper part of this tufa body is formed from the waterfall facies (Fig. 9B), with hanging tufa curtains and primary cavities (cf., Pedley et al., 2003;Özkul et al., 2010).Below the waterfall, the distal slope is covered with subhorizontally bedded detrital tufas.Today, tufa precipitation is restricted to the Değirmenler subsite (Horvatinčić et al., 2005).Older tufa and travertine samples (KT-2, OT-3), collected near Karateke (Fig. 9C), yielded U-Th ages of 519.4 ka and 543.6 ka (Table 1).Two other samples (OT-1, OT-2) from the travertine quarry at the Obruk Tepe nearby Karateke (Fig. 1A) yielded U-Th ages of 164.1 ka and 175.1 ka (Table 1).

Water chemistry
Today, travertine precipitating spring waters belong to four groups (I to IV) based on their physico-chemical features and locations in the basin: (I) Karahayıt and Çukurbağ, (II) Pamukkale, (III) Kaklık cave and Kelkaya, and (IV) Honaz (Table 2).The reservoir rocks for the thermal waters in the Karahayıt and Çukurbağ sites are Palaeozoic marble, whereas the main aquifers supplying warm and mineralized water to the Pamukkale thermal springs are Palaeozoic marbles and Mesozoic limestones (Şimşek et al., 2000).
Thermal waters from the springs and wells discharge at various temperatures.Relatively higher temperature thermal waters (32.9-57.1 °C) arise along the northern margin of the basin at Karahayıt, Pamukkale and Çukurbağ, whereas the waters with lower temperature (18.7-23.7 °C) appear in and around Kaklık cave and Kelkaya to the southeast and Honaz to the south (Fig. 1A, Table 2).
The pH of waters in the study area varies from 6.0 to 7.4 (Table 2) and increases up to 7.8 towards the distal parts of discharge aprons at Pamukkale (Kele et al., 2011).The electrical conductivity (EC) values of the thermal waters vary between 2770 and 3090 μS/cm in Karahayıt and Çukurbağ, 2360-2400 μS/cm at Pamukkale and 1769-1932 μS/cm at Kelkaya-Kaklık.Honaz waters have lower EC values (685-1147 μS/cm).Electrical conductivity, almost all ions, SiO 2 and Sr values of the waters in the basin increase with increasing water temperature (Table 2).The waters in the springs at Pamukkale and Pınarbaşı and associated wells are of the Ca-Mg-HCO 3 -SO 4 type.In contrast, the waters from the Çukurbağ thermal spring, well KH-3 in Karahayıt, Kaklık cave and Kelkaya are of the Ca-Mg-SO 4 -HCO 3 type (Table 2).Basin-wide, the δ 18 O and δD values of thermal and cool waters are between − 9.0 and − 8.1‰ and between − 51.8 and − 58.9‰, respectively (Table 2) (Fórizs et al., 2011).The values indicate a meteoric origin for the waters with residence times of about 20-30 years (Şimşek, 2003).The R/RA and mantle-derived CO 2 are higher in the eastern part of Büyük Menderes Graben (Karakuş and Şimşek, 2013).The δ 13 C analyses from the waters show that the CO 2 required for travertine deposition was derived largely from the decomposition of carbonate rocks (Filiz, 1984), along with a substantial contribution of magmatic CO 2 .

Travertine mineralogy
Based on the XRD analysis of ~50 travertine samples, calcite is the most common mineral at all sites (Table 3).Aragonite and dolomite are, however, also present in the travertines found on the northern basin margin at Çukurbağ (Table 3,Figs. 10,11) and Gölemezli, respectively.Although micrite (grains b 4 μm long) dominate most deposits, the vertically banded travertine, crystalline crusts and secondary pore fills are formed largely of coarse spar calcite (Figs. 11D,12).The white, recently precipitated crystalline crust travertine (Fig. 12A-D), precipitated on the slope in front of Jandarma spring at Pamukkale, is formed mainly of dendritic calcite (Fig. 12D) (Jones and Kahle, 1986; see also Kele et al., 2011).There, minor amounts of aragonite (b1%) are also present on the distal part of the discharge apron.Some samples contain detrital minerals, including quartz, gypsum, mica, goethite, smectite and kaolinite in very small amounts (Table 3).
At Pamukkale and Çukurbağ, aragonite, in varying amounts, is associated with the vertically banded, ooid, and coated gas bubble travertine (Table 3;Figs. 10,11,13).A white, compact and finely crystalline crust sample (ÇB-6) (Fig. 10A) formed entirely of aragonite (Fig. 10A-D) was found adjacent to the orifice of Çukurbağ spring, where mesothermal waters are discharged at a temperature of 57 °C.Some of the vertically banded travertine from the Çukurbağ fissure ridge (Fig. 11A) is formed by fibrous aragonite and rhombohedral calcite (Fig. 11B-D), each being segregated into alternating laminae, as confirmed also by cathodoluminescence analysis.Several aragonite samples from the fine crystalline crust and vertically banded travertines show rosette forms that are radially arranged (Fig. 10B-D).
Some of the samples from Gölemezli contain up to 14% dolomite grains that are up 200 μm long (Fig. 14).The dolomite is typically found with quartz and mica grains (Fig. 14A, B) in mm-scale layers, lenses, and pockets that lie between calcite laminae (Fig. 14A).Some of the grains were removed from their places leaving behind mouldic cavities (Fig. 14C, D).This association of the dolomite with the quartz and mica indicates that it is probably of detrital origin.

Major and trace elements
Based on 55 samples that were analyzed geochemically (Table 4), the Ca concentrations in the travertines range from 329,857 to 405,214 ppm whereas the Mg varies from 120 ppm at Çukurbağ to 8160 ppm at Gölemezli (site 1).Calcite vein travertine at Gölemezli yielded Mg values of 20,940 ppm.The minimum and maximum Sr values in calcite are from Ballık (184 ppm) and Pamukkale (3028 ppm), respectively.Aragonite at Çukurbağ (Pamukkale-site 2), which comes from the vertically banded travertine (ÇB-6) along the central fissure of the Çukurbağ ridge, contains up to 7392 ppm Sr (Table 4).
Fe and Mn values of the calcite are lower in the light coloured travertines (e.g., ~280 ppm Fe, 70 ppm Mn, in the white crystalline calcite travertine precipitated on the slope facies of Pamukkale).The Fe (37,209 ppm) is high in the reddish-brown calcite from the Akköy travertine ridge (Table 4).The Mn content in the calcite is 1471 ppm from the dark brown coloured travertine from Ballık (site 3).In the same site, an abnormal Mn value of 27,640 ppm is from secondary enrichment in a cavity.Although there are some exceptions, basin-wide there is a positive correlation (R 2 = 0.58) between Fe and Mn (Table 4).

Stable carbon and oxygen isotope composition
The stable carbon (δ 13 C) and oxygen (δ 18 O) compositions of the travertine deposits, derived from 66 samples, vary from one site to another in the basin (Table 4, Fig. 15).The δ 13 C values range from − 4.0 to + 11.7‰ PDB, whereas δ 18 O values range from − 16.6 to − 7.8‰ PDB.Along the northern boundary (e.g., Gölemezli, Pamukkale, Çukurbağ, and Akköy) and around Kocabaş, the recent and fossil travertines have high δ 13 C values (+ 3.7 to + 11.7‰).In contrast, the lowest δ 13 C values (− 4.0‰) are from the tufas that are located along the southern boundary (e.g., Honaz) where both positive and negative values of the δ 13 C have been measured.Similar δ 13 C values (− 3.3 to + 0.8‰) were obtained from Kömürcüoğlu quarry, which is located east of the Ballık site (Table 4, Fig. 15).

Variations in depositional setting of the travertine
The depositional architecture of the travertines varies from locality to locality in accord with the conditions that existed at each site when the water was actively flowing.At Ballık, the horizontally bedded travertines, exposed in the lower part of the succession, grade upward into the low angle slope facies and then to the tufa horizon.The uppermost tufa horizon is a dome-like structure formed from numerous mound and waterfall bodies, which have coalesced with each other in all directions (Özkul et al., 2002).The mound facies is comparable to the 'reed mound facies' found in Rapolano Terme, Italy (Guo and Riding, 1998).The waterfall facies is similar to those found at other localities (Pedley et al., 2003;Özkul et al., 2010).The lower part of the succession formed in a shallow lake or depression environment that was fed by warm thermal waters that were modified by rainwater.This part of the succession is comparable to the travertines found at Tivoli, near Rome (Chafetz and Folk, 1984;Minissale et al., 2002;Faccenna et al., 2008), Rapolano Terme in Tuscany, central Italy (Guo and Riding, 1998), and Süttő, which is located on the Danube River, ~60 km northwest of Budapest, Hungary (Sierralta et al., 2010).
The recent travertines at Pamukkale have developed largely on smooth and terraced slope environments that are fed by warm thermal springs (T = ~34.5 °C at the spring orifice).Downslope variations in the thermal waters and associated precipitates are well documented (Ekmekçi et al., 1995;Şimşek et al., 2000;Kele et al., 2011).The slight downslope increase in the δD and δ 18 O water values is attributed to progressive evaporation (e.g., Fouke et al., 2000) and decrease in water temperature.The downslope increase in δ 13 C travertine is linked to the amount of CO 2 degassing (Kele et al., 2011).
Fissure ridges are one of the most prominent features in the region.They have been considered as important tools for tectonic investigations (Altunel and Hancock, 1993a,b;Hancock et al., 1999;Brogi and Capezzuoli, 2009;De Filippis et al., 2012) because local stress directions can be inferred from them (Hancock et al., 1999).At Pamukkale, for example, the east-west-and northwest-southeast-trending fissure ridges are products of local N-S and NE-SW stress directions, respectively.
Along the southern boundary of the basin, travertine and tufa deposits developed along the Honaz boundary fault.Travertines formed in these areas display slope and ridge-like depositional morphologies, whereas the tufa deposits are evident in the paludal and waterfall facies (Horvatinčić et al., 2005).The coexistence of tufa and travertine at adjacent localities at the western end of the Honaz fault probably  resulted from shallow and deeply circulated waters in the fault damage zone (Brogi and Capezzuoli, 2009).

Occurrence of aragonite versus calcite
Precipitation of the calcite and aragonite found in these travertines was controlled by a complex set of interrelated parameters (Jones and Renaut, 2010).Aragonite and calcite precipitation depends on many factors including water temperature, growth inhibitors, supersaturation with respect to CaCO 3 caused by CO 2 degassing and/or evaporation (Chafetz et al., 1991), Sr content (Malesani and Vanucchi, 1975), Mg/Ca ratio (Folk, 1994), and various biological factors (Kitano, 1962;Busenburg and Plummer, 1986;Renaut and Jones, 1997;Pentecost, 2005;Jones and Renaut, 2010;Rodríguez-Berriguete et al., 2012).Based on a survey of spring deposits, Folk (1994) suggested that water temperature and the Mg:Ca ratio are the main controlling factors.Thus, he argued that aragonite is precipitated from waters with a temperature > 40 °C, whereas calcite precipitates when the temperature is b40 °C.There are, however, many exceptions.At Angel Terrace, Mammoth Hot Springs in Yellowstone National Park, for example, aragonite is precipitated at temperatures > 44 °C, whereas aragonite and calcite co-precipitate between 30 and 43 °C, and only calcite precipitates below 30 °C (Fouke et al., 2000).At Egerszalók, Hungary, where the water temperature is ~70 °C, almost pure calcite is precipitated around a borehole (e.g., spring vent), whereas farther downslope, various amounts of aragonite (5-35%) are being precipitated where the water temperatures are 45-50 °C (Kele et al., 2008).Similarly, calcite was precipitated directly from waters with temperatures of >90 °C in Kenya (Jones and Renaut, 1995) and New Zealand (Jones et al., 1996).In natural settings, identifying the factor(s) that control the precipitation of these polymorphs is difficult because all the environmental factors are operating simultaneously.
In the Denizli Basin, calcite is the dominant mineral found at all travertine sites.Aragonite is found in two different settings that clearly illustrate the diverse settings where aragonite can develop.
• Some of the banded travertines, found along fault planes and extensional fissures, are formed from alternating calcite and aragonite laminae.Uysal et al. (2007) showed that the aragonite and calcite found at Pamukkale were original precipitates with no evidence that the calcite had formed by inversion of the aragonite.In other springs, aragonite precipitation has been attributed to periods of rapid CO 2 degassing and the consequent high levels of supersaturation (Jones and Renaut, 2010).Occasionally, aragonites include needle aggregates, some of them may have microbial nuclei such as bacteria and pollen, on which the needles grew (Guo and Riding, 1992).The aragonite in the fine crystalline crust-and vertically banded travertines examined in this study is formed from needle aggregates that do not appear to include any microbial nucleus (Fig. 10B-D).Spherulitic aragonite growth, without microbial nucleus, has been ascribed to high disequilibrium conditions (Jones and Renaut, 1995) or inorganic processes that include rapid CO 2 degassing (Pentecost, 1990).• Aragonite ooids and crusts formed in and around the margin of the Çukurbağ hot spring pool.Similar aragonitic ooids, up to 5 cm in diameter, have also been reported from Tekkehamam (called as 'Tekke Ilıca' in Richter and Besenecker, 1983), close to Sarayköy town southwest of the Denizli Basin, which is one of the main geothermal fields in the Denizli province.There, aragonitic ooids may have formed from the thermal waters (up to 99.7 °C) that are of Na-SO 4 -HCO 3 type (Şimşek, 2003), high supersaturation level (Sc: 0.6-0.7)with respect to CaCO 3 , and rapid CO 2 degassing (Ali Gökgöz, unpublished data).

Occurrence of dolomite
Dolomite is rare in travertine.Barnes and O'Neil (1971) found dolomite in a few Californian travertines and trace amounts of dolomite have been found and in some of the Japanese (Kitano, 1963) and Italian hot springs (Folk, 1994).Minor amounts of dolomite were found with calcite and aragonite around hot spring orifices at Chemurkeu on the western shore of Lake Bogoria (Renaut and Jones, 1997).
In the Denizli Basin, dolomite was found in some of the banded travertine at Gölemezli along with quartz and mica grains.There, the underlying bedrocks are formed from dolomite and dolomitic limestone.The association of the dolomite with the quartz and mica suggests that the dolomite is probably of detrital origin.

Strontium distribution at the travertine sites
In the Denizli Basin, the Sr in the hot waters and travertines ranges from 1.16 to 9.09 ppm and 184 to 7392 ppm, respectively (Tables 2, 4).Travertines found at the northern sites (i.e., Gölemezli, Yenice, Karahayıt, Pamukkale, Çukurbağ and Akköy) contain more Sr than the travertines found in the east, southeast and south localities (i.e., Ballık, Kelkaya and Honaz).The highest Sr contents in the northern sites are from the vertically banded-and crystalline crust travertines.The Sr in the modern crystalline calcite travertines at Pamukkale decreases significantly from the proximal to distal areas on the slope aprons (Kele et al., 2011).A  similar trend was reported from Angel Terrace, Mammoth Hot Springs (Yellowstone National Park, U.S.A.) (Fouke et al., 2000).
In central Italy, the Sr concentration of inactive travertines (up to 1500 ppm at Viterbo) and Sr concentration of present thermal springs are higher to the west of the Tiber Valley than those to the east (Minissale et al., 2002, their Fig. 8).The elevated concentration of Sr and SO 4 in thermal springs west of the Tiber Valley is probably caused by the interaction of the circulating groundwater with the gypsum in the Triassic Burano Formation found at the base of the Mesozoic limestone sequence (Minissale et al., 2002).
In the Denizli Basin, the Late Triassic Kızılyer Formation (Gündoğan et al., 2008), which is formed from evaporites and dolomites (Alçiçek et al., 2003), crops out as a tectonic slice at the eastern end of the Honaz fault near Kızılyer, may be the source of Sr found in the travertines.The deeply circulated thermal waters in the basin could have interacted with the buried part of the Kızılyer Formation.Some Sr may also have come from the Neogene strata in the Denizli Basin that includes gypsum intercalations in the areas around Sarayköy and Gölemezli (Alçiçek et al., 2007).

Basinal variations of the stable isotopic composition
Stable isotopic compositions of travertines have been linked to regional fluid flow, active tectonics, and palaeoenvironmental and palaeoclimatic changes during the Late Quaternary (Chafetz and Lawrence, 1994;Guo et al., 1996;Hancock et al., 1999;Uysal et al., 2007Uysal et al., , 2009;;Sierralta et al., 2010;Kele et al., 2011).For example, there are consistent differences in the geochemical and isotopic signatures of modern and fossil travertines, associated thermal spring waters, and gas vents found on the east and west sides of Tiber Valley (central Italy).On the west side of Tiber valley, δ 13 C values of fossil travertines are higher than those to the east side (e.g., the Ancona-Anzio line).Fossil travertines to the east have characteristics typical of meteogene origin (e.g., tufa including abundant plant casts and organic impurities) (Minissale et al., 2002).
In the Denizli Basin, the stable isotope compositions of the travertine deposits show some variations based on the location within the basin.The δ 13 C values of recent and fossil travertines found along the northern boundary (i.e., Pamukkale, Karahayıt and Gölemezli) yielded more positive δ 13 C values than those from the other sites (Table 4, Fig. 15A).More positive δ 13 C values like these have been attributed to the contribution of CO 2 liberated by thermometamorphic processes associated with magmatic activity (Kele et al., 2011).The shift in the δ 13 C values, up to +11.7‰ PDB, are thought to arise from the more rapid CO 2 -degassing that was associated with the fast flowing water on the steeper parts of the downslope sections (Kele et al., 2011).In contrast, the less positive and more negative values obtained from other precipitates possibly resulted from mixing between deeply sourced CO 2 and soil-derived CO 2 (Crossey et al., 2006).

Palaeotemperature calculations
Calcite oxygen isotope palaeothermometry generally uses equations that implicitly assume equilibrium isotope fractionation during carbonate precipitation (e.g., Friedman and O'Neil, 1977 [Eq.(1)], Kim and O'Neil, 1997 [Eq.(2)]): where α = (δ 18 O calcite + 1000) / (δ 18 O water + 1000) and 10 3 lnα c-w ≈ δ 18 O calcite − δ 18 O water .For these equations, palaeotemperature calculations are based on the δ 18 O calcite values and the δ 18 O water .In the case of fossil travertine deposits, however, the δ 18 O water is not known and must therefore be inferred.This is further complicated by the fact that isotopic equilibrium is rarely maintained under natural conditions (e.g., Coplen, 2007;Kele et al., 2008;Demény et al., 2010;Tremaine et al., 2011).At Pamukkale, for example, there is a systematic positive shift in the Δ(calcite-water) values downslope (Kele et al., 2011).For travertines precipitated around spring orifices, deviation from the equilibrium can cause the calculated temperature to be 8-9 °C lower than the true temperature (Kele et al., 2011).Most of the samples are composed of calcite.Some samples, like those from Çukurbağ and Kocabaş (Table 3), however, contain significant up to 100% aragonite (Table 3).Zhou and Zheng (2003), Kim and O'Neil (2005), and Kim et al. (2007) showed that the difference between calcite-water and aragonite-water fractionation can produce a bias of up to 5 °C (Kele et al., 2008).Herein, palaeotemperature calculations (Table 5) were based on the equilibrium equations of Friedman and O'Neil (1977) and Kim and O'Neil (1997) and modified using the observations of Kele et al. (2011).For these the calculations, we used δ 18 O water values of recent springs that are located closest to the travertine body and it was assumed that the oxygen isotope composition of the palaeosprings was similar to those of the current springs.At Pamukkale, it was possible to test the reliability of the equilibrium equations (Eqs.(1), ( 2)) for palaeotemperature calculations, because there are recent deposits and the parent waters allow comparison of the measured and calculated temperatures of deposition (Kele et al., 2011, their Fig. 16).These samples showed that the use of the Friedman and O'Neil (1977) and Kim and O'Neil (1997) equilibrium equations produced underestimations of 4.2-7.1 °C compared to the real (observed) temperature (Table 5).This demonstrates that precipitation of the calcite was not in isotopic equilibrium with the spring waters.
The highest calculated temperatures came from Gölemezli (51-73 °C), Çukurbağ (37-69 °C) and Kocabaş (47-56 °C), whereas medium temperatures came from Akköy fissure ridge (26-45 °C) and Ballık (23-39 °C), and the lowest temperatures came from Honaz (22-30 °C) and Kelkaya (22-26 °C).These temperature ranges are based on all δ 18 O travertine values because it is generally impossible to precisely locate the orifice of palaeospring systems.For the fissure ridge systems, samples from the central part of the ridge should be located closest to the orifice and should provide the highest palaeotemperature data.In other words, samples providing the highest calculated palaeotemperatures (Table 5) could have been precipitated from the warmest waters (i.e., closest the orifice of the palaeospring).
Using the U-Th age of the travertines and comparing the temperature of the current springs with the calculated temperature values of the palaeosprings a slight overall decrease in the average temperature of the hydrothermal system seems to have taken placed between the Pleistocene and the Holocene.It should be noted, however, that the δ 18 O water values of the palaeosprings could have been 3‰ lower than the recent ones during glacial periods.If so, the calculated temperatures would be high.

Origin of the banded travertines
The banded (or vein) travertines, one of the most important components of the travertines in the Denizli Basin, typically fill spaces along the normal fault planes and fissure spaces that are found in the travertine ridges (Altunel and Hancock, 1993a,b;Hancock et al., 1999;Özkul et al., 2002).The best examples are found in the Çukurbağ and Akköy fissure ridges around Pamukkale, Gölemezli and Yenice to the north-northwest and at Kocabaş to the east.In some cases, they are found as veins that cut across bedded travertines and bedrocks (Rihs et al., 2000;Uysal et al., 2009).They are composed of calcite and/or aragonite.
The vein travertines with high Sr content (up to 6822 ppm, Table 4) are typically found closest to linear spring discharges along the fissures.The high Mg content (4560 to 8160 ppm) of the Gölemezli samples (Table 4) is probably related to the detrital dolomite that came from the underlying dolomitic bedrock.The δ 13 C values of the vein travertines, compiled from this study and others (Uysal et al., 2007;De Filippis et al., 2012), are highly positive (+3.7 to +5.8‰) (Fig. 15A, B; Supplementary Table 1) and display a uniform distribution for each locality as in Gölemezli, Çukurbağ, Akköy and Kocabaş where the δ 18 O values range from +13.8 to 19.5‰ (Fig. 15B).Similar ranges of values were obtained from vein deposits at Pamukkale and in the Gediz graben (Uysal et al., 2007, their Fig. 2).The positive δ 13 C values of the vein travertines have been attributed to a thermogene origin (Uysal et al., 2007(Uysal et al., , 2009)).Data obtained in this study indicate that many of the other lithotypes are also of thermogene origin (Table 4, Fig. 15A, B).According to Uysal et al. (2007), the banded travertines may have been generated by rapid precipitation from meteoric waters that were enriched in CO 2 of deep origin and mobilised during intense seismic activity.

Geochronology of the travertine occurrences and possible influence of palaeoclimate on precipitation
Although the number of qualitative and quantitative data for the ages of the travertines in the Denizli Basin has increased (Altunel, 1996;Özkul et al., 2004;Altunel and Karacabak, 2005;Erten et al., 2005;Uysal et al., 2007;Kappelman et al., 2008;Uysal et al., 2009;De Filippis et al., 2012), the age of the earliest travertine deposition and the complete chronology of different travertine occurrences remain debatable.Nevertheless, compilation all the age data from this study (Table 1) and previous works allows comparison between development of the springs in the Denizli with global and regional palaeoclimate trends (Fig. 16, Supplementary Table 2).
The travertine deposits are located at elevations ranging from 1092 m north of Ballık (site 3) to 370 m near Kocabaş (site 5).A travertine sample from the elevation of ~1000 m at Ballık gave a thermoluminescence (TL) age of 510 ± 50 ka (Özkul et al., 2004), whereas one from a lower elevation (~370 m) near Gürlek (site 5a) yielded an U-Th age of 108.7 ± 2.0 ka.Likewise, an U-Th age of the travertine sample OT-3 from Obruk hill (510 m asl), near Karateke (site 6a) is 544 ± 111 ka.These ages indicate that the travertine deposition becomes younger from the graben's margins north and south towards the centre.It should be noted, however, that in some localities (e.g., Pamukkale) travertine deposition was active during many different time intervals (Fig. 16).
There is only limited number of palaeoclimate records for the Late Quaternary in Turkey (Nicoll and Küçükuysal, 2013).Thus, the high resolution records derived from the marine benthic record (Lisiecki and Raymo, 2005), the speleothems from the eastern Mediterranean region and China (Bar-Matthews et al., 1999, 2003;Wang et al., 2001Wang et al., , 2008)), and the combined records of Peqiin and Soreq speleothems in Israel (Bar-Matthews et al., 2003) have been used in an effort to decipher the relationship between travertine deposition and palaeoclimate in the study area.
In this study, fifteen travertine samples have been U-Th dated (Table 1).The oldest age of 613 ka came from Gölemezli (site 1), whereas the youngest age came from Akköy ridge.Another vein sample from Gölemezli yielded an age of 345 ± 18 ka, which corresponds to marine isotope stage (MIS) 10.The high error margin associated with this sample, however, means that it could partly overlap into MIS 9 (Fig. 16).
Eight samples with ages between 27.5 and 58.31 ka coincide with the younger δ 18 O part of MIS 3. One bedded travertine sample from Çukurbağ with a U-Th age of 60 ± 1 ka (De Filippis et al., 2012) corresponds to the lower boundary of MIS 4.

Discussion
The depositional architecture and geochemical characteristics of the travertine deposits are controlled by the balance between extrinsic (e.g., tectonics, seasonal climatic variations) and intrinsic (e.g., composition and flow pattern of spring water) factors (Jones and Peng, 2012).These factors, which operate over all scales and vary through time, are collectively responsible for the tufa and travertine that form around spring vents (Guo and Riding, 1998;Minissale et al., 2002;Jones and Renaut, 2010).To separate the effect that each of these factors may have had on the development of ancient travertines is, however, difficult.One possible way of doing this is to compare different travertine deposits that occur in a single basin like the Denizli extensional basin (Fig. 1).
The Denizli Basin, which is one of the main geothermal provinces in Turkey (Ekmekçi et al., 1995;Şimşek, 2003;Mutlu et al., 2008), is still seismically active today (Ates and Bayülke, 1982;Altunel and Barka, 1996).In this region, the upper crust is cut by numerous water saturated cracks and fluid pressure is high (Kaypak and Gökkaya, 2012).High mountains to the north and south are the main recharge areas for the hydrothermal system (Özler, 2000) and the deep-and shallow sourced waters in the basin are mainly of meteoric origin (Table 2).As the thermal waters ascend to the surface, they mix with the shallow cool groundwater (Dilsiz, 2006;Crossey et al., 2009).The CO 2 involved in travertine precipitation comes from thermometamorphic processes associated with magmatic sources in the area (e.g., Kele et al., 2011).
Thermal waters (Group I, II) that come to the surface along the northern boundaries (e.g., Karahayıt, Pamukkale and Çukurbağ) are derived from deeply sourced waters, i.e. endogenic waters mixed with meteoric water in different ratios (Crossey et al., 2006;Fórizs et al., 2011), and have high water temperature, high amounts of free CO 2 , and high saturation levels (Table 2).The waters in Karahayıt, Çukurbağ and Kelkaya springs are below saturation level with respect to calcite (Table 2) but become saturated as CO 2 degasses from the water while it flows downslope (Kele et al., 2011).In this part of the basin, travertines precipitated from hot waters of Group I, II and fossil equivalents (i.e., Karahayıt and Pamukkale) have higher δ 13 C and Sr values than the others (Tables 2, 4), and are composed mostly of calcite with only small amount of aragonite locally (Table 3).
The spring waters in Group III and IV (Table 2) to the east, southeast, and south (i.e., Kaklık cave, Kelkaya and Honaz), with low temperature (~19-24 °C) and high discharge rates, are characterized by groundwater circulation and mixing at shallow depths (Gökgöz, 1998;Horvatinčić et al., 2005;Dilsiz, 2006).Tufa and travertine precipitates that formed during different time periods are found along the southern boundary at the western end of the Honaz fault (Table 1).These variations in travertine and tufa precipitation may reflect temporal variations in water supply, water chemistry, mixture of deeply derived endogenic waters and epigenic waters, CO 2 levels, tectonic/seismic activity and/or climatic controls (Jones and Peng, 2012).
In extensional and transtensional provinces, faults and associated fissures served as natural conduits for emerging thermal waters (Altunel and Hancock, 1993a;Hancock et al., 1999;Şimşek, 2003;Dilsiz, 2006;Brogi and Capezzuoli, 2009;De Filippis et al., 2012).Consequently, tectonic activity can significantly influence the depositional architecture of travertine precipitation at regional and local scales.At a local scale, various depositional morphologies exist (e.g., slope, waterfall, depression fill, fissure ridge, channel) during a particular period of time.Along boundary faults, the thermal springs emerge directly onto the slopes and lead to the formation of smooth and terraced slope deposits like those seen at Pamukkale and Kelkaya (Özkul et al., 2002;Kele et al., 2011).In these settings, dendritic calcite is commonly precipitated as the CO 2 -rich spring water rapidly degases during their flow downslope (Kele et al., 2011;Jones and Peng, 2012).Similar modern and fossil examples of slope depositional systems have been reported from Mammoth Hot Spring in Yellowstone National Park, Wyoming (Chafetz and Folk, 1984;Pentecost, 1990;Fouke et al., 2000), Rapolano Terme, central Italy (Guo and Riding, 1998), and the Denizli Basin (Özkul et al., 2002;Kele et al., 2011).
Warm springs, like those at Ballık (site 3), which produced the largest travertine deposit over a volume of ~0.94 km 3 , are characterized by horizontally bedded travertines that were precipitated in depressions and/or large pools.
Similarly, warm spring waters issuing into depressions resulted in laterally extensive deposits, like those found at Bagni di Tivoli, east of Rome (Chafetz and Folk, 1984;Faccenna et al., 2008) and Rapolano Terme (Guo and Riding, 1998).These deposits have been described as 'shallow lake-fill travertines' (Chafetz and Folk, 1984).In those deposits, steepening (evolving from depression to slope facies) and/or levelling upward (evolving from slope to depression or from mound to depression depositional systems) are possible (Guo and Riding, 1998).
At Ballık, the travertine sequence gradually evolved from a depression depositional system, represented by horizontally bedded travertines (Özkul et al., 2002), to a slope depositional system that was characterized by low angle smooth slope facies and waterfall or cascade facies (cf., Guo and Riding, 1998).The depression system that evolved upwards into slope deposition (cf., Guo and Riding, 1998) has been attributed to increased flow rates that caused an increase in deposition that promoted transformation of the area into a slope system (Jones and Renaut, 2010).
Fissure ridges are elongate, wedge like structures that formed as travertine was precipitated from hot waters that ascended along a fracture or fault plane (Altunel and Hancock, 1993a;Guo and Riding, 1999;Hancock et al., 1999;Atabey, 2002;Pentecost, 2005;Brogi and Capezzuoli, 2009;Selim and Yanik, 2009;De Filippis et al., 2012).The hydrostatic pressure needed to form the largest mounds is considerable, approaching 7 kg cm −2 at ground level.These pressures, however, can be realised in artesian systems.Mound height must be limited by hydrostatic head.High supersaturation levels lead to rapid deposition around the vent and development of a steep mound (Pentecost, 2005).
The fissure ridges in the Denizli Basin are typically found at the ends of normal fault segments, which are step over or releasing zones, comprising a fracture network that are natural pathways for thermal waters (Çakır, 1999;Özkul et al., 2002;Altunel and Karabacak, 2005).These configurations are common along the northern boundary (e.g., Yenice and Pamukkale) and the Kocabaş area to the east (Fig. 1).In these areas, the physicochemical parameters like temperature, pCO 2 , saturation level with respect to CaCO 3 of the present springs, and Sr are higher than those at other sites (Table 2).Maximum travertine deposition takes place along the central axis, possibly because of rapid CO 2 degassing that maintains the high levels of supersaturation with respect to CaCO 3 .A hot spring, located at the eastern end of the Çukurbağ fissure ridge near Pamukkale (Fig. 4), for example, has the highest temperature, saturation levels, and Sr of all springs that were examined (Table 2).The formation of the banded travertines, however, is open to debate.Uysal et al. (2009) suggested that banded travertine formed during dry, cold periods and De Filippos et al. ( 2012) argued that they may have been formed from deep geothermal waters that had been released during seismic activity.Data compiled during this study, however, show that formation of the banded travertines cannot be attributed to specific climate conditions because they developed equally under cold, dry conditions and warm, wet conditions (Fig. 16).Some other studies have also shown that vein travertines formed during warm periods, such as those that existed during MIS 1, MIS 5, and MIS 7 (Rihs et al., 2000;Kampman et al., 2012).In summary, the available data shows that travertine deposition in the Denizli Basin was probably caused by episodic fluxes in the deeply derived CO 2 that was related primarily to seismicity as a consequence of neotectonic activity (Crossey et al., 2011).
The highest calculated palaeotemperatures came from deposits found in the northern (e.g., Gölemezli, Çukurbağ) and eastern (around the Kocabaş fissure ridges) areas, whereas medial temperatures came from the Akköy fissure ridge and Ballık sites, and the lowest temperatures came from Honaz and Kelkaya sites in the south (Table 5).Overall, there appears to have been a slight decrease in the average temperature of the hydrothermal system from the Pleistocene to the Holocene.Palaeotemperature values show a similar distribution with respect to those of some other parameters (i.e., stable isotope, pCO 2 , Sr, saturation level).

Conclusions
Quaternary travertine deposits of the Denizli extensional basin have been studied and compared in six locations with the view of determining the extrinsic and intrinsic factors that influenced their genesis.Travertine deposits in these locations, with thickness from 30 to 75 m, each cover an area between 1 and 34 km 2 .Up to 1 km 3 of travertine volume is present in the largest deposit found at Ballık.
Today, spring waters found in the northern part of the Denizli Basin have the highest temperatures, electrical conductivity, dissolved CO 2 , Sr, and CaCO 3 saturation levels.Collectively, the data indicate that these waters probably have deeper flow paths than elsewhere in the basin.
The travertines were precipitated in various depositional settings, including fissure ridges, slope systems, and depression systems.The fissure ridges, one of the main depositional morphologies, are restricted largely to the northern boundary.Present day travertine deposition continues on the terraced and smooth slopes at Pamukkale.The depression (or lake-fill) travertines are widespread around Ballık in the northeast.In this area, the depositional system evolved progressively from a horizontal depression, to low angle slope travertines, and finally to mound and waterfall systems at the top.
Although calcite is the dominant mineral at all sites, aragonite is present in some of the vertical banded, crystalline crust, raft and pisoid travertines from the Çukurbağ site (Pamukkale) in the north.The aragonite bearing samples rich in Sr are found mostly around the spring orifices.There are considerable variations in the stable isotope compositions of travertines throughout Denizli Basin.The δ 13 C values of travertines found along the northern boundary are more positive than elsewhere (up to + 12‰ PDB).Vein travertines precipitated in the fissure spaces and fractures of bedrocks, with a narrow range in δ 13 C values from +3.7 to +5.8‰ (PDB) for each locality, display more uniform conditions during the precipitation.In contrast, the δ 18 O values have a wide range.
The travertines have been precipitated in warm and wet periods as well as in cold and dry periods.The spring activity and therefore travertine precipitation in the Denizli Basin is not related to climate and appears to be largely a function of tectonic activity.Palaeotemperature calculations, similar to the present case, show higher values for the spring waters at the northern locations.However, from Pleistocene to Holocene a slight overall decreasing has been recorded in the spring temperatures.
Table 5 Comprehensive table of palaeotemperature calculations for Denizli travertines including the stable oxygen isotope compositions of travertines and thermal springs.

Fig. 2 .
Fig. 2. Field views of Gölemezli travertine site.(A) Vein travertines with network appearance located along steeply dipping fault planes and fractures in metamorphic bedrock, exposed in quarry wall.U-Th dates are shown.View to northeast.(B) Cut surface showing cross cutting relationships between different generations of vein travertine.(C) Interpretation of surface shown in panel B, indicating vein generations 1, 2 and 3.

Fig. 3 .
Fig. 3. Field photographs from Karahayıt-Pamukkale. (A) Thermal water, with a temperature of ~53 °C, discharging from a pipe that has been placed in spring vent at Karahayıt.Note reddish coloured travertine and green microbial algal mat on surface.(B) Surface of travertine partly covered by green microbial algal mat.(C) Stepped slope formed from travertine, slope apron in front of the Jandarma spring, Pamukkale.(D) Terraced slope travertines at Pamukkale, terrace pools filled with water.(E) Terrace pool lined by rimstone with lime mud on floor of pool.

Fig. 4 .
Fig. 4. Field photographs of Çukurbağ.(A) Westward view of the Çukurbağ fissure ridge, partly exposed by quarrying.(B) Vertical banded and associated bedded travertine in central part of ridge with U-Th date.(C) Vertical bands expanding upward, eastern end of the quarry shown in panel A. (D) Recent coated gas bubbles, rafts and microbial mat on the pool surface around Çukurbağ hot spring, eastern end of ridge.(E) Recent to subrecent, rafts and associated coated gas bubbles.

Fig. 5 .
Fig. 5. Field photographs of travertines in the Akköy fisssure ridge showing which veins.Here bt: bedded travertine, lt: lithoclast, r: raft, and v: vein.(A) Cross-section view perpendicular to the central fissure axis.Bedded travertines gently dip away opposite sides with respect to the ridge axis in the middle.View from the northwest.(B) Fissure space, 3 m wide, bounded by vertical bands on both sides.(C) Multistage fissure fill with sharp contact.(D) Multistage vertical veins and raft clusters attached to the fissure wall.(E) Horizontal ledges (white) on fissure wall, resulting from progressive accumulation of rafts.(F) Bedded travertines composed of crystal shrubs.(G) Dendritic crystal shrub layers.(H) Coated gas bubbles in bedded travertines, north flank of Akköy fissure ridge.

Fig. 6 .
Fig. 6.Field photographs of Ballık site.(A) Horizontally bedded travertines (tr) at the bottom, overlain by green mudstones (gm) and sandstone (ss), west end of Çakmak quarry, southwest part of the area.(B) Close view of horizontally bedded travertines in A. (C) Tufa mound formed mainly of reed cluster with large cavities, quarry face is ~5.5 m high.(D) Reed casts, in growth position, in tufa mound.

Fig. 7 .
Fig. 7. Field photographs of Kelkaya site, situated at the foot of northern slope of the Kelkaya Hill.(A) Light coloured travertines were deposited on a gentle slope that prograded to the north.The uppermost bench (~12 m high) is underlain by a palaeosol layer (ps).Keltepe Hill, in the background, is composed of Mesozoic limestone.(B) Terrace pools and rims (r) on cut surface showing successive rims aligned on the same line (arrowed).Flow to the right.

Fig. 8 .
Fig. 8. Field photographs of crystalline vein travertines exposed in quarry faces around Kocabaş. (A) Light coloured, gently inclined crystalline beds, up to 60 cm in thickness, penetrated as an interlayer between the bedded travertines.Box labelled B indicates area shown in panel B. (B) Crystalline calcite bed in the inset area in A. (C) Brecciated crystalline vein travertine on the quarry face.(D) White crystalline vein beds penetrated along the bedding plane of the dark travertines.(E) Cross-cut relationship between the vertical and horizontal veins on quarry wall cut in fissure ridge, the Kuşgölü area, north of the Highway 320.Box labelled F indicates area shown in panel F. (F) Close view of the cross-cut relationship, inset area in E. Many vertical and horizontal veins cut of each other repeatedly.

Fig. 9 .
Fig. 9. Field photographs of the Gürlek and Honaz sites.(A) Light and dark coloured, horizontally bedded travertines with palaeosol horizon, quarry face (5 m high), near Gürlek, Kocabaş showing U-Th date.(B) Waterfall facies with hanging tufa curtain (~20 m high), Kayaaltı sublocality, west end of the Honaz fault zone, south of the basin.(C) Waterfall tufa exposure near the Karateke village, with a U-Th age of 520 ± 110 ka, west end of the Honaz fault zone.(D) Parallel bedded travertine sequence tilted to the southeast due to faulting, Gökpınar area, southwest boundary of the basin (see Fig. 1 for locations).

Fig. 11 .
Fig. 11.Hand specimen photo and SEM images of vertically banded travertine sample (ÇB-11a) from Çukurbağ fissure ridge, Pamukkale.(A) White and brown layers on cut surface formed from calcite and aragonite in different ratio, confirmed by XRD measurements.(B) Alternation of fibrous aragonite (ar) and rhombohedral calcite (ca) layers.Close views of (C) fibrous aragonite and (D) rhombohedral calcite crystals.

Fig. 12 .
Fig. 12. (A) Front surface of rimstone dam formed from crystalline calcite, proximal slope in front of Jandarma spring, Pamukkale.Length of the hammer: 32 cm.(B) Microterrace pools developed on the proximal slope in front of Jandarma spring, Pamukkale.(C) Hand specimen of modern crystalline crust travertine developed perpendicular to the depositional surface (sample PK-12).(D) SEM image from sample PK-12 shown in panel C.The calcite dendrites are in growth position.

Fig. 13 .
Fig. 13.Aragonite ooids from Çukurbağ, Pamukkale.(A) Hand specimen of the ooids, precipitated from bubbling mesothermal waters of 58 °C in Çukurbağ spring orifice.(B) SEM image of concentrically laminated ooids in different size.Some grains, ranging in shape from spherical through irregularly rounded, have been subjected to plastic deformation.(C) Thin section image of a single ooid displaying regular concentric lamination, formed in bubbling water.(D) Close view of concentric laminae.(E) Concentric laminae, up to 12 μm thick, composed of aragonite needles.(F) Close view of aragonite needles with sizes of 2-3 μm.

Fig. 14 .
Fig. 14.Thin section and SEM images of the vein travertine (sample GL-20) from Gölemezli.(A) Thin calcite laminae with thickness of 1.0-1.5 mm below the sandy layer formed mainly of detrital dolomite, quartz, and mica.(B) Enlarged thin section image from the sandy field in A including detrital dolomite (d) arrowed.(C) Moulds appeared after removing a sand size grain.(D) A close view from one of the moulds in C.

Fig. 15 .
Fig. 15.(A) Basin scale distribution of stable carbon and oxygen isotope compositions of travertine samples in the Denizli Basin.(B) Stable carbon and oxygen isotope compositions of vein travertines.See SupplementaryTable 1 for complete list of data.Data from this study (Table 4) are from Uysal et al. (2007) and De Filippis et al. (2012).

Fig. 16 .
Fig. 16.Comparison of U-Th ages of the travertine deposits from the Denizli Basin with regional and global palaeoclimate records.High δ 18 O values are from stalagmites from Hulu and Soreq caves(Wang et al., 2001;Bar-Matthews et al., 2003).Age data from this study are combined with those from previous works(Altunel, 1996;Özkul et al., 2004; Altunel  and Karabacak, 2005;Uysal et al., 2007Uysal et al., , 2009;;De Filippis et al., 2012).Global glacial periods (white bars and even numbers) and interglacial periods (shaded bars and odd numbers) are shown by marine isotope stages (MISs) which are fromLisiecki and Raymo (2005).

Table 4
Element and stable isotopic compositions of travertine samples from the different sites in the Denizli Basin.