Introduction

Tufa is found as common continental carbonate deposits in a wide range of environmental depositional, climatic, and tectonic settings throughout the world (Henning et al. 1983; Magnin et al. 1991; Baker et al. 1993; Ford and Pedley 1996; Pedley et al. 1996; Guo and Riding 1998; Hancock et al. 1999; Arenas et al. 2000; Glover and Robertson 2003; Martìn-Algarra et al. 2003; Andrews 2006; Ozkul et al. 2010; Brasier et al. 2011; Domínguez-Villar et al. 2011; Kosun 2012; Capezzuoli et al. 2014; Özkul et al. 2014; Henchiri 2014a; Della Porta 2015). Tufa is considered to be subaerial deposits produced from ambient cool waters and contains the remains of micro- and macrophytes, gastropods and bivalves and bacteria (Pentecost and Viles 1994; Pedley et al. 2003; Andrews 2006; Ozkul et al. 2010). It is a common continental carbonate deposit in the Holocene and present-day depositional systems of Eastern Europe and the Mediterranean region (Pedley 1990; Ford and Pedley 1996; Guo and Riding 1998; Arenas et al. 2000; Andrews 2006; Ozkul et al. 2010; Capezzuoli et al. 2014; Arenas et al. 2014; Orhan and Kalan 2015; Toker 2017; Pla-Pueyo et al. 2017; Sancho et al. 2015; Karaisaoglu and Orhan 2018; Pisciotta et al.2018). Recently, tufa sediments have been classified by using parameters such as the type of encrusting of the macrophytes, carbonate buildup and the oncolites (Arenas et al. 2000, 2007, Arenas-Abad et al. 2010; Sanders et al. 2011; Henchiri 2014b; Orhan and Kalan 2015; Toker 2017; Pla-Pueyo et al. 2017). Tufa generally contains low-Mg calcite and its deposition is related to the biogenic and physicochemical processes. The textures of the tufa deposits are controlled by the climatic and environmental conditions (Pedley 1990; Guo and Riding 1999; Brasier et al. 2011; Peng and Jones 2013; Richter et al. 2015).

The purpose of this paper is to describe and interpret the depositional and geochemical characteristics of tufa facies in an active tufa deposition site in the Yerköprü area (South of Konya, Turkey) in terms of sedimentological and stable isotopic data.

Geological setting

The study area is located south of Konya (Turkey) within the central Tauride (Fig. 1), which is highly folded and faulted. The climate in the study area is continental, with cold and snowy winters, and hot and dry summers.

Fig. 1
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Geological map of the study area (Delikan and Mert 2014)

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A rare geological structure known as natural bridge or tufa/travertine bridge is present in the Yerköprü area (Figs. 1 and 2). The Yerköprü tufa deposition took place in relation to this natural bridge. The recent Yerköprü tufa sediment was deposited with angular unconformity on the Aladağ terrace tufa (~ 90–350 ka, Delikan et al. 2017; Fig. 3f), the Permian carbonate rocks (Taşkent formation) and Triassic meta-olistostrome (Zindancık meta-olistostrome).

Fig. 2
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Field view of the Yerköprü waterfall. The Karasu spring water flow down from the cliff and mixes with the Göksu River flowing beneath the natural bridge

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Fig. 3
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Measured stratigraphic section and field pictures of different facies. a Vertical stems (Lvs) and macrophytes (Lmp), developed at the cascade setting. b Vertical stems (Lvs) and macrophytes (Lmp) facies resulted from coating solely tree and plant roots. c Approximately, 18 m-thick sequence of intercalated macrophytes (Lmp) and bryophytes (Lbr) tufa facies and phytoclastic tufa facies (Lpc) resulted from flooding. d Detailed appearance of the bryophytes tufa facies. e Stromatolitic tufa facies (Ls) observed generally at the bottom part of the sequence. f The fossil tufa formed by phytotherm tufa facies unconformably underlying the recent tufa deposits

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There are two dip slip normal faults which are parallel to each other on both sides of the Göksü valley: the Yerköprü fault dipping to the south on the north side of the valley and the Karasu fault dipping to the north on the south side of valley (Fig. 1; Delikan and Mert 2014). These faults were developed as steps. The Göksü River flows in the graben between these two faults. At the Yerköprü area, it flows about 500 m beneath an old tufa block fallen from the Göksü valley (Figs. 2 and 4c, d). Besides this, the Karasu spring discharging from the Karasu fault on the south-west part of the tufa deposition area flows over an old tufa block, forming a natural bridge. Bicarbonate-rich water of the Karasu spring drops over a 20-m-high waterfall and mixes with the Göksu River beneath the waterfall (Figs. 2, 3c and 4b, d).

Fig. 4
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a CO2 degassing of bicarbonate-rich water of the Karasu spring. b Macrophytes in the Karasu spring water. c Cave entrances at the east side of Göksu River. d The waterfall of the Karasu spring and the output of the Göksu River from the cave. e Active micro-cascade environment. f Carbonate crust covering the roots of trees (facies Lvs)

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Materials and methods

During the field observations, the lithofacies and their relation were determined, and 78 samples were collected for lithological and geochemical analysis (δ18O and δ13C stable isotope, major and trace elements). Samples were collected systematically from places where the tufa sequence was thick. Also, random sampling was realized from different lithologies (in 2013 and 2014). 15 water samples were collected on April 4, 2013 and analyzed with Dionex ICS-1000 in the Water Control Analysis Laboratory of State Hydraulic Works of Turkey. Thin sections and acetate peels from samples were prepared for determining the petrographic and sedimentological characteristics of different facies.

Stable isotope (oxygen and carbon) analyses of 19 carbonate powder samples were performed at the Institute for Geological and Geochemical Research, Hungarian Academy of Sciences, Budapest, Hungary, using an automated carbonate preparation device (Gasbench II) and a Thermo Fisher Scientific Delta Plus XP continuous flow mass spectrometer. Standardization was conducted using laboratory calcite standards calibrated against the NBS 18 and NBS 19 standards. All samples were measured at least in duplicate and the mean values expressed in the conventional δ notation in parts per thousand (‰) on the V-PDB scale (δ13C, δ18O) and VSMOW scale (δ18O). Reproducibility was better than ± 0.1‰ for δ13C and δ18O values of carbonates.

The mineralogical composition of representative samples was determined by Bruker-D8/Advance/X-ray diffraction (XRD) and Zeiss Evo/Ls10-Scanning Electron Microscope (SEM) methods at ILTEK laboratory in Selçuk University.

Chemical analysis (major oxide, trace and rare earth element contents) of ten samples was performed at the ACME analytical laboratories (Vancouver, British Columbia, Canada) by inductively coupled plasma emission and mass spectrometry (ICP-ES–MS) devices.

Results

Facies characteristics

Tufa sediments, which were actively deposited in the Yerköprü area, were defined by following the classification of Pedley (1990), Ford and Pedley (1996), Arenas et al. (2000) and Vazquez-Urbez et al. (2012), based on the field observations and sedimentological and textural characteristics of tufa deposits. Four different lithofacies were determined in the Yerköprü area (Table 1). These facies are the following.

Table 1 Principal features and interpretation of tufa facies in the Yerköprü region
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Phytotherm framestone tufa facies

Description

This facies is the most common sediment in the study area and reaches up to 18 m of thickness. It is characterized by encrusted macrophytes (Lmp), bryophytes (Lbr), coated vertical roots, branches and trunks (Lvs) (Figs. 3a, b and 4e, f) and leaf fossils and is highly porous. Concentric layers comprising alternate coarse sparry calcite and micritic layers are common (Fig. 5a, b). The space between encrusted roots is filled with micrite. This facies starts with the formation of crust by water flowing over the branches, roots and plants, such as Adiantum capillus-veneris hanging from the slopes (Figs. 3, 6a, b and 7b–d). At the space behind this facies, speleothems were formed by seeping water. The encrusted macrophytes are generally overlain by encrusted bryophytes, which have been repeated several times through tufa sequences (Figs. 3, 7a and 8g, h).

Fig. 5
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a Low-Mg calcite developed as concentric rings around the tree roots (facies Lmp), b macrophyte facies formed by concentric micritic and sparitic rings encrusting around leaves. c Thin section appearance of bryophytes, d micritic lamina separated by coarse spray calcite (stromatolitic facies, Ls), e pelloidal facies (micritic tufa facies, Lm) deposited in small pools, f coarse radial calcite grows within the pelloidal facies

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Fig. 6
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Formation phases of the Lmp facies developed by downward vertical growing Adiantum capillus-veneris in cascade. a The three phases of crust development on living Adiantum capillus-veneris and caves behind them. b Umbrella looking Lmp facies developed at three phases

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Fig. 7
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Field photographs of the recent tufa. a Lmp facies formed on the vertical waterfalls with curtains of Adiantum capillus-veneris. b Detailed appearances of Lvs facies. c Tree roots coated by concentric rings (facies Lmp). d Radial aragonite overgrowths around the tree roots and leaves (polished section of facies Lmp). e Actual bryophytes growing on the tufa where the fast-moving water falls. f detailed view of bryophytes and leaves fossil. g Stromatolitic tufa deposits were formed in the watermill channels. h, i Stromatolite (facies Ls) with intercalated dark and light lamina

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Fig. 8
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Bacteria and fossils within the recent and old tufa sediments. a, b Single cell colonies; prokaryotes, c, d cyanobacteria filaments, e, fDiatom sp. and Cal: calcite crystals, g, h detailed view of moss facies

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Interpretation

In modern and recent tufa environments, bryophytes develop along the waterfalls, barriers and in water-splash areas beside active channels and pools (Pedley 1990; Ford and Pedley 1996; Arenas et al. 2000; Pentecost 2005¸ Arenas et al. 2007, 2014). Macrophytes tufa facies (Lmp) are generally deposited at the slopes having dense roots and plants and high energy condition. Deposition starts with crust formation around the plants while the water flow is turbulent. After flooding, ponds were formed on the flatness around the waterfall. During these periods, the terrestrial algae growing on the branches and in the pond bring about the development of bryophytic tufa under both turbulent and stagnant conditions (Figs. 5c, 7e, f). Adiantum capillus-veneris and the trunk and roots of plants cause the formation of vertical stem tufa (Fig. 6).

Boundstone tufa facies (stromatolitic tufa facies—Ls)

Description

This facies is one of the common facies among the tufa deposits. It is represented by 5 cm to 30 cm thick intercalated light and dark laminae and occurs as domes and columns, planar laminae and slightly crenulated forms (Figs. 3e, 5d and 7h, i). Bedding is generally undulated and comprises hemispheric and kidney-shaped structures. Stromatolitic tufa was observed in the water course channels of the Karasu spring and watermill canals. About 5-m-high wall of stromatolitic tufa was formed in the watermill canals (Fig. 7g). Stromatolitic tufa facies laterally grades into phytotherm framestone facies, as seen in other places (Pedley 2009; Ozkul et al. 2010; Vazquez-Urbez et al. 2012).

Interpretation

Stromatolites may have developed under low energy condition in shallow ponded areas and partially isolated fluvial channels as well as in fast-flowing channels (Pedley 1990; Ford and Pedley 1996; Sanders et al. 2011; Arenas and Jones. 2017). Stromatolitic tufa facies in the study area is deposited at the pond rims, flood plains, channels and cave walls (Figs. 3e, 5d, 7h and 8a–f). The alternation of light and dark lamina in this facies is interpreted as having resulted from seasonal changes and depositional rate (Pentecost and Whitton 2000; Andrews and Brasier 2005; Pedley 2009; Gradzinski 2010; Jones and Renaut 2010; Arp et al. 2010). The thick deposition in the watermill canal indicates fast sedimentation and resulted from fast-flowing water (Fig. 7g).

Micro detrital tufa facies (micritic tufa facies—Lm)

Description

This facies (Lm) is present within the recent and old pools on the natural bridge and the slopes of the valley near the waterfall. It is represented by thick bedded lime mud deposited within these ponds and micro-pools (Figs. 5e, 9b). It contains voids that resulted from decaying of organic material. Some of these voids were filled with coarse aragonite crystals (Fig. 5f). Their thickness ranges from 10 to 83 cm. It has lenticular shape and rests conformably on the stromatolitic facies (Fig. 9a–c). Micritic sediments are generally surrounded by macrophyte tufa or stromatolites developed at the corner of the pools.

Fig. 9
figure 9

Field views of tufa formation. a Micro-pools formed at the slopes and micritic carbonate deposition within them. b Close view of sediment in micro-pools (the pan is 14 cm.). c Sediment deposited within a large pool bordered by a fault just behind the slope. d Lenticular facies Lpc comprising fragments derived from facies Lmp during flooding

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Interpretation

Micritic tufa facies has been interpreted as being deposited in quiet ponded areas by direct precipitation of fine calcite and/or fine particles derived from erosion of the surrounding and nearby facies (Ford and Pedley 1996; Arenas et al. 2007; Melón and Alonso-Zarza 2018).

Macro detrital tufa (phytoclastic tufa facies—Lpc)

Description

This facies consists of weakly consolidated fragments of highly variable size and shape (Fig. 9d). It is commonly associated with broken phytoclasts and stromatolites within the micrite matrix or spar cement. It has a thickness ranging from 15 to 48 cm and short lateral extension.

Interpretation

This facies was developed as channel fills and has lenticular shape. It contains mainly fragments of previously deposited facies, but the fragments of basement rocks and broken pottery are also present in lesser amounts. This facies represents the high discharging periods of the Karasu spring, causing flooding in the area, and originated from erosive processes related to flooding events that affected previously deposited facies. Their thickness and extension are a function of the power and duration of the flooding.

Chemical composition of Karasu spring and Göksu River

Water samples collected from both the Göksu River and the Karasu spring were subjected to chemical analysis (Table 2). The bicarbonate (H2CO −13 ) contents of the Karasu spring water samples taken from the upstream, on and downstream of the waterfall were measured as 506,3 mg/L, 549 mg/L and 231,8 mg/L, respectively. On the other hand, the bicarbonate (H2CO −13 ) content of the Göksu River water taken before reaching the natural bridge was 131 mg/L. The pH of the water was slightly basic. The temperatures of the Göksu River and Karasu spring water were 20.3 °C and 16.5 °C, respectively (Table 2).

Table 2 Chemical composition of the Karasu spring water and the Göksu River
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Geochemical composition of the tufa deposits

Mineralogical analysis indicates that all tufa samples are nearly pure calcite (Fig. 10). The element contents of tufa samples collected from different locations and facies are given in Table 3. The Mg content ranges from 2400 to 9780 with an average of 3786 ppm. The MgCO3 of the tufa deposits ranges from 0.836 to 3.4067 mol% with an average of 1.3187 mol%, which indicates that tufa samples are composed of low magnesian calcite (Arenas et al. 2000; Ozkul et al. 2010). The Ba and Sr contents of the tufa samples were 31–61 ppm and 2384–4558 ppm, respectively. The Ba and Sr contents were slightly higher than those in the fluvio-lacustrine tufas in the central Ebro Depression, NE Spain (Arenas et al. 2000; Ozkul et al. 2010), while Ba content in the Quaternary tufa stromatolite from central Greece also had similar values (Andrews and Brasier 2005; Ozkul et al. 2010).

Fig. 10
figure 10

The XRD diffractometry of the Lv facies (a) and the Ls facies (b) was generated by pure calcite

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Table 3 Major and trace element contents (in ppm) and MgCO3 contents (in mol%) of the Yerköprülü Tufa deposits
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Stable isotope data

The δ13C (V-PDB) and δ18O (V-PDB) values were obtained mainly from recent phytotherm framestone facies. The δ13C content of the tufa deposits ranged from 1,6 to − 3,3 and δ18O contents from − 9,6 to − 11,5 (Table 4). These values closely resemble the values obtained from fluvial tufa sediments in the Denizli-Sarıkavak region (Toker 2017).

Table 4 Stable isotope values of the Yerköprü tufa deposits
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Discussion

The field observation and chemical analysis of waters from the Karasu spring and the Göksu River showed clearly that the tufa deposition resulted only from the carbonate-rich water of the Karasu spring (Table 2). The water from Karasu spring flows on the natural bridge where the Göksu River flows beneath. The distance from the source of the Karasu spring to the waterfall is 700 m, but the tufa formation took place only on and around the waterfall where the water flow was fast and turbulent (Jet flow), causing aeration and low pressure on the carbonate-rich Karasu water. This increases the rate of degassing of CO2. Tufa deposition in the waterfall area resulted from degassing of CO2 due to sudden hydrological changes rather than due to the effects of organisms, evaporation and sediment–water interaction. This caused the water to reach oversaturation and accelerate tufa deposition. This fast deposition of tufa on the waterfall is known as waterfall effect (aeration effect, low pressure effect and jet flow effect) (Zhang et al. 2001; Chen et al. 2004).

The rate of deposition depends on the seasonal changes and is greater during spring and summer (warm season, few cm/year) than during autumn and winter (cool season). The Karasu River flows in a relatively narrow channel (about 700 m) up to the waterfall area, where it branches out into small bars in a v-shape on the flat part of the natural bridge. Pools with various shapes and sizes developed as a result of spreading of water on the flatness close to the waterfall or redirection of the watercourse by humans. The facies Lm, Ls and Lmp were deposited within these pools. Water from pools and watercourse flows toward the waterfall or toward both sides of the Göksu River and mixes with the Göksu River. Tufa formation took place only on the Yerköprü natural bridge from the water of the Karasu spring. No tufa deposition site was observed on the upstream and downstream sites of the Göksu River.

Cascade formation took place by carbonate deposition in the micro-pools, at both sides of the waterfall and by carbonate coating of roots and branch of the trees (Figs. 3 and 6). Facies Lmp, Lbr and Lvs (Figs. 3d, 5a) were deposited in the cascade environment, and facies Lm and Ls in the micro-pools (Fig. 7a). Facies Lpc formed within the flooding channels and stromatolitic facies within small canals built for irrigation. The cascade sediments comprise calcite-coated moss mats at the vertical waterfall and various plants growing on the valley slope, stalactite and stalagmite in the cavities beneath the waterfall and stromatolitic tufa deposit on the damp walls of caves.

Cascade sediments formed as downward hanging masses often collapse due to their heavy weight. This sometimes changes the watercourse and causes the formation of different depositional settings (Fig. 9c). The collapsed bodies are often covered by new tufa deposits which are mainly controlled by the size and shape of the collapsed body and the vegetation covers growing on them.

Tufa-depositing spring waters emerging from karstic massifs are commonly of the Ca–HCO3 and Ca–HCO3–SO4 types and are supersaturated with respect to calcite (Ozdemir and Nalbantcilar 2002; Ozkul et al. 2010, Khalaf 2017; Nicoll and Sallam 2017). Calcium and bicarbonate are the dominant ions in the Karasu spring water (Table 2). Therefore, the water of Karasu spring is classified as a Ca–HCO3 type.

Field observations show that tufa sediments are only deposited on the pathway of the Karasu spring. No tufa sedimentation has been observed along the Göksu River itself. The H2CO −13 content of the water sample taken from the orifice of the Karasu spring, where the degassing of CO2 starts (Fig. 4a), is 5063 mg/L. But it was measured as 549 mg/L at the edge of the waterfall and as 2318 mg/L at the base of the waterfall where the water mixes with the Göksu River. The distance between the orifice and the waterfall edge is 700 m.

The calcite saturation indices (SIC) is 0,18 at the orifice, 0,59 on the top of the waterfall and 0,45 at the bottom of the waterfall (Table 2). It is minimum at the orifice, but decreases rapidly along the flow path due to CO2 degassing and reaches the maximum at the top of the waterfall where dense tufa deposition has taken place.

The δ18O and δ13C values obtained from the studied tufa samples, which is located at about 725 m altitude, resemble those from the tufa deposits in the Grand Canyon, Arizona, USA (Fig. 11; Andrews 2006) and fall in the ‘High mountain tufa’ area on the plot (Fig. 12) constructed by Andrews (2006). “High mountain tufa” represents the tufa deposition from the karstic water flowing out along a fracture in a high-altitude (700–800 m) setting (perched springline tufa, Andrews 2006). The characteristics of the studied tufa deposits such as the altitude (725 m) and source of carbonate-rich waters (the dissolution of older carbonate rocks) correspond exactly to the descriptions by Andrews (2006). The interpretation introduced by Teboul et al. (2016) also supports the idea that the bicarbonate in the Karasu water was derived from dissolution of older carbonate rocks (Fig. 13).

Fig. 11
figure 11

The distribution δ18O and δ13C valoes of the Yerköprü tufa deposits on the plot introduced by Andrews (2006)

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Fig. 12
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Regional setting of the Yerköprü tufa deposits based on the δ18O and δ13C stable isotope data plot of Andrews (2006). The tufa deposits in this study correspond to ‘high mountain tufa’. See for details Andrews (2006)

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Fig. 13
figure 13

The distribution of δ18O and δ13C values of the studied samples on the combined δ18O (‰PDB) and δ13C (‰PDB) plot for recent to modern calcitic or aragonitic travertine and tufa (CATT; Teboul et al. 2016)

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The stable isotope values of the tufa deposits are controlled by different parameters such as biological activities, evaporation, water temperature, turbidity, bed configuration and altitude (O’Brien et al. 2006; Gandin and Capezzuoli 2008; Arenas-Abad et al. 2010; Hassan 2014; Toker 2017). The δ18O values (− 9.6 to − 11.5) of the tufa sediment in the studied area resemble those of many other places (Vazquez-Urbez et al. 2012; Arenas et al. 2000, 2007; Zamarreno et al. 1997, Ozkul et al. 2010; Kosun 2012; Orhan and Kalan 2015; Toker 2017), while the δ13C values are much higher (1.6 to − 3.3). These high values of δ13C point to the dissolution of the carbonate aquifer (Andrews et al. 1993). As can be seen in Fig. 14, the isotopic values of the tufa samples fall in the travertine area. The field measurements show that the Yerköprü tufa sediments were deposited from cold water. The carbonate-rich water derived from dissolution of carbonate aquifer has the hydrochemical signals of deep circulation, which is similar to the tufa formation “travitufa” in a specific setting (Capezzuoli et al. 2014; Toker 2017). The drop in δ18O is related to the decrease in temperature and prevalence of the freshwater, which can be seen clearly in the Yerköprü tufa deposits (Andrews 2006; Toker 2017).

Fig. 14
figure 14

Combined plot of δ18O (‰PDB) and δ13C (‰PDB) values derived from carbonates presently forming in terrestrial (travertine, calcareous tufa, speleothems) and marine (skeletal-ooid sediments and pelagic muds) environments and from lithified marine limestones (Gandin and Capezzuoli 2008)

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Conclusion

This is a first study on the sedimentological and geochemical characteristics of the recent tufa deposits in the study area which is tectonically active. The following conclusions were introduced after evaluating field, sedimentological and geochemical data.

  1. 1.

    The field observation and chemical analysis data showed that the tufa deposition resulted from the carbonate-rich water of the Karasu spring.

  2. 2.

    Based on the thickness data obtained from the deposition in an artificial canal in the area, the tufa sedimentation rate is at least 1 cm/year.

  3. 3.

    The Yerköprü tufa deposition took place in a small graben as a natural bridge, which is a rare structure. Its formation in the area is related to the neotectonic activities. Therefore, the deposition is topographically controlled.

  4. 4.

    There is an active cascade formation in the study area. Even though most of the tufa deposition took place in the cascade setting, some deposits were in small lakes and ponds around the waterfall. Fluvial deposits occur as lenticular phytoclast sediment. Except for the fluvial and oncolitic facies, other representative facies (Lmp, Lvs, Lbr, Ls, Lm, Lpc; see Table 1). are present in the studied area.

  5. 5.

    The δ18O and δ13C values of the Yerköprü tufa sediments is classified as “High mountain tufa” of Andrews (2006) which was deposited from karstic water discharging on a fracture. The high δ13C values (1.6 to − 3.3) of tufa sediments in the Yerköprü area show that the bicarbonate in the Karasu spring was derived from the dissolution in the carbonate rocks.

  6. 6.

    The isotopic values of the studied tufa samples fall in the travertine area in Fig. 14, but the Karasu water is interpreted as cold water. Therefore, it can be classified as “travitufa”.