Sedimentology of Pleistocene palustrine tufas and associated deposits of the Ebrón Valley (Iberian Ranges, Spain)

Stratigraphy

The studied deposits in the Los Santos area (Figs. 2, 4, 5 and 6) are dominated by limestones over clastic sediments (i.e., consisting of extraclasts). The record showed a small proportion of polymictic clastic deposits composed of conglomerates, sandstones and occasionally mudstones, overlain by the dominant carbonate deposits that consist of varied sedimentary facies (Figs. 5 and 6). Three stratigraphic sections were measured (Fig. 5). These sections have been divided into several units for the sake of clarity in description. These units are based on lithology and/or textural characteristics. Individual facies description and facies labels in the following summary are found in Table 1.

Deposits close to and in the village of Los Santos (Fig. 7)

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No attempt was made to measure stratigraphic sections given the vertical topography (i.e., cliffs) and/or poor exposures in this part of the studied area. The deposits close to the village consist of ca. 6 m of thick greyish carbonate sands with interbedded cm-thick peaty layers that are recognized over extensive areas, of hectometre lateral continuity, despite most parts being covered, e.g., by fine detritals from younger overlaying sediments (Figs. 7A, 7B, 7C). The deposit outcropping in the village is formed of ca. 4 m thick of tufa limestone consisting of conspicuous up-growing stems arranged in extensive palisades cm to dm high and ca. 7.5 m visible wide, and associated carbonate sands and phytoclastic tufa (Fig. 7D, 7E). In general, these deposits conform tabular geometries.

Figure 7.  Field views of deposits outcropping close to the village of Los Santos (A, B, C) and in the village of Los Santos (D, E). (A, B, C) Dominant carbonate sands with interbedded fine phytoclastic rudstones and peaty layers. Note in A the orange color due to overlying silt dropping. (D, E) Tabular deposits consisting mostly of up-growing stem boundstones, and associated carbonate sand and fine phytoclastic rudstones. Legend for facies labels in Figure 5 and Table 1.

Sedimentology

A variety of sedimentary facies has been found (carbonate, detrital and peaty; Table 1 and Figs. 8, 9 and 10). The main mineralogy of the carbonate facies was calcite, as determined through microscopy and calcimetry analyses (see Materials and methods). In a few cases, these facies contain minor amounts of clay- to sand-size siliceous grains. A wide array of carbonate facies has been distinguished in the studied area based on texture and biotic composition (Table 1 and Fig. 8). The most abundant facies are the phytoclastic rudstones (Lph, Lphf), carbonate sands with gastropods and ostracods (Sb) and up-growing stem boundstones (Lst1). Less common are moss boundstones (Lbr), down-growing stem boundstones (Lst 2) and bioclastic limestones (Lb). In general, plant stems are coated with several concentric laminae (Fig. 11C, 11D), which in most cases contain bacterial evidence, i.e., cyanobacterial fan- and bush-shaped calcite bodies, as well as isolated filament moulds (Fig. 11E, 11F). Most of these facies were formed in multiple sub-environments, except for the occasionally occurring curtains of hanging stems, which formed in waterfalls. For example, up-growing stems developed in fluvial banks, shallow pools and floodplains; phytoclasts were ubiquitous, thus phytoclastic rudstones are found in almost every sub-environment; bryophyte layers formed in waterfalls, barrages and jumps.

Detrital (extraclastic) facies are characterized based on texture and sedimentary structures (Table 1 and Fig. 9). Conglomerates and gravels consist of angular to rounded, poorly sorted, carbonate and minor siliceous extraclasts, mainly from Mesozoic rocks, and at times including Quaternary tufa clasts, which together are embedded in a polymictic sandy-gravel matrix. They mostly form structureless deposits (Gm), but locally can show cross stratification (Gt). Sandstones and siltstones are also of polymictic nature, including variable amount of tufa intraclasts. All these clastic deposits occur at the base of the outcropping record and represent first incision and then deposition in low sinuosity channels during high energy events (Gm, Gt, St). Mudstones, consisting mostly of siliciceous grains (Fm, Fh), and marls (Mm) may be associated with thin peaty layers (facies C), giving evidence of periods of stagnant water on the floodplain and in ponds (Figs. 10; 11J, 11K).

Facies Associations (FA) and depositional interpretations

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The sedimentary facies recognized in the Los Santos area of the Ebrón Valley are associated vertically into simple vertical sequences or facies associations (FAs) that recorded the sedimentary processes that occurred in a particular sedimentary area through time. In the field, five facies associations were recognized (Fig. 12).

Figure 12.  Some of the most common facies associations (FAs) in the Los Santos area of the Ebrón river Valley. Explanation in the text.

Proposal of a sedimentary facies model

The proposed sedimentation model was constructed based on the stratigraphic correlation and the various facies associations that resulted from progradation, aggradation and lateral migration of environments through time. The studied deposits represent the distal termination of a high-slope fluvial tufa system, in which the decreasing slope downstream favoured the development of palustrine areas, shallow ponded areas and small cascades. Moreover, the increase in width of the valley downstream favoured extensive development and preservation of the palustrine facies (i.e., up-growing calcite-coated stem boundstone). Therefore, the model corresponds to a low-slope, wide stretch dominated by palustrine conditions at the end of a stepped, cascade-barrage fluvial system (Fig. 13).

This model includes detrital sediments (extraclasts) that were deposited in shallow channels during incision periods due to high water discharge events, leading to gravel and sand deposits. Typically, channel incision during high discharge occurred at the initial stages of the Middle-Late Pleistocene record and was followed by gravel and sand fill and then by floodplain deposits, at places with accumulation of fine-grained carbonaceous matter (FA 1). Afterwards, carbonate sedimentation turned to be dominant throughout. Most flat areas became then site for palustrine vegetation with sluggish water flowing across extensive areas. The flooded action from some high-energy episodes eroded the up-growing stem phytoherms and associated deposits (e.g., carbonate sands) over the palustrine zones, producing phytoclasts that were deposited along the channels (e.g., at the beginning of FA 2 and 3) and on the palustrine areas. Some phytoclastic accumulations favoured the formation of small cascades and barrages-cascades along the channel, with deposition of fine sediment (i.e., carbonate sand and granule size) in the upstream dammed areas and moss layers downstream (FA 4). Progradation of these cascades and barrages-cascades structures gave rise to small hanging calcite-coated stem boundstones (e.g. as in FA 5). Hydrophilous vegetation and pooled areas with sandy sediment became dominant downstream in flat zones (FA 2 and 3), where aggradation of carbonate sand and lime mud in slow flowing and ponded areas occurred during low energy discharges (FA 2, 3 and 5). In this context, the hygrophilous plants were partially to fully submerged, then becoming coated with calcite and transformed into boundstone of up-growing stems. Also present is the abundant phytoclastic rudstone, which originated from the breakage of calcite-coated hydrophytes and broken stems accumulated in the palustrine environments. Water stagnation of some zones in these areas favoured accumulation of marly, silty and sandy sediment and preservation of peaty matter (FA 3).

Factors controlling extensive palustrine deposition and preservation

Considering the large-scale sedimentary context of the Ebrón Valley, the different pre-Quaternary carbonate and non-carbonate rocks are incised by the present river as it flows down the decreasing slope gradient. There is a remarkable contrast between the upstream and downstream sedimentological features in the Pleistocene outcrops. The large cascade-barrage structures (formed of moss and stromatolite bioherms) and associated thick dam deposits upstream (consisting of carbonate sands and phytoclastic rudstones), e.g., near Castielfabib (as described in Lozano et al., 2012Lozano, M.V.; Sancho, C.; Arenas, C.; Vázquez-Urbez, M.; Ortiz, J.E.; Torres, T.; Pardo, G.; Osacar, M.C. & Auqué, L. (2012). Analisis preliminar de las tobas cuaternarias del Río Ebrón (Castielfabib, Valencia, Cordillera Ibérica). Geogaceta, 51: 51-54., and Sancho et al., 2015Sancho, C.; Arenas, C.; Vázquez-Urbez, M.; Pardo, G.; Lozano, M.V.; Peña-Monné, J.L.; Hellstrom, J.; Ortiz, J.E.; Osácar, M.C.; Auqué, L. & Torres, T. (2015). Climatic implications of the quaternary fluvial tufa record in the NE Iberian Peninsula over the last 500 ka. Quaternary Research, 84 (3): 398-414. https://doi.org/10.1016/j.yqres.2015.08.003 ), became much less prominent downstream (e.g., near Los Santos). There, cascade-barrage structures were smaller and overall “small-slope and slow-water facies” were dominant (e.g., carbonate sands and silts, phytoclastic rudstones and stem phytoherms). Interestingly, stromatolites, which commonly form in fast-flowing water areas along moderate to high slope zones (e.g., Violante et al., 1994Violante, C.; Ferreri, V.; D’Argenio, B. & Golubic, S. (1994). Quaternary travertines at Rochetta a Volturno (Isernia, Central Italy). Facies analysis and sedimentary model of an organogenic carbonate system. PreMeeting Fieldtrip Guidebook, A1. International Association of Sedimentologist, Ischia ‘94, 15th regional meeting, Italy, 3-23.; Arenas et al., 2014bArenas, C.; Vázquez-Urbez, M.; Auqué, L.; Sancho, C.; Osácar, C. & Pardo, G. (2014b). Intrinsic and extrinsic controls of spatial and temporal variations in modern fluvial tufa sedimentation: A thirteen-year record from a semi-arid environment. Sedimentology, 61: 90-132. https://doi.org/10.1111/sed.12045 ), are absent in the Los Santos area of the Ebrón Valley. Moreover, most moss mounds form discrete bodies that represent small jumps (Fig. 5). Another remarkable attribute in the Los Santos study area is the lack or limited presence of features denoting large or deep erosional processes within the tufa sequence, which supports deposition in overall slow to moderate flow condition.

These depositional changes coincide with a general decrease in slope and an increase in width downstream through the valley during the Pleistocene. This change was conditioned by variations of bedrock lithology, from Mesozoic carbonate rocks upstream to Neogene alluvial rocks downstream (e.g., as shown in Fig. 2). The gentle sloping topography of the Los Santos area and the lack of vigorous erosional processes at the distal termination of the fluvial system enabled a dominant palustrine condition to exist on the sluggish streams, along stream banks and in pools within wide floodplains.

The Los Santos tufa sequence is much thinner (up to 19 m) than the Castielfabib sequence (up to 77 m), which is consistent with the distal deposition from the carbonate source (springs upstream), leading to reduction of calcite precipitation downstream, due to the decrease of the dissolved calcium carbonate content and/or the diminishing mechanical CO₂-loss, related to lesser water turbulence. This feature has been shown in some modern fluvial tufa systems (Arenas et al., 2015Arenas, C.; Auqué, L.; Osacar, M.C.; Sancho, C.; Vázquez-Urbez, M. & Pardo, G. (2015). Current tufa sedimentation in a high discharge river: A comparison with other synchronous tufa records in the Iberian Range (Spain). Sedimentary Geology, 325: 132-157. https://doi.org/10.1016/j.sedgeo.2015.05.007 ). Remarkable is the fact that the present day water composition of the Ebrón River shows, mostly in the downstream area, minor decreasing trends in Ca content and alkalinity values, but the values of the Saturation Index with respect to calcite usually remains higher than +0.6, suggesting the existence of tufa sedimentation in the distal part of the river, though reduced compared to upstream deposition rates (Arenas et al., 2015Arenas, C.; Auqué, L.; Osacar, M.C.; Sancho, C.; Vázquez-Urbez, M. & Pardo, G. (2015). Current tufa sedimentation in a high discharge river: A comparison with other synchronous tufa records in the Iberian Range (Spain). Sedimentary Geology, 325: 132-157. https://doi.org/10.1016/j.sedgeo.2015.05.007 ). Moreover, present day sedimentation studies indicate that the tufa deposition rates in each fluvial subenvironment of the Ebrón River were mainly controlled by the CO₂-outgassing intensity linked to local flow conditions and the biological substrate type (Arenas et al., 2015Arenas, C.; Auqué, L.; Osacar, M.C.; Sancho, C.; Vázquez-Urbez, M. & Pardo, G. (2015). Current tufa sedimentation in a high discharge river: A comparison with other synchronous tufa records in the Iberian Range (Spain). Sedimentary Geology, 325: 132-157. https://doi.org/10.1016/j.sedgeo.2015.05.007 ). In the Pleistocene study case, a decrease in CO₂-outgassing intensity would be expected, as the area corresponded to the downstream, low-slope area. As a matter of the fact, thickness of deposits on tablets monitored every six months along that river also showed decreasing depositional rates downstream, coinciding with the area of Los Santos. Additional water inputs through springs that introduce CO₂ in the carbonate system may cause decreases in calcite precipitation, as in the case of the present Ebrón River (Arenas et al., 2015Arenas, C.; Auqué, L.; Osacar, M.C.; Sancho, C.; Vázquez-Urbez, M. & Pardo, G. (2015). Current tufa sedimentation in a high discharge river: A comparison with other synchronous tufa records in the Iberian Range (Spain). Sedimentary Geology, 325: 132-157. https://doi.org/10.1016/j.sedgeo.2015.05.007 ). This type of water input was occasional in the downstream stretch. In the Pleistocene, although spring CO₂-inputs may have accounted for the small tufa accumulation at the downstream part of the system, this type of inputs is not expected to have been dominant, given the extensive palustrine tufa deposits. Thus, springs, if present, were small or occasional.

Many other ancient sedimentary fluvial systems with slope and facies changes along the valley path have been found in the Quaternary record (e.g., Violante et al., 1994Violante, C.; Ferreri, V.; D’Argenio, B. & Golubic, S. (1994). Quaternary travertines at Rochetta a Volturno (Isernia, Central Italy). Facies analysis and sedimentary model of an organogenic carbonate system. PreMeeting Fieldtrip Guidebook, A1. International Association of Sedimentologist, Ischia ‘94, 15th regional meeting, Italy, 3-23.; Pedley et al., 1996Pedley, H.M., Andrews, J., Ordóñez, S., González-Martín, J.A., García Del Cura, M.A. & Taylor, D. (1996). Does climate control the morphological fabric of freshwater carbonates? a comparative study of Holocene barrage tufas from Spain and Britain. Palaeogeography, Palaeoclimatology, Palaeoecology, 121: 239-257. https://doi.org/10.1016/0031-0182(95)00080-1 ; Arenas et al., 2014aArenas, C.; Vázquez-Urbez, M.; Pardo, G. & Sancho, C. (2014a). Sedimentology and depositional architecture of tufas deposited in stepped fluvial systems of changing slope: Lessons from the Quaternary Anamanza valley (Iberian Range, Spain). Sedimentology, 61: 133-171. https://doi.org/10.1111/sed.12053 ; Capezzuoli et al., 2014Capezzuoli, E.; Gandin, A. & Pedley, M. (2014). Decoding tufa and travertine (freshwater carbonates) in the sedimentary record: The state of the art. Sedimentology, 61: 1-21. https://doi.org/10.1111/sed.12075 ; Huerta et al., 2016Huerta, P.; Armenteros, I.; Merino Tomé, Ó.; Rodríguez Gonzálvez, P.; Silva, P.G.; González Aguilera, D. & Carrasco-García, P. (2016). 3-D modelling of a fossil tufa outcrop. The example of La Peña del Manto (Soria, Spain). Sedimentary Geology, 333: 130-146. https://doi.org/10.1016/j.sedgeo.2015.12.013 ; Toker, 2017Toker, E. (2017). Quaternary Fluvial Tufas from Sarikavak Area, Denizli, Southwestern Turkey: Facies and depositional systems. Quaternary International, 437: 37-50. https://doi.org/10.1016/j.quaint.2016.06.034 ). Several other Quaternary examples with palustrine environments come from central Italy (Buccino et al., 1978Buccino, G.; D’Argenio, B.; Ferreri, V.; Brancaccio, L.; Panichi, C. & Stanzione, D. (1978). Il travertini della bassa Valle del Tanagrio (Campania): Studio geomorphologico, sedimentologico e geochimico. Bolletino della Societá Geologica Italiana, 97: 617-646.), Israel (Heimann & Sass, 1989Heimann, A. & Sass, E. (1989). Travertines in Northern Hula Valley, Israel. Sedimentology, 36: 95-108. https://doi.org/10.1111/j.1365-3091.1989.tb00822.x ), central Spain (Pedley et al., 2003Pedley, H.M.; Ordóñez, S.; González Martín, J.A. & García del Cura, M.A. (2003). Sedimentology of Quaternary perched springline and paludal tufas: Criteria for recognition, with examples from Guadalajara Province, Spain. Sedimentology, 50: 23-44. https://doi.org/10.1046/j.1365-3091.2003.00502.x ), southern Turkey (Özkul et al., 2010Özkul, M.; Gökgöz, A. & Horvatincic, N. (2010). Study from the Denizli Province, Western Turkey springline tufa deposits and associated spring waters: A case study from the Denizli Province, Western Turkey. In: Tufas and Speleothems: Unravelling the Microbial and Physical Controls (Pedley, H.M. & Rogerson, M., Eds.), Geological Society, London, Special Publications, 336: 245-262. https://doi.org/10.1144/SP336.13 ) and southwestern Tunisia (Henchiri, 2014Henchiri, M. (2014). Quaternary paludal tufas from the Ben Younes spring system, Gafsa, southwestern Tunisia: interactions between tectonics and climate. Quaternary International, 338: 71-87. https://doi.org/10.1016/j.quaint.2013.12.024 ). However, situations with extensive low-slope palustrine tufa areas at the most downstream fluvial stretch, as in the case study of Los Santos, are not so commonly preserved.

Arenas et al. (2014a)Arenas, C.; Vázquez-Urbez, M.; Pardo, G. & Sancho, C. (2014a). Sedimentology and depositional architecture of tufas deposited in stepped fluvial systems of changing slope: Lessons from the Quaternary Anamanza valley (Iberian Range, Spain). Sedimentology, 61: 133-171. https://doi.org/10.1111/sed.12053 described a moderate-slope fluvial tufa model for Holocene deposits in the Iberian Ranges in which some portions with dominant palustrine facies could be similar to those in the Los Santos studied example. Thick palustrine deposits have also been described in the lower reach of the Pleistocene Mesa River (Váquez-Urbez et al., 2012Vázquez-Urbez, M.; Arenas, C. & Pardo, G. (2012). A sedimentary facies model for stepped, fluvial tufa systems in the Iberian range (Spain): The quaternary Piedra and Mesa valleys. Sedimentology, 59: 502-526. https://doi.org/10.1111/j.1365-3091.2011.01262.x ). Henchiri (2014)Henchiri, M. (2014). Quaternary paludal tufas from the Ben Younes spring system, Gafsa, southwestern Tunisia: interactions between tectonics and climate. Quaternary International, 338: 71-87. https://doi.org/10.1016/j.quaint.2013.12.024 related the dominant palustrine conditions of Holocene tufas in southwestern Tunisia to bedrock configuration and the flat topography of the area where the springs emerged, prone to form a marshy context.

In the Los Santos studied system, the large dammed areas, opposed to minor and small cascades-barrages, formed as a result of gentle gradient in the wider valley, thus vertical growth of barrages was limited, while favouring lateral migration of the fluvial environment. Stagnant conditions in the pooled areas could be established due to the likely long periods without water renewal and lack of vigorous flow, which together favoured accumulation and preservation of organic matter such as peat (e.g. as described by Pedley et al., 2003Pedley, H.M.; Ordóñez, S.; González Martín, J.A. & García del Cura, M.A. (2003). Sedimentology of Quaternary perched springline and paludal tufas: Criteria for recognition, with examples from Guadalajara Province, Spain. Sedimentology, 50: 23-44. https://doi.org/10.1046/j.1365-3091.2003.00502.x ; Arenas et al., 2014aArenas, C.; Vázquez-Urbez, M.; Pardo, G. & Sancho, C. (2014a). Sedimentology and depositional architecture of tufas deposited in stepped fluvial systems of changing slope: Lessons from the Quaternary Anamanza valley (Iberian Range, Spain). Sedimentology, 61: 133-171. https://doi.org/10.1111/sed.12053 ; Henchiri, 2014Henchiri, M. (2014). Quaternary paludal tufas from the Ben Younes spring system, Gafsa, southwestern Tunisia: interactions between tectonics and climate. Quaternary International, 338: 71-87. https://doi.org/10.1016/j.quaint.2013.12.024 ).

The palustrine tufa of the Ebrón River is a typical example illustrating the distal evolution of a carbonate-rich riverine environment in a wide low-layering area that, in the downflow direction, progressively reduces or stops its capacity to precipitate calcite due to the decrease of the dissolved calcium carbonate and/or the diminishing mechanical CO₂-loss, related to lesser water turbulence in overall low-slope surfaces. In other settings, this calcite precipitation reduction has been related to the system entering other hydrological system with different hydro-geochemical characteristics (see Toker, 2017Toker, E. (2017). Quaternary Fluvial Tufas from Sarikavak Area, Denizli, Southwestern Turkey: Facies and depositional systems. Quaternary International, 437: 37-50. https://doi.org/10.1016/j.quaint.2016.06.034 ). In these settings, carbonate deposits are rapidly replaced by detrital tufa or mixed clastic successions (Ortiz et al., 2009Ortiz, J.E.; Torres, T.; Delgado, A.; Reyes, E. & Díaz-Bautista, A. (2009). A review of the Tagus river tufa deposits (central Spain): Age and palaeoenvironmental record. Quaternary Science Reviews, 28: 947-963. https://doi.org/10.1016/j.quascirev.2008.12.007 ; Martini & Capezzuoli, 2014Martini, I. & Capezzuoli, E. (2014). Interdigitated fluvial clastic deposits and calcareous tufa testifying an uplift of the catchment area: An example from the Pianizzoli area (southern Tuscany, Italy). Sedimentary Geology, 299: 60-73. https://doi.org/10.1016/j.sedgeo.2013.11.001 ). However, this is not the case of the Pleistocene example studied herein.

Together, the above discussion indicates the complex interplay of factors that control fluvial tufa deposition (geology, topography, hydrology, hydrochemistry, biology, climate; see Goudie et al., 1993Goudie, A.S.; Viles, H.A. & Pentecost, A. (1993). The late-Holocene tufa decline in Europe. The Holocene, 3(2): 181-186. https://doi.org/10.1177/095968369300300211 ; Pedley et al., 2003Pedley, H.M.; Ordóñez, S.; González Martín, J.A. & García del Cura, M.A. (2003). Sedimentology of Quaternary perched springline and paludal tufas: Criteria for recognition, with examples from Guadalajara Province, Spain. Sedimentology, 50: 23-44. https://doi.org/10.1046/j.1365-3091.2003.00502.x ; Pentecost, 2005Pentecost, A. (2005). Travertine. Springer-Verlag, Berlin, 445 pp.; Valero Garcés et al., 2008Valero Garcés, B.L.; Moreno, A.; Navas, A.; Mata, P., Machín, J.; Delgado Huertas, A., González Sampériz, P., Schwalb, A.; Morellón, M.; Cheng, H. & Edwards, R.L. (2008). The Taravilla lake and tufa deposits (Central Iberian Range, Spain) as palaeohydrological and palaeoclimatic indicators. Palaeogeography, Palaeoclimatology, Palaeoecology, 259: 136-156. https://doi.org/10.1016/j.palaeo.2007.10.004 ; Arenas-Abad et al., 2010Arenas-Abad, C.; Vázquez-Urbez, M.; Pardo-Tirapu, G. & Sancho-Marcén, C. (2010). Fluvial and Associated Carbonate Deposits. In: Carbonates in continental setting (Alonso-Zarza, A.M. & Tanner, L.H., Eds.), Developments in Sedimentology, Elsevier, 61: 133-170. https://doi.org/10.1016/S0070-4571(09)06103-2 ). As cited above, the factors that favour the formation of tufa palustrine systems are: the reduced slope and large amplitude of the sedimentary substrate, the high saturation of water in calcium carbonate and the reduced and slow water level changes, along with the absence of strong erosional processes.

Subsidence is a principal factor that favours accumulation and preservation of different types of deposits. In the investigated environment, for instance, thick tufa, oncolite-bearing and bioclastic limestones formed in the middle-late Miocene of the Ebro Basin; thickness and preservation were clearly linked to subsidence induced by evaporite-solution of underlying bedrocks (Arenas et al., 2000Arenas, C.; Gutiérrez, F.; Osácar, C. & Sancho, C. (2000). Sedimentology and geochemistry of fluvio-lacustrine tufa deposits controlled by evaporite solution subsidence in the central Ebro Depression, NE Spain. Sedimentology, 47: 883-909. https://doi.org/10.1046/j.1365-3091.2000.00329.x ). In this case, the corresponding beds showed typical geometry and thickness variations. Similarly, thick and varied facies deposits formed westward in the Ebro Basin, in relation to subsidence associated to a blind thrust rooted in the Iberian Range during the middle-late Miocene (Vázquez-Urbez et al., 2013). However, thick dominantly palustrine tufa successions preserved in basins affected by tectonic activity and subsidence have been reported only in a few cases (Ellera Basin; Pazzaglia et al., 2013Pazzaglia, F.; Barchi, M.R.; Buratti, N.; Cherin, M.; Pandolfi, L. & Ricci, M., (2013). Pleistocene calcareous tufa from the Ellera basin (Umbria, central Italy) as a key for an integrated paleoenvironmental and tectonic reconstruction. Quaternary International, 292: 59-70. https://doi.org/10.1016/j.quaint.2012.11.020 ).

Due to the concurrence of such varied conditions, extensive areas of palustrine tufa at the downstream areas of fluvial systems form only rarely and related to local situations. For instance, in arid conditions (Ben Younes tufa - Gafsa area, Tunisia) Henchiri (2014)Henchiri, M. (2014). Quaternary paludal tufas from the Ben Younes spring system, Gafsa, southwestern Tunisia: interactions between tectonics and climate. Quaternary International, 338: 71-87. https://doi.org/10.1016/j.quaint.2013.12.024 described extensive palustrine tufa deposits related to repeated occurrence of springs able to lead supersaturated waters. Tufa deposits can also represent the distal portion of thermal systems, where typical travertine facies are transitional to cooler, plant-rich tufa facies and especially of suitable environments as lakes, alluvial plains or marshes (Della Porta, 2015Della Porta, G., (2015). Carbonate build-ups in lacustrine, hydrothermal and fluvial settings: Comparing depositional geometry, fabric types and geochemical signature. In: Microbial Carbonates in Space and Time: Implications for Global Exploration and Production (Bosence, D.W.J.; Gibbons, K.A.; Le Heron, D.P.; Morgan, W.A.; Pritchard, T. & Vining, B.A., Eds.), Geological Society, London, Special Publications, 418: 17-68. https://doi.org/10.1144/SP418.4 ; Della Porta et al., 2017Della Porta, G.; Croci, A.; Martini, M. & Kele, S. (2017). Depositional architecture, facies character and geochemical signature of the Tivoli travertines (Pleistocene, Acque Albule Basin, Central Italy). Rivista Italiana di Paleontologia e Stratigrafia, 123: 487-540.; Brogi et al., 2017Brogi, A.; Capezzuoli, E.; Kele, S.; Baykara, M.O. & Shen, C-C. (2017). Key travertine tectofacies for neotectonics and palaeoseismicity reconstruction: effects of hydrothermal overpressured fluid injection. Journal of the Geological Society, 174(4): 679-699. https://doi.org/10.1144/jgs2016-124 ; Mancini et al., 2019Mancini, A.; Capezzuoli, E.; Erthal, M. & Swennen, R. (2019). Hierarchical approach to define travertine depositional systems: 3D conceptual morphological model and possible applications. Marine and Petroleum Geology, 103: 549-563. https://doi.org/10.1016/j.marpetgeo.2019.02.021 ; Mohammadi et al., 2020Mohammadi, Z.; Claes, H.; Capezzuoli, E.; Mozafari, M.; Soete, J.; Aratman, C. & Swennen, R. (2020). Lateral and vertical variations in sedimentology and geochemistry of sub-horizontal laminated travertines (Çakmak quarry, Denizli Basin, Turkey). Quaternary International, 540: 146-168. https://doi.org/10.1016/j.quaint.2018.11.041 ).

In brief, from the above discussion, the factors that favour the formation of tufa palustrine systems are: the reduced slope and large width of the sedimentary substrate, the water saturation in calcium carbonate and the reduced and slow water level changes, along with the absence of strong water discharge provoking erosion. Preservation is expected to be greatly affected by subsidence, as in any other subaerial contexts. Whether subsidence could control the characteristics of the studied palustrine system in the Ebrón valley is unknown.