Sedimentological and hydrochemical parameters of the River Piedra (northeast Spain) were monitored every six months (from 1999 to 2012) at 24 sites, at which tablets were installed all along the river. The river water is of HCO 3 -SO 4 -Ca type and is notably influenced by inputs from upstream karstic springs. Tufa deposition was first detected 8 km downstream of these springs and greatly increased from there, primarily along the steeper stretch (i.e. within the Monasterio de Piedra Natural Park); then, deposition decreased through the most downstream stretch, with smaller ground water inputs. The spatial evolution of the tufa thickness, with parallel variations of PWP (Plummer, Wigley, Parkhurst) rates, was thus determined by the river water pCO 2 which was controlled by ground water inputs and by the river bed slope. Five fluvial subenvironments and seven sedimentary facies were characterized. The water flow conditions are the primary factor responsible for the distinct deposition rates of facies, mainly through CO 2 -outgassing. Stromatolites and moss-tufa and alga-tufa had the highest rates, whereas loose tufa formed in slow-flowing water and tufa of spray areas had thinner deposition. A six-month pattern in the deposition rate was detected through thickness measurements. That pattern was parallel to the seasonal PWP rates. The increased deposition during warm periods (spring and summer; mean: 5Á08 mm) compared with cool periods (autumn and winter; mean: 2Á77 mm) is linked chiefly to temperature, which controlled the seasonal changes in the physico-chemical and biological processes; this finding is supported by a principal components analysis. Seasonal variations of insolation and day duration also contributed to such a deposition pattern. Large discharge events, which provoked erosion of tufa deposits and dilution of water, caused the reversal of the seasonal deposition rate pattern. Stromatolites are likely to preserve the most complete sedimentary record. Although tufas are a potentially sensitive record of climate-related parameters, erosion is an intrinsic process that may overwhelm the effects of such parameters. This issue should be considered in palaeoclimatic studies based on the tufa record, particularly in semi-arid conditions.Keywords Fluvial tufa facies, present hydrochemistry and deposition rate monitoring, sedimentary processes, semi-arid climate, Spain. INTRODUCTIONThe study of present-day tufa sedimentation in the fluvial environment has become a matter of great interest because of the ability of tufas to record environmental (for example, climatic and hydrological) changes at different time scales. The high-deposition rates of fluvial tufas (i.e. as much as 16 to 17Á5 mm yr À1 ; Pentecost, 1978; V azquez-Urbez et al., 2010) makes possible the short-term monitoring of physical, chemical and biological parameters, which allows the factors that control the tufa sedimentation to be analyzed. Most studies of present fluvial tufas deal with stable-isotope geochemistry Kawai et al., 2009;...

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The tufa record and hydrochemical characteristics of the River Piedra in the Monasterio de Piedra Natural Park (NE Spain) were studied for 6 years. The mean discharge of this river was 1.22 m 3 /s. The water was supersaturated with calcium carbonate. The HCO 3 -, Ca 2? and TDIC concentrations decreased along the 0.5-km-long studied stretch, whereas the calcite SI showed no systematic downstream or seasonal variation over the same stretch. Several sedimentary subenvironments exist in which four broad types of tufa facies form: (1) Dense laminated tufa (stromatolites), (2) Dense to porous, massive tufa, (3) Porous, coarsely laminated tufa with bryophytes and algae, and (4) Dense, hard, laminated deposits in caves. The halfyearly period thickness and weight of sediment accumulated on 14 tablets installed in several subenvironments showed that the deposition rate was greater in fast flowing river areas and in stepped waterfalls, and lower in slow flowing or standing river areas and in spray and splash areas. Mechanical CO 2 outgassing is the main factor controlling calcite precipitation on the river bed and in waterfalls, but this process does not explain the seasonal changes in depositional rates. The deposition rates showed a halfyearly period pattern recorded in all fluvial subenvironments persistent over time (5.26 mm, 0.86 g/cm 2 in warm periods; 2.26 mm, 0.13 g/cm 2 in cool periods). Mass balance calculations showed higher calcite mass values in warm (21.58 mg/L) than in cool (13.68 mg/L) periods. This biannual variation is mainly attributed to the seasonal differences in temperature that caused changes in inorganic calcite precipitation rate and in biomass and the correlative photosynthetic activity. Tufa sedimentation was therefore controlled by both physicochemical and biological processes. The results of this study may help test depositional rates and their environmental controls and thus assess the climatic and hydrological significance of ancient tufas in semi-arid conditions, in particular in the Quaternary.

This contribution deals with the External Sierras and a part of the foreland Ebro Basin related to the southern Pyrenean thrust front. The structure of the External Sierras consists of a south‐verging thrust system developed from middle Eocene to early Miocene times. Since the end of the early Oligocene, a regional‐scale detachment anticline (the Santo Domingo anticline) developed, folding the original thrust system and creating new thrust units. The molassic fill in this part of the Ebro Basin (Uncastillo Formation) mainly corresponds to an extensive, composite distributary fluvial system, termed the Luna system, which drained the uplifted Gavarnie Unit to the north. Small, marginal alluvial fans originated along the External Sierras and coalesced in the proximal‐middle portions of the Luna system. Three tecto‐sedimentary units (TSU), late Oligocene to early Miocene in age, comprise the Uncastillo Formation. Lateral relationships and areal distribution of lithofacies through time have been used to establish sedimentary models for the marginal alluvial fans and the Luna fluvial system. Their sedimentary evolution was controlled by tectonics affecting the drainage basins, and based on mapping and stratigraphic relationships of the TSU, the temporal succession of the marginal alluvial fans and their relationships with each thrust system in the south Pyrenean front can be shown. Alluvial fan formation evolved through time from west to east, in accord with the progressive eastward growth of the Santo Domingo anticline as a conical fold. The fluvial network of the Luna system appears to have been mainly radial, but near the basin margin its architecture was influenced by the syndepositional Fuencalderas and Uncastillo anticlines developed within the Ebro Basin. These low‐amplitude folds originated by layer‐parallel shearing caused by rotation of the southern flank of the Santo Domingo anticline. Progressive uplift of these anticlines constrained part of the fluvial discharge to synclinal areas parallel to the basin margin; these areas where characterized by meandering sandy channels. At the peripheral tips of the anticlines the channel system flowed basinward.

The Pleistocene and Holocene tufas of the Añamaza valley (stepped build‐ups, up to 70 m thick, along the valley) consist of several depositional stages separated by erosional surfaces. Eight associations of tufa and related carbonate facies, plus minor polygenic detrital facies, represent the processes that occurred in different fluvial and related environments. The bedrock lithology and structure controlled the location of the knickpoints along the valley and allowed separation of two stepped stretches with distinct conceptual facies models. The moderate‐slope model includes extensive standing‐water areas dammed by barrage‐cascades. In the lakes, bioclastic silts, sands and limestones along with phytoclastic and marly, at places peaty, sediments formed. Abundant stem phytoherms account for extensive palustrine areas. The high‐slope model consists of smaller dammed areas between close‐up cascades and barrage‐cascades, which were composed primarily of moss phytoherms and phytoclastic tufas. An outstanding feature is the extensive steep reach with phytoclastic and polygenic detrital sediments, and stepped cascades consisting of stromatolitic and moss phytoherms. There, the steep slope limited the preservation of stem phytoherms and favoured erosion. The geometry and thickness of the sedimentary fill (wedge‐shaped units composed of cascade and barrage‐cascade deposits downstream, and dammed and gentle‐sloped channel deposits upstream) are therefore different for each model. Multi‐storey wedges are a distinctive feature of the high‐slope model. The initial knickpoint geometry and the tufa aggradation/progradation ratio on such steep surfaces (for example, related to changes in discharge) controlled the growth style of the cascades or barrage‐cascades and, hence, the extent, thickness and vertical evolution of the upstream deposits. The sedimentological attributes and stable‐isotope composition of the carbonate facies suggest a higher and more variable precipitation/evaporation ratio during the Pleistocene than during the Holocene, consistent with an overall decrease in the river discharge. This evolution was coupled with warm conditions, which prevailed during the stages of tufa formation. These results may help to assess architectural patterns in interpreting other basins, and underscore the significance of tufas as records of past hydrology and climate.

Stratigraphic and sedimentological analyses of the Quaternary tufa and associated deposits in the Piedra and Mesa river valleys allowed a number of stages of their sedimentary evolution to be characterized, and a depositional sedimentary model for this north-central sector of the Iberian Range (Spain) to be established. The proposed sedimentary facies model may explain tufa arrangements in other medium to high gradient, stepped, fluvial tufa systems with narrow transverse profiles occurring in temperate, semi-arid areas, in both recent and past scenarios. There are several tufa deposits within the Piedra and Mesa river valleys that, over a maximum thickness of about 90 m, record one or more stages of tufa deposition produced following the fluvial incision of the bedrock or previous tufa deposits. Each depositional stage begins with coarse detrital sedimentation. Six fundamental, vertical sequences of tufa facies with small amounts of detrital material reveal the sedimentary processes that occurred in different fluvial environments: channel areas with: (i) free-flowing water; (ii) barrages and/or cascades; and (iii) dammed water and palustrine floodplains. The proposed sedimentary model involves narrow, stepped, fluvial valleys in which tufa cascades were common. Alternating intervals of bryophyte and stromatolite facies commonly formed at some cascades. Many of these represented barragecascade structures that consisted of phytoclast rudstones, thick phytoherms of mosses and associated stromatolites, and curtain-shaped phytoherms of stems. Upstream of these structures, dammed areas with bioclastic sands and silts developed and palustrine vegetation grew. The channel stretches between barrages and/or cascades were loci for extensive stromatolite growth in fast flowing water. The palustrine floodplain was home to pools and drainage channels. The model also explains the growth of some barrages in the River Piedra that surpassed the height of the divide, with the diffluence of the main channel into a secondary course forming other tufa deposits in the area. The distribution and abundance of certain types of tufa facies in fluvial basins may be an indicator of differences in their gradients. The facies studied in this work suggest that the gradient of the ancient River Piedra was steeper than that of the ancient River Mesa. Assuming similar scenarios for climate and hydrology, the depositional settings mentioned above and their dimensions would have been determined mainly by the gradient and width of the associated river valleys. This sedimentary model may also be useful for inferring variations in other river basin slopes, as well as accounting for the presence of tufas in areas that normally have no permanent water input.

The drainage area of the Iberian Ranges (NE Spain) houses one of the most extensive Quaternary fluvial tufaceous records in Europe. In this study, tufa deposits in the Añamaza, Mesa, Piedra and Ebrón river valleys were mapped, stratigraphically described and chronologically referenced from U/Th disequilibrium series, amino acid racemization and radiocarbon methods. Tufa deposits accumulated in cascades, barrage-cascades and related damming areas developed in stepped fluvial systems. The maximum frequency of tufa deposition was identified at 120 ka (Marine Oxygen Isotope Stage [MIS] 5e), 102 ka (MIS 5c), 85 ka (~ MIS 5a) and 7 ka (MIS 1), probably under warmer and wetter conditions than today. Additional phases of tufa deposition appear at ~ 353 ka (~ end of MIS 11), 258–180 ka (MIS 7) and 171–154 ka (MIS 6). Although most tufa deposition episodes are clearly correlated with interstadial periods, the occurrence of tufa deposits during the penultimate glaciation (MIS 6) is remarkable, indicating that the onset of this stage was climatically favourable in the Iberian Peninsula. Biostatic conditions and the dynamics of karstic systems regulating tufa deposition seem to be sensitive to the precipitation regime, controlled by shifts in the position of North Atlantic atmospheric belts, and summer insolation, regulated by orbital forcing.