Sedimentary processes related to the groundwater flows from the Mesozoic Carbonate Aquifer of the Iberian Chain in the Tertiary Ebro Basin, northeast Spain

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Sedimentary Geology 129 (1999) 201–213

Sedimentary processes related to the groundwater flows from the Mesozoic Carbonate Aquifer of the Iberian Chain in the Tertiary Ebro Basin, northeast Spain J.A. Sa´nchez a , P. Coloma a , A. Pe´rez b,* a b

Geodynamics Area, Department of Earth Sciences, University of Zaragoza, 5009 Zaragoza, Spain Stratigraphy Area, Department of Earth Sciences, University of Zaragoza, 5009 Zaragoza, Spain Received 20 July 1998; accepted 26 February 1999

Abstract In this work, the importance of groundwater in the formation and evolution of evaporitic lacustrine facies in the Iberian Chain and Ebro Basin contact (Spain) is studied. There are outcrops of geological materials from Palaeozoic to Quaternary times. These materials have been classified into eight hydrostratigraphic units. The Jurassic ‘carniolas’ and limestones in the Iberian Chain (HU 4 and 5) are the materials which have the best hydraulic properties for underground water catchment and for forming the regional aquifer. The Triassic gypsum and marl of HU 3 form the impermeable substratum of the overlying Jurassic and Cretaceous carbonated aquifers, and they are therefore the impermeable base of the aquifers being studied. The equipotential line map of the regional aquifer shows the pattern of the groundwater flowing to specific points which mark the springs producing the underground drainage of the Iberian Chain. The volume of water throughout the aquifer is currently evaluated at approximately 250 hm3 =year, which discharge through springs with high flowrates, or through diffuse discharges in the riverbeds and in wetlands (saline lakes). The water discharged in these springs has a high mineralisation, with a dry residue of over 1000 mg=l. Calcium sulphate compositions dominate, originating in the presence of soluble anhydrous materials within the Lias and Keuper formations. The current sedimentation in relation to the groundwater flows from the Iberian Chain can only be found in the areas of diffuse discharge where the evaporitic sedimentation can be observed because of the frequently endorheic character. On the right bank of the Ebro River more than 60 depressions are known, where very mineralised lakes form. These are locally referred to as ‘saladas’. During the Miocene, the hydrogeological functioning would be similar to the present one, leaving the groundwater and the dissolved salts in a non-marine basin. They would therefore accumulate in large areas of diffuse discharge, creating lakes where the evaporites would precipitate. The deep regional groundwater flows would ascend to the surface from the overthrust lamina fronts, going through thick Tertiary alluvial deposits in the Ebro Basin, and therefore the discharges were mainly diffuse.  1999 Elsevier Science B.V. All rights reserved. Keywords: groundwater; regional groundwater flows; saline lakes; Iberian Chain; Ebro Basin; Spain

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author. E-mail:[email protected]

0037-0738/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 9 9 ) 0 0 0 1 6 - 0

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1. Introduction The importance of groundwater in the formation and evolution of evaporitic lacustrine facies is well documented (Matter and Tucker, 1978; Eugster and Hardie, 1978; De Deckker, 1988; Fleet et al., 1988; Salvany et al., 1994; Renaut and Last, 1994); however, water supply from underground drainage in large sedimentary basins is a fact which is rarely discussed, some known cases being those set out by Williams (1970), Rouse and Sheriff (1980), Burne et al. (1980), Magaritz (1987), and Martı´nez Gil et al. (1989). A good example of these cases can be shown in a large sector of the Iberian Chain and the Ebro Basin (Fig. 1). The Iberian Chain is an intraplate mountain range which occupies the northeastern part of the Iberian Peninsula and forms the southern margin of the Ebro Basin. This geological terrain is made up of rocks which cover most of the Phanero-

zoic stratigraphic record; however, Jurassic and Cretaceous carbonated formations predominate in the Iberian Chain (Fig. 2). The Ebro Basin is a foreland basin which developed during the Palaeogene period (Mun˜oz-Jime´nez and Casas-Sainz, 1997). The Neogene deposits onlap the Palaeogene units of the southern margin of the basin, and overlie unconformably the Palaeozoic and Mesozoic Formation of the Iberian Chain. The Neogene deposits comprise mainly conglomerates and mudstones in the margin of the basin, and gypsum and carbonates in the central part, with a thickness of over 1000 m. The relatively high altitude of the northern side of the Iberian Chain (2313 m), its pluviometry which in the highest areas reaches over 1000 mm=year of average rainfall, and the existence of large and thick carbonated outcrops, account as a whole for exceptionally elevated groundwater rates in the area. The fact that the main springs are found along the

Fig. 1. Situation of the Tertiary Ebro Basin between the Pyrenean and Iberian chains. The study area is situated along the contact between the Iberian Chain and the southern margin of the Ebro Basin.

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Fig. 2. Hydrostratigraphic units defined in the contact between the Iberian Chain and the Tertiary Ebro Basin.

geological contact between the Iberian Chain and the Tertiary Ebro Depression is also noticeable. This study brings together a series of investigations carried out by Martı´nez Gil et al. (1989), Sa´nchez Navarro et al. (1987), Villena et al. (1995), and Coloma et al. (1997), giving a regional compre-

hensive view of the hydrogeology of the study area. This study includes new information which allows an interpretation of the influence of groundwater flows in the formation of the small saline lakes currently developed in the Tertiary formations of the Ebro Basin. The hydrogeological pattern also provides a

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basis for the interpretation of the evaporitic facies of a playa-lake type which form a considerable part of the Miocene formations of the basin.

2. Hydrostratigraphy A hydrostratigraphic unit is defined as a body of rock distinguished and characterizer by its porosity and permeability. A hydrostratigraphic unit may occur in one or more allostratigraphic, pedostratigraphic, or lithodemic units and is unified and delimited on the basis of the nature, extent, and magnitude of interstices in any body of sedimentary, metamorphic, or igneous rock (Seaber, 1982, 1986). According to this definition, the materials which constitute the Iberian Chain are classified into eight hydrostratigraphic units (Fig. 2). 2.1. HU 1: Palaeozoic–Triassic Palaeozoic materials are widely represented in the study area. The Palaeozoic formation ranges from the Lower Cambrian to the Permian (Fig. 2) and forms a series of over 12,500 m in thickness, made up almost exclusively of quartzites, orthoquartzites, sandstones, siltstones and shales. There are some small outcrops of limestone of Late Ordovician age, whereas the Devonian materials consist of intercalations of limestones and marls within a thick series which is predominantly detrital. Because of their intense lithification, the Palaeozoic materials, have a typical hydrogeological behaviour as hard rocks, that is, a high heterogeneity due to the fact that their permeability is mainly related to fissuration, this resulting in a low or very low permeability. Therefore, there are numerous springs with a low flowrate (less than 0.5 l=sec), with notable seasonal variations. During rainy periods, the Palaeozoic outcrops, mainly give rise to surficial runoff. The Triassic materials (Buntsandstein facies) are made up of red sandstones and conglomerates. Their high lithification makes they have a hydrogeological behaviour similar to the Palaeozoic rocks, and they are therefore included in the same hydrostratigraphic unit.

2.2. HU 2: Triassic in Muschelkalk facies Dolomites and limestones characteristic of the Muschelkalk facies of the Triassic ranging from 25 to 159 m in thickness appear as discontinuous and isolated outcrops in narrow bands stretching NW– SE. They behave like a karstic aquifer with a diffuse flow, and therefore have a permeability which is notably higher than the underlying Palaeozoic materials. However, because of their limited extension and the reduced groundwater flow, they act as low-efficiency aquifers of very local importance. 2.3. HU 3: Triassic in Keuper facies On top of the Muschelkalk dolomites and limestones, there are red and green mudstones with intercalations of gypsum, attributed to the Keuper facies, with a thickness of up to 120 m. These materials crop out widely in the Iberian Chain, and they are found with great continuity in deep drillings carried out in the Ebro Depression. The clays in Keuper facies stand out in the whole study area because of their very low permeability, forming the impermeable substratum of the overlying Jurassic and Cretaceous carbonated aquifers. 2.4. HU 4: Late Triassic–Early Jurassic The carbonates included in this unit belong to Rhaetian and Lias formations (Renales Group, which is made up of the Imo´n Dolomites, Cortes de Tajun˜a Breccias and Cuevas Labradas Dolomites, as defined by Goy et al., 1976), which in places exceed 300 m in thickness. These materials have a high transmissivity as a result of strong karstification and fissuration, and they possess also a high storage coefficient. Therefore these materials constitute an excellent aquifer producing most of the underground drainage of the Iberian Chain, and the largest springs are related to them. Within these materials, a few levels of cavernous breccia are especially interesting. Deep under the surface, these relate laterally to anhydrites. The active underground flows, on dissolving the anhydrites, create voids which increase the porosity and permeability of these materials, thus forming the facies of breccias known locally as ‘carniolas’. The ‘carniolas’ in

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the Iberian Chain are the materials which have the best hydraulic properties for groundwater catchment. 2.5. HU 5: Early Jurassic (Pliensbachian) to the Early Cretaceous (Aptian) The Middle and Upper Jurassic is formed of several hundred metres of mainly carbonated materials which alternate with marls which have been defined as of the Ablanquejo Group and Turia Group (as defined by Goy et al., 1976). The formations from the end of the Jurassic–Early Cretaceous are irregularly distributed in the study region. In the eastern sector the outcrops of the HU5, made up of marls, sandstones, mudstones and limestones, are discontinuous, reaching a maximum thickness of 300 m. On the contrary, in the western sector, the materials which form this unit are approximately 8000 m in thickness and comprise sandstones, mudstones and carbonated intercalations which are included in the Tera and Oncala groups (as defined by Tischer, 1969). As a whole, this unit forms a karstic aquifer with high transmissivity but low storage capacity. 2.6. HU 6: Early Cretaceous (Albian) The materials which form this unit correspond to the so-called Utrillas Formation. This consists predominantly of sands (kaolin sands) and interbedded mudstones with a variable thicknesses of between 10 and 200 m. They are widely found throughout the whole Iberian Chain, whereas they have been rarely identified in wells drilled in the Ebro Basin. These materials have a variable permeability: the sandy deposits have a high permeability, but as a whole the clayey interbedded make it a low-efficiency aquifer. 2.7. HU 7: Late Cretaceous This unit comprises mainly carbonated materials which are generally strongly karstified, and therefore have a high permeability. Nevertheless, the reduced extent of Late Cretaceous carbonates in this part of the Iberian Chain and their distribution in the upper relief areas imply that they do not influence the underground drainage of the Iberian Chain.

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2.8. HU 8: Tertiary The Tertiary materials in the continental facies range from Late Eocene to Late Miocene in age and make up the filling of the Ebro Basin. The thickness of the Tertiary deposits varies from 750 to 3000 m. They are composed of conglomerates and sandstones on the basin margin, and mudstones, limestones and gypsum in the more central areas. They are low-permeability materials, especially the mudstone and evaporitic facies.

3. Structural aspects The most outstanding structural aspect is the way in which the Iberian Chain and the Tertiary Ebro Basin relate to each other (Fig. 3). We can differentiate three sectors, according to the structural style of the contact between the two geological units. The western sector includes the hydrographical basins from the Jubera River to the Alhama River (Fig. 4). Geophysical information and mechanical drillings (IGME, 1987; ITGE, 1990; Guimera` and Alvaro, 1990; Casas Sa´inz, 1993) confirm the existence of an overthrust contact with major displacement. This structure can be observed in the outcrop of the Keuper facies (Fig. 3a). The central sector extends from the Queiles River to the Huerva River. The contact between the Iberian Chain and the Tertiary Ebro Basin is neat and corresponds to an overthrust structure known as the ‘Northiberian fault’. This structure has been deduced by using geophysical methods and mechanical drilling, as the surface is covered by Tertiary materials (Fig. 3b,c). The eastern sector extends from the Huerva River to the Guadalope River, and the contact is characterised by a series of anticlines and overthrusts with a predominantly N120 direction, which sometimes can be observed in outcrops, whereas at other times it is fossilised by Tertiary materials, in such a way that when the Mesozoic materials crop out, they constitute palaeoreliefs of limited extension, hindering direct observation of the structure (Fig. 3d–f).

206 J.A. Sa´nchez et al. / Sedimentary Geology 129 (1999) 201–213 Fig. 3. Hydrogeological sections of the contact between the Iberian Chain and the Tertiary Ebro Basin. The sections have been made using data obtained from seismic reflection and electric prospecting, research drilling for oil, groundwater investigation and collection and surface geology.

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4. Hydrogeological pattern At the contact between the Iberian Chain and the Tertiary Ebro Basin, Sa´nchez Navarro et al. (1990, 1992) describe a group of springs (Fig. 4) with a constant high flowrate. The spring waters have high mineral contents (over 1 g=l), a predominantly calcium sulphate mineral content, an abnormally high temperature on emergence, and a low tritium content. These features are typical of groundwater derived from regional flows such as those characterised in models of Toth (1972). The most relevant characteristics of these springs are shown in Table 1. The water in these springs comes from the thick, extensive carbonated aquifers (HU 4 and 5) through which most of the water infiltrating the Iberian Chain is drained. This water moves downwards until it reaches the uppermost Keuper materials (HU 3), which form an impermeable substratum, and circulates mainly through the Lias aquifer (HU 4) which can be consequently considered as a ‘regional drainage aquifer’. When this aquifer is bounded laterally by low permeability materials — such as in the Northiberian overthrust — the groundwater moves upwards and flows out in the springs located in the contact between the Iberian Chain and the Ebro Basin. The movement of groundwater is conditioned by the hydrodynamic characteristics (transmissivity, storage capacity and hydraulic gradient) of the aquifer and by its surface and underground hydraulic boundaries. The surface boundaries, such as rivers and watersheds, are the result of the interaction between the topographic and the piezometric surfaces, this interaction being variable in time. On the contrary, the underground hydraulic boundaries are hydrostratigraphical and structural, and these remain stable for millions of years. The equipotential contours map of the regional aquifer (Fig. 4) shows the pattern of the groundwater flowing to specific points which mark the springs that produce the groundwater drainage of the Iberian Chain. The regional character of this drainage is demonstrated by the existence of many underground water transfers between the hydrographic basins (e.g. from the Huecha and Huerva basins to the Jalo´n River, and from the Aguas Vivas basin to the Martı´n River).

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The rivers which start to flow in the topographically high areas of the Iberian Chain lose their water at the time they reach the outcrops of the Mesozoic carbonated aquifer (HU 4 and 5). This water is incorporated into the underground drainage, leaving long stretches of the rivers dry for a long time. Downstream, the presence of the above-mentioned springs gives the rivers their base flow. The volume of water throughout the aquifer (HU 4 and 5) is currently evaluated at approximately 250 hm3 =year. This volume is either discharged through springs with high flowrates or through diffuse discharges in the riverbeds, wetlands and saline lakes.

5. Physico-chemical characteristics of the water The hydrochemistry of the waters discharging from the Iberian Chain into the Ebro River has been defined in a number of previous studies (De Miguel et al., 1989; San Roma´n, 1994; Coloma et al., 1995; Coloma, 1997). Analytical data are listed in Table 1 and represented graphically in Fig. 4. In general, the waters discharged in these springs have a high mineralisation, with a dry residue of over 1000 mg=l. Calcium sulphate compositions dominate, which provide evidence of the presence of soluble anhydrous materials within the Lias and Keuper formations. Some few discharge points have slight traces of sodium chlorate (Fig. 4), as in the Virgen de la Magdalena spring. The highest concentration of salts occurs in the western sector, specifically in the thermal springs of Fitero and Arnedillo, with values of between 4500 and 7500 mg=l of dry residue and a sodium chloride composition. Both the clearly distinct hydrochemistry of the waters in these springs and their thermal nature are indicative of their deep circulation and the long residence time in the aquifer. In all the springs studied, the chemical composition is constant and the tritium content is very low (IGME, 1982; De Miguel et al., 1989). Besides these localised discharges in springs, there are diffuse discharges which create wetlands where saline lakes form. Table 1 shows the characteristics of the waters in these lakes, the diffuse discharges which appear in the Tertiary materials, and drillings. The water is salty, with 50 and 60

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Table 1 Main parameters of springs and saline lakes related to regional groundwater flows from the Iberian Chain to the Tertiary Ebro Basin Denomination

h.a.s.l. (m)

Q (l=s)

RS (mg=l)

T (ºC)

Regional flow springs 1 Arnedillo 2 Fitero 3 San Juan 4 Borja 5 S. Ana y Heras 6 Pontil y Toron˜el 7 Virgen de Muel 8 Virgen de Magdalena 9 La Cultia 10 Virgen de Arcos 11 Los Fontanales 12 Font Calent

710 490 490 450 430 290 400 310 355 440 570 430

20 50 200 500 40 500 100 200 50 800 1000 150

6728 4689 492 806 3426 1024 776 1100 2700 2406 1130 623

52.5 46.0 18.0 18.0 20.0 23.0 18.0 23.0 18.0 22.0 18.0 22.0

Wells 13 Z34 14 Z39 15 Sondeo Fuentes

511 448 320

9154 44558 45422

22.9 22.0 19.0

Saline lakes 16 Magallo´n 17 Mediana 18 Salada Grande 19 Salada Alcan˜iz

342 335 335 350

34470 55320 82546 23236

Diffuse discharges 20 21 Santa Fe spring

342 320

0.1 0.5

52320 33984

16.0 15.5

g=l of dissolved salts but also values of 300 g=l can be reached, the pH is high and the composition varies from magnesium–sodium sulphates to sodium chlorides.

6. Sedimentary consequences related to groundwater Because of its hydrogeological characteristics, the flowing groundwater in the Iberian Chain gives place to a widespread regional process of mobilisation and transportation of dissolved salts. The annual volumes of water that emerge from the springs and the concentration of salts in these waters (Table 1)

SO24 (meq=l)

Cl (meq=l)

2.9 2.9 3.4 3.0 5.0 3.6 5.4 3.9 2.8 3.4 3.5 4.5

31.4 27.9 3.5 3.7 39.0 10.6 3.7 8.4 37.5 25.9 9.7 6.0

98.5 44.8 1.4 2.7 7.7 2.2 1.2 5.6 4.5 2.6 0.3 0.3

3.0 4.4 0.3

97.5 69.8 151.0

3.7 4.1 4.7 3.9 17.3 6.9

CO3 H2 (meq=l)

Ca2C (meq=l)

Mg2C (meq=l)

NaC (meq=l)

22.5 24.7 5.2 5.0 34.0 10.4 7.2 7.2 20.6 26.0 10.2 7.2

5.8 8.1 1.6 2.0 8.0 3.0 2.2 4.8 21.6 4.4 3.6 3.2

103.5 43.9 1.8 2.6 10.9 2.3 1.1 6.5 5.8 2.7 0.3 0.1

0.7 0.7 0.1 0.0 0.2 0.1 0.1 0.1 0.4 0.1 0.1 0.1

42.4 642.8 140.0

30.0 84.0 26.0

32.0 10.0 14.0

74.8 616.3 296.4

1.8 2.0 2.1

311.5 726.0 508.3 261.5

226.4 146.0 850.5 96.0

15.0 25.0 40.0 35.0

235.0 293.0 210.0 155.0

273.9 552.0 1130.4 183.9

2.6 4.5 2.6 0.4

835.0 439.6

101.0 21.7

23.0 5.0

536.0 117.0

421.3 326.1

3.3 0.8

KC (meq=l)

can be used to estimate the mass of salts (approximately 50,000 m.t.=year) which the groundwater flows are capable of dissolving and evacuating from the geological materials of the Iberian Chain. This calculation has been done using the volume of water in the springs multiplied by their concentration of salts. At present, most of these dissolved salts are leached to the Mediterranean Sea via the Ebro River which acts as a collector for the streams flowing from the Iberian Chain. The bulk of these salts is calcium sulphate. Part of the groundwater flows from the Iberian Chain does not discharge in the aforementioned springs, but rather move through the Tertiary materials (HU 8) of the Ebro Basin and emerge diffusely in

Fig. 4. Potentiometric contour lines and directions of the regional flow map in the Iberian Mesozoic carbonated aquifer, and Stiffs diagrams of the main springs and saline lakes.

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the riverbeds, or form the wetlands which maintain the saline lakes. Sedimentation directly related to groundwater flows from the Iberian Chain takes place in areas of diffuse discharge resulting in precipitation of evaporites that are preserved because of the frequently endorheic character of those areas. On the right side of the Ebro River valley there are more than 60 depressions where very mineralised lakes are currently formed. These are locally referred to as ‘saladas’. All these lakes show evaporitic sedimentation derived from brines of Cl–SO4 –Na–(Mg) composition. The surficial sediment is produced by precipitation of chlorides (halite), sulphates (gypsum, mirabilite– thenardite and bloedite) and small quantities of carbonates (Pueyo, 1979; Auque´ et al., 1995; Pe´rez et al., 1998). These precipitates form efflorescent crusts capping a sedimentary sequence made up of clay, dolomite, quartz and gypsum occurring as lenticular crystals. The absence of fluvial streams reaching the small lakes supports the idea that the origin of the water is the combined effect of rainfall and discharge of groundwater flows into the lakes. The current lakes are considered to be relics of large saline lakes which formed in the Ebro Basin during the Miocene, and so, during the Early and Middle Miocene, sedimentation in the Ebro Basin took place under a continental regime not connected to the sea. The sedimentation around the margins of the basin was detritic, originating from alluvial systems associated distally with large saline mudstone plains. These plains were linked with extensive saline and carbonate lacustrine systems. The existence of playa lake systems of up to 100 km long and 30–50 km wide developing in the centre of the basin has been interpreted by Pe´rez et al. (1989), Salvany et al. (1994), Villena et al. (1995) and Ingle´s et al. (1998). Three concentric subenvironments are differentiated in these lakes: (1) central lake areas with generation of gypsum and halite facies; (2) intermediate areas with pervasive sedimentation of laminated gypsum and anhydrite as well as diagenetic glauberite, and (3) marginal lake areas with nodular diagenetic anhydrite which gradually pass into distal alluvial plains. In very marginal areas which were isolated from the central lacustrine systems, small evaporitic lakes of a few kilometres maximum size developed. The deposits accumulated in these lakes

consist mainly of carbonate, gypsum and anhydrite with abundant diagenetic chert nodules. This association suggests precipitation from moderately concentrated brines (Salvany et al., 1994). The evaporitic successions for the central lake reach thicknesses of over 200 m. From the palaeogeographic point of view, the Ebro Basin was formed at the end of the Palaeogene as a foreland basin of the Pyrenean and Iberian Mountains. Therefore, at the beginning of the Miocene, the structural accidents which affect the Iberian Chain and the base of the Ebro Basin (Fig. 5) have already taken shape, and from this moment the covering with alluvial and lacustrine sediments in the Middle and Upper Miocene begins (Casas Sa´inz, 1993). Considering that during the Middle and Upper Miocene the Ebro Basin had no outlet to the sea, all the salts dissolved in the water remained in the basin. On the other hand, as the current Ebro River did not exist, neither its organised current and marked fluvial network, the discharges of groundwater mainly occurred diffusively in a large central lake (Fig. 5) and other surrounding lakes. These marginal lakes corresponded to the emergence of deep groundwater flows which would rise from the overthrust fronts that had already formed. Because these flows had to pass through thick detrital deposits from the Tertiary, they did not give rise to large localised springs as they do now, but rather to more or less deep wetlands, which formed the marginal lakes. As the volume of water which emerged in these marginal areas was very low, most of the water would emerge in the large central lake. Also this central lake would receive superficial runoff from the alluvial systems on the southern bank of the basin and also on the northern bank, which periodically could be responsible for the dissolution of the brine in the marginal lakes and the central saline lake.

7. Conclusions The water springs located in the contact between the Iberian Chain and the Tertiary Ebro Basin are fed by the thick, extensive carbonate aquifers (HU 4 and 5) which carry the water which infiltrates in the Iberian Chain. This water moves downwards until

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Fig. 5. Hydrogeological patterns of the contact between the Iberian Chain and the Tertiary Ebro Basin, comparing the present day situation and that deduced for the Miocene.

it reaches the uppermost Keuper materials (HU 3), which constitutes the impermeable substratum for flow, and circulates mainly through the Lias aquifer (HU 4) which therefore behaves as a ‘regional drainage aquifer’. When this aquifer is bounded laterally by materials of low permeability — such as in the Northiberian overthrust — the groundwater moves upwards, giving rise to the springs. The volume of water throughout the aquifer (HU 4 and 5) is currently evaluated at approximately 250 hm3 =year. The waters discharged in these springs are highly mineralised, with a dry residue of over 1000 mg=l. Calcium sulphate compositions dominate, which suggests extensive dissolution of anhydrous materials within the Lias and Keuper formations. Besides local discharge in springs, there are diffuse discharges which create wetlands where saline lakes have been formed. This is salt water, with values of between 30 and 60 g=l of dissolved salts,

which can reach values of 300 g=l, the composition varies from magnesium–sodium sulphates to sodium chlorates. The current lakes are considered to be relics of large saline lakes which formed in the Ebro Basin during the Miocene. Three concentric subenvironments are differentiated in these lakes: (1) central lake areas with a generation of gypsum and halite facies; (2) intermediate areas with pervasive sedimentation of laminated gypsum and anhydrite as well as diagenetic glauberite; and (3) marginal lake areas with nodular diagenetic anhydrite which gradually pass into distal alluvial plains. In very marginal areas which were isolated from the central lacustrine systems, small evaporitic lakes of a few kilometres maximum size developed. The deposits accumulated in these lakes consist mainly of carbonate, gypsum and anhydrite with abundant diagenetic chert nodules. This association suggests precipitation from moderately concentrated brines.

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During the Miocene, the hydrogeological functioning would be similar to the present one, leaving the groundwater and the dissolved salts in a nonmarine basin. They would therefore accumulate in large areas of diffuse discharge, creating lakes where the evaporites would precipitate. The deep regional groundwater flows would ascend to the surface from the overthrust lamina fronts, going through thick Tertiary alluvial deposits in the Ebro Basin, and therefore the discharges were mainly diffuse.

Acknowledgements This study was financed by projects PB89=0344 (Spanish Ministry of Education and Science), PMA 0694 and P122=97 (Government of Aragon). Our sincere gratitude to J.P. Calvo Sorando and to an anonymous reviser for their comments on this article.

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