Geomorphology of the Tertiary gypsum formations in the Ebro Depression (Spain)

Geoderma 87 Ž1998. 1–29

Geomorphology of the Tertiary gypsum formations in the Ebro Depression žSpain / Mateo Gutierrez Elorza, Francisco Gutierrez Santolalla ´ ´

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Departamento de Ciencias de la Tierra, Facultad de Ciencias, UniÕersidad de Zaragoza, 50009 Zaragoza, Spain Received 21 September 1996; accepted 11 September 1997

Abstract This paper reviews the current knowledge of the mainly karstic geomorphological features developed in the evaporitic formations of the Ebro Depression Žnorthern Spain.. Special emphasis is given to the recently published and unpublished scientific advances. The gypsum formations, of Tertiary age, have an extensive outcrop area within the Ebro Depression. Here, their morphogenesis is controlled mainly by processes of surface and subsurface dissolution acting on the gypsum. Outstanding landforms in the gypsum terrain include saline lakes developed in flat bottom dolines Žsaladas.. Other characteristic morphologies include karren and gypsum domes, which occur on a decimetre scale. Where the gypsum is covered by Quaternary alluvial deposits the karstification processes are especially intense and cause subsidence phenomena. Karstic subsidence affects stream terraces, mantled pediments and infilled valleys, which in the region are called vales. Dissolution-induced synsedimentary subsidence has produced interesting geological features, which include significant thickening and deformation of the alluvial deposits. In contrast to the rapid removal of gypsum by dissolution, the amount of gypsum removed by erosion is low. Water erosion studies carried out on gypsiferous slopes of the Ebro Depression, indicate that the sediment yield ranges from 0.59 to 7.82 trharyear. This low yield results from the high infiltration capacity of the soils. Subsidence caused by gypsum dissolution has important socioeconomic consequences in the Ebro Depression. The active alluvial karstification of the gypsum causes numerous sinkholes that are harmful to linear structures Žroads, railway lines, irrigation channels., buildings and agricultural land. Unforeseen catastrophic subsidence also puts human lives at risk. The benefits of such terrains include thickened alluvial deposits which act as valuable water reservoirs and which form excellent sources of aggregates. Fluvial valleys in this gypsiferous terrain commonly show an asymmetrical geometry with prominent gypsum scarps at one side. These gypsum scarps are affected by numerous landslides. These slope movements are hazardous,

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Corresponding author. Fax: q34-76-761088; E-mail: [email protected]

0016-7061r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 6 - 7 0 6 1 Ž 9 8 . 0 0 0 6 5 - 2

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may dam rivers and cause flooding of the alluvial plains. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Ebro Depression; geomorphology; gypsum karst; subsidence; sinkholes; soil erosion

1. The evaporitic formations in the evolution of the Tertiary Ebro Basin The Ebro Basin is one of the three main Tertiary depositional basins in the Iberian Peninsula. The basin lies in a triangular topographic depression, bounded by the Pyrenees to the north, the Iberian Range to the south-west and the Catalan Coastal Range to the south-east Ž Fig. 1. . The Ebro River cleaves the entire Depression from WNW to ESE, crosses the Catalan Coastal Range, and flows into the Mediterranean Sea. The Ebro Basin is the southern foreland basin of the Pyrenees, an alpine orogene developed in a continental collision zone. From the geodynamic viewpoint, the basin was generated by flexure of the continental lithosphere induced by vertical loading of the Pyrenees orogenic wedge. Like most foreland basins, the Ebro Basin has a marked asymmetrical geometry. The maximum thickness of the sedimentary fill Ž up to 7000 m in the western sector. is associated with the northern margin of the Ebro Basin, where allochthonous units of the southern Pyrenean nappes overthrust the Tertiary fill. The crustal kinematics and the stratigraphical record of the basin are intimately related to the evolution of the Pyrenean orogenic wedge. Throughout the tecto-sedimentary evolution of the Basin, the main evaporitic formations have developed in the most actively subsiding depocenters. These depocenters have migrated progressively from north to south ŽOrtı, ´ 1990. driven by continued convergence Ž . and forebulge translation Fig. 1 . This resulted in a general onlap of successively younger stratigraphical units onto the foreland. During the initial sedimentary stage Ž Paleocene–Eocene. , the basin was open to marine transgressions with continental and marine sedimentation that took place in exorheic conditions Ž Riba et al., 1983. . Most of these sedimentary units were subsequently incorporated into the Pyrenean orogenic belt. The second stage in the sedimentary evolution of the Ebro Basin Ž Upper Eocene–Miocene. began during the Priabonian regression Ž Upper Eocene. . During this regression potassic evaporites were deposited in the shallow marine environments of the Navarra and Cataluna ˜ sub-basins ŽRiba et al., 1983.. After this period the Ebro Basin was no longer occupied by the sea, but became an individual endorheic basin surrounded by topographic highs. During this endorheic stage, the erosion of the surrounding tectonically uplifted areas provided the basin with detritus Žmolasse. deposited as alluvial fans. These alluvial fans were distally related to shallow lacustrine environments with evaporitic Ž playalakes. and carbonate sedimentation. The palaeogeographic distribution of the lacustrine systems was controlled by the variable location of the depocenters.

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Fig. 1. Distribution of the Tertiary evaporitic formations in the Ebro Depression. ŽElaborated from Ortı, ´ 1990 and I.T.G.E., 1995..

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The continental evaporite formations of the Ebro Basin were formed in two main types of evaporitic lacustrine systems: high and low concentration systems ŽOrtı´ et al., 1989; Ortı, ´ 1990.. High concentration lacustrine systems Žchloride– sulphate lakes. were located in the distal areas of the basin. They are typified by thick and extensive gypsiferous formations with halite intercalations. The low concentration lacustrine systems Ž sulphate lakes. are associated with the Iberian and Catalan margins. They are represented by evaporitic units of limited extent and thickness. The evaporitic marine sedimentation of the potassic Navarra and Cataluna ˜ basins gave way to the continental sedimentation of the Barbastro Gypsum Žcentral and eastern sectors. and the Puente de la Reina Gypsum Žwestern sector., both Upper Eocene–Lower Oligocene in age ŽSaez ´ and Salvany, 1990; Salvany et al., 1994.. These formations crop out close to the Pyrenean margin ŽFig. 1. in anticline cores with Tertiary and Quaternary diapiric activity Ž Sole´ Sabarıs, ´ 1953a,b; Riba and Llamas, 1962b; Pena, ˜ 1975, 1983; Sancho, 1988, 1989.. During the Oligocene, two depocenters with greater amounts of subsidence formed in the western ŽNavarra–La Rioja. and eastern Ž Cataluna ˜ . sectors of the Ž Ebro Basin. In the western sub-basin a thick evaporitic body greater than 1500 m. was deposited. This includes the Falces Gypsum Ž Middle Oligocene. and the Lerın ´ Gypsum ŽUpper Oligocene–Lower Miocene. ŽSalvany, 1989, 1990; Salvany et al., 1994. . These formations generally have a gently folded structure with a strike parallel to the axis of the basin. The Falces Gypsum that crops out in the cores of anticlines is associated with long-sustained diapiric activity recorded in the overlying Ž Tertiary and Quaternary. sediments Ž Bomer and Riba, 1962; Riba, 1964; Benito and Casas, 1987a,b; Leranoz, 1989, 1993; Casas et al., ´ . 1994 . The slightly deformed Lerın ´ Gypsum crops out along the limbs of the folds. Next to the Iberian margin the Autol Gypsum Ž Upper Oligocene–Lower Miocene. was deposited, it is a correlative of the Lerın ´ Gypsum ŽMunoz ˜ and Salvany, 1990; Salvany et al., 1994. . In the western sub-basin, the depocenter migrated progressively throughout the Oligocene towards the centre of the basin. At the beginning of the Miocene it was placed in the central or Aragonian sector. The sedimentation in the eastern sector ceased, but in the western sector it continued in synclinal troughs ŽRiba et al., 1983. . In the central sector an extensive playa-lake was developed and the Zaragoza Gypsum Ž Upper Oligocene?–Lower Miocene. was deposited ŽQuirantes, 1978; Riba et al., 1983; Ortı, ´ 1990.. It is a thick sequence comprising up to 3000 m of gypsum Ž with anhydrite in depth. and includes halite units up to 120 m thick ŽTorrescusa and Klimowitz, 1990. . The Zaragoza Gypsum crops out in a wide area round Zaragoza Ž Fig. 1. . Several evaporitic units of Lower Miocene age occur around the southern margin of the Ebro Basin. These include the Ribafrecha Gypsum, Monteagudo Gypsum, Borja Gypsum, Vinaceite Gypsum and Calanda Gypsum. These evaporitic bodies, of limited extent

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and thickness, were deposited in shallow and low concentration saline lakes. These lakes formed in the distal areas of alluvial fans developed at the foot of the Iberian margin, or in inter-fan zones ŽMunoz ˜ and Salvany, 1990; Salvany et al., 1994.. The most recent evaporitic sedimentation recorded in the Ebro Basin corresponds with the Cerezo Gypsum Ž Upper Miocene. , deposited in the Bureba corridor at the extreme west of the basin. This formation reaches 250 m in thickness and is composed of gypsum and anhydrite with intercalations of detrital material, carbonate, glauberite and mirabilite beds Ž Anadon, ´ 1990.. All these Miocene evaporitic units have a general subhorizontal structure. Possibly at the end of the Miocene, or the beginning of the Pliocene, a proto Ebro River captured the depression, which consequently lost its endorheic character. The change to exorheic conditions coincided with the relative uplift of the Iberian Peninsula and downward movement of the Mediterranean Sea. In these circumstances, with semiarid morphoclimatic conditions, new alluvial systems developed. These selectively eroded the sedimentary fill of the basin and generated the main geomorphological features of the Ebro Depression. These include structural reliefs and stepped sequences of mantled pediments and terraces ŽGutierrez and Pena, ´ ˜ 1994.. The transition from endorheic to exorheic conditions was a crucial event in the development of gypsum karst. From that time, large volumes of evaporites were dissolved and removed by the new drainage systems. In addition, recent diapiric activity in some of the evaporite formations has produced structural landforms and deformational structures in Pleistocene and Holocene morphologies and deposits Ž Fig. 2. .

Fig. 2. Tilting caused by the diapiric activity of the Falces Gypsum affecting a Quaternary terrace of the Aragon ´ River in Caparroso ŽPhotograph by B. Leranoz ´ ..

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The most elevated areas of the Ebro Depression, with altitudes up to about 1000 m, correspond to the resistant conglomeratic beds located around its edges. The central carbonate platforms are also high, reaching 800 m. The lowest topographic point in the depression is the outlet of the Ebro River just 45 m above sea level. The climate of the basin is Continental Mediterranean with strong Atlantic influences in the western area. The area of maximum aridity is located in the lower zones of the central sector Ž Zaragoza and Lerida areas. , ´ where the mean annual precipitation is below 350 mm. The aridity decreases progressively towards the surrounding mountain ranges. Essentially gypsiferous evaporites crop out extensively in the Ebro Depression Ž Fig. 1. . The genesis and evolution of landforms in the gypsiferous terrains are mainly controlled by the surface and subsurface dissolution processes acting on this highly soluble rock. The specific soil and vegetation characteristics of these areas also play a significant role in landform development Ž Herrero, 1991. .

2. The process of gypsum dissolution The solubility of gypsum in water depends on the chemical composition of the aqueous solution, the temperature and pressure. Since we are interested in the role that gypsum dissolution plays in landform generation, the temperature and pressure conditions at the earth’s surface are considered. The solubilities of gypsum and halite in water, at 258C and 1 atmosphere pressure, are respectively 2.4 grl and 360 grl ŽFord and Williams, 1989. . According to Jackus Ž 1977. , in distilled water at 208C, gypsum and halite are respectively 183 and 25,000 times more soluble than calcite. These high solubilities explain the greater intensity and rate of landscape development in salt and gypsum karst compared with that of limestone karst. Temperature experiments show that at one atmosphere pressure, gypsum has its maximum solubility between 358C and 408C ŽHardie, 1967; Blount and Dickson, 1973; Sonnenfeld, 1984; White, 1988. . Another factor that influences gypsum solubility is the type and concentration of the dissolved ions in the aqueous solution. The solubility is strongly influence by saline and common ion effects ŽSonnenfeld, 1984; Mandado et al., 1984. . The saline effect produces an increase in the solubility of gypsum by the high ionic concentration Žionic strength. of the solution, this causes a decrease in the activity of the SO42y and Ca2q ions. Several authors have shown that the presence of NaCl in the solution may substantially enhance the solubility of gypsum ŽPonsjack, 1940; Schreiber and Schreiber, 1977.. Ponsjack Ž 1940. showed that NaCl concentrations of between 75 and 200 grl, increase the solubility of gypsum by 3 to 4 times over that in pure water. However, if the dissolved ions in water include Ca2q andror SO42y, the common ion effect

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occurs and the solubility of gypsum decreases. With these ions already present in solution, the product of ionic activity equals the solubility product produced by a lower dissolution level. In addition to the solubility of gypsum and its controlling factors noted above, there is also the problem of the dissolution rate and the processes that control it. According to White Ž 1984. , the rate at which the dissolution processes occurs depends on a chemical component and a hydrodynamic component. The chemical component relates to the equilibrium and kinetics of the dissolution reaction. Numerous authors consider that gypsum dissolution rates are controlled by diffusional transport across a boundary layer and that gypsum dissolves according to first order kinetics Ž Liu and Nancollas, 1971; Kemper et al., 1975; James and Lupton, 1978. : dCrdt s KA Ž Cs–C . From this it can be seen that the rate of gypsum dissolution ŽdCrdt . is a function of: 1—the degree of saturation of the solution Ž Cs–C., 2—the area exposed to the aqueous phase Ž A., 3—the rate of solution for unit area Ž K . . This coefficient K is inversely proportional to the thickness of the diffusion layer and depends on the diffusion coefficient of gypsum in the diffusion layer. The hydrodynamic component relates to the aqueous phase as a static or a moving medium which influences the procedure and the rate of mass transfer from the solid phase to the liquid phase Ž Trudgill, 1986. . If the water is static, the solute flux takes place by diffusion. If the water is moving, mass transfer in the moving water, termed convection, is added to the diffusion and increases the dissolution rate Ž Trudgill, 1986. . Berner Ž1978. differentiated two types of dissolution systems. The first is the surface-reaction controlled system; this applies to low solubility compounds, in which the dissolution rate is essentially controlled by the dissolution reaction Žthe chemical component.. The second is the transport-controlled system, which applies to high solubility compounds, in which the dissolution rate depends largely on the flow velocity and regime Ž the hydrodynamic component. . Most literature on gypsum dissolution concurs that gypsum dissolves in a transport-controlled dissolution system Ž Liu and Nancollas, 1971; Berner, 1978; White, 1988.. Consequently, the rate of gypsum dissolution and the intensity of the karstification processes depend greatly on the flow velocity and the flow regime, whether it is laminar or turbulent Ž Kemper et al., 1975; James and Lupton, 1978; White, 1988. . However, experimental studies by Raines and Dewers Ž1997., for gypsum in aqueous solutions at 258C with low ionic strengths and a range of saturation states, showed both a mixed surface-reaction and a transport control for the gypsum dissolution kinetics. In this experimental study, under laminar flow conditions, the dissolution rates were limited by transport through the diffusion layer. The transition from laminar to turbulent flow causes a sharp increase in the dissolution rate. Under a turbulent hydrody-

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namic regime, gypsum dissolution shows a surface-reaction control. The mixed behaviour has the effect that non saturated solutions can penetrate much further than one would predict for a transport controlled system; this favours the development of conduits in gypsum rocks. In the development of karst in gypsum formations, the physical–chemical factors that control the dissolution process are directly related to a complex suite of interacting geological and environmental factors. Some of the most relevant factors are listed below 2.1. Geological factors Lithological: texture, size and geometry of the crystals andror particles, internal structure, porosity, presence of non soluble components. Stratigraphical: thickness of the gypsum formations, intercalations of non soluble bodies, existence of halite beds, presence of adjacent aquifers. Structural: Structure of the gypsum formations, discontinuity planes Ž joints, faults, stratification planes. and their geometrical characteristics Žwidth, extension, density, orientation. , neotectonics. Geomorphological: relief configuration, time of development. Hydrogeological: type of flow Ž fissure flow, interstitial flow, flow in an adjacent aquifer. , flow velocity, flow regime Ž laminar or turbulent. , residence time of the water in the karstic system, chemical composition of water, water table fluctuations. 2.2. EnÕironmental factors The environmental factors mainly relate to climate, especially rainfall and temperature. The karstic systems developed in gypsum are very sensitive to climatic variations. For this reason, the study of the variable karstification process intensity, through time, could be an interesting tool to use in the reconstruction of palaeoclimatic conditions. The rapid evolution of gypsum karst also means that recent human activity can also considerably influence the genesis of some gypsum karst features such as sinkhole formation.

3. The water erosion of gypsiferous slopes In addition to the removal of gypsum by dissolutional karstic processes, water erosion at the surface also occurs. Experimental work has been undertaken near Zaragoza to quantify the effects. The Zaragoza Gypsum of Miocene age outcrops over a large area Ž11,000 km2 . in the central sector of the Ebro Depression. Here, the erosion processes that acted on the gypsum throughout the Quaternary have produced a badland landscape with a dense network of flat

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bottom infilled valleys. Studies of runoff and sediment yield from the gypsiferous slopes have been undertaken. Two representative experimental areas have been used near Zaragoza, at La Puebla de Alfinden ´ and Mediana de Aragon ´ ŽGutierrez et al., 1995.. At each location, two closed plots were installed on the ´ north and south facing slopes respectively. These slopes have a xeric vegetation with strict gypsiferous communities, although there are different associations at each location ŽDesir et al., 1995. . The south facing slopes are steeper than the north facing ones and show higher percentage of lichens and stone cover. The north facing slopes display a higher proportion of vegetation cover and organic matter. The experimental plots were instrumented with automatic raingauges and collector devices including two Geib type divisors for runoff and sediment collection. Rainfall simulation studies were carried out in both experimental areas. With a high rain intensity of 40–60 mmrh, the maximum infiltration velocity for these gypsiferous soils was found to be 30–35 mmrh ŽGutierrez et al., 1995.. ´ At the La Puebla de Alfinden ´ plots, 56 rainfall events giving rise to sediment production were recorded over a four years period Ž from July 1991 to July 1994.. A relationship between sediment yield and total rainfall was observed for each plot. The sediment yield was found to be generally higher for the south-facing plot than for the north-facing one. For the south-facing slope 62% of the recorded events produced more sediment than the north-facing slope. Furthermore, if only the events with significant sediment yield are considered, the south-facing plot generated more sediment than the north facing one for 83% of the events. The measured data show that the mean annual erosion rates for the north and south-facing slopes respectively are 7.53 and 10.42 trharyear. From data such as this it is possible to obtain the erosion rates, and to establish their relationships with the different vegetation associations. Because the study areas are representative of the region, which has a morphological, edaphological and vegetational homogeneity, it may be possible to extrapolate the relationships to larger units of the central Ebro Depression.

4. Karst in gypsum Despite the Ebro Depression being a semi-arid environment, the high solubility of gypsum facilitates the development of gypsum karst phenomena. Two contrasting karstification situations occur, uncovered karst and karst covered with mainly alluvial deposits. 4.1. UncoÕered karst In areas where bare gypsiferous formations crop out, without any type of cover, the karst landforms are mainly limited to karren, dispersed dolines and

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enclosed depressions Ž Gutierrez and Pena, ´ ˜ 1994.. The karren are mainly of the rillenkarren and napfkarren types which develop preferentially on nodules of alabastrine gypsum. The most striking examples of enclosed depressions are found on the plain of Bujaraloz. Here there are about one hundred scarped-edge dolines which measure up to a maximum of several kilometres in axial length. The karst here is developed in alternating subhorizontal limestones and gypsum and it is controlled by the predominant WNW–ESE joint direction Ž Quirantes, 1965.. Some of these closed depressions are flat bottomed and seasonally occupied by ephemeral salt lakes. In this region these salt lakes are called saladas. They are the most northern European playa-lake environments where evaporitic precipitation is currently occurring Ž Pueyo, 1979; Pueyo and de la Pena, ˜ 1991.. A genetic and evolutionary model for these saline closed depressions was proposed by Sanchez et al. Ž 1989, 1998. . They considered that ´ dissolution caused by meteoric water infiltration produced the initial dolines. They then deepened until their bases intercepted the water table and small lakes prone to periodic flooding formed. The exposure of the water table at the surface, with the consequent evaporation, creates topographically controlled water table gradients towards the saline lakes. The dolines then extend laterally with the marginal detritus, and the salts supplied by ascending groundwater, partially removed by the wind, the predominant direction of which coincides with the joint trend. These exceptional salt lakes constitute very sensitive ecosystems of great ecological value. Proposed plans to irrigate the land and change its use would alter the hydrochemical characteristics of these playa-lake environments. This would result in the disappearance of their exceptional flora and fauna and the replacement of it by another ordinary one ŽSuarez ´ et al., 1991.. An example of salt lake in a doline is La Sulfurica de Mediana de Aragon, ´ ´ located to the SE of Zaragoza; here the karstification is also structurally controlled Ž Van Zuidam, 1976. ŽFig. 3. . In the Barbastro anticline, there are also large closed depressions developed on a hectometre to kilometre scale. They are generated by the karstification of the Barbastro Gypsum and are strongly controlled by the folding direction ŽGutierrez et al., 1985; Sancho, 1988. . ´ A characteristic morphology, found on the bare gypsum surfaces in the central sector of the Ebro Depression, are gypsum domes. These cannot be considered as true karst landforms, although the dissolution processes takes part in their generation. These gypsum domes are bulge structures, subcircular in plan view and up to 1 m in diameter Ž Fig. 4. . The bulged gypsum layer may reach 20 cm thick and the cavity inside the domes can be up to 30 cm high. Frequently, the central parts of the domes collapse resulting in crater-like morphologies. Artieda Ž1993. studied these features using field and thin section petrographic techniques. He has attributed the genesis of these gypsum cupolas to in situ dissolution and associated precipitation of the surficial gypsum. After dissolution, the secondary precipitation of gypsum crystals takes place in the

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Fig. 3. Aerial view of the Salada de la Sulfurica de Mediana, an ephemeral salt lake in a doline ´ developed in gypsum. The picture was taken in summer when the flat bottom of the lake was covered by salts precipitated from the evaporated brines.

Fig. 4. Collapsed gypsum dome showing its crater-like morphology Žhammer for scale, 26 cm long..

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pores generated by the dissolution. This produces an increase in volume of the outer layers so that the crystallization pressures cause the gypsum to expand laterally resulting in bulging and the generation of the domed morphology. Radial and concentric cracks then develop until the top of the bulge eventually collapses. 4.2. Mantled gypsum karst associated with alluÕial systems The karstification processes are particularly pronounced where the gypsum is covered by alluvial deposits. This karstification can cause severe subsidence hazards with their associated social and economic consequences. Subsidence phenomena caused by the karstification of gypsum below alluvial deposits commonly affect the stepped levels of mantled pediments, terraces and infilled valleys which are called vales in Aragon. ´ The vales produce a typical landscape in the gypsiferous terranes with hilly ridges separating an intricate network of infilled valleys ŽGutierrez and Pena, ´ ˜ 1994. ŽFig. 5.. In the Ebro Depression, the mantled pediment and terrace deposits overlying the Tertiary gypsum formations show significant thickness variations. The deposit of an alluvial level may change in thickness over short distances and vary rapidly from less than 10 m to more than 110 m thick. The boundary between the Quaternary alluvial sediments and the gypsum bedrock generally

Fig. 5. Aerial view of flat bottom infilled valleys Žvales. dissecting the Miocene gypsum near to Zaragoza city.

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shows a very irregular geometry. These thickened alluvial deposits fill ‘basins’ and commonly have bed fans at the margins. In the places where the sedimentary deposits are thickened, the ratio of flood plain facies to channel facies is considerably higher and palustrine facies Ž marls and peat. commonly occur. In addition, the Quaternary alluvial cover shows abundant ductile and brittle deformation. This contrasts with the underlying gypsum that remains undeformed. This fact belies a tectonic origin for this type of thickening and deformation. In some instances, the deposits of different terrace levels may be superimposed and bounded by either angular or parallel unconformity. Despite this field evidence, some thickened terrace deposits of the Ebro River, that overly evaporites, have been explained as paleovalley infills ŽSoriano and Simon, ´ 1995.. An evolutionary model that involves synsedimentary karstic subsidence has been proposed to explain the anomalous features observed in the alluvial deposits above the evaporitic formations Ž Benito, 1989; Benito and Perez´ Gonzalez, 1990, 1993, 1994; Gutierrez, 1994a,b, 1995, 1996; Arauzo and ´ ´ Gutierrez, 1995; Gutierrez and Arauzo, 1997; Benito et al., 1996, 1998. . ´ ´ Water flowing in alluvial deposits that overly evaporitic bedrock causes karstification of the soluble substratum. As the evaporite is removed, the alluvial cover subsides, either in a ductile way by passive bending, or in a brittle way by collapse. The processes of differential karstification of the evaporite, and subsidence in the alluvial cover, can produce closed depressions Ž dolines. that are of variable size and geometry. Within these closed depressions, where the water table is close to, or above, the surface, palustrine environments develop ŽFig. 6.. As these subsiding depressions develop they break the equilibrium profile of the alluvial surface. The alluvial system consequently responds by aggrading in the subsiding areas until it reaches a condition of dynamic equilibrium, in which the subsidence rate is balanced by the aggradation rate. The subsiding areas act as ‘traps’ for sediment, with both bed load and suspended load deposition. This synsedimentary karstic subsidence causes localized thickening and deformation of the alluvial deposits. The sedimentary fill of these ‘solution-induced basins’ has a typical basin type of structure with bed fans developed at the margins. In addition, there is an upwards attenuation in the degree of deformation, typical of such karstic basin deposits. In these alluvial sediments affected by synsedimentary subsidence it can be seen that subsidence controls the dynamics of the alluvial system. It triggers aggradation and degradation processes and controls channel patterns changes, migration and avulsion processes. When the fluvial system has become entrenched, the deposits of a subsequent terrace level can be inset in the bedrock or superimposed to the deposits of older terrace levels. The different sedimentary units can be bounded by angular unconformities at the margins of the areas affected by subsidence, or parallel unconformities towards the depocenter of the solution induced basins. In spite of

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Fig. 6. Aerial view of a swamp with palustrine vegetation developed in an alluvial doline in the Ebro valley near to Casetas village. The Remolinos–Juslibol scarp can be seen in the background of the view.

the karstic subsidence, the morphogenetic surfaces of the different terrace levels can be inset and stepped. Variations in the thickness of the alluvial sedimentary sequences at different locations record temporal and spatial variations in the magnitude of the causative subsidence. The evolutionary pattern of this phenomenon, over space and time, can be controlled by internal factors Žstructural, hydrogeological, hydrochemical, lithological. and external factors Ž climate and neotectonics. . Climatic variations over time can cause temporal variations in the water supply and the resultant karstification. Extensional neotectonic activity can generate and open discontinuity planes in the evaporitic rocks. This can induce accelerated karstification and affect the subsidence in both a temporal and a spatial way. Examples of the interaction between karstic subsidence and alluvial sedimentation are presented by Benito Ž1989., Benito and Perez-Gonzalez ´ ´ Ž1990, 1993, . Ž . 1994 and Benito et al. 1996, 1998 . Along the lower reach of the Gallego ´ River they have recorded overlapping deposits belonging to several terrace levels. A map of alluvium isopachs shows that these deposits are up to 110 m thick in the greatest depocenter and they fill several solution-induced basins and troughs developed in the Zaragoza Gypsum. Here, two thickening periods related to greater subsidence have been dated by palaeomagnetic reversals as Matuyama Žpre-0.78 ky. and Brunhes Žpost-0.78 ky. . These periods of greater karstification and subsidence have been related to intervals of high water supply.

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Ž1993. also observed anomalous thickening of the Ebro River terraces Leranoz ´ deposits where they overly the Oligocene gypsum formations in Navarra. The deposits belonging to some terrace levels of the Jalon ´ and Huerva Rivers also show thickening of at least 60 m. These thickened alluvial deposits are economically important since they constitute valuable water reservoirs and rich sources of aggregates. Synsedimentary subsidence induced by alluvial karstification of gypsum has also been observed in infilled valleys that drain into the main Ebro Valley and its tributaries. These vales form a dense network of ephemeral creeks that dissect the gypsiferous formations Ž Fig. 5. . Within the sedimentary fill in some of the vales several morpho-sedimentary units can be differentiated. The deposits in the vales are essentially formed of gypsiferous silts produced by the weathering and slope runoff from the surrounding gypsum. Chronological information has been obtained by radiocarbon dating and pottery remains found within the deposits ŽVan Zuidam, 1975, 1976; Burillo et al., 1985; Soriano and Calvo, 1987; Soriano, 1989; Pena 1994. . In ˜ et al., 1993; Arauzo and Gutierrez, ´ some of the infilled valleys, the deposits are locally thickened and deformed due to synsedimentary subsidence. The thickened sediments include a high proportion of carbonate and carbonaceous facies. They show the structures of small basins with bed fans at their margins. These facies were deposited in palustrine environments developed in ‘solution induced depressions’ Ž Arauzo and Gutierrez, ´ . 1994, 1995; Gutierrez and Arauzo, 1997 . Several outcrops show the superposi´ tion of different sedimentary units separated by angular unconformities ŽFig. 7.. In addition to the observations seen at outcrop, modern depositional examples reinforce the interpretation. Palustrine environments Žponds. have been observed in several of the infilled valleys where they have formed in actively subsiding dolines developed on a hectometre scale ŽGutierrez et al., 1985; Arauzo and ´ Gutierrez, 1994.. ´ In the Ebro Depression, a common feature of the main fluvial valleys developed on gypsiferous lithologies is that they are commonly assymmetrical. They occur with a sequence of stepped terraces at one margin and a prominent scarp at the other. The genesis of these escarpments relates to the overall downcutting and lateral migration of the rivers during their Quaternary evolution. These scarps generally show numerous ancient and currently active rotational slides, topples and falls. In general, the main factors that contribute to the generation of these slope movements are unloading, the structurally controlled karstification of the gypsum and river undercutting at the scarp foot Ž Ibanez ´˜ and Mensua, 1976; Pellicer et al., 1984; Faci et al., 1988a,b; Ayala et al., 1988; Leranoz, 1993; Gutierrez et al., 1994.. Where these scarps have retreated ´ ´ rapidly, for example at the structurally controlled Alfajarin scarp, impressive hanging valleys have developed Ž Fig. 8. . In 1874 a catatrophic slope movement, with a volume of several thousands of m3, occurred in Azagra Ž Navarra. . This slide mass killed and buried more than a hundred of people Ž Faci et al., 1988a..

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Fig. 7. Two morpho-sedimentary units of the Torrecilla Val bounded by angular unconformity. The calcareous lower unit is deformed by synsedimentary subsidence whereas the adjacent gypsum remains undeformed.

Fig. 8. Hanging valleys in the Alfajarın ´ gypsum scarp.

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In some places these landslides have dammed the rivers and caused flooding of the alluvial plains ŽAyala et al., 1988. . This is a modern geological hazard for some reaches of the Ebro River, such as below the Remolinos–Juslibol scarp ŽFig. 9.. Dolines especially occur in the Quaternary alluvial deposits which include mantled pediments, terraces, infilled valleys and alluvial fans where they overlie evaporitic formations. The dolines show a wide variation in their geometry and morphology. These alluvial dolines, or sinkholes, preferentially occur in the lower alluvial levels and where the alluvial deposits are thin and unaffected by synsedimentary karstic subsidence. Around the outskirts of Zaragoza sinkhole activity is high and numerous sinkholes are being actively generated. This high activity has produced dense doline fields in areas such as Casetas and Utebo ŽVan Zuidam, 1976; Gutierrez et al., 1985; Soriano, 1986, 1988, 1990, 1992; ´

Fig. 9. Rotational landslide in the Remolinos–Juslibol gypsum scarp. The slided masses have the potential to dam the Ebro River, cause flooding of the alluvial plain and avulsion of the river channel.

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Simon ´ et al., 1991; Soriano et al., 1994; Soriano and Simon, ´ 1995., the areas of Ž Alfajarın, ´ La Puebla de Alfinden ´ and Villamayor Benito, 1987, 1989; Benito and Gutierrez, 1987, 1988. and the left bank of the Gallego River ŽBenito and ´ ´ Perez ´ del Campo, 1991.. The identification and mapping of subsidence depressions in the Ebro valley is a extremely difficult task which is complicated by human activity, notably the excavation of illegal aggregate pits. In many instances the inhabitants villages close to Zaragoza have reported that material removed from gravel pits has been used to fill the dolines. The misidentification of gravel pits as dolines, and the removal of dolines by filling, may lead to the generation of erroneous geomorphological maps and questionable interpretations about the geological controls of doline formation. In many outcrops, where the boundary between the Quaternary alluvium and Tertiary gypsum is exposed, it is possible to observe palaeodolines. These can be collapse dolines ŽFig. 10., bending dolines or a gradation between both

Fig. 10. Palaeocollapse affecting the deposits of a mantled pediment located to the south of Zaragoza city Žrucksack for scale, 60 cm high..

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Fig. 11. Processes involved in alluvial doline development.

extremes. In exposures it can be seen how the detrital alluvial deposits fill cavities Žpipes, funnels, fissures. caused by the karstification of the gypsum ŽFig. 10.. This palaeokarst information provides details about the subsurface processes that cause the sinkholes. By integrating the geomorphological evidence, with the palaeokarst recorded in the alluvial deposits, it is possible to understand the processes and controlling factors involved in the generation of alluvial dolines. Three linked processes control the development of alluvial dolines, karstification, subsurface erosion and subsidence Ž Benito and Perez ´ del Campo, 1991. ŽFig. 11.. 4.2.1. Karstification of eÕaporitic bedrock oÕerlain by alluÕial deposits In this situation the water flowing through the alluvial aquifer dissolves the substratum. As a result, the karstification process may lower the surface of the

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gypsum bedrock. If the karstification is structurally controlled, the progressive widening of discontinuities in the gypsum may also produce voids of different geometries including pipes, funnels or cavities. 4.2.2. Subsurface mechanical erosion of the alluÕial coÕer and migration of the detrital material through the Õoids generated by the karstification processes (piping) This downward transport of material may occur by gravity flow of the dense loose water-saturated sediment. The high pore fluid pressure in the sediment reduces both the effective normal stresses and the intergrain friction. As a consequence, the sediment–water mixture lacks resistance to shear and behaves like a viscous fluid. The subsurface flow of water through the alluvial–karstic system may also erode and remove material to enhance cavity migration ŽGutierrez, 1996.. As a consequence of the erosion and downward transport of ´ the alluvial material, the top of the cavity may advance, or pipe, towards the surface reducing the cover thickness. 4.2.3. Subsidence of the alluÕial coÕer When the effective weight of the alluvial cover exceeds its mechanical strength, subsidence occurs and an alluvial doline forms. Two types of subsidence mechanism occur: bending Ž ductile behaviour. causes dolines with diffuse

Fig. 12. Bending doline formed in the Portazgo industrial area Žoutskirts of Zaragoza.. This currently active subsidence depression severely affected a factory which was finally demolished.

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Fig. 13. Collapse affecting a track in the proximity of Zaragoza.

edges ŽFig. 12., collapse Žbrittle behaviour. generates sinkholes with scarped edges Ž Fig. 13.. In practice a complete gradation between both extremes can be found.

5. Geological hazards associated with gypsum karst In the evaporitic terranes of the Ebro Depression, the great majority of human development including urban areas, communication routes, irrigation channels and agriculture occur on the Quaternary alluvial deposits. In many areas the active subsidence is a geohazard that causes environmental problems that have a social and economic impact. There is a reciprocal interaction between anthropic activities and sinkhole generation. Many human activities can accelerate and trigger the processes involved in the generation of sinkholes.

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The effects of subsidence are particularly harmful to linear constructions and buildings. Numerous roads, motorways and railways have been damaged causing temporary disruption to these communication routes Ž Benito and Gutierrez, ´ 1987, 1988; Perez ´ del Campo and Lanzarote, 1988; Perez ´ del Campo, 1989; Soriano et al., 1994; Soriano and Simon, ´ 1995; Benito et al., 1995.. Catastrophic collapses in roads and buildings can occur with potentially fatal consequences. Problems with buildings are quite frequent in some areas. For example, around Casetas and Utebo where several buildings have been damaged Ž Gutierrez et al., ´ 1985; Soriano, 1988. . In the Portazgo industrial estate some factories have been pulled down ŽFig. 12. and a gas explosion was also attributed to the breakage of a gas pipe caused by subsidence. Locally the water system is also disrupted by subsidence and 20,000 inhabitants hereabouts commonly loose their water supply. The most striking example of subsidence affecting development comes from the recently built village of Puilatos, in the Gallego Valley. This was ´ severely damaged by subsidence and abandoned before it could be occupied ŽBenito and Gutierrez, 1987, 1988.; US$92,000 were invested in this village ´ ŽAgriculture and Environment Department, D.G.A., personal communication. . Collapses also affect irrigation channels and result in substantial economic losses. These include the costs of repairing the damage, the costs of interruptions in the water supply and the costs associated with the loss of agricultural production ŽLlamas, 1962; Riba and Llamas, 1962a,b. . In 1996 a collapse cut the important Canal Imperial at Gallur village. The cost of the repair work alone was estimated to be US$625,000 Ž Jose Antonio Martınez, C.H.E., personal ´ communication., the consequential losses were not quantified. Though not directly visible, sinkholes also form in the submerged beds of the river channels. On December 19th, 1971, a bus fell from a bridge into the Ebro River at Zaragoza, where the so-called ‘San Lazaro well’ is located. Ten people lost their lives in this accident, nine of them where never found. People suggested that they were drawn by suction into this sinkhole. Years later, a skin-diver explored the sinkhole up to 16 m in depth without reaching its bottom. Certain human activities favour the generation of alluvial dolines. Benito and Ž 1987, 1988. observed that dolines preferentially form in close Gutierrez ´ proximity to unlined canals. The supply of irrigation water to crop fields can also play an important role in sinkhole generation ŽVan Zuidam, 1976; Llamas, 1962; Benito, 1989, 1993; Benito and Perez ´ del Campo, 1991; Soriano, 1992; Soriano et al., 1994; Soriano and Simon, ´ 1995; Benito et al., 1995.. The variations in the water table induced by water pumping are also another human-induced triggering factor that causes dolines. As the water level declines it causes a loss of buoyant support to the ground, increases the flow gradient and water velocity, facilitates the aquifer recharge and reduces the geomechanical strength of the cover ŽNewton, 1984a,b; Lamoreaux and Newton, 1986. . In addition, a vacuum suction effect may operate ŽBenito, 1987, 1989, 1993. .

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Fig. 14. Doline used for dumping waste in the Barbastro anticline ŽAlcampel..

Another subsidence triggering factor are static or dynamic loading on metastable areas. Dolines are also associated with other environmental problems. These sinkholes are frequently used as areas to dump uncontrolled industrial and domestic waste ŽFig. 14. . Because of the direct connection between them and the aquifer, such uncontrolled dumping can cause rapid chemical and bacterial groundwater pollution ŽSancho, 1988; Benito, 1993; Benito et al., 1995. .

6. Final considerations The recent research has greatly improve our knowledge about the different geomorphological aspects of the Tertiary gypsum formations in the Ebro Depression. Nevertheless, more research is required to solve outstanding scientific puzzles and apply the results in a social and economic way. From a scientific viewpoint, dissolution-induced subsidence, if not recognized and considered, can lead to serious interpretational errors in many different geological disciplines, including applied geology, geomorphology, tectonics, stratigraphy and paleontology. For example, the deformation structures in some Quaternary alluvial sediments, overlying an evaporitic substratum, have been interpreted as neotectonic structures and used to infer regional stress fields. Although karstification and subsidence may be structurally controlled, the

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primary dynamic origin of these gravitational deformations is the lack of basal support caused by the karstification of the soluble bedrock. Another potential problem arises when fossiliferous beds are found in alluvial sequences affected by dissolution subsidence. The fossils are usually attributed to the morphological alluvial level immediately above them. However, they may correspond to a much older sedimentary unit covered by the deposits of a younger alluvial level. The karstic subsidence that affects the Ebro Depression alluvial systems is a very complex process of great relevance to applied geology. Benefits include thickened alluvial deposits that act as water reservoirs and provided constructional aggregate. Unfortunately, the problems caused by gypsum karst-related subsidence are large, involving considerable amounts of destruction to buildings, roads and canals with the consequent large financial losses. The most important challenge for scientists working on gypsum dissolution subsidence in the Ebro Depression is to map the subsidence hazards and make the information available for land use planning. This is an extremely difficult task because it is problematical to produce realistic maps of subsidence features in populated areas. This is because the dolines are commonly filled soon after they form. In addition they may be confused with the abundant gravel pits that are present in the alluvial deposits. This information can be supplemented with records of building damage and problems with roads and services. The observations of the subsidence features at the surface are complemented, and can often be explained, by the investigation of paleokarst and paleodoline features seen in the outcrops of older deposits. The present morphology explains the surface effects and the study of sections through the older deposits gives a good key to the understanding of the dissolutional processes that occur at the bedrockrcover interface. By integrating morphological mapping with geological and hydrogeological investigation the foundations of hazard assessment and planning in the Ebro Basin have been laid. The challenge ahead is to finish the work and apply it in a beneficial, social and economic way.

Acknowledgements The authors would like to thank the contributions and comments from Dr. Anthony Cooper ŽBritish Geological Survey, Nottingham, UK. and an anonymous reviewer which have led to a considerably improved manuscript. The revision of the aspects related to gypsum dissolution by Professor Luis Oro ŽChemistry Department, Zaragoza University. is also gratefully acknowledged. Ž AgriWarm thanks to Jose Antonio Martınez ´ ŽC.H.E.. and Arancha Gutierrez ´ culture and Environment Department, D.G.A.. for provided data about the cost of damages caused by subsidence.

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