Evaporite formations in Spain

The Iberian Peninsula constitutes a microplate located between the convergent European and African plates. Broadly, the geology of this microplate includes the Hercynian (Variscan) Massif, made up of pre-Mesozoic metamorphic and igneous rocks, and Alpine orogens and Tertiary basins dominated by Phanerozoic sedimentary rocks (Fig. 1). The vast majority of evaporite formations, both marine and continental in origin, occur in these orogens and basins. The evaporite outcrops in Spain cover more than 35,000 km2, approximately 7% of the total country area (∼500,000 km2) (Macau and Riba 1962). These figures explain the significant impact of the environmental problems related to evaporite karst in Spain. Marine evaporite sedimentation in Spain covers a wide time span, from the Triassic to the present-day, whereas most of the continental evaporites were deposited in lake environments during the Tertiary. Geochemical and isotopic studies demonstrate that the Tertiary lacustrine evaporites were derived from the recycling (dissolution and reprecipitation) of Mesozoic marine formations (Utrilla et al. 1992). Most of the evaporitic rocks are made up of Ca-sulfate (gypsum and anhydrite) or Ca-sulfate and halite. Some marine and continental formations include K-Mg-chlorides and Na-sulfates (glauberite and thenardite), respectively.

Fig. 1
figure 1

Distribution of the main evaporite outcrops in Spain and principal fluvial systems affected by synsedimentary subsidence phenomena caused by evaporite dissolution. The small sketch shows the main geological domains in the Iberian Peninsula

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Gypsum, with a solubility of 2.4 g/l, is commonly the only evaporitic mineral that crops out. Although, the highly soluble chloride salts and Na-sulfates rarely crop out, their dissolution by groundwater has played an instrumental role in the development of some karstic phenomena. Most of the outcropping gypsum corresponds to secondary gypsum coming from the hydration of anhydrite and, in the case of the continental evaporite bearing Na-sulfates, the incongruent dissolution of glauberite. Primary gypsum has been preserved only in some Neogene marine and terrestrial formations (Ortí 1989; Ortí et al. 1992). A great proportion of the exposed evaporite formations were deposited in Tertiary basins linked to the synorogenic and postorogenic evolution of the Alpine ranges (Fig. 1). Some of these basins underwent a progressive transition from open-marine conditions to a continental endorheic regime. Broadly, the Paleogene evaporites are affected by compressional deformations, whereas the Neogene evaporites commonly show a subhorizontal structure.

Marine Mesozoic and Tertiary evaporitic formations

The most widespread episodes of evaporitic sedimentation took place during Triassic and lower Liassic times in shallow-marine platform environments (lagoons, sabkhas) affected by a rifting process (Ortí et al. 1996). Triassic evaporites are made up of Ca-sulfate and halite units up to 400 m thick embedded in variegated marls and shales. This formation occurs in numerous outcrops dispersed over the Alpine orogens, locally forming diapiric structures (Fig. 1). The lower Liassic evaporites (anhydrite zone), up to 800 m thick, are composed of Ca-sulfates associated with dolomites (Pérez-López et al. 1996). Borehole data reveal the presence of several Jurassic and Upper Cretaceous anhydritic units more than 100 m thick in some sectors of the Iberian Peninsula. In the Iberian Range and in the Pyrenees there are also gysiferous units deposited in transitional and probably continental environments with a poorly constrained late Cretaceous-Paleocene age (Garum facies). Jurassic and Cretaceous Ca-sulfate units, with a limited outcrop extent, are frequently represented close to, and at, the ground surface by collapse breccias generated by interstratal karstification of the evaporites and brecciation of the associated carbonate rocks, forming the so-called carniolas (Gutiérrez et al. 2001).

During the Paleogene, marine evaporitic sedimentation was restricted to the southern foredeep of the Pyrenees, where evaporite deposition took place during two regressive phases: the Middle Eocene phase (Lutetian), and the Upper Eocene phase (Priabonian). Evaporite deposition of the first phase, confined to the eastern sector, is represented by the Beuda Gypsum, composed of 100 m of anhydrite/gypsum and minor halite deposits at depth (Ortí and Rosell 1997). The second phase developed in two sub basins, probably linked in the initial stage to form a single sedimentary trough, 300 km long, called the South Pyrenean Potash Basin. These sediments are composed of Ca-sulfate and halite with a substantial amount of K-Mg chlorides, mainly sylvite and carnallite. The Cardona Saline Formation (eastern subbasin), with 300 m of chlorides, remains in the autochthonous zone of the Ebro Basin, whereas the Guendulain Formation (western subbasin), up to 100 m thick, has been incorporated into the allochthonous structural units of the Pyrenees (Rosell and Pueyo 1997).

Neogene marine formations occur in the Penedés Basin (Catalan Coastal Range) and in a large number of intramontane basins of the eastern sector of the Betic Cordillera. The internal basins of the Betic Cordillera (Lorca, Fortuna, Guadalentín, Granada) host Tortonian-early Messinian evaporitic sequences up to several hundred meters thick made up of Ca-sulfate and halite that record a transition from marine to continental conditions (Playà et al. 2000). In the external basins of the Betic Cordillera (Sorbas, Almería, Nijar-Carboneras, San Miguel de Salinas, Palma de Mallorca), the marine evaporites, Upper Messinian in age, are composed of 12–14 cyclic layers of primary selenitic gypsum with a total thickness of 70–130 m (Rosell et al. 1998).

Continental Tertiary evaporitic formations

Most of the main Spanish Tertiary basins contain extensive and thick continental evaporitic formations. The Ebro Basin, subsequent to the Priabonian potassic phase, evolved into an endorheic condition giving way to the deposition of extensive Ca-sulfate and halite lacustrine evaporitic formations of Upper Eocene-early Oligocene age. These are the Barbastro Gypsum and the Puente de la Reina Gypsum, located in the central-eastern and the western sectors of the basin, respectively. They reach 300–400 m in thickness and crop out in the core of several salt anticlines (Salvany 1997). From the Middle Oligocene to the early Miocene, two thick evaporitic formations were deposited in the western sector of the Ebro Basin, the Falces and the Lerín Formations. The tightly folded Falces Formation reaches more than 1,000 m thick in the core of diapiric anticlines and is made up of Ca-sulfate, glauberite, and halite. The Lerín Formation, 500–1,000 m in thickness, is made up of Ca-sulfate, glauberite, halite, and polyhalite (Salvany 1997). At the beginning of the Miocene, the basin depocenter shifted to the central sector of the Ebro Basin, where the Zaragoza Formation (upper Oligocene?-lower Miocene) was deposited. This formation, 800 m thick, crops out around Zaragoza city and includes, close to the surface, halite and glauberite units more than 150 and 30 m thick, respectively (Salvany et al. 2007). The youngest evaporitic unit corresponds to the Cerezo Gypsum (Upper Miocene), located in the Bureba corridor, which links the Ebro and the Duero basins. This unit, about 200 m thick, bears Ca-sulfates and glauberite (Anadón 1990). In addition to these formations deposited in central high-salinity lakes, the Ebro Basin fill also contains several minor gypsum units deposited in marginal lakes, like the Paleogene evaporites associated to the Catalan Coastal Range and the Oligo-Miocene units located along the Iberian margin of the basin (Ortí 1997).

Evaporite sedimentation in the Tertiary Duero Basin is recorded by a few Middle–Upper Miocene gypsum units less than 100 m thick with a significant proportion of insoluble sediments (Mediavilla et al. 1996). The Tertiary Tajo Basin is composed of two subbasins: the western sector or the Madrid Basin, and the eastern sector or the Loranca Basin. During the Paleogene, evaporite deposition was restricted to the Madrid Basin, represented by folded Ca-sulfate units. This basin also contains an extensive Miocene evaporitic succession constituted by two units: the Saline Unit and the overlying Intermediate Unit. The Saline Unit, several hundred meters thick, bears substantial amounts of halite and Na-sulfates in the subsurface (Orti et al. 1979; García del Cura et al. 1979), whereas the Intermediate Unit is primarily made up of gypsum. The Loranca Basin hosts several Miocene gypsum units less than 100 m thick (Torres et al. 1985).

In the Calatayud Basin (Iberian Range), the evaporitic sediments crop out in Calatayud and Barrachina areas. In Calatayud area the evaporitic sequence is composed, in ascending order, of a halite unit more than 350 m thick of probable Oligo-Miocene age, a Ca-sulfate and Na-sulfate unit more than 200 m thick, and an upper gypsum unit locally more than 150 m thick (Ortí and Rosell 2000). In this area borehole data indicate the presence of a Neogene Ca-sulfate formation more than 150 m thick with halite beds in the subsurface (Sanz-Rubio et al. 2003). Also in the Iberian Range, the Teruel Graben hosts several Miocene to Pliocene evaporite units (Orrios Gypsum, Tortajada Gypsum, Libros-Cascante Gypsum, and Aljezares Gypsum) formed mainly by primary gypsum that locally reach more than 150 m in thickness. Gypsum units less than 100 m thick are known in the Neogene infill of the Granada and Baza basins in the Betic Cordillera.

Karstification of Mesozoic evaporitic rocks

Borehole data and field stratigraphic evidence demonstrate that the Mesozoic evaporitic formations commonly thin towards the ground surface due to interstratal karstification processes caused by groundwater flow. Triassic evaporites, commonly with thick halite units at depth, are made up of gypsum and shales at the surface. Jurassic and Cretaceous formations, composed of Ca-sulfates and carbonates at depth, typically pass into dissolution-collapse breccias (carniolas) near the surface. The wedging out of evaporitic sequences and the collapse breccias reveal that large volumes of evaporites have been evacuated progressively in solution by underground flows causing the subsidence of overlying sediments. These flows were one of the sources for the brines of the lake systems where the Tertiary evaporites were formed by a recycling process (Coloma et al. 1997; Sánchez et al. 1999). Although not studied specifically in Spain, these interstratal karstification processes and the consequent subsidence phenomena may have relevant geological implications: (1) Karstic gravitational deformations affecting sediments underlain, now or in the past, by evaporites, may be erroneously interpreted as tectonic in origin. (2) The dissolution-induced subsidence phenomena may have controlled thickness variations in sedimentary units deposited synchronically with the removal of the evaporites (synsedimentary subsidence). (3) Uncertainties related to the previous existence of evaporitic units removed in solution, and the unknown original thickness of some formations, may make some lithostratigraphic and paleogeographic interpretations difficult. (4) Since some evaporitic formations (primarily Triassic) constitute a major detachment level in some thrust belts, their subjacent karstification may have influenced the kinematics and style of these contractional structures.

The frequent occurrence of springs of the sodium chloride and calcium sulfate hydrochemical facies associated with Mesozoic evaporitic formations demonstrate that the deep-seated karstification is a currently active process. A remarkable example corresponds to the lower Liassic collapse breccias in the Iberian Range. This hydrostratigraphic unit, with a high secondary permeability related to interstratal karstification of anhydrite, constitutes a major regional aquifer with a great economic and environmental significance. Locally, hot springs related to the rapid rise of groundwater through this highly porous formation have propitiated the development of wetlands (e.g. Ojos de Pontil in the Ebro Basin), the source of rivers (e.g. Ginel River in the Ebro Basin), and the construction of spas with a highly positive influence on the local economy (e.g. Fitero and Arnedillo villages in the western sector of the Ebro Basin) (Coloma et al. 1997). Triassic evaporites, which crop out in numerous areas throughout the Alpine ranges, are the Mesozoic units that display the best-developed karst systems.

Karst in the evaporite Triassic rocks of the Betic Cordillera

The Antequera karst

The most outstanding evaporite karst systems developed in the western sector of the Betic Cordillera are found in some halokinetic structures developed in the so-called “Antequera Trias” (Fig. 1). This geological unit corresponds to a chaotic megabreccia with gypsum bodies at the surface and halite and Ca-sulfate masses at depth, derived from Triassic formations in Miocene times by olistostromic processes. A close relationship between the geological structure and the karst morphology and hydrochemistry has been documented in several evaporitic outcrops like in Gobantes-Meliones and Salinas-Fuente Camacho (Calaforra and Pulido-Bosch 1999a). The Gobantes-Meliones outcrop consists of two dome structures with sandstones, limestones and ophites in the outer zones, and evaporites in the core. Here, the collapse sinkholes and the calcium-sulfate springs are concentrated in the central portion of the halokinetic structures, whereas the outer zones are characterized by broad subsidence depressions and springs of the sodium chloride hydrochemical facies. The rise of these salt structures has induced the development of deeply incised karstic canyons, like the Guadalhorce River canyon and the Martín Arroyo (Durán 1984), and perched springs and caves like the El Aguila Cave (Calaforra 1998). This cave, developed at the contact between gypsum and carbonate rocks, reaches 120 m in depth and contains a chamber 25 m high and about 200 m2 in area.

The existence of halite at depth is evidenced by the hydrochemistry of the spring waters. The Meliones Spring, with a mean discharge of 1–2 l/s, has an electrical conductivity higher than 200,000 μS/cm. This spring, located in the upstream sector of the Guadalhorce River reservoir, issues around 5,000–10,000 tons of sodium chloride per year, causing a severe degradation of the reservoir waters that supply Malaga city. Several measures have been attempted to mitigate the problem, but with no success. An attempt was made to prevent the water outflow by drilling boreholes directly into the spring. Additionally, several dolines and cave entrances, including the El Aguila Cave, were sealed with compacted clays and concrete to reduce water recharge of the karstic aquifer (Fig. 2a). Obviously, this measure did not reduce the discharge in the Meliones Spring, fed by deep underground flows, and it caused a serious adverse impact on the karst environment and its protected subterranean fauna. Recently, the administration has proposed construction of a desalinization plant downstream of the Guadalhorce Dam.

Fig. 2
figure 2

a Sealing with compacted clays and concrete of the sinkhole that gives access to the El Águila Cave to prevent water recharge. The natural entrance to the cave was deteriorated with construction of a concrete structure. b Sinkhole resulting from the upward stoping of cavities generated by solution mining in the Triassic evaporites of the Polanco Diapir. c Mio-Pliocene sediments in Teruel Graben (Iberian range) affected by a monoclinal flexure generated by the interstratal karstification of Triassic evaporites. d General view of Gallocanta Lake (Iberian Range)

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The Antequera Triassic outcrops also contain a large number of ephemeral lakes of great environmental value constituting the so-called “Betic endorheism” (Durán and López-Martínez 1999). The origin of these closed depressions is largely related to subsidence phenomena caused by rising groundwater flows. The most outstanding example corresponds to the Fuente de Piedra Lake, included in the Ramsar Convention of Wetlands (Fig. 1). This saline lake, with sodium-chloride waters, covers 13.6 km2 and hosts the largest breeding colony of flamingos (Phoenicopterus rubber) in the Iberian Peninsula.

The Vallada karst

The Vallada karst, located in the transitional zone between the Betic Cordillera and the Iberian Range, is developed in a diapiric structure made up of the Upper Triassic Keuper facies. Outcropping sediments include massive gypsum, shales, marls and dolomite beds. The structural evidence of diapirism and the presence of saline springs indicate the existence of halite in the subsurface. The most remarkable feature corresponds to the 210 m deep El Sumidor Cave, which is the second deepest gypsum cave in the world. The large shafts of this cave are mostly carved in gypsum units juxtaposed against steeply dipping dolomite beds. Although, calcium-sulfate waters flow through the El Sumidor Cave, the spring that drains the system, located 400 m beyond the lowermost accessible point of the cave, issues water of the sodium-chloride hydrochemical facies. This hydrochemical change, together with an increase in the discharge and temperature of water in the spring, is attributed to incorporation along the last 400 m long reach of the flow path of NaCl-rich upward flows coming from deep halite bodies (Calaforra 1998).

Evaporite karst in Triassic formations of the Pyrenees

Outcrops of Triassic evaporites in the Pyrenees are usually situated in the core of anticlinal structures and diapirs. The circulation of groundwater through the upper part of diapiric bodies, that commonly bear large volumes of halite at depth, has resulted in the development of thick caprocks devoid of sodium chlorides. This indicates that both diapirism and opposing dissolution-induced subsidence phenomena have operated in the evolution of these salt structures. According to borehole data, Quaternary alluvium underlying the Nervión River floodplain in the central sector of the Orduña Diapir reaches more than 80 m in thickness, indicating that the fluvial system has been affected by synsedimentary subsidence caused by karstification of the bedrock (Arrate and Sanz de Galdeano 2002). The La Muera Spring, located in the lowest point of the diapir next to the Nervión River, issues around 1,000 tons of solutes per year.

The occurrence of sinkholes is a relatively frequent process in some diapers, like Orduña, Salinas del Oro, and Estella. In the Estella Diapir, Eraso (1959) has documented gypsum caves several tens of meters long (Longinos Cave), bedrock collapse sinkholes up to 35 m deep and 50 m across, and a saline spring with a mean discharge of 100 l/s. In most cases, the highly concentrated waters of the springs have a detrimental effect, causing degradation of the surface waters (Ega and Nervión Rivers). In some cases, these springs are used to produce salt (Salinas de Añana) or for therapeutic purposes (Orduña, Estella), thus constituting a highly valuable resource for the local economy. In the Polanco Diapir there is a group of sinkholes that have resulted from the upward propagation of cavities generated by solution mining (Fig. 2b). According to Cendrero and González-Lastra (1980), the old mining operations in this diapir, which started in 1907, generated cavities at depths of 34 m. The most notable karst feature in Triassic evaporite outcrops of the central-eastern sector of the Pyrenees corresponds to the Estaña Lakes. They are a group of dolines and uvalas that host three permanent lakes with calcium-sulfate waters whose sedimentary fill has been used in paleoenvironmental and paleohydrological investigations (Riera et al. 2004).

Evaporite karst in Mesozoic rocks of the Iberian Range

In Teruel Graben, Neogene sediments are locally affected by conspicuous gravitational deformations caused by interstratal karstification of the underlying Triassic evaporites (Gutiérrez 1998a). North of Teruel city, synsedimentary karstic subsidence phenomena have been recorded in Neogene alluvial fan deposits that show basin structures with cumulative wedge outs and tufaceous facies in the core. These structures correspond to cover sagging sinkholes up to several hundred meters in length that hosted palustrine environments with calcium carbonate precipitation. This paleokarst constitutes stratigraphic evidence of subsurface dissolution processes involved in the recycling process that lead to deposition of Mio-Pliocene gypsum formations in the Teruel Graben (Gutiérrez 1998a). Also in this sector, postsedimentary subsidence caused by the subjacent karstification of Triassic evaporites has generated numerous deformations in Neogene sediments, including tilting, passive bending folds and periclines, and transtratal collapse breccias (Gutiérrez 1998a). The Río Seco monocline, 1.5 km long and 150 m in amplitude, affects a Mio-Pliocene sequence that includes sediments selected for the formal definition of the Turolian stage (Calvo et al. 1999) (Fig. 2c). The concordant slope underlain by the upper anticlinal fold of this structure shows fresh uphill-facing fault scarps (sackung) that make evident the dissolution-induced subsidence and spreading movements that affect this gravitational structure (Gutiérrez 1998a).

In Orihuela del Tremedal village, built on dolomitic collapse breccias underlain by Triassic evaporites, numerous buildings are severely damaged by subsidence and some have been demolished. According to the CEDEX (1998), subsidence is related to episodic reactivation of old buried sinkholes generated by the collapse of dolomitic breccias into cavities detected by means of boreholes and gamma ray logs in the underlying evaporites. The authors of this report attribute the current subsidence to breakdown, suffusion, and compaction processes, and indicate a good temporal correlation between the main subsidence events-intervals and high rainfall periods.

The Palancia River Depression close to Segorbe village is one of the areas with a high number of exokarstic landforms associated with Triassic evaporites (Garay 2001). Here, mining experience indicates that gypsum gives way to anhydrite at a depth of about 30 m. The frequent tumuli that develop on the floor of gypsum quarries are attributed to the volume increase caused by hydration of anhydrite (Garay 2001). The Prado de Lagunas Polje, 1.1 km long and 0.5 km wide, is located 1 km south of Segorbe village. The bottom of this depression, occasionally flooded, hosts several ponors (swallow holes) in its southeastern edge. Also in this area, the upward stoping of cavities developed in the Triassic evaporites overlain by dolomites produce cylindrical bedrock collapse sinkholes up to 25 m deep and 50 m in diameter. These dolines, called “clotes” by the local people, reach a density higher than 20 sinkholes/km2 in the Tío Cabrera Clotes area. According to an inventory of sinkholes compiled by Garay (1991) in the eastern sector of the Iberian Range, most of the new sinkholes occur in areas where the natural hydrogeological conditions have been altered by human activities. In addition to those found in Triassic evaporites, sinkholes and large subsidence depressions also have been documented in Upper Cretaceous–Paleogene gypsum outcrops, like in areas to the north of Cañamares village and south of Paredes village. In Paredes area, the Madrid–Valencia high-speed railway runs very close to an old bedrock collapse sinkhole 15 m deep and 75 m in diameter.

Clear evidence of the active dissolution processes that affect Triassic evaporites is the presence of saline lakes and springs associated with these formations. The Gallocanta Lake, 13 km2 in area, is located at the bottom of a limestone karst polje whose deepening by corrosion processes ceased when the underlying shales and evaporites were reached, propitiating the development of this saline lacustrine system of paramount environmental interest (Gracia et al. 2001) (Fig. 2d). The waters of La Sima spring in Santa Cruz de Moya rise through a karstic conduit developed at the foot of a slope flowing into the Turia River. In 1984, the development of several sinkholes around the spring triggered a landslide of 100,000 tons (Durán and Del Val 1984). Subsequent to this event, water of the Turia River, which supplies Valencia City, was not drinkable for a few months, due to increased salinity.

Karstification of Tertiary evaporitic rocks

General geological and environmental implications

The subsidence phenomena caused by karstification of evaporites is the topic that has received a wider attention in the Tertiary basins. These are particularly frequent in the sectors where the evaporitic bedrock is overlain by Quaternary alluvial deposits which may behave as perched aquifers (terraces and mantled pediments) or as discharge areas fed by upward groundwater flows (floodplains). This alluvial karst occurs in reaches of the main Spanish fluvial systems where they traverse evaporitic outcrops (Gutiérrez and Gutiérrez 1998; Benito et al. 2000; Gutiérrez et al. 2001; Guerrero et al. 2007) (Fig. 1). Commonly, in these areas the alluvial deposits show sharp thickenings, locally reaching more than 100 m. The thickened alluvial deposits fill complex dissolution-induced basins up to several tens of kilometers long generated by synsedimentary subsidence. These thickenings are generally larger in areas where the evaporites contain halite and glauberite in the subsurface. Recently, a novel morpho-stratigraphical model of fluvial evolution controlled by different subsidence/aggradation ratios has been proposed for areas affected by this phenomenon (Guerrero et al. 2007). From an economic perspective, the thickened alluvial deposits constitute highly valuable aquifers and a substantial source of aggregates. On the other side, as a consequence of the synsedimentary subsidence, deposits of different alluvial levels may be superimposed and bounded by angular unconformities (Fig. 3a). These peculiar arrangements have given rise to erroneous interpretations. Some authors have attributed a tectonic origin to the unconformities and ascribed a Tertiary age to the deformed lower units.

Fig. 3
figure 3

a Two superimposed terrace units of the Pancrudo River bounded by an angular unconformity (Barrachina, Calatayud Graben). Note the synformal structure of the lower unit. b Dissolutionally enlarged joints filled with gravels derived from the overlying detrital cover. The circular bodies of gravel correspond to the transverse section of inclined alluvium-filled conduits (Madrid Basin, R-3 highway). c Rock-fall occurred on June 1997 destroying a recently built house at the foot of Calatayud gypsum escarpment. d Cave passage partially destroyed by gypsum mining in Sorbas. e Carbonate and detrital Neogene sediments (covered by vegetation) collapsed within halite- and glauberite-bearing evaporites (Calatayud Graben). f Solution notches in the human-induced Del Riu Cave in the Cardona salt diapir. g Saline lake developed in a solution doline in the Bujaraloz Platform (Ebro Basin). h Collapse sinkhole resulting from the reactivation of a buried doline formed on May 23, 2006, next to the 232 highway on the outskirts of Zaragoza city

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Another characteristic of alluvial sediments underlain by Tertiary evaporites is the presence of numerous gravitational deformations. Some of these features were erroneously interpreted as periglacial cryoturbations by several authors (Imperatori 1955; Johnson 1960; Brosche 1978) (Fig. 3b). These structures may affect solely the alluvial mantle, or both the cover and the bedrock, depending on whether they result from a rockhead or interstatal karstification, respectively. Deep-seated interstratal karst seems to be particularly common in areas where the evaporitic sequence bears halite and glauberite units in the subsurface (Guerrero et al. 2004a, 2007). These deformational structures, together with dissolutional features found in paleokarst exposures, are the best source of information to understand the subsidence processes involved in the generation of sinkholes: sagging, suffusion, and collapse (either by brecciation or through the development of well-defined failure planes) (Gutiérrez 2004; Gutiérrez et al. 2008a).

The current activity of dissolution and subsidence processes is revealed by the formation and reactivation of sinkholes that show a wide variety of sizes and geometries, largely depending on the dominant subsidence mechanism. Sinkholes that intercept the water table host lakes that may have a notable environmental interest. Although, some of these wetlands are protected zones, commonly they are used for disposal of waste material or are filled to transform them into arable or urban land (Gutiérrez et al. 2007). On the other hand, sinkhole activity caused by the karstification on Tertiary evaporties constitutes a geological hazard of great economic impact in some sectors of the Spanish territory, like in the outskirts of Zaragoza city, in Calatayud, in the southeastern sector of Madrid metropolitan area, and in Oviedo. As an example, in Oviedo city, partially built of Tertiary evaporites, the direct economic losses caused by a sinkhole event triggered by water withdrawal in 1998 were estimated at 18 million euros (M. Gutiérrez-Claverol, personal communication).

Generally, the sinkholes show a higher probability of occurrence (hazard) in the lower alluvial levels, coinciding with areas where development and human activity tend to concentrate resulting in high-risk situations (Gutiérrez et al. 2001, 2004a). Commonly, the most effective mitigation strategy is avoidance of existing sinkholes and high susceptibility areas. Unfortunately, the sinkhole hazard analyses are rarely incorporated in the local planning process. Another frequent characteristic of the fluvial valleys excavated in Tertiary evaporites is the presence of unstable gypsum escarpments with hanging valleys, triangular facets, and numerous mass movements (Gutiérrez et al. 1994, 2001). Slope movements that have produced the highest number of deaths have occurred in this type of gypsum scarps, weakened by karstification processes acting along discontinuity planes (Guerrero et al. 2004b). Four rock-fall events from a gypsum cliff occurred in 1856, 1874, 1903 and 1946; they killed a total of 106 people in the village of Azagra, located in the western sector of the Ebro Basin (Ayala et al. 1988).

The Sorbas Gypsum karst (Betic Cordillera)

The Sorbas gypsum karst has been developed in a 120 m thick sequence made up of alternating gypsum and marl layers. It hosts more than 1,000 cavities in an area of 12 km2. According to Calaforra and Pulido (2003), speleogenetic evolution started with development of phreatic conduits in gypsum layers at the contact with the impervious marl layers. Subsequently, this multilevel system changed progressively into vadose conditions, leading to the formation of passages with triangular sections largely carved by mechanical erosion in the marl layers. The Agua Cave, 8.5 km long, is the longest gypsum cave in Spain. Covadura Cave, more than 4 km long, contains unique gypsum hollow stalagmites (Calaforra and Forti 1990). The most representative exokarstic landforms include a large number of bedrock collapse sinkholes, and gypsum tumuli generated by bulging of the uppermost gypsum layer due to dissolution and reprecipitation processes (Calaforra and Pulido-Bosh 1999b). The Molinos del Río spring, with a relatively constant mean discharge of about 70 l/s, constitutes the main outlet for waters that flow through the partially confined karstic system and the overlying low permeability sediments.

Despite the fact that the Sorbas karst was declared a Natural Landscape in 1988 by the Andalusia Government, there is conflict between the preservation of this exceptional natural heritage and the open-cast mining of gypsum. One of the quarries, currently under exploitation, is considered the second largest gypsum quarry in the world. The impacts caused by this economic activity include alteration of the natural landscape, destruction of exokarstic and endokarstic landforms (Fig. 3d), and negative changes in the surface and underground hydrology (Pulido-Bosch et al. 2004).

Calatayud Neogene Graben (Iberian Range)

The most evident manifestation of subjacent karstification of evaporites in Calatayud Graben, both in Calatayud and in Barrachina areas, are the subsidence phenomena recorded in the overlying sediments: Neogene sedimentary units of the basin fill and Quaternary alluvial deposits. To the southeast of Calatayud city there are two areas covering 4.4 and 12 km2 where the supra-evaporitic carbonate and detrital units have subsided more than 200 m due to interstratal karstification of the halite- and glauberite- bearing evaporites (Fig. 3e). The strongly deformed collapse sediments, foundered within the subhorizontal evaporites, show numerous brittle and ductile deformations with a very chaotic arrangement (Gutiérrez 1996). In Barrachina area, Tertiary sediments underlain by evaporites also show abundant dissolution-induced gravitational deformations.

In Calatayud area, terrace deposits of the Jalón and Jiloca Rivers shows thickenings in excess of 100 m, superimposed terrace units, and numerous deformations related to the development of dissolution-induced basins and paleosinkholes. The magnitude and spatial distribution of the thickenings identified in different terrace levels indicate that synsedimentary subsidence has diminished and migrated in a downstream direction through time (Gutiérrez 1996). In nearby Barrachina village, the terrace deposits of the Pancrudo River also show thickenings, superimposed units, and sagging and collapse paleosinkholes (Fig. 3a).

Current subsidence activity in Calatayud area has a clear influence on the dynamics of the fluvial systems and causes numerous damages to buildings and infrastructures. The Jalón River floodplain shows diffuse-edged and swampy subsidence depressions up to 1 km long that locally control the trajectory and sinuosity of the river channel. Collapse sinkholes are particularly frequent in the vicinity of irrigation ditches and canals. The initially projected trace of the Madrid-Barcelona motorway (E-90) was changed to avoid a sinkhole-prone area. A 12 km long stretch of the highly vulnerable Madrid–Zaragoza high-speed railway has been built on soft and water-saturated floodplain alluvium underlain by karstified evaporites. The city of Calatayud is located at the foot of a gypsum escarpment on an alluvial fan and the Jalón River floodplain. Most of the historical buildings of Calatayud, declared Historical Monuments in 1967, are severely damaged by subsidence. Integrated analysis of the spatial distribution of subsidence damage, and the characterization of underlying sediments, indicate that subsidence in Calatayud is primarily due to karstification of the evaporitic bedrock and hydrocompaction of the gypsiferous silts of the alluvial fan deposits (Gutiérrez 1998a; Gutiérrez and Cooper 2002). In November 2003, the structure of a five-storey building with pad foundations was seriously damaged by a catastrophic collapse sinkhole that resulted from upward propagation of a cavity more than 600 m3 in volume. The building was finally demolished and the direct losses caused by this single subsidence event have been estimated at 4.8 million euros (Gutiérrez et al. 2004b). Frequent rock-falls and rock-topples derived from the gypsum escarpment restrict urban development and cause frequent road cuts and damage to buildings. In 1988, one person was killed by a rock-fall and several buildings have been destroyed by slope movements (Fig. 3c).

Teruel Neogene Graben (Iberian Range)

The dissolution-induced subsidence phenomena have affected alluvial deposits of the Alfambra fluvial system in different sectors of the Teruel Graben. In the northern sector (Villalba Alta-Escorihuela), the suballuvial karstification of the Orrios Gypsum has given rise to thickenings in terrace and pediment deposits, and paleosinkholes with synsedimentary deformations that host palustrine facies (Moissenet 1989). Downstream, in the area where the Alfambra River crosses the Tortajada Gypsum (Cuevas Labradas-Tortajada), deposits of the terrace levels situated at 75, 60–55, and 44–38 m above the river channel, reach 45, 60, and 40 m in thickness, respectively. The main thickening affects the 60–55 m terrace, filling a dissolution trough 3.2 km long in the western margin of the valley (Gutiérrez 1998a, b). The most spectacular paleosinkholes are found in an artificial railway cut located in Villalba Baja village. Deformed alluvium in this exposure shows gravel pockets interpreted as liquefaction-fluidization structures induced by catastrophic collapse processes (Gutiérrez 1998a, b). Several active sinkholes associated with old subsidence structures have been detected at this site, suggesting that the distribution of paleosinkholes can be used for the spatial prediction of sinkholes (Gutiérrez 1998a, 2004). North of Teruel city, a Middle Pleistocene terrace of the Alfambra River, located at 60–50 m above the river channel, reaches more than 55 m in thickness. Synsedimentary subsidence recorded by these deposits may be related to the combined effect of karstification of the underlying Triassic evaporites and neotectonic activity (Gutiérrez 1998a).

The Beuda Gypsum karst (Eastern Pyrenees)

Although, outcrops of this formation are very limited, its interstratal and suballuvial karstification has produced a considerable amount of endo and exokarstic features. The most significant manifestation corresponds to the Bañolas Lake, 1.12 km2 in area (Fig. 1). This lacustrine system has been developed in a group of coalescent collapse sinkholes generated by upward flows derived from a deep carbonate aquifer confined by the Beuda Gypsum (Sanz and Trilla 1982; Canals et al. 1990). According to Bischoff et al. (1994), dissolution of the gypsum in this karstic aquifer is favored by dedolomitization reactions that reduce the calcium concentration in the groundwater. During rainy periods, the upward discharge through a trop plein located 40 m above the Bañolas Lake gives rise to an ephemeral lake, 4 ha in area, called the Clot d’Espolla (Vila et al. 1989). Some sudden changes in the water level of Bañolas Lake have been attributed to collapse events in its bottom (Brusí et al. 1992). The occurrence of sinkholes is relatively common in the surroundings of the lake, mainly during low discharge periods. On the other hand, the numerous gravitational deformations and paleosinkholes that display the Plio-Pleistocene lacustrine sediments in the Bañoles-Besalú sedimentary basin reveal the significant role played by karstic subsidence phenomena on its morpho-sedimentary evolution (Fleta et al. 1996; Ros et al. 1996).

In Sant Miquel de Campmajor valley, Pallí and Trilla (1979) have identified 88 active and relict sinkholes, some of them hosting ponds. In Borró River valley, a group of funnel-shaped sinkholes up to several tens of meters across connect to the 1,315 m long Bores de Borró Cave system (Miret and García 1999). In Beuda village area, the 962 m long La Mosquera Cave, with excellent examples of scallops in its walls, has been severely spoiled by the disposal of excrement from a pig factory (Cardona 1989–1990; Miret and García 1999). The 920 m long Rotgers Cave, in Borredá village, is fed by a swallow hole located in the bottom of a stream. Some buildings in Besalú village have been irreversibly damaged by the formation of sinkholes. In this area, terrace deposits of the Fluviá River show numerous deformations and thickenings higher than 60 m that record subsidence phenomena caused by karstification of the Beuda Gypsum (Solá et al. 1996).

Evaporite karst in the Ebro Tertiary Basin

The Cardona salt karst

Piercing of the overburden by salt in the Cardona Diapir has given rise to the largest salt outcrop in Western Europe, covering around 0.9 km2. This salt stock, with a NE-SW trending ellipsoidal geometry in plan view, is crossed by a meander of the Cardener River in its northeastern edge. Underground mining of potassium salts and halite has caused dramatic geomorphological and hydrological changes in the karst system (Cardona 1989–1990; Gutiérrez et al. 2001; Cardona and Viver 2002; Lucha et al. 2008a). The diapir has a well-developed endokarstic system, including historical caves whose genesis has been induced by the mining operations. The 680 m long Forat Mico Cave, discovered in 1967, has been for 15 years the longest known salt cave in the world. It is composed of two levels. The upper passage displays unusual scallops of aeolian origin. Generation of the 280 m long Del Riu Cave was due to inflow of fresh water from the surrounding sandstone aquifer; this inflow was caused by the excavation of a ventilation gallery (Fig. 3f). The 335 m long Riera Salada Cave is carved in the halite debris of a slag heap built between 1925 and 1972. This cave was primarily generated by sewage waters infiltrated in an artificial depression generated by the slag heap. In March 1998, the interception of a phreatic conduit by a shallow mine led to the inflow of fresh water from the Cardener River into the mine galleries, resulting in generation of the 4,300 m long Salt Meanders Cave, which is the third longest explored salt cave in the world. This flooding event caused a sudden decline in the piezometric level of the karstic aquifer and massive dissolution of salt with the consequent generation of a large number of sinkholes which caused severe damage in roads, buildings, and the mine infrastructure. Lucha et al. (2008a), based on an inventory of 178 sinkholes, have estimated minimum probability of occurrence values of 4.7 and 8 sinkholes/km2 year, respectively for the time intervals previous and subsequent to the 1998 mine flood event.

The main exokarstic landforms found in the salt outcrops and the halite slag heaps are dolines and different types of karren, mainly rillenkarren, spitzkarren, and salt pedestals. Measurements carried out in solution flutes (Mottershead et al. 2006) and pedestals (hoodoos) have yielded lowering rates of several centimeters per year. The Bofia Gran (meaning big sinkhole), located in the southwestern edge of the diapir, is a 300 m long and 220 m wide polygenetic karstic depression with nested bedrock collapse sinkholes and swallow holes. In 1986, collapse of the Riera Salada Cave roof generated a subcircular collapse sinkhole 50 m across in the slag heap.

The flow of saline water into the Cardener River has caused a significant hydrochemical degradation of the river waters. To partially overcome this problem, a 100 km long pipe was constructed in 1989 to divert the brine to the sea. Subsequent to the 1998 mine flood event, the Cardener River meander, affected by numerous sinkholes, was cut-off by means of a tunnel to prevent inflow of the river water into the mine galleries.

The Barbastro Formation evaporite karst

Although, evaporites that form the core of the Barbastro Anticline bear a substantial amount of halite in the subsurface, deformations and thickenings in terrace deposits of the transverse rivers that cross the structure are not very common. In the Noguera-Ribagorzana valley, the deposit of the terrace level located at 161–176 m above the river channel reaches 110 m in thickness, filling a 2.5 km long dissolution-induced basin. The terrace levels situated at 95–50 and 59–49 m above the channel of the tributary Lo Reguer Stream also show anomalous thickenings caused by synsedimentary karstic subsidence reaching 100 and 50 m, respectively (Lucha et al. 2008b). According to Lucha et al. (2008b), the absence of thickenings in the Cinca River terraces underlain by halite-bearing evaporites could be related to uplift caused by the halokinetic upward flow of the salt towards the valley. This hypothesis is supported by deformations that are displayed in some terraces. Terraces situated 92–62 and 63–30 m above the channel show an upwarping structure about 30 m in amplitude and a conspicuous backtilting towards the valley flank, respectively (Lucha et al. 2008b).

Subsidence activity is particularly active in areas where human activities involve an additional input of water to the ground (Lucha et al. 2008b). Serviceability of the Ariéstolas and Aragón-Cataluña irrigation canals is frequently interrupted by the occurrence of sinkholes. This problem is being partially ameliorated by the injection of cement and clay mixtures (grouting). The abundant deformations that show structures in Ivars village, built on a 2 m thick terrace of the Noguera-Ribagorzana River underlain by evaporites, have been attributed to karstification of the bedrock. The water supply pipe network of the village has been replaced to reduce the infiltration of water into the ground from leakages. An additional environmental implication of evaporite karst in the Barbastro Anticline is the substantial increase in the salinity of rivers that traverse the structure. Lucha et al. (2008b) estimate that the underground flows supply about 300,000 and 100,000 tons of NaCl and CaSO4 per year to the Cinca River, respectively.

Geotechnical problems related to the dissolution of gypsum also affect the mantled pediments which contain a high proportion of gypsum particles south of the Barbastro Anticline. In January 2001, an earth dam located in the vicinity of Altorricón village failed catastrophically during the first filling test producing a flood event. Failure of the dam, built on gypsum-rich pediment deposits, has been attributed to subsidence induced by karstification of the alluvial mantle and piping processes affecting the dispersive clays of the dam core and embankments (Gutiérrez et al. 2003).

Evaporite karst in the Zaragoza Formation

The main landforms developed by dissolution of this formation in bare karst settings include several types of karren (rillenkarren and napfkarren), and solution dolines that locally host saline lakes of great environmental value (Gutiérrez and Gutiérrez 1998). The Bujaraloz structural platform, capped by gypsum and limestone beds, contains around 100 depressions with a prevalent WNW–ESE orientation that coincides with the direction of the prevailing winds. The flat bottom of some of these basins, up to several kilometers long, is occupied by lakes that constitute the northernmost playa-lakes with evaporite deposition in Europe (Fig. 3g). The origin of these basins has been attributed to the combined effect of karstification and deflation processes (Sánchez et al. 1998; Gutiérrez-Elorza et al. 2001).

Most of the karstic manifestations documented in the central sector of the Ebro Basin are related to subsidence phenomena caused by dissolution of the halite- and glauberite-bearing Zaragoza Formation in alluvial karst settings. The dissolution-induced subsidence has controlled the evolution of several fluvial systems, giving rise to substantial thickenings in terrace deposits (Gutiérrez and Gutiérrez 1998; Benito et al. 2000). Alluvium in the lower reach of the Gállego River valley fills a dissolution trough 30 km long and 8 km wide, composed of several basins up to 110 m deep (Benito et al. 1998). Borehole data indicate that the Quaternary fluvial deposits in the Ebro valley locally reach more than 60 m thick (Gutiérrez et al. 2007). In the Huerva River, downstream of Cuarte village, deposits of the terrace located 60 m above the river channel change abruptly from less than 4 m to more than 60 m in thickness, filling a 5 km long dissolution trough. The coincidence between the increase in the halite thickness at depth and the alluvium thickening strongly suggests that the synsedimentary subsidence is largely related to the interstratal karstification of halite units (Guerrero et al. 2007). A 50 m thick fluvio-lacustrine tufa deposit in the Jalón River valley shows several onlaped basin structures with cumulative wedge-out arrangements that record spatio-temporal variations of a synsedimentary karstic subsidence probably controlled by paleosprings (Arenas et al. 2000). Thickenings caused by the development of subsidence basins coeval with deposition have also been studied in the dissected infill of small creeks like the Torrecilla Stream. Here the sediments of a thickened and deformed alluvial level show sagging paleodolines with calcareous and carbonaceous facies (Gutiérrez and Arauzo 1997).

In addition to the abrupt changes in thickness, Quaternary deposits underlain by the Zaragoza Formation commonly show a wide variety of gravitational deformations and paleosinkholes (Gutiérrez et al. 2008). Initially, some of these structures were interpreted as relict cryotubations developed under pre-existing periglacial conditions (Johnson 1960; Brosche 1978). Subsequently, these deformations were related to gypsum karstification. Recent studies, based in borehole data, paleokarst exposures, and hydrochemical evidence, attribute a significant contribution to the interstratal karstification of halite and glauberite to explain the deformational structures that affect both the alluvial mantle and the bedrock strata (Guerrero et al. 2004a, 2006; Gutiérrez et al. 2007).

The outskirts of Zaragoza city are very probably the area in Europe where the sinkhole hazard due to evaporite dissolution has a higher economic impact. The sudden occurrence of collapse sinkholes in buildings, railways and motorways have made clear the serious threat that this process poses to human safety (Fig. 3h). In the Gállego River valley, the village of Puilatos was abandoned in 1980 and demolished 5 years later due to structural failures caused by karstic subsidence on the buildings. In the Ebro valley, the sinkhole types and activity show marked differences.

Upstream of Zaragoza city, the lower terraces are primarily affected by sagging and collapse sinkholes typically several tens of meters in diameter (Soriano and Simón 1995). The interstratal karstification of halite and glauberite beds seems to play a significant role in the generation of these dolines. Here, subsidence damage is caused mainly by the activity and reactivation of artificially filled dolines, rather than to the generation of new sinkholes. Van Zuidam (1976), using aerial photographs from different dates estimated a minimum probability of occurrence of 0.1 sinkholes/km2 year in a sector covering 21 km2.

Downstream of Zaragoza city, Gutiérrez et al. (2007) have estimated a probability of occurrence of 45 sinkhole/km2 year in a portion of the lower terrace of the Ebro River. Here the sinkholes, largely induced by irrigation, are commonly 1.5–2 m in diameter and result from the downward migration of cohesive alluvial mantle into dissolutional conduits (cover collapse sinkholes). Probably, one of the main challenges from the risk management perspective is to produce reliable sinkhole susceptibility maps showing the relative probability (relative or quantitative) of sinkhole occurrence. Preliminary susceptibility maps have been produced for a stretch of the Ebro valley downstream of Zaragoza analyzing the statistical relationships between the known sinkholes and a set of conditioning factors. Checking of the models through the application of quantitative validation techniques indicates that they provide reasonably good predictions (Galve et al. 2007).

The Madrid Tertiary Basin

The oldest evidence of evaporite karst in Madrid Basin corresponds to extensive paleokarst surfaces associated with major sedimentary breaks in the Miocene basin fill (Rodríguez-Aranda et al. 2002; Cañaveras et al. 1996). These stratigraphic discontinuities show cavities, paleosinkholes, collapse breccias, karren features, and karstic residues. Their genesis has been related to the karstification of gypsum deposits in saline lakes during desiccation episodes. Locally, Miocene sediments of the basin fill underlain by evaporitic units also show numerous postsedimentary deformations caused by interstratal karstification processes, like the examples exposed in the cuts of the M-45 highway.

Dissolution subsidence phenomena have controlled the Quaternary evolution of the Tajo River and its tributaries, the Jarama, Tajuña, Manzanares, and Henares Rivers. In the Tajo River valley, the deposit of the terrace level situated 60–65 m above the channel changes abruptly from 3 m to more than 60 m in thickness in the sector where detrital bedrock gives way to an evaporitic substratum (Pinilla et al. 1995). In the Jarama River, Pérez-González (1971) recognized a deformed and thickened terrace deposit superimposed by a younger terrace unit. In the lower reach of the Manzanares River, alluvium of the oldest terrace, 16–22 m above the river channel, shows conspicuous deformations and an anomalously high thickness (>20 m) (Silva 2003). Even though the thickenings and most of the deformations that display the terrace deposits in the evaporitic areas of the Madrid Basin have been attributed to gypsum karstification (Fig. 3b), it would be interesting to explore the role played by the dissolution of halite and glauberite beds at depth. Development of sinkholes has caused numerous geotechnical problems in some urban areas, like Rivas-Vaciamadrid and in the M-45 highway. These problems may increase substantially in the next few years as the metropolitan area of Madrid expands towards the evaporitic outcrops to the southeast (Fig. 1)

The Pedro Fernández or Estremera endokarstic system is a maze cave controlled by two orthogonal joint sets that reach 4 km in length and 64,000 m3 in volume (Almendros and Antón 1983). This outstanding cave, with clear similarities to the gypsum caves of western Ukraine (Klimchouk 2000), was declared an Artistic and Historical Monument to protect its Neolithic and Bronze Age archaeological sites (Eraso 1995).