Geological and environmental implications of the evaporite karst in Spain

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954 Environ Geol (2008) 53:951–965 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) 123 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
Environ Geol (2008) 53:951–965 955 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 m 2 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 lS/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. 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 Fig. 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) Ramsar Convention of Wetlands (Fig. 1). This saline lake, with sodium-chloride waters, covers 13.6 km 2 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 123
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Geological and environmental implications of the evaporite karst in Spain

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