Geological and environmental implications of the evaporite karst in Spain

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960 Environ Geol (2008) 53:951–965 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 km 2 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 km 2 . This salt stock, with a 123 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/km 2 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
Environ Geol (2008) 53:951–965 961 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 123
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Geological and environmental implications of the evaporite karst in Spain

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