Geological and environmental implications of the evaporite karst in Spain

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956 Environ Geol (2008) 53:951–965 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 123 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 dissolutioninduced 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 eventsintervals 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/km 2 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
Environ Geol (2008) 53:951–965 957 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 km 2 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 dissolutioninduced 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. 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 welldefined 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 123
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Geological and environmental implications of the evaporite karst in Spain

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