Geological and environmental implications of the evaporite karst in Spain

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
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