Geological and environmental implications of the evaporite karst in Spain

Environ Geol (2008) 53:951–965
DOI 10.1007/s00254-007-0721-y
ORIGINAL ARTICLE
Geological and environmental implications
of the evaporite karst in Spain
F. Gutiérrez Æ J. M. Calaforra Æ F. Cardona Æ
F. Ortí Æ J. J. Durán Æ P. Garay
Received: 15 October 2006 / Accepted: 6 March 2007 / Published online: 12 April 2007
Ó Springer-Verlag 2007
Abstract In Spain, evaporite outcrops cover approximately
7% of the total area of the country. Most of the
evaporitic formations are made up of Ca-sulfates (gypsum/
anhydrite) or Ca-sulfates and halite. Certain Paleogene
marine evaporites also contain K-Mg-chlorides, and some
Tertiary continental formations bear substantial amounts of
Na-sulfates in the subsurface (glauberite and thenardite).
Mesozoic evaporitic formations commonly wedge out towards
the ground surface, passing into condensed sequences
and dissolution-collapse breccias. Some of these
highly porous breccias constitute major regional aquifers.
In several areas, interstratal karstification of the evaporites
has given rise to gravitational deformations such as basin
structures, monoclines, and collapse structures covering
F. Gutiérrez (&)
Dpto. de Ciencias de la Tierra, Universidad de Zaragoza,
C/. Pedro Cerbuna, 12, 50009 Zaragoza, Spain
e-mail: fgutier@unizar.es
J. M. Calaforra
Dpto. de Hidrogeología, Universidad de Almería,
04120 La Cañada; Almería, Spain
F. Cardona
Espeleo Club de Gràcia, C/. Astúries, 83,
Barcelona, Spain
F. Ortí
Dpto. de Geoquímica, Petrología i Prospecció Geológica,
Universidad de Barcelona, 08028 Barcelona, Spain
J. J. Durán
Instituto Geológico y Minero de España,
C/. Rios Rosas, 23, 28003 Madrid, Spain
P. Garay
Dpto. de Geología, Universitat de Valencia,
46100 Burjassot, Valencia, Spain
several square kilometers that record a cumulative subsidence
in excess of 200 m (Teruel and Calatayud Grabens).
A widespread consequence of evaporite dissolution processes
in Spain is the hydrochemical degradation of surface
waters. Some of the largest and most outstanding lake
systems, from an environmental perspective, occur in
karstic depressions developed in evaporitic formations
(Fuente de Piedra, Gallocanta, Bujaraloz, and Bañolas
lakes). Sinkhole activity is a major geohazard in several
evaporite karst areas. The sinkhole risk has a particularly
high impact in sectors where Tertiary evaporites are
overlain by Quaternary alluvial aquifers (Calatayud, Zaragoza,
and Madrid areas). Some of the detrimental effects
of subsidence include severe damage to historical monuments
(Calatayud), the demolition of a whole village (Puilatos),
or the derailment of a freight train (Zaragoza area).
The deepest gypsum caves are found in Triassic diapiric
structures (El Sumidor Cave, 210 m deep), and the longest
ones are developed in horizontally lying Neogene sequences
(Sorbas caves, and Estremera maze cave). The
Cardona diapir hosts salt caves up to 4,300 m long whose
genesis is related to flooding of mine galleries caused by
the interception of a phreatic conduit. The main anthropogenic
impacts on the endokarstic systems are related to
the disposal of wastewaters and the destruction of caves
by quarrying. The fluvial valleys that cross Tertiary
evaporitic outcrops commonly show peculiar geological
characteristics related to dissolution-induced synsedimentary
subsidence phenomena: (1) Thickened alluvium filling
dissolution basins up to several tens of kilometers long and
more than 100 m deep. The largest thickenings are found
in areas where the bedrock contains halite and glauberite.
(2) Superimposed alluvial units locally bounded by angular
unconformities. (3) Abundant deformational structures and
paleosinkholes related to the rockhead and/or interstratal
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952 Environ Geol (2008) 53:951–965
karstification of the substratum. These fluvial valleys typically
are flanked by a prominent gypsum escarpment.
Rock-falls favored by the dissolutional enlargement of
joints derived from these scarps are the type of mass
movement which has caused the highest number of casualties
in Spain.
Keywords Evaporite karst Geohazards Impacts
Spain
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 km 2 , approximately 7% of the total
country area (~500,000 km 2 ) (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
Fig. 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
123
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-Mgchlorides
and Na-sulfates (glauberite and thenardite),
respectively.
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.

Environ Geol (2008) 53:951–965 953
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 Tortonianearly
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 Casulfates
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
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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
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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

958 Environ Geol (2008) 53:951–965
Fig. 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 humaninduced
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
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 km 2 . 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
123
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,

Environ Geol (2008) 53:951–965 959
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 km 2 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 m 3 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).
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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

962 Environ Geol (2008) 53:951–965
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/km
2 year in a sector covering 21 km 2 .
Downstream of Zaragoza city, Gutiérrez et al. (2007)
have estimated a probability of occurrence of 45 sinkhole/
km 2 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
123
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 m 3 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).

Environ Geol (2008) 53:951–965 963
Acknowledgments The authors would like to thank Prof. J.P. Calvo
(Spanish Geological Survey) and Dr. Pablo Silva (Salamanca University)
for information provided on the evaporite karst in Madrid
Basin. We are also thank Dr. Kenneth S. Johnson (Oklahoma Geological
Survey) and Mr. Pedro Lucha (EUPLA) for the thorough review
of the manuscript. This work has been co-financed by the
Spanish Education and Science Ministry and the FEDER (project
CGL2004–02892/BTE).
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