ORIGINAL ARTICLE

A genetic classification of sinkholes illustrated from evaporitepaleokarst exposures in Spain

Francisco Gutierrez Æ Jesus Guerrero ÆPedro Lucha

Received: 15 September 2006 / Accepted: 6 March 2007 / Published online: 12 April 2007

� Springer-Verlag 2007

Abstract This contribution analyses the processes in-

volved in the generation of sinkholes from the study of

paleokarst features exposed in four Spanish Tertiary basins.

Bedrock strata are subhorizontal evaporites, and in three of

the basins they include halite and glauberite in the sub-

surface. Our studies suggest that formation of dolines in

these areas results from a wider range of subsidence pro-

cesses than those included in the most recently published

sinkhole classifications; a new genetic classification of

sinkholes applicable to both carbonate and evaporite karst

areas is thus proposed. With the exception of solution do-

lines, it defines the main sinkhole types by use of two terms

that refer to the material affected by downward gravita-

tional movements (cover, bedrock or caprock) and the main

type of process involved (collapse, suffosion or sagging).

Sinkholes that result from the combination of several

subsidence processes and affect more than one type of

material are described by combinations of the different

terms with the dominant material or process followed by

the secondary one (e.g. bedrock sagging and collapse

sinkhole). The mechanism of collapse includes any brittle

gravitational deformation of cover and bedrock material,

such as upward stoping of cavities by roof failure, devel-

opment of well-defined failure planes and rock brecciation.

Suffosion is the downward migration of cover deposits

through dissolutional conduits accompanied with ductile

settling. Sagging is the ductile flexure of sediments caused

by differential corrosional lowering of the rockhead

or interstratal karstification of the soluble bedrock. The

paleokarsts we analysed suggest that the sagging mecha-

nism (not included in previous genetic classifications)

plays an important role in the generation of sinkholes in

evaporites. Moreover, collapse processes are more signifi-

cant in extent and rate in areas underlain by evaporites than

in carbonate karst, primarily due to the greater solubility of

the evaporites and the lower mechanical strength and

ductile rheology of gypsum and salt rocks.

Keywords Sinkholes � Sinkhole classification �Paleokarst � Subsidence mechanisms � Evaporite karst

Introduction

Sinkholes or dolines are closed depressions with internal

drainage that are characteristic features of karst landscapes.

They display a wide range of morphologies (cylindrical,

conical, bowl or pan-shaped) and are up to several hundred

meters across and tens of meters deep (Williams 2003). The

term doline, derived from the Slavic dolina, is used chiefly

by European geomorphologists, whereas sinkhole is the

most common term in North America and in the interna-

tional literature dealing with engineering and environmental

issues. Several similar genetic classifications of sinkholes

have been recently published (Williams 2003; Beck 2004;

Waltham et al. 2005). These classifications distinguish two

main categories of sinkholes—those resulting from dissol-

utional lowering of the surface and those created by internal

erosion and deformational processes caused by subsurface

karstification. The first group is represented by solution

sinkholes, which are generated by the differential dissolu-

tional lowering of the ground where karst rocks are exposed

at the surface or merely soil-mantled (bare or uncovered

karst). The second group includes four different types,

F. Gutierrez (&) � J. Guerrero � P. Lucha

Earth Science Department, Edificio Geologicas,

Universidad de Zaragoza, C/. Pedro Cerbuna, 12,

50009 Zaragoza, Spain

e-mail: fgutier@unizar.es

123

Environ Geol (2008) 53:993–1006

DOI 10.1007/s00254-007-0727-5

depending on the type of material affected by downward

gravitational movement (bedrock, caprock and unconsoli-

dated cover) and the subsidence mechanism (collapse or

suffosion). Bedrocks and caprocks can suffer brittle

collapse, whereas cover deposits can be affected by both

collapse and suffosion. This second group is the most

important from an engineering perspective. Sinkholes gen-

erated by the upward propagation (stoping) of cavity rock

roofs by breakdown processes over dissolutional voids are

designated as bedrock collapse or caprock collapse sink-

holes, depending on whether the cavity migrates through

karst or non-karst lithologies, respectively. Caprock and

bedrock collapse sinkholes are commonly cliffed around the

sides, metric to decametric in scale and typically appear in a

catastrophic manner. However, in most karst areas they have

a low to very low probability of occurrence (Beck 2004;

Waltham et al. 2005), largely due to the formation of stable

cupola arches (Bogli 1980; White and White 2000)

and/or roof support by piles of breakdown beneath them

(Andrejchuk and Klimchouk 2002; Klimchouk and

Andrejchuk 2005). Cover subsidence sinkholes or suffosion

sinkholes result from the downward migration (suffosion,

ravelling) of cover material through dissolutional voids and

its ductile settling. These sinkholes are commonly funnel- or

bowl-shaped depressions and may reach more than 10 m in

diameter. Cover collapse (or dropout) sinkholes form by the

upward propagation of breakdown cavities through cohesive

and brittle but unconsolidated cover above dissolutional

voids. Such sinkholes frequently develop with catastrophic

rapidity and have cliffed or overhanging sides. Old cover

collapse sinkholes may be difficult to differentiate from

suffosion sinkholes since they tend to degrade into conical or

bowl shapes. Finally, the classifications incorporate the term

buried sinkhole for those dolines filled by sediments; com-

paction of the fills may lead to the generation of compaction

sinkholes (Waltham et al. 2005).

These classifications explain the main processes and

mechanisms generating sinkholes in carbonate karst ter-

rains (dissolution, collapse and suffosion). However, our

study reveals that sinkhole development in evaporite karst

areas can involve a wider range of processes. This paper is

based on extensive literature review and the study of a

large number of paleokarst exposures in four Spanish

Tertiary basins containing subhorizontally lying Neogene

evaporite deposits. A new genetic classification of sink-

holes is proposed that is applicable to both carbonate and

evaporite karst areas.

Geological setting

The dissolution and subsidence structures of concern are

found in natural and artificial exposures in the Ebro, Tajo,

Calatayud and Teruel Tertiary basins in northeast and

central Spain (Fig. 1). The Ebro Basin is the southern

foreland basin of the Pyrenees; the Tajo Basin is an

intraplate basin bounded by alpine orogens; and the Ca-

latayud and Teruel basins are post-orogenic grabens

developed within the Iberian Range, an intraplate Alpine

orogenic belt. The paleokarst features result from dissolu-

tion of Tertiary evaporites and the consequent gravitational

deformation of the overlying sediments, which may be

Tertiary evaporites, Neogene caprock units and/or Qua-

ternary alluvium. The evaporites were deposited under

endorheic conditions in continental playa-lake environ-

ments developed in depocentral sectors of the basins. The

evaporites of the Ebro Basin (Zaragoza Gypsum), Calat-

ayud Graben (Calatayud Gypsum) and Tajo Basin (Lower

Unit or Saline Unit) consist primarily of secondary gypsum

with marl and shale intercalations in the outcrop, and Ca-

sulphates (gypsum and anhydrite) with significant amounts

of halite and Na-sulphates (mainly glauberite) in the sub-

surface (Ortı 1988, 2000; Gutierrez et al. 2001, 2004). The

absence of any halite and glauberite beds in exposures is

attributed to interstratal dissolution during entrenchment of

the drainage networks (Gutierrez et al. 2001, 2004). Pres-

ence of halite and Na-sulphates in the subsurface is a

crucial factor in the development of dissolution-induced

subsidence phenomena due to their high solubility; while

the equilibrium solubility of gypsum at 25�C is only 2.4 g/

l, halite and glauberite have solubilities of 360 and 118 g/l,

respectively (Ford and Williams 1989). The Zaragoza

Gypsum in the Ebro Basin (upper Oligocene–lower Mio-

cene in age) is around 800 m in thickness. The exposed

upper 300 m consists of secondary gypsum with marl and

shale intercalations, but borehole data reveal the presence

of thick halite and glauberite units close to the surface

(Torrescusa and Klimowitz 1990; Ortı and Salvany 1997).

Similarly, the Miocene Calatayud Gypsum is made up of

200 m of secondary gypsum with thin marl partings in the

outcrop and some 300 m of Ca-sulphates (gypsum and

anhydrite) with a high proportion of halite and glauberite in

the subsurface (Ortı and Rosell 2000). The Lower or Saline

Unit in the Madrid city area (Lower–Middle Miocene in

age) is a thick (c. 500 m) evaporite succession composed of

~30 m of secondary gypsum in the outcrop and gypsum/

anhydrite, halite and Na-sulphates at depth (Garcıa del

Cura et al. 1996). The paleokarst features we investigated

in the Teruel Graben are associated to the Tortajada Gyp-

sum (Upper Miocene–Lower Pliocene), which is more than

150 m of primary gypsum deposited in a saline lake with

low ionic concentration so that there was no precipitation

of the most highly soluble minerals (Hernandez and Ana-

don 1985). All of these evaporitic formations have sub-

horizontal structure and are affected by subvertical joints

and small-throw faults.

994 Environ Geol (2008) 53:993–1006

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From Middle Miocene to Pliocene times, these land-

locked endorheic basins were captured by external

drainage networks, thus changing progressively from en-

dorheic to exorheic conditions (Garcıa-Castellanos et al.

2003; Gutierrez et al. 2007b). This change in the morpho-

hydrological regime of the basins played a decisive role in

the history of dissolution and subsidence because large

volumes of evaporites could now be evacuated in solution

by the exorheic drainage (Gutierrez 1998; Gutierrez et al.

2001). The new river channels selectively dissected the

basin infillings, generating stepped sequences of terraces

and mantled pediments that record the Plio-Quaternary

evolution of the main fluvial systems: Ebro River and

tributaries in the Ebro Basin and Calatayud Graben, Tajo

River and tributaries in the Tajo Basin and Alfambra

River in the studied area of the Teruel Graben (Fig. 1). In

the investigated sectors, alluvial sediments covering

evaporites display some anomalous characteristics indi-

cating subsidence processes due to karstification of the

bedrocks (Gutierrez 1996; Gutierrez and Gutierrez 1998;

Benito et al. 2000; Gutierrez et al. 2001, 2004; Guerrero

et al. 2004, 2007): (1) the alluvium shows abrupt thick-

enings and locally filled solution basins up to several tens

of kilometres long and more than 100 m deep, generated

by synsedimentary karstic subsidence; (2) the deposits of

a terrace or pediment level may be superimposed by

angular unconformity to the thickened and deformed

sediments of older alluvial levels; (3) the alluvium

underlain by evaporitic sediments commonly shows

abundant synsedimentary and post-sedimentary gravita-

tional deformations.

The basic hydrogeology of the areas is characterised

by two interconnected aquifers—the Quaternary alluvial

deposits and the underlying karstified evaporitic bedrock.

The karstic aquifers have relatively large recharge areas

including limestone-capped mesas, perched alluvial

deposits underlain by the evaporitic formations and bare

evaporite outcrops. Downward vadose flow dominates in

these areas. The highly mineralised waters of the karstic

aquifer then discharge upwards into the floodplain allu-

vium, with a consequent increase in the ionic concentra-

tion in these alluvial aquifers (Gutierrez et al. 2007a). The

entrenchment and lateral migration of the fluvial systems

have led to important changes in the hydrogeological

behaviour of some sectors of the karstic aquifers. Evap-

orites overlain by perched fluvial terraces that were once

floodplain discharge areas fed by upward phreatic flow

now function as recharge areas with downward vadose

flow. Very likely, a significant proportion of the many

dissolutional conduits found in evaporites underlying the

fluvial terraces are phreatic passages inherited from the

discharge stage, now modified by vadose flow. Conse-

quently, the progressive river entrenchment and conse-

quent water table lowering may control the development

of the multilevel and multiphase karst systems (Ford

2000; Palmer 2000; Osborne 2000, 2002). Former phreatic

passages are commonly modified by vadose processes

such as the dissolutional excavation of the floor and walls,

and ceiling breakdown (Loucks 1999). Breakdown pro-

cesses under vadose conditions are enhanced by the dis-

solutional enlargement of joints, the removal of cave fills

by vertical downward flow and the increase in the effec-

tive weight of the cavity roofs caused by the loss of

buoyant support (White 1988; White and White 2000).

Presently, most of the new sinkholes forming in the

investigated areas occur in the poorly cemented lower

Fig. 1 Distribution of the

investigated Tertiary evaporitic

formations in central and

northeast Spain

Environ Geol (2008) 53:993–1006 995

123

alluvial levels favoured by the presence of upward

discharge flows or the artificial recharge of water to

the ground (Benito et al. 1995; Gutierrez et al. 2004;

Gutierrez et al. 2007a).

Processes involved in the development of sinkholes

and a genetic classification

This section presents an analysis of the processes involved

in the generation of the dissolution and subsidence fea-

tures observed in the studied paleokarst exposures and

proposes a genetic classification of sinkholes. With the

exception of solution dolines, this new systematisation

follows the methodology of Beck’s (2004) sinkhole clas-

sification and the most widely used landslide classifica-

tions (e.g. Cruden and Varnes 1996; Dikau et al. 1996) by

utilizing two terms to define each main sinkhole type: the

first describes the material affected by downward gravi-

tational movements (cover, bedrock or caprock), while the

second indicates the principal process involved (collapse,

suffosion or sagging) (Fig. 2). In practice, more than one

material type and several processes can be involved in the

generation of some sinkholes. In the proposed classifica-

tion, these complex examples can be described using

combinations of the different terms with the dominant

material or process followed by the secondary one (e.g.

cover and bedrock collapse sinkhole, cover suffosion and

sagging sinkhole).

Cover sinkholes

Cover sagging sinkholes

In mantled karst settings, the progressive corrosional low-

ering of the rockhead may lead to the gradual settlement of

the overlying cover by passive sagging or bending (Figs. 2,

3a, 4a, b). In the exposures we studied, differential disso-

lution at the top of the bedrock resulted in the development

of an irregular rockhead and a karstic residue sandwiched

between deformed surficial deposits and the undeformed

substratum. The karstic residue, made up of variegated

marls and shales with gypsum fragments, usually shows a

pseudostratification subparallel to the rockhead; its thick-

ness tends to be proportional to the amount of evaporites

removed in solution. An important applied aspect is that

the generation of these sinkholes does not require the

existence of cavities, because continuous accommodation

of the overburden material inhibits the formation of voids.

The flexure of the cover results in the development of a

basin structure with centripetal dips. These basin structures

show cumulative wedge out arrangements when the sag-

ging process has operated synchronously with the deposi-

tion of the cover (synsedimentary subsidence).

Sinkholes produced by this mechanism are commonly

shallow and have poorly defined margins; they can be

several hundred meters across (Gutierrez 1998; Gutierrez

et al. 2005; Gutierrez et al. 2007a). Although the analysed

outcrops show that passive sagging of the cover plays an

Fig. 2 The main subsidence

sinkhole types. Solution

sinkholes generated by the

corrosional lowering of the

ground surface are not included

996 Environ Geol (2008) 53:993–1006

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important role in the development of a large proportion of

sinkholes in evaporite karst, this mechanism is not con-

sidered in any of the recently published genetic classifi-

cations of sinkholes because they are concerned primarily

with carbonate karst terrains (Williams 2003; Beck 2004;

Waltham et al. 2005). In our classification scheme these

dolines are designated as cover sagging sinkholes (Fig. 2).

Although they do not constitute a direct threat to human

lives due to their slow subsidence rates, they may cause

severe damage to buildings and infrastructure.

Cover suffosion and cover collapse sinkholes

The circulation of water through a soluble bedrock overlain

by an unconsolidated deposit tends to concentrate along

discontinuity planes and their intersections, generating di-

solutionally enlarged pipes (Ford 2000; Lauritzen and

Lundberg 2000). In recharge areas under vadose condi-

tions, the enlargement of conduits by downward flow in-

duces a positive feedback that tends to accelerate

karstification (Williams 1983; Klimchouk and Andrejchuk

1996; Klimchouk 2000; Palmer 2000). Preferential disso-

lution of the rockhead at joint junctions and similar dis-

continuities leads to the formation of a drawdown cone

around the widened structures in the overlying mantling

deposits. This results in the re-orientation of the local

hydraulic gradient towards the karstic drain, accentuating

its enlargement. In contrast, in discharge areas of upward

groundwater flow, the enlargement of joints progresses

from the bottom to the top (Klimchouk 2000; Birk et al.

2003). The resulting widened fissures (cutters) and pipes

may be vertical or may display multiple orientations and

branches (Fig. 4c, d, e). In steep outcrops, inclined con-

duits with subcircular cross-sections may be seen. These

may taper downwards, locally forming a pinnacled rock-

head that may ultimately connect with deeper passages.

Unconsolidated cover deposits may migrate downward

into the fissures and conduits in the rockhead by action of a

wide range of processes collectively designated as suffo-

sion or ravelling (White 1988; Ford and Williams 1989;

Beck 1988, 2004; Sowers 1996; Waltham et al. 2005). The

principal mechanisms are: (1) down washing of particles

by percolating water; (2) cohesionless granular flow, like

the tapping of sand in a hourglass; (3) viscous gravity flow

(non-newtonian flow) of clay-rich deposits (Jancin and

Clark 1993); and (4) fall of particles detached from the

base of the overburden or from cavity roofs. The downward

transport of the cover material through corrosionally en-

larged pipes may produce two main types of sinkholes,

depending on the rheological behaviour of the mantling

deposits (Williams 2003; Beck 2004; Waltham et al. 2005).

(1) Where the cover is a ductile or loose granular deposit,

erosion into dissolutional conduits may cause the

accommodation of the cover (Figs. 2, 3b). Particles

may be downwashed progressively, undermining the

cover and allowing its gradual settlement. The

transport of a cohesionless sand cover by granular

flow would produce a funnel-shaped sinkhole with a

slope determined by the angle of repose of the sand

(Waltham et al. 2005). A clay-rich deposit may mi-

grate downwards as a viscoplastic flow generating

sheath folds with concentric structure (Fig. 4f). In

COVER SINKHOLES

A CBSagging Suffosion Collapse D ESagging+Suffosion Sagging+Collapse

Fig. 3 Diagram showing the main subsidence mechanisms involved

in generation of cover sinkholes. a Passive sagging of the cover

caused by differential corrosional lowering of the rockhead (coversagging sinkhole). Synthetic and antithetic bending-moment failure

planes may develop when the failure point is reached (cover saggingand collapse sinkhole). b Ductile sagging of the cover caused by the

migration of particles through dissolutional conduits (cover suffosion

sinkhole). c Upward propagation of cavities through a cohesive and

brittle cover above dissolutional conduits. d Passive sagging of the

cover accompanied by suffosion into karstic pipes (cover sagging andsuffosion sinkhole). e Passive sagging of the cover combined with the

stoping of a soil cavity linked to a corrosional pipe (cover saggingand collapse sinkhole)

Environ Geol (2008) 53:993–1006 997

123

agreement with William’s (2003) and Waltham’s

et al. (2005) classifications, the term cover suffosion

sinkhole is proposed for this type (Fig. 2). We con-

sider that ‘suffosion’ provides a better genetic

description than ‘subsidence’, as was used in Beck’s

(2004) classification. The term ‘subsidence’, which is

commonly used to group cover collapse and cover

suffosion sinkholes (Beck 2004; Waltham et al.

2005), has a broad meaning that encompasses a wide

range of mechanisms. In the ‘Glossary of Geology’, it

is defined as ‘the sudden sinking or gradual down-

ward settling of the Earth’s surface with little or no

Fig. 4 a Sagging synform affecting a terrace deposit of the Jalon

River in the Calatayud Graben. The material underlying the gravel

deposit is a karstic residue of marls generated by corrosional lowering

of the gypsum rockhead. This insoluble residue displays pseudostr-

atification roughly concordant with the overlying deformed beds. bMantled pediment deposits in the Ebro River valley showing a

synformal structure generated by passive sagging. c Subvertical

solutionally enlarged joints filled with pediment deposits in the Ebro

River valley. d Inclined dissolutional pipes filled with gravel deposits

derived from an overlying terrace in the Ebro Basin (Jalon River

valley). e Subvertical karstic conduit filled with a massive mantled

pediment deposit in the central sector of the Ebro Basin. f Tight

synform (sheath fold) generated by the downward plastic flow of

terrace deposits through underlying karstic conduits (Huerva River

valley, Ebro Basin). g Synsedimentary collapse structure controlled

by well-defined subvertical planes in terrace deposits of the Alfambra

River (Teruel Graben). h Gravel-filled conduit in thickened terrace

deposits of the Huerva River. i Gravel-filled pipe in thickened

mantled pediment deposits of the Alfambra River valley. j Synform

cross-cut by a conduit connected to a gravel-filled, bowl-shaped

depression

998 Environ Geol (2008) 53:993–1006

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horizontal motion. The movement is not restricted in

rate, magnitude or area involved’ (Jackson 1997).

Generally, cover suffosion sinkholes do not form

catastrophically, show a funnel- or bowl-shaped

geometry and are typically a few meters in diameter.

(2) When the cover consists of cohesive deposits with

brittle rheology, an arched cavity may develop over

any karst conduit. This void can migrate upwards by

successive soil arch (or roof) failures. The fallen

material accumulates at the floor of the cavity or is

evacuated through the dissolutional conduit (Figs. 2,

3c). The upward propagation of the cavity may cease

temporarily when the breakdown pile supports the

roof due to bulking of the fallen material where there

is no effective removal, or when it reaches a more

resistant bed such as a caliche, causing its lateral

growth. Eventually, collapse of the cavity roof may

intercept the ground surface, abruptly creating a vis-

ible sinkhole. The mechanics of upward propagation

of soil cavities has been analysed by Tharp (1995).

Voids developed over dissolutional pipes may pene-

trate through thick sequences of drift material, as

revealed by the gravel-filled collapse chimneys more

than 60 m deep and <1 m wide that are seen in terrace

and pediment deposits of the Alfambra and Ebro river

valleys (Gutierrez 1998; Guerrero et al. 2004, 2007).

The gravels filling these breakdown pipes show

conspicuously reoriented fabrics with the major axes

parallel to the margins of the chimneys (Fig. 4h, i). In

some cases, the collapse process does not occur by

progressive stoping, but by the en bloc downdrop of

the cover controlled by cylindrical failure planes with

almost no internal deformation (Fig. 4g).

These sinkholes commonly appear suddenly, have

cliffed or overhanging sides and are typically <10 m across.

Mass wasting on their steep margins tends to transform

them into funnel- or bowl-shaped basins (Fig. 4j). Follow-

ing Beck’s (2004) classification, the term cover collapse

sinkhole is proposed for this type (Fig. 2). The term ‘col-

lapse’ is preferred to ‘dropout’ as used in William’s (2003)

and Waltham’s et al. (2005) classifications. In our opinion,

‘collapse’ describes the process better than ‘dropout’, a

term which is scarcely used in any earlier literature.

Bedrock and caprock sinkholes

In all the areas studied except the Teruel Graben, where the

evaporite sequence lacks the highly soluble halite and

glauberite, paleokarst sections reveal that a large propor-

tion of the subsidence structures (paleosinkholes) are

associated with interstratal dissolution. The frequent

occurrence of voids associated with halite and glauberite

beds in boreholes also suggests that deep-seated interstratal

karstification plays an important role in the generation of

sinkholes (Gutierrez and Cooper 2002; Guerrero et al.

2004, 2007). Studied exposures indicate that the subsidence

mechanisms depend on the mechanical strength of the rock

mass, which is largely determined by the density and ori-

entation of discontinuity planes (joints, bedding planes),

the geometry and size of the cavities (span width) and the

hydraulic conditions (either vadose or phreatic) that will

determine the mode of groundwater flow, increases in

plasticity of the evaporitic bedrock (Bell 1994; Dashnor

et al. 2006) and the effective weight of cavity roofs

(buoyant support).

The two extremes of mechanical strength are massive

bedrock devoid of jointing and thinly bedded and densely

jointed strata. When the bedrock is barely affected by

jointing, the subsidence mechanism is controlled primarily

by the geometry and size of the cavities. The roofs of

stratiform voids with wide span tend to be subject to sag-

ging (Fig. 5a), whereas cavities with significant height and

a limited width are commonly affected by collapse pro-

cesses controlled by dome-shaped failure planes (Fig. 5b).

When the bedrock has a dense network of joints, collapse

and brecciation controlled by the numerous pre-existing

failure planes is the dominant mechanism (Fig. 5c). This

brittle deformation may be accompanied by sagging when

the cavities are also of large width (Fig. 5e). Consequently,

interstratal karstification of evaporitic bedrocks in the areas

studied leads to two main subsidence mechanisms: sagging

and collapse (Fig. 5). These processes can affect the

evaporitic bedrock and also other overlying bedrocks such

as limestones, which in this case function as caprocks.

Bedrock and caprock sagging sinkholes

The passive sagging process is related mainly to prefer-

ential interestratal dissolution of particular beds, probably

halite or glauberite in many cases (Fig. 5a). The pro-

gressive stratigraphically controlled dissolution may be

accompanied by continuous flexure of the overlying strata,

so that cavities do not necessarily develop beneath the

sagging structures. Such sagging deformation may affect

the evaporitic bedrock, caprock units and overlying cover

deposits (Fig. 6a, b, c). Sagging is favoured by the pres-

ence of interstratified marl and shale beds. The process

results in the development of basin structures with cen-

tripetal dips. These structures frequently show an upward

decrease in the amount of deformation, which may be

explained by the gravitational detachment of successive

beds from one another, favoured by the fact that bonding

across bedding planes is much weaker than the strength

within the beds (Ford and Williams 1989). When the dis-

solution-induced sagging of the bedrock occurs at different

Environ Geol (2008) 53:993–1006 999

123

sites, it may produce a series of basins and domes (egg-

carton type of structure), which appears as a sequence of

synforms and antiforms in cross-section. This type of

subsidence commonly produces shallow, vaguely edged

sinkholes that may reach several hundred meters in length

(Guerrero et al. 2004, 2007; Gutierrez et al. 2007a). The

terms bedrock sagging sinkhole and caprock sagging

sinkhole are proposed for these types of dolines (Fig. 2).

Fig. 5 Diagram showing the main subsidence mechanisms involved

in the generation of bedrock and caprock sinkholes. a Passive sagging

of bedrock strata caused by stratigraphically controlled sheet

dissolution. At some stage, the bedrock strata affected by flexure

may develop bending-moment faults. b Upward propagation of roof

cavities and development of breccia pipes. Dissolution transforms the

chaotic packbreccias into matrix-rich floatbreccias. c Collapse and

brecciation of densely jointed bedrock overlying a stratigraphically

controlled dissolution zone. d Sagging of bedrock strata and

development of a trans-stratal breccia pipe. e Sagging and collapse

brecciation of densely jointed bedrock caused by sheet interstratal

karstification

Fig. 6 a and b Sagging

structures caused by interstratal

karstification of evaporitic

bedrock affecting gypsum

sediments and pediment

deposits (high-speed railway SE

of Zaragoza city). The lineindicates the boundary between

the alluvium and the gypsum

bedrock. c Sagging synform in

marly limestones generated by

karstification of the underlying

evaporites in the Madrid Basin

(M-45 highway)

1000 Environ Geol (2008) 53:993–1006

123

Bedrock and caprock collapse sinkholes

Development of collapse structures is favoured by the

following factors: (1) high joint density, so that the bedrock

does not have enough mechanical strength to bridge cavi-

ties (Bogli 1980; Kerans 1988; White 1988; Ford and

Williams 1989; Loucks 1999; White and White 2000;

Williams 2003); (2) the absence of voids with a span large

enough to initiate sagging; and (3) vadose conditions under

which the cavity roofs have a greater effective weight

(White and White 1969, 2000; White 1988; Ford and

Williams 1989).

When the width of an opening is small enough to pre-

vent sagging, the cavity roof tends to propagate upwards by

progressive failure (Fig. 5b). The deflection of gravita-

tional stresses around the cavity creates a tension zone over

the roof that is overlain by an arched compression zone

(Waltham et al. 2005). This tension zone determines the

development of cupola-shaped failure planes and the gen-

eration of arched roofs (Fig. 7a). The failure mechanism

has been studied by Tharp (1995) who points out that the

rock breaks by means of crack propagation in the direction

of maximum compression, with failure occurring when

crack surfaces coalesce. Collapse of the roof produces a

breakdown pile on the floor, in which the size of the blocks

depends largely on the density of discontinuity planes. As

the cave stopes upwards, the breakdown pile grows gen-

erating a transtratal breccia pipe (also called collapse

chimney, breakdown column or geological organ)

(Klimchouk and Andrejchuk 1996, 2005) that may reach

several hundred meters in height (Johnson 1989; Ford

1997; Lu and Cooper 1997; Warren 1999). These highly

porous evaporite breccia pipes may then act as zones of

preferential groundwater flow, which can accelerate rates

of dissolution. Such rapid karstification tends to transform

the initial clast-supported chaotic angular breccia (packb-

reccia) into a matrix-supported mass of corroded blocks

embedded in a karstic residue (floatbreccia) and finally into

a massive karstic insoluble residuum devoid of clasts

(Kerans 1988; Loucks 1999; Warren 1999). The breccia

pipes in the Ebro Valley attain more than 30 m in height

and are commonly filled by massive floatbreccia of dis-

persed gypsum clasts embedded in a marly residue

(Fig. 7b, c). As noted, whether a stoping void can reach the

surface or not depends chiefly on the size of the opening,

the overburden thickness and the bulking effect of the

breakdown (Ege 1984; Andrejchuk and Klimchouk 2002).

The stoping process may cease temporarily or permanently

if the cavity roof and the breakdown pile meet. Another

option is the en bloc foundering of the bedrock sediments

overlying the cavity with almost no internal deformation

(Fig. 7d; and see Christiansen 1971; Christiansen and

Sauer 2001; Guerrero et al. 2004, 2007). In our study areas,

these collapse processes lead to the sudden formation of

steep-walled sinkholes, commonly several tens of meters in

diameter. The terms bedrock collapse sinkhole and caprock

collapse sinkhole are proposed for this type of sinkhole

(Fig. 2), whether it is at the top of a breccia pipe or of a

foundered block. The main difference between these two

types is that dissolution processes cannot affect the breccia

pipes in the caprock collapse sinkholes if the caprock is

composed of non-soluble lithologies.

Fig. 7 a Dome-shaped failure plane in evaporitic sediments of the

Calatayud Graben. b Strongly karstified transtratal breccia pipe in a

cut of the high-speed railway SE of Zaragoza. c Passive sagging

synform cross-cut by a karstified breccia pipe filled with gypsum

blocks embedded in a fine-grained karstic residue (floatbreccia).

d Bedrock collapse structure controlled by well-defined failure planes

in the central sector of the Ebro Basin. The downthrown block

incorporates gravel-filled karstic conduits surrounded by a karstic

residue

Environ Geol (2008) 53:993–1006 1001

123

Complex sinkholes

A large proportion of paleosinkholes observed in the

studied karst result from the combination of two or more

subsidence processes, indicating that a spectrum of sink-

hole types exists between the simple main types discussed

above. Moreover, when dissolution operates at some depth

within the evaporitic substratum (interstratal karstification),

subsidence processes may affect both the unconsolidated

cover and the rock formations (evaporitic or caprock). In

our proposed classification scheme, these complex sink-

holes are described with combinations of the different

terms listing the dominant material or process first and then

the secondary ones. The genesis and characteristics of the

most frequently observed types are described below.

One widespread situation is found where there is con-

comitant differential dissolutional lowering of the rockhead

and formation of conduits. This karstification leads to

progressive accommodation of the cover by passive sag-

ging coupled with downward migration of the cover

material into the solutionally enlarged pipes (Fig. 3d, e).

This internal erosion may lead to local settling of the cover

(suffosion; Fig. 8a, b) or to upward stoping of voids (col-

lapse). The resulting depressions may be classified as cover

sagging and suffosion sinkholes or cover sagging and

collapse sinkholes, depending on the rheological behaviour

of the cover above the conduits. If sagging affects large

areas, the subsidence landforms can be described as cover

collapse or cover suffosion sinkholes nested in a cover

sagging sinkhole.

In some cover sagging sinkholes, as flexure proceeds,

the cover may start to behave in a brittle manner and de-

velop failure planes. Commonly, tensile stresses acting at

the margins of the subsiding basins generate subvertical

normal faults, whereas compression in the inner part of the

structure related to space limitations produces antithetic

reverse faults (bending-moment ring faults, Fig. 3a; see

Gutierrez 1998; Ge and Jackson 1998; Dias and Cabral

2002). Dolines produced by this combination of processes

can be described as cover sagging and collapse sinkholes

(Fig. 8c). Synformal structures affected by normal and

reverse bending-moment faults have been also observed in

bedrock sediments (bedrock sagging and collapse sink-

holes) (Figs. 5a, 9a).

When there is interstratal karstification in a densely

jointed substratum, the overlying strata tend to subside by a

combination of passive sagging and collapse processes

(Figs. 5e, 9b). In this case, collapse takes place by the

relative displacement of blocks along pre-existing failure

planes (bedrock brecciation) leading to the formation of

stratiform collapse breccias roughly concordant with the

bedding (crackle breccias), indicating that cracking, dis-

placement and rotation have not been intense. These

breccias then degrade to clast-supported mosaic breccias

according to Loucks’ (1999) and Warren’s (1999) classi-

fications. As dissolution proceeds in this highly porous

material, the packbreccia is transformed into a floatbreccia

made up of corroded clasts embedded in a fine-grained,

largely insoluble residue (Fig. 9c). Obviously, dissolution

of the breccia subsequent to collapse may contribute sig-

nificantly to subsidence of the ground. In our study sites,

the size of the breccia fragments is up to 4 m on the major

Fig. 8 a Passive sagging and suffosion of pediment deposits in the

Ebro Basin. Note the karstic residue between the deformed alluvium

and the undeformed gypsum bedrock. b Sagging and suffosion of

terrace deposits in Madrid Basin. The subcircular gravel mass at the

left corresponds to the section of an inclined gravel-filled conduit.

c Sagging synform affected by normal faults in the limbs and

antithetic reverse faults in the inner part of the fold (Alfambra River

valley)

1002 Environ Geol (2008) 53:993–1006

123

axis and the karstic residue consists of marls, shales and

minor gypsum clasts. In the cuts along the high-speed

railway south of Zaragoza, these tabular breccias associ-

ated with sagging structures are very common and coincide

with large shallow depressions (bedrock sagging and col-

lapse sinkholes) similar to those produced by a purely

bedrock sagging mechanism (equifinality).

The combination of interstratal sheet dissolution and the

creation of large cavities by focused karstification within

the bedrock causes the passive sagging of the overlying

strata and the upward propagation of the cavities by roof

collapse (Figs. 5d, 7c). Once sagging has begun, the

location of new dissolutional cavities may be controlled by

the generation of new fracture planes; these tend to con-

centrate in the hinge zones of the sagging synforms (zones

of maximum dissolution and deformation). This sequence

of subsidence has been observed by Andrejchuk and

Klimchouk (2002) in the gypsum caves of Russia, where

sagging of the cavity roofs preceded and accompanied

failure. The coeval or sequential operation of bedrock

sagging and upward stoping of roof cavities produces

synformal structures interrupted by breccia pipes filled by

chaotic packbreccias or strongly karstified floatbreccias

with a high proportion of insoluble residue (Fig. 7c). The

morphology of these bedrock sagging and collapse sink-

holes is characterised by large, gently sloping depressions

that have steep-sided hollows in their floors.

When bedrock that is subject to sagging with collapse

brecciation is overlain by unconsolidated deposits, the

cover tends to accommodate concordantly with the sub-

stratum by ductile deformation (Fig. 9d). The downward

migration of the cover deposits through open spaces in the

underlying breccia (locally widened by corrosion) may

emplace cover suffosion or cover collapse sinkholes within

the earlier cover and bedrock sagging and collapse sink-

holes. This complex suite of deformation mechanisms is

recognised in the Ebro valley, where it is associated with

either thickened terrace deposits or with hectometre- to

kilometre-scale closed depressions.

Discussion and conclusions

Aside from the solution sinkhole class, our proposed ge-

netic classification defines the principal types by two terms;

first, the material being displaced by downward gravita-

tional movements (cover, bedrock or caprock); second, the

chief subsidence process involved (collapse, suffosion or

sagging; Fig. 2). The complex sinkholes that result from

some combination of several subsidence mechanisms are

described using combinations of the different terms, with

the dominant material or process followed by the second-

ary one (e.g. cover and bedrock sagging sinkhole, bedrock

sagging and collapse sinkhole). This classification is

applicable to both carbonate and evaporite karst terrains

because it includes all the genetic types described in car-

bonate karst areas and adds new terms characteristic of

evaporite karst settings. The term ‘buried sinkhole’ has

been excluded from the classification since it does not

describe the origin of sinkholes but their condition.

The names adopted for the types of materials are the

same as those applied in the most recent classifications of

sinkholes by others (Williams 2003; Beck 2004; Waltham

et al. 2005). ‘Cover’ refers to residual soil material or

allogenic unconsolidated deposits, ‘bedrock’ to karst rocks

and ‘caprock’ to non-karst rocks. The subsidence mecha-

nisms identified in the analysed paleokarst exposures in-

clude sagging, suffosion and collapse. ‘Sagging’ refers to

the ductile flexure of sediments caused by the lack of basal

support. The differential corrosional lowering of the

rockhead may cause the passive sagging of the cover

(Fig. 4a, b), whereas interestratal karstification may result

Fig. 9 a Sagging synform in gypsum strata affected by antithetic

reverse faults and brecciation on the hinge zone (Calatayud Graben).

b Combination of passive sagging and collapse brecciation. Note the

incipient failure planes developed at the margin of the flexure.

c Karstified collapse breccia (floatbreccia). d Gypsum bedrock

affected by sagging and collapse and sagged alluvium overlying a

thick karstic residue. Images b, c and d are from the cuttings along the

high-speed railway track SE of Zaragoza

Environ Geol (2008) 53:993–1006 1003

123

in the sagging of the overlying bedrock strata and cover

deposits (Fig. 6) In our analysed exposures, sagging caused

by interstratal karstification is particularly frequent in

evaporitic sequences containing halite and glauberite units

in the subsurface. This process commonly produces large

shallow and diffuse-edged sinkholes. Although in our study

areas this mechanism plays an instrumental role in the

generation of a great proportion of the sinkholes, it has not

been emphasised in previous classifications based primarily

on observations in carbonate karst. Passive sagging of

cover and bedrock material has been documented in studies

of paleokarst features and sinkholes in other evaporite karst

areas of the world (e.g. Cooper 1986; Ackermann et al.

1995; Forbes and Nance 1997; Jassim et al. 1997; Kirkham

et al. 2002). Ford and Williams (1989) indicate that sag-

ging is found where there is interstratal solution of evap-

orate rocks at shallow depth. ‘Suffosion’ is the downward

migration of cover deposits through dissolutional conduits

and its ductile settling (Fig. 4c, d, e, f). This mechanism

only affects unconsolidated surficial deposits, where it

tends to produce bowl- or funnel-shaped sinkholes of

limited extent. The term suffosion as used in Williams’

(2003) and Waltham’s et al. (2005) classifications is pre-

ferred to ‘subsidence’, which is used in Beck’s (2004)

classification. ‘Collapse’ is the brittle gravitational defor-

mation of cover, caprock and bedrock materials; this pro-

cess includes upward stoping of cavities through bedrock

(Fig. 7a–c) and cover sediments by progressive roof fail-

ure, the development of well-defined failure planes in

bedrock and cover sediments (Figs. 4g, 7a, c, 8c) and the

brecciation of rocks (Fig. 9). The resulting sinkholes gen-

erally have well-defined, steep sides. The term ‘cover

collapse sinkhole’, as used in Beck’s (2004) classification,

is preferred to ‘dropout sinkhole’ (Williams 2003; Wal-

tham et al. 2005) because the latter is unknown in the

previous literature.

The Spanish paleokarst features reveal that the subsi-

dence processes involved in the generation of sinkholes in

evaporite karst terrains display some significant qualitative

and quantitative differences compared to those documented

in carbonate terrains. The sagging mechanism, infrequent

in carbonate karst settings, plays an important role in the

generation of sinkholes in some evaporite karst areas.

Collapse occurs to a greater extent and at higher rates in

evaporite karst terrains. These differences are primarily

related to the greater solubility and lower mechanical

strength of the evaporites. Gypsum dissolution rates

>1 mm/year have been reported in caves by Klimchouk

et al. (1996) and Klimchouk and Aksem (2005). The rapid

differential lowering of the rockhead in mantled evaporite

karst settings may lead to the progressive sagging of the

cover, creating cover sagging sinkholes at a significant rate.

On the other hand, gypsum rock is more ductile than the

carbonate rocks; Young’s modulus for limestone is com-

monly three times greater than that of gypsum (Waltham

1989; Salinas 2004). Geomechanical tests indicate that

gypsum undergoes plastic–elastic–plastic deformation due

to crystal reorganisation, migration of water molecules to

capillary spaces and the small content of varying amounts

of muddy material (Bell 1994; Karacan and Yilmaz 2000;

Dashnor et al. 2006). This explains why interstratal kars-

tification of evaporites may cause passive sagging in

overlying evaporite beds, generating basin structures with

centripetal dips. According to Waltham et al. (2005) wide

cave chambers are unknown in gypsum due to roof col-

lapse and plastic deformation. The closure of voids by

deformation of gypsum beds has been reported in salt

mines (Ege 1984) and in natural caves (Andrejchuk and

Klimchouk 2002). Finally, gypsum commonly fails more

readily than carbonate rocks because the mean compressive

and tensile strength of limestone are more than ten and

three times greater (Waltham 1989; Selby 1993; Salinas

2004). Additionally, the evaporitic rock masses may un-

dergo a significant reduction in mechanical strength due to

the rapid corrosional widening of joints.

An effective sinkhole hazard mitigation requires a good

understanding of the subsurface processes involved in

sinkhole generation. The selection of corrective measures

may be largely dependent on the type of karst (e.g. subsoil,

interstratal, phreatic and vadose), the material affected by

subsidence (cover, bedrock and caprock) and the subsi-

dence mechanisms (sagging, suffusion and collapse). Study

of paleokarst may provide a great deal of information about

these practical aspects. The proposed classification may

help identify different genetic types of sinkholes in a par-

ticular area and allow selection of the most appropriate

mitigation strategy for each one.

Acknowledgements The original manuscript has been substantially

improved thanks to the reviews of Prof. Derek Ford, Dr. Barry Beck

and Dr. Tony Waltham. This work has been partially co-financed by

the Spanish Education and Science Ministry and the FEDER (project

CGL2004-02892/BTE).

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