A New View on Karst Genesis by Radulovic M. Milan

7/29/2019 A New View on Karst Genesis by Radulovic M. Milan

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A NEW VIEW ON KARST GENESIS

A new view on karst genesis

Milan M. Radulovi

M. M. Radulovi, PhD

Faculty of Civil Engineering, University of Montenegro,Cetinjski put bb, 81000 Podgorica, MontenegroEmail: [email protected]

The final publication is available at http://link.springer.comCopyright 2013, Springer-Verlag. All Rights Reserved

Citation: RaduloviMM (2013) A new view on karst genesis.Carbonates and Evaporites doi:10.1007/s13146-012-0125-2

The final publication is available at http://link.springer.com. Copyright 2013, Springer-Verlag. All Rights Reserved1

Abstract Karst terrains and their specific landforms, suchas sinkholes and caves, have been thoroughly studied fromthe second half of the nineteenth century. However, karst

genesis remains a puzzling issue to this day. The results ofthe recent studies of ocean floor and the results obtained bydrilling deep oil boreholes have raised doubts about theexisting explanations of the karst landforms developmentand encouraged the emergence of new views on thissubject matter. According to the new hypothesis, themajority of karst landforms were formed at great depthsbeneath sea level where dissolution of carbonates increasesdramatically. Underwater dissolution first caused theformation of karst depressions and the primary network ofkarst conduits elongated along the existing fractures. Thisprocess was followed by further expansion of the conduitsand the formation of spacious caves due to the water

regression and the action of turbulent flows. It isconsidered that the introduction of the new concept wouldaccelerate the development of karstology and improve theprinciples and methods for solving numerous practicalproblems such as the abstraction of quality drinking waterand the research of oil, gas and bauxite deposits.

Key words Karst genesis Landforms New hypothesis Dinarides

Introduction

Karst specific features have always attracted the

attention of researchers and lead them to think about itsgenesis. From the late nineteenth century, starting from thepioneering work of Cviji (1893), widely regarded as thefather of modern karstology, to this date a largecontribution has been made to understand karst from thegeomorphological, hydrological, geological andhydrogeological point of view. The most comprehensivesynthesis of previous karst studies is given by Ford andWilliams (1989, 2007). Also, a great contribution wasgiven by the following authors: Sweeting (1972); Jennings(1985); Dreybrodt (1988); White (1988); LaMoreaux andLaMoreaux (1998); Palmer et al. (1999); Klimchouk(2000); Gunn (2004); Bakalowicz (2005); Waltham et al.(2005); Andreo et al. (2010). Introduction and furtherdevelopment of karstology have expanded the circle of

scientists and experts who take part in finding answers to amyriad of practical problems, from drinking watersupplies, construction of dams and reservoirs, to water

protection in general.Karst covers approximately 12 % of the earths land

surface and about 25 % of the world population depend onkarst aquifers for water supply. The specific nature of karstterrains and complex geological composition, have alwaysposed difficulties to exploring and solving scientificproblems in this field. The problems are complexespecially when studying highly karstified terrains, and tothis date no appropriate methods have been establishedwhich would deal with some of the key issues related towater circulation in these terrains. The researchers apply anumber of scientific techniques that enable terrain analysis

from the surface, from the inside (speleology) and from adistance (remote sensing). However, the complex nature ofthese problems usually generates insufficiently reliableresults and problematic solutions in practice.

The paper presents new views on the karst developmentwhich have been encouraged by the discovery of sinkholesand karstified formations far below the present sea level(Land et al. 1995; Michauda et al. 2005; Milovanovi1965). The areas at such great depth could not have beenexposed to the effects of atmospheric water which havebeen so far considered a major agent in the development ofmost surface and subsurface karst features.

According to the new hypothesis, karst depressions

(sinkholes, uvalas and poljes) and some karst conduits areformed beneath sea level, more precisely beneath thelysocline where carbonate solubility dramaticallyincreases.

The hypothesis presented in this paper is mainlyexemplified by classic karst of the Dinaric area (4145N,1319E). This is due to the marked diversity of karstlandforms found in these terrains, but also to facilitate thecomparison with the existing fundamental postulate onkarst development which is based precisely on the Dinarickarst (Cviji 1893, 1918). Thickness of the carbonate rocksin the External Dinarides can reach more than 5 km, and

the continuous development of karstification to greatdepths has been registered as well.For analysis of genesisof other karst types (cockpit karst, hypogenic karst, karst in


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hot arid and cold regions, karst in evaporites) there was noenough space, but it will be probably done in one of thefollowing papers.

The subject matter covered by this paper is selected as auniversal problem. This concept is not in favour ofnumerous scientific hypotheses so it should not be fitted at

all costs in other unproven postulates.

Karst landformstheoreticalbackground

Karst is a geological term which refers to a set ofspecific morphological forms of landscape that are theresult of interaction between a number of factors, primarilywater and watersoluble rocks. Therefore, karst forms aredeveloped only in terrains made of soluble rocks,commonly limestones and dolomites, but also in terrainmade of gypsum, anhydrite and halite rocks. Due to the

solubility of carbonate rocks (limestones and dolomites),tectonic faults are expanded and secondary porosity ofrocks is increased. It causes high permeability of rocks anddisappearing of surface water from karst terrains. Theabsence of a drainage network from surface separates karstterrains from other terrains composed of impermeablerocks. Instead of normal river valleys, specific surface(karren, sinkholes, uvalas, poljes, dry valleys) andsubsurface (caves) geomorphologic features are formedconditioning water runoff mostly below the terrain surface.

Karst is a subject of numerous scientific fields, whereasJovan Cviji and his followers founded a separate

discipline called karstology dealing particularly with karstterrains.

Surface karst forms

The surface of karst terrains is usually characterized bya diversity of karst landforms that provide for a speciallook of these terrains. Most typical and often describedkarst forms are: karren, sinkholes, uvalas, poljes and dryvalleys.

Karren

Karren are small-scale and most typical landforms onkarst surfaces formed mainly due to the chemical action ofatmospheric water. Recent solution rate of carbonate rocksin the Dinaric area ranges from 30 to 100 m3 CaCO3/km

2(or the equivalent lowering of the surface in one millionyears for 30100 m) as estimated by Gams (1966, 2000).Karren appear in various shapes and sizes, often on slopingsurfaces. According to the morphogenetic principle, karrencan be roughly divided into the following two groups:hydraulically controlled karren and fracturecontrolledkarren.

Hydraulically controlled karren occur on compactrocks. They are much shallower and the bottoms of theirchannels are smooth and usually properly elongated. It isconsidered that these small forms are exclusively linked to

limestones, i.e. they occur primarily due to the corrosiveeffects of atmospheric water. These karren most commonlyoccur on inclined surfaces when they occur in a lineararrangement (Fig. 1). Their depth in the Dinaric area mayrange from a few mm to about 0.5 m.

Fracturecontrolled karren are formed by the

expansion of systems of fractures and bedding planesthrough the chemical and partly mechanical action ofwater. In some cases, such karren continue into shortelongated abysses, although these karren commonlydescend down to 5 m. Carbonate rock surfaces dominatedby karren are called karren fields.

Fig.1 Hydraulically controlled karren on compact carbonate rock(key at the top of the photo may be used to assess the scale)

Littoral karren (micro pits) should be mentioned as aspecial group. They are represented by circular smallforms. Unlike most other karren they are not produced bysolution actions of atmospheric water. Their formationoccur due solution actions of standing water. Littoralkarren could be clearly observed in zones of seasonal lakelevel oscillations where they may develop to high densities(Fig. 7b).

Karren can also appear beneath the soil cover in theepikarst zone. Epikarst represents the shallow subsurfacezone that has a higher porosity than the deeper parts of thevadose zone. Soil thickness and quality, the content ofbiogenic acids and humidity concentration in thepedological layer have also influence on the developmentof epikarst, i.e. underground solution forms similar tokarren on the terrain surface.

Sinkholes

Sinkhole (doline) is very frequent surface karstlandform (Fig. 2). There are several types of this karstlandform (solution, collapse, subsidence sinkholes), yetthey all relate to differently shaped depressions incarbonate rocks, most often being funnel, saucer, bowl andwellshaped. Diameters and depths of these landforms also

vary, most often in metric to hectometric scale. Theirbottom is usually filled with soil, but can also be rocky orfilled with coarsegrained material.


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It is thought that the sinkholes are commonly associatedwith the system of fractures and faults. Sinkholes are oftenelongated along faults, and they also commonly occur atthe intersection of two or more faults.

Sinkholes are most common in karst plateaus. Terrainsdensely pitted with sinkholes are often called polygonal

karst (Williams 1972; Ford and Williams 2007). Morewords about sinkholes will be in the following sections.

Fig.2 A funnelshaped sinkhole in Kui karst plateau

Uvalas

Uvalas are surface karst forms which are larger in sizethan sinkholes but smaller than poljes. These aredepressions of hectometric size, often irregularly shaped.

Bottoms of uvalas can be also covered with soil, mostoften with terra rossa. The longer axis of uvalas is often inparallel with the direction of faults and synclines. This isthe case of most uvalas in the Dinarides which are mostlyelongated in NWSE direction.

Short and periodical surface water streams can emergein uvalas; however, the drainage of uvalas usually takesplace underground.

Poljes

Topographically closed karst depressions larger thansinkholes and uvalas are called poljes. The floors of poljes

are usually flattened, but small hills may also occur. Poljeis usually covered with lacustrine, moraine, glaciofluvialand alluvial sediments and, apart them, thick complexes ofother sediment rocks such as marl, clay and coal can befound. Poljes are most commonly predisposed by tectonicsas suggested by the direction of the longer axis of most ofthe poljes of the External Dinarides (NWSE).

The specific features of polje are reflected in thefunctioning of the hydrogeological system. The upstreamof polje is characterized by strong karst springs from whichsurface water streams are formed. Polje drainage takesplace through numerous swallow holes (ponors) usuallylocated along downstream edges. In addition to waterbreaking out onto the surface, most poljes are alsocharacterized by deeper permeable horizons through which

groundwater directly flows to the erosion basis. Poljes canoften be flooded when water inflow exceeds the capacityof swallow holes. Flooding can also happen due to the riseof groundwater level above the level of polje bottom whenoutflows also appear in numerous estavelles.

Hydrotechnical reservoirs have been built in many

poljes of the External Dinarides, preceded by extensivehydrogeological, engineering and geological research(Vlahovi 1975; Milanovi 2006). Due to highpermeability of the limestones, great sinking losses ofaccumulated water represent the main problem even to thisday. The opening of new swallow holes often occurs aftercharging and discharging of reservoirs.

Dry valleys

Dry valleys are represented by elongated karstdepressions. The bottom of dry valleys is often intersectedby crests of sinkholes and uvalas. Dry valleys lack surfacewater streams, except in rare cases when streams emergeonly for a short period and for a short distance. In additionto dry (abandoned) valleys, karst terrains are alsocharacterized by the valleys with permanent or temporarystreams such as blind valleys, through valleys, gorges andcanyons.

Subsurface karst forms

One of the basic features of karst terrains is theirsubsurface morphology encompassing different types ofcavities developed in soluble rocks. In a large number of

cases, subhorizontal and subvertical conduits areintersected, thus creating caves systems. Caves commonlyrelate to subsurface karst forms that are large enough forhuman entry. However, subsurface karst forms shouldinclude all underground cavities, including the smallestones which are created through solvent and mechanicalaction of water as well as through accompanying collapseactions. Therefore, subsurface karst forms also include alltypes of karst conduits and caverns of a different size andspatial orientation.

Caves develop mainly along faults, fractures, andbedding planes. These forms are especially linked todirections of fault intersections.

Based on hydrographic characteristics, Cviji (1895a)distinguished two types of proper caves (referring to karstconduits in general): river caves and dry caves. Apart fromhydrographic characteristics, these cave types also differgenetically. Cviji considers that river caves wereprimarily formed by mechanical erosion of major streams,while dry caves developed due to the corrosive effects ofatmospheric water. However, according to Cviji (1895a)there are examples of dry caves created mostly by rivererosion, and vice versa, some conduits of present caveswith running water also genetically belong to dry caves.Cviji further states that detailed research should beconducted in each case individually to determine if a cavegenetically belongs to river caves or dry caves.


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Although caves are typically influenced by chemicaland mechanical water action, two main types of cavescould be genetically distinguished by following Cvijisdivision and simplifying Bellas classification (1998):

- corrosion (solution) cavesformed primarily bychemical erosion, and

- fluvial cavesformed mainly by erosion of strongturbulent streams.

Fluvial caves are characterized by various erosion formssuch as potholes, subsurface canyons and natural bridges,as well as by the presence of fluvial materials (gravel andsand). In the case of corrosion conduits, larger erosionforms are absent, channels have more regular crosssections, walls are usually coated with speleothems crust,and floor are often covered with the layers of soil, withoutlarger rounded materials.

Corrosion caves are much more numerous andwidespread, but generally they have far smaller diameters

and length of individual segments as compared to fluvialcaves. Solution conduits form a part of the subsurface karstnetwork which is often well connected with systems offractures. Such small conduits and caverns are often foundwhile performing exploratory drilling when drillingequipment dropdown and loss of drilling fluid occurs.Solution forms also include primary vadose caves withblind vertical conduits (Fig. 3a) which are largely presentmainly on karst plateaus. Bgli (1980) considers thatprimary vadose caves are caused by corrosive effects ofrain and snow melt water. However, it is assumed that theycan also appear due to the gradual lowering of groundwaterlevel (drowdown vadose caves) (Waltham 1981). Vertical

corrosion caves can be plain with a simple blind conduit orin step formation in which case their shape is partiallyadapted to major horizontal discontinuities. Verticalcorrosion caves are found in all karst plateaus inMontenegro while the number of these caves in Triestekarst is extremely large (even 5070 caves per 1 km2).

Fluvial caves are longer and have more spaciousconduits compared to the conduits of solution forms. This

group includes caves with subsurface streams as well asswallow holes typically formed on the edges of uvalas andkarst poljes, at the end of blind valleys or in sinking pointsof allogeneic streams that flow from impermeable terrains.Fluvial cave may be completely dry when they are abovethe maximum groundwater table, but many indicators

suggest that flow existed in these caves. These caves wereprobably formed along the direction of existing solutionconduits, which are additionally expanded by the action ofstrong turbulent flows with a bedload. The Slivlje swallowhole on the southeastern edge of the Niki polje(Montenegro) is a typical example of a fluvial cave. Theopening of this swallow hole is about 25 m long and 18 mwide. A vertical shaft of a constant diameter descends 45m from the surface. Via several steps divided by terraceswith potholes, this cave descends 176 m (Fig. 3b). TheObod cave, from which the water of the Crnojevia spring(Montenegro) periodically flows out of, also belongs to

this group of caves.In the case of some caves it is difficult to discernwhether their occurrence was caused due to chemical ormechanical erosion, since these forms were formed by thecombined action of these two types of erosion. Erosion isalso intensified by accompanying collapses of some partsof the ceilings and walls of the caves.

From the previously mentioned it can be concluded thatkarst conduits of different origins and size are interwoventhrough a karstic underground. Conduits formed by thesoluble actions of water create a primary networkspreading along fractures and faults, and are occasionallypenetrated by vaster fluvial caves with a higher

permeability.It is very difficult to detect spatial distribution of caves

and conduits, and hence a number of such phenomenaremain unknown to researchers. New conduits withunknown surface entries and exits are often revealed whenbuilding tunnels or other structures. Speleologicalresearches, if possible, still provide the most reliable dataon the distribution and morphology of caves.

Fig. 3a Crosssection of thesolution cave Bujna Jama on thePiva plateau (after Ljeevi

2004), b crosssection of fluvialcave Slivlje on the southeasternedge of the Niki polje (afterPetrovi 1968)


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Overview of some dilemmas

The development of sinkholes according tothe existing hypotheses

Since thorough research of karst geomorphology hasbegun, scientific understanding of the origin of sinkholeshas been marked by inconsistencies and disagreementsmainly related to the very creation of sinkholes and thetimeframe. Generally speaking, disagreement has occurredusually between representatives of the idea of the erosionorigin of sinkholes and advocates of the idea of thecollapse of cave ceilings.

In his monograph "Karst", Cviji (1895b) states thatAustrian researchers (Tietze 1880) who investigated thetypical karst of Kranjska Gora and the western part of theBalkan Peninsula, and linked geological knowledge of theAustrian and Montenegrin karst, concluded that sinkholes

occur due to the collapse of cave ceilings. With theseresults they challenged earlier concepts present in Austrianliterature at the time that sinkholes are formed by surfaceerosion. Contrary to this view is Mojisisovics opinionexpressed in his research of western Bosnia (Mojsisovic etal. 1880). In his opinion, erosion landforms in purelimestone, which he calls karsttricker (probably referringto typical small sinkholes), belong to a group of geologicalorgans. Diener (1886) generally agrees with this view andlists a number of evidence against the collapse theory.Although the research conducted in Kranjska Gora and inthe Cevennes considerably contributed to the knowledge of

karst terrains, researchers would later still come back withdifferent conclusions. Kraus (1887) who conductedresearch in Kranjska Gora, concluded that collapse causesthe creation of sinkholes and it occurs becausegroundwater dissolves and washes out limestone; also, allsinkholes are connected with caves. Martel (1890) came upwith a completely opposite conclusion after conductingresearch of karst terrains in the Cevennes. According toMartel, small sinkholes are not connected with caves.From 40 examined sinkholes only seven were connectedwith welldeveloped cave conduits and a subsurface river,whereas only one sinkhole could have been caused by acollapse. At the time, the British and American researchers

disagreed with the theory of collapse, often stating thatsinkholes are formed by dissolving action of atmosphericwater and erosion along fractures and faults.

In his monograph "Karst", Cviji (1895b) also presentedhis view of sinkhole formation, with the explanation thatcollapses do occur but are rare and most sinkholes are

created due to the effects of atmospheric conditions.Temperature changes and chemical dissolution extendedvertical and horizontal fractures in limestone; water isabsorbed through these fractures which causes theiradditional expansion since water directly or indirectlydissolves limestone with its carbon dioxide. Due todifferent effects of surface water, these fissures expand andturn into a funnel. Cviji called such phenomena normaldoline (solution sinkholes) considering them the mostcommon ones, but also acknowledging that this is not theonly type of sinkhole formation.

Cramer (1941) contributed to distinction between

solution (normal) and collapsed sinkholes. He comparedtopographic maps of various karst terrains around theworld and made a morphometric description of the terrainsconcerned. Sinkholes and collapses, elongated along onedirection, were often linked to groundwater streams.Stepanovi (1965) states that a series of sinkholes along adry valley axis indicates that a karst conduit goes along thesame direction, whereas fresh collapses of sinkhole bottomshow that a groundwater stream occasionally circulatesthrough the karst conduit undermining the bottom ofsinkholes from below.

Many authors accepted Cvijis view of the formationof solution sinkholes. Ninety years later, Paul Williams

(1983) provided a model of the relation between sinkholesand epikarst which resembles Cvijis typical profile ofsinkholes, as noticed by Ford (2007). Williams (1983)considers that terrain subsidenceoccurs gradually througha lasting process of diffuse recharge of epikarst hangingaquifers with atmospheric water which is accumulated inthe subsurface zone and concentrically infiltrated deeperunderground through a central vertical fracture (Fig. 4).

Also, it is often stated that solution sinkholes can occurfollowing the erosion of overlay impermeable layer fromwhich streams, while passing on karst terrains, graduallybegin to sink and disorganize the terrains by forming a

large number of sinkholes known as pointrecharge

Fig. 4 Cviji 1893 conception of theform of a dissolutional doline comparedwith Paul Williams 1983 modelshowing the relationship of doline andepikarst. Cviji diagram is based upon acrosssection exposed in a railwaycutting near Logatec, Slovenia. Ford(2007) believed that Cviji wouldapprove Williams of elaboration.Springer and the EnvironmentalGeology, 51(5), 2007, 657684, Ford D,Jovan Cviji and the founding of karstgeomorphology, Fig. 3, Copyright 2007.

Reprinted with kind permission fromSpringer Science and Business Media


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dolines (Sauro 2012).In recent years, certain "reconciling" theories have

emerged such as the theory of the expanded conduit(shaft) at the bottom of the epikarst zone(Klimchouk 2000), which is an attempt to reconcile thecollapse and solution theories. According to

this conceptual model, discharge of hanging epikarstaquifers results in the gradual expansion of verticalfractures from below and formation of a natural shaftclosed with the upper layer of carbonate rocks. Due to thecorrosion of terrain surface and shaft expansion frombelow, the upper layer gets thinner which at one pointresults in thecollapse and opening of the shaft towards theterrain surface. According to this concept, the final phaseincludes shaping of the shaft sides and formation of afunnelshaped sinkhole.

A number of scientists acknowledge several ways inwhich sinkholes can be formed and based on that they

develop classifications which are in most cases similar tothe one provided by Waltham et al. (2005). According tothem sinkholes were classified as: solution, collapse,dropout, buried, caprock and suffusion sinkholes.However, these authors agree that solution sinkholes aredominant whereas natural collapse is an extremely rarephenomenon.

In recent years, sinkholes have been discovered on thedeep ocean floor that has never been exposed toatmospheric conditions. Since each of the mentionedhypotheses assumes atmospheric water as an agent,through them is not possible to explain the genesis of thesephenomena.

Dilemmas about the development of cavesystems

Along with the emergence of ideas about the genesis ofsurface karst forms and carbonate aquifers, in the first halfof the twentiethcentury, a number of hypotheses about thegenesis of the caves appeared, from which Ford and Ewers(1978) highlight the following: vadose hypothesis, deepphreatic hypothesis, and watertable cave hypothesis.

One of the first major hypotheses, known as the "vadosehypothesis", was advocated by the French speleologist

Martel (1921). Having in mind the small solvent capacityof water enriched with CO2 from the atmosphere, Martelconsidered that such water could have an impact only inthe vadose zone. He also believed that mechanical erosioncaused by strong streams with bedloads has an importantrole in the expansion of channels only in the zone abovewater table. According to this hypothesis, the action ofmechanical erosion does not affect the phreatic zone,which consists of only narrow channels created by thesolvent capacity of slow flowing waters.

With the increasing number of surveyed caves, it wasfound that there were caves located deep below the

regional water table. By using empirical evidence fromcave maps and sections, the famous Americangeomorphologist Davis (1930) proposed a diametrically

opposite hypothesis (deepphreatic hypothesis), in whichthe caves are developed slowly at random depths beneaththe regional water table.

By recognizing the importance of soil CO2 as a boosterof limestone solubility, there was a weakening of thepreviously mentioned two explanations. Swinnerton (1932)

introduced a new hypothesis according to which the caveswere formed along the water table, but also in theepiphreatic zone, which was similar to a previousexplanation given by Cviji (1918). Though, deep phreaticcaves have remained an enigma for this hypothesis.

However, later experiments performed by Weyl (1958)found that infiltrating waters would quickly becomesaturated with calcite (after a few meters), so they do notcontinue to dissolve and thus caves could not be formed. Itwas a crisis period for each of the previously statedhypotheses. Since the existence of caves is a fact, analternative explanation was necessary. In that period the

effects of sulphides (Howard 1964) and mixing corrosion(Bgli 1964) on the development of caves wereconsidered. But, based on newer laboratory experiments itwas concluded that the dissolution rate of carbonates isnonlinear and that there is large reduction in the rate asthe chemical equilibrium is approached (Berner and Morse1974; Plummer and Wigley 1976). The results have givenrise to a revival of earlier hypotheses that are reconciled bythe forstate model provided by Ford and Ewers (1978).

During the last two decades, a number of studiesdealing with the genesis of caves have been performed(Knez 1996; Lowe 2000; Dreybrodt and Gabrovek 2000;Palmer and Audra 2003; Worthington 2004).

Many dilemmas regarding the development of cavesystems exist in todays karstology. According to Ford andWilliams (2007), no single theory of genesis has been ableto encompass all caves except at a trivial level ofexplanation.

This section will review only two positions that arisefrom the above hypotheses:

- phreatic caves are developed slowly at randomdepths beneath the regional water table,

- dissolution and caves development cannot takeplace deep below sea level, i.e. below freshwatersaltwater interface.

One of the major scientific debates in this field isrelated to the clarification of the development of phreaticcaves with conduits descending far beneath the water table.To this day, only a small number of this type of caves havebeen explored around the world, primarily because theirmapping is extremely difficult since they are completelyfilled with water and speleodiving is required for theirresearch. Some of these caves have been investigated onthe Montenegrin coast such as the Risan, Sopot, Gurdicand Ljuta caves (Dubljevi 2001; Milanovi 2007) withconduits descending below 100 m under the sea level.Potholes have been registered in the passages of the Risancave (Milanovi 2010) which can classify this type of cavein the group of fluvial caves. Thus, the discovery ofpotholes at the bottom of the phreatic caves has created


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doubt in the previous explanations about the slowdevelopment of these channels.

Karst conduits detected in deep oil boreholes also posea dilemma. For instance, exploration drilling in Romaniaregistered caves filled with water at the depth of around3,000 m. According to Ford and Williams (2007), some of

these caves may be deep phreatic (bathyphreatic) cavesbut, as authors note, it is likely that many deepinterceptions are of swallower types of caves that havebeen dropped downwards by tectonic activity. Also, deepdrilling performed along the Adriatic coast (ExternalDinarides) showed that cavernous limestones exist in someareas at levels below 2,000 m under the sea level(Milovanovi 1965). Thus for instance, due to the deepdrilling performed near Ulcinj it was established thatcavernous limestonedolomite masses descend down toover 1,500 m under the sea level. At Bijela Gora, highlykarstified (cavernous) limestones have been found at the

depth from 1,560 to 1,579 m. In a deep borehole in RavnaKorita, periodic and often intensive loss of drilling fluidoccurred up to the depth of 2,000 m. In Rovinjsurroundings, cavernous dolomites lie in the range of1,3502,330 m where a loss of drilling fluid has beenregistered. By performing deep drilling on the island ofVis, karstified layers of limestone, dolomite and limestonebreccia, in depth range of 732 to 2,040 m beneath sealevel, have been registered (periodically losses of drillingfluid would occur). Despite extensive research, noindicators of hypogene speleogenesis caused by watercontaining H2S have been detected in this area. What ispuzzling about this region is that geologists (Waisse 1948;

Grubi 1975) believe that in the past it was affected byorogenesis, i.e. terrain uplifting instead of lowering. So thequestion arises as how the karstification of subsurfacezones lowered to such great depths.

New concept

Numerous shortcomings of the existing explanations ofkarst development have created a need to establish a newconceptual model that would explain the karst phenomenain a more acceptable manner.

This section will first explain the terms related to

submarine dissolution (lysocline and calcite compensationdepth), followed by the elaboration of the arguments infavour of the hypothesis that the majority of karstlandforms have been formed underwater, and anexplanation of the mechanism of their formation.

Lysocline and calcite compensation depth

Shallow seawater is generally supersaturated in calcitebut as depth, i.e. hydrostatic pressure increases calcitesaturation of seawater decreases. Thus, calciumcarbonateshells are not subject to dissolution in seawater until theysink below the lysocline, which represents the depth in theocean below which water becomes aggressive to calcite

and the rate of calcite dissolution increases dramatically(Berger 1967; Wise 2003).

Below the lysocline there exists the calcitecompensation depth (CCD) which is the ocean depth atwhich the rate of supply of mineral calcite (CaCO3) equalsthe rate of its dissolution (Bramlette 1961; Pytkowicz

1970; Wise 2003). In the present conditions, below suchdepth deposition of carbonate sediments is not possible.Therefore, the lysocline depth and calcite compensationdepth depend on the solubility of calcite or, more precisely,of the parameters that influence its dissolution such astemperature, hydrostatic pressure and chemicalcomposition of water, especially the content of carbonateions and dissolved carbon dioxide (CO2) in water.Solubility of calcium carbonate increases with decreasingtemperature and increasing hydrostatic pressure. It alsoincreases with decreasing content of carbonate ions andincreasing partial pressure of CO2.

The depth of the lysocline varies and usually rangesbetween 3,7004,000 m (Wise 2003). The variations ofCCD have been also observed, but it can be stated that itsaverage value is around 4,500 m. Variations of thesedepths are the function of the circulation patterns withinthe oceans (Berger 1967).

The genesis of sinkholes

This subsection first elaborates on three facts whichindicate the underwater formation of sinkholes followed bythe explanation of the genesis of these phenomenaaccording to the new concept. In this paper, the term

sinkhole is mainly related to solution sinkhole i.e. doline.

Existence of sinkholes deep below sea level

The development of new techniques intended fordetailed bathymetric surveying of topography and featuresof sea floor (multibeam echosounders) led to the discoveryof a number of closed circular depressions that resemblekarst landforms such as sinkholes, uvalas and poljes. Thesephenomena are also often registered at great depths so it isconsidered that they could not have been formed on thesurface of terrains due to the influence of atmosphericconditions (Land et al. 1995). A rather spaciousunderwater area clustered with sinkholes and uvalas ofmultikilometre diameters (Fig. 5) formed in carbonatesediments has been detected near the Galapagos Islands(045055S; 89509050W), at the depth between1,500 and 2,600 m (Michauda et al. 2005). The dispersionpattern of the underwater sinkholes detected in this area isvery similar to the dispersion pattern of surface sinkholes,as indicated by nearest neighbour indexes (NNI) (Clarkand Evans 1954). NNI calculated for the mentioned areaclustered with underwater sinkholes amount 1.36. Nearestneighbour statistics have been done for many terrainsworldwide where the polygonal karst is developed on the

surface, as clearly presented by Ford and Williams (2007).Based on this data it can be observed that NNI for 10 out


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Fig. 6 The crosssection of the solution sinkhole exposed in aroad cutting on the Herceg NoviTrebinje road. The polygonalkarst is developed in this area

sinkholes because terrain morphology has enabled watercollection and accumulation. However, the emergence ofexpanded fracture is not a necessary condition for thegenesis of dissolution sinkholes; expanded fractures andswallow holes are actually often a secondary phenomenoncaused by the drainage of sinkholes. On the other hand,

enlarged fractures and vertical vadose caves appear atmany places in karst terrains which are not linked todepressions. Entrances of vertical vadose caves can belocated even on the crest between two sinkholes, which isthe case with Bujna cave (25 m deep, Fig. 3a) in the Pivaregion (Ljeevi 2004). Therefore, in this case sinkholesare not concentrated around the main conductive zone(vertical vadose cave) but are formed on more compactrocks. The origin of elongated sinkholes along faults fallsunder a separate case which is easier to explain with thenew concept of underwater corrosion compared to previousviews on the genesis of sinkholes caused by the action of

atmospheric water. This will be, however, discussed morein the section dealing with the genesis of uvalas and poljes.

Existence of microforms of sinkholes

Littoral karren (micro pits), formed due to the corrosiveaction of lake water, occur in many coastal zones. They areusually densely clustered in a polygonal pattern and

morphologically reminiscent of polygonal karst consistingof sinkholes at many karst plateaus (Fig. 7).Vajoczki andFord (2000) studied the appearance of littoral karren in thecoastal part of the Georgian Bay (Lake Huron, Canada) tothe depth of 25 m and they found an exceptionally goodcorrelation between the depth of littoral karren and waterdepth (r2 = 0.93). Interestingly enough, the researchers ofthe previously mentioned underwater sinkholes near theGalapagos Islands (Michauda et al. 2005) also managed toestablish some correlation between ocean depth and thedepth of karst depressions. Based on this, it can beconcluded that the processes governing the formation of

littoral karren and underwater sinkholes are very similar,although quantitatively different. Littoral karren aredistributed on compact rocks more or less densely and

mainly in polygonal pattern, whereas rocks that are cut bya fissure, karren are distributed along that predisposeddirection (Simms 2002). Such is the case with thedistribution of sinkholes and uvalas in karst terrains.

Discussion about the development of sinkholes

Based on these facts, it can be concluded that theformation of circular karst depressions most probablyoccurred underwater below the lysocline, where thedissolution of carbonate sediments is increased. Thishypothesis of underwater dissolution of carbonatesediments may have been first put forward and favouredamong several others (pockmark origin, sediment creeping,paleotopography, effects of subbottom currents, and bothmarine and subaerial karstic origins) by the researcherswho studied karst depressions on the ocean floor near theGalapagos Islands (Michauda et al. 2005). However, theirexplanations focused only on the genesis of the submarinesinkholes that are now beneath the ocean. According to thenew concept, the development of the karst depressionswhich are now on the surface occurred in the same way,during the period when the surface was still under the sea.

Most probably the processes of deposing and dissolvingthe rocks altered successively. In this way it is alsopossible to explain the development of fossil sinkholes andother paleokarst forms. The lysocline was lowering due tothe regression of the sea; hence the deepest karstdepressions are reasonably found under the present sealevel, i.e. in areas that have been most exposed tounderwater dissolution.

According to the new concept, the impact ofatmospheric water on the development of karst landformsis limited only to the development of hydraulicallycontrolled and fracturedcontrolled karren (not includingthe littoral karren). Actually, atmospheric water has adominant influence on the expansion of existing fracturesonly in the surface zone, since atmospheric water has thehighest potential for dissolution at the beginning ofinfiltration. The result of this, is the existence of a lowerborder of epikarst to which the effect of atmospheric wateris evident. This border is usually located to the depth of afew meters. It is often clearly distinguishable from the

deeper parts of the carbonate rocks which have a muchlower degree of karstification. The appearance of epikarst(subcutaneous zone) was well observed by Williams(1983), but according to the new concept, epikarst shouldnot be linked to the genesis of sinkholes since sinkholescan also occur in places where epikarst is not developed.Epikarst represents a zone consisting only of karren(mostly fracturedcontrolled karren) which can often becovered with soil.

Collapse sinkholes are not common, and though thefinal rock collapse may be almost instantaneous, naturalcollapse events are extremely rare (Waltham et al. 2005).Karst collapses in solid rock masses are the result of a

collapse of previously formed cave passages with ceilingextending almost to the surface. Suffosion and dropout


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sinkholes are formed through the processes occurring inthe overlay sediments deposited over carbonate rocks withwelldeveloped karstic network. Hence, the genesis ofthese landforms should be linked to the secondaryprocesses taking place after a more dominant phase of

karstification.

The genesis of uvalas, poljes and dryvalleys

As mentioned earlier in this paper, uvalas and poljes aretypically closed karst depressions larger than dissolutionsinkholes. By studying poljes in the western Bosnia andHerzegovina, Cviji (1900, 1918) realized that all majorpoljes are linked to the tectonic rifts, faults or the synclineaxis mostly of the Dinaric direction (NWSE). He alsorealized that tectonic depressions are just a starting point

for erosion action because all tectonic forms areconsiderably narrower than the poljes which are formed bythe marging of uvalas. Therefore, the new conceptacknowledges that uvalas and poljes are commonly (butnot always) linked to faults or structural tectonic forms.However, according to the new concept, the corrosionwhich expanded and shaped tectonic forms is being linkedto a far more intensive underwater dissolving action asdescribed earlier in this paper. Clay sediments with marland coal interbeds often cover the bottom of poljes in theDinarides, which is another indicator that these karstdepressions used to be underwater.

Dry valleys are nothing else than a series of linearly

arranged and partly shaped sinkholes and elongated uvalaswhich morphologically resemble normal river valleys. Themajor difference is that dry valleys do not have acontinuous slope; instead their bottom is intersected bycrests of uvalas and sinkholes. Such series of elongateduvalas and sinkholes are classified as a blind valley if itdoes not descend to the erosion basis (most common case)but ends with divideof the lowest uvala in the series whoseedges are covered with swallow holes. The areas aroundthe swallow holes distributed across the downstream edgeof blind valleys, uvalas and poljes are often accompaniedby potholes (Cviji 1960). This indicates the presence of

strong and turbulent water movements conditioned by alarge hydrostatic pressure of water that karst depressions

were filled with. These flows could also have a certaininfluence on shaping of karst valleys.

The genesis of subsurface karst forms

Karst conduits were registered while performingexploration drilling on the Adriatic coast at depths of morethan 2000 m below sea level (Milovanovi 1965), asdescribed above. Therefore, the genesis of thesecorrosionkarst conduits also cannot be explained by dissolventaction of atmospheric water or water containing H2S. Inthe conditions of underwater dissolution below thelysocline, the expansion of the fractures earlier formed bytectonics is also possible; hence the primary network ofkarst corrosion conduits was most likely developedunderwater. In the following phase, while the water wasregressing, water retention in karst depressions occurred,and once the necessary hydraulic gradient was reached,water began to run off through earlier formed corrosionconduits. Due to the turbulent movements of groundwater,some corrosion conduits were extended and fluvial cavesappeared along their direction, usually much spacer thanthe conduits of the primary network.

Water reduction from karst depressions first causes theoccurrence of swallow holes in the upper part of edge ofuvala or polje, extends the upper channels of thesubsurface karst network and runs off downstream at thelevel of lower water that has withdrawn faster (Fig. 9d).Further withdrawal of flood water leads to the formation oflower swallow holes, extension of the lower horizon of

conduits, emergence of springs and possibly spring cavesat lower levels as compared to the previous dischargepoint. The lowest swallow holes along the downstreamedges of uvalas or poljes are often functional to this day(Fig. 9e). The lowest discharge zone probably occurredonce the hydraulic gradient was formed and it was furtherextended once the water had completely withdrawn. Thelowest fluvial caves that are most recent are typically thelargest in size, and they can descend even lower than thelevel of the discharge point while forming phreatic caveswith loops and vauclusian (ascending) type of springs. Inthe case of conduits of descending type, cave exits are

typically characterized by potholes, the collapse of ceilingsand the occurrence of similar forms indicating that water

Fig. 7a Polygonal karst withdense distribution of sinkholeson the mountain Golija(Montenegro) photographedfrom an aircraft; b Littoralkarren (micro pits) on the shoreof Skadar Lake (Montenegro)


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circulated through these channels under extremely highpressures.

In areas with a series of uvalas and poljes distributed instep formation, swallow holes typically occur on thedownstream edges of almost all major karst depressions.The appearance of swallow holes is not necessarily linked

to their successive (upstream or downstream) shift alongdry valleys as it has been explained by many authors,starting with Cviji (1960). Their formation is morerealistically linked to the previously described processwhich first occurred at the edges of uvalas at higheraltitudes, then at the edges of lower uvalas or poljes,ending with the lowest depression located just above theregional erosion basis.

Vauclusian (ascending) type of springs and phreaticcaves with loops are formed in areas with a high hydraulicgradient which enables water from upstream karstdepressions to flow through corrosion conduits to below

the sea level, i.e. below the lower water level. At certaindepths, when the hydrostatic pressure of lower water washigh enough to prevent further penetration of phreaticcaves, groundwater is forced to break out onto the surfaceand create lifting forms of exit passages.

A connected system of reservoirs "Viegrad" and"Bajina Bata" on the Drina River could be considered asan experimental evidence of a rapid expansion of karstconduits due to changes in hydrostatic pressure (Fig. 8).These reservoirs are arranged in step formation. The"Viegrad" reservoir is located upstream of the "BajinaBata" reservoir. The "Viegrad" dam is 79.5 m high andthe difference between the levels (H) of these two

reservoirs is theoretically within the range from 28.8 to 69m (Vukovi et al. 2004). The gorge of the river Drina inthe area of the "Viegrad" dam cuts into limestones anddolomites considered to be reasonably impermeable priorto the dam construction. Since the Drina riverbedrepresents the regional erosion basis below which thedegree of karstification should be small, greater lossesfrom the reservoir were not expected. However, after theinitial charging of the reservoir three submarine springsemerged downstream of the dam. Infiltration losses wereestimated to be 1.4 m3/s after a year, around 6 m3/s after 7years, and around 15 m3/s after 20 years following the

construction of the reservoir "Viegrad" (Milanovi 2009).Hydrogeological and speleodiving research (Milanovi2009) discovered a swallow hole with 4.5 m diameter atthe bottom of the "Viegrad" reservoir near the dam, whichwas not registered in earlier research performed prior to theconstruction of the dam. The discharge through a numberof submarine springs occurs below the level ofdownstream reservoir, about 300 m away from the dam.Mathematical modelling estimated that karst conduits haveknee shape and they descend below the dam body and thegrout curtain to the depth of around 150 m. Some of theseconduits were also confirmed by drilling. The author of the

Report concludes that the formation of swallow holes andsubsurface conduits occurred due to the expansion of faultsintersecting this area (Milanovi 2009).

This example clearly indicates that the extension of thepreviously formed primary network of karst conduits andformation of phreatic caves are possible even if thehydraulic gradient is much lower than the one which mighthave existed between the upstream karst depressions anddownstream vauclusian springs.

Fig. 8 Schematic overview of the genesis of the extend karstconduit beneath "Viegrad" dam. a Karstified porousenvironment consisting of the primary network of karst conduitsprior to the construction of the "Viegrad" dam on the Drina

River; b swallow hole, fluvial cave and sublacustrine springformed in postconstructional period (19852009) as a result ofincreased hydrostatic pressure in the back of the dam

The presented concept of the speleogenesis could alsobe applied to simplify the explanation of the existence ofoverburden karst, i.e. commonly called paleokarst. Inaddition to karst depressions, overburden karst can alsoconsist of the primary network of corrosion karst conduitswhich is often detected by performing deep drilling.Underwater dissolution of carbonate sediments has beenoften "competing" with the sedimentation of carbonates orother sediments. Hence, some periods were dominated by

dissolution when karst depressions and networks ofcorrosion conduits were formed, followed by periods ofsedimentation when previously formed karst forms becamecovered. In this way it is also much easier to explain theunderwater formation of carbonate bauxites.

Dissolution of carbonate sediments below the lysoclineis the chemical process completely different from theprocess so-called hypogene speleogenesis caused by watercontaining H2S. The link between these two types ofkarstification has not yet been established, but in any caseit is necessary to conduct more extensive research in orderto further clarify this issue.


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Karst development sequences according tothe new concept

The development of the karst as known nowadays wasnot influenced by atmospheric conditions; its developmentmainly occurred beneath the water surface having been

caused by the subsequent actions of strong turbulent flowsresulting from water withdrawal.

Before the start of the formation of karstic networks byunderwater dissolution of carbonate sediments, it wasnecessary that the fractured carbonate rocks occur at greatdepths below sea level, i.e. below the lysocline. Theauthors of the previous hypothesis of the underwaterdissolution of carbonate rocks below the lysocline(Michauda et al. 2005) also indicate that, prior to theintensive karstification process, a heterogeneous porositynetwork in the carbonate sediments already existed.

The following might be an explanation of one of the

possible ways as to how consolidation and appearance offractures within carbonate sediments occur before thebeginning of underwater dissolution. Carbonate sedimentsdeposited above the lysocline were initiallyunconsolidated, with a porosity of about 4080 %.Immediately after deposition, submarine cementationbegan; moreover, the deposition of new layers of carbonatesediments caused the compaction of lower layers, so thatthese sediments became more consolidated. By tectonicmovements, the fractures and faults were formed withinthe lower consolidated layers. Due to the quick andsignificant increase in water level, carbonate sedimentsbecame exposed to a very intensive underwater dissolutionbelow the lysocline. The unconsolidated upper layers werethe first to be rapidly dissolved. From the moment ofexposure of the lower consolidated fractured carbonatesediments to underwater dissolution, the primary networkof karstic channels and elongated karst depressions start tobe created as mentioned in previous sections. Dissolutionwill continue to occur until some other insoluble materialscover carbonate sediments or until the sea level decreasesto the level where carbonate sediments are above thelysocline again. Cementation and compaction of carbonatesediments could partially continue above the lysocline, andalso partially after exposure of carbonates to atmospheric

conditions. This could result in closure of one smallernumber of the karstic conduits.

Underwater dissolution and the genesis of karst, whichis the main topic of this paper, most likely occurred in afew sequences, as shown and described in Fig. 9.

Conclusion

Compared to the existing hypotheses, the presentedconcept provides a more simple explanation of the genesisof karst landforms and karst in general.

Most karst landforms developed underwater due to the

dissolution of carbonate sediments at greater depths.Solvent action of atmospheric water causes only theformation of the majority of smaller karst forms such as

some karren. By determining the deepening rate of karrenand their average depth in compact rocks, the exposuretime of carbonate rocks to atmospheric conditions could beassessed.

Dissolution sinkholes exist only in the areas that havebeen exposed to underwater dissolution. Therefore, their

occurrence is not realistically expected in places which arethe result of the later erosion (such as steep canyon sides),

Fig. 9 Karst development phases according to the new concept atectonically disrupted carbonate rocks are located at greaterdepths beneath the ocean, more precisely at depth below thelysocline; b creation of sinkholes and uvalas by underwaterdissolution. Extension of fractures and inception of the primarykarst network of corrosion conduits; c continuation ofdissolution. Uvalas are being extended and poljes are created.Further extension of fractures occurs; d water withdrawal. Highhydraulic gradient is created by water retention in karstdepressions. Swallow holes are formed as well as fluvial caves in

the present aeration zone, and upper springs occur downstream; ecomplete water withdrawal. The lowest horizon of fluvialphreatic caves has been previously formed. Deposited sedimentsremain in poljes. Water of atmospheric origin is infiltratedthrough already formed conduits and creates karst aquifers asknown today


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which could not have existed beneath the lysocline. Thishypothesis could be checked by mathematical modelling ofthe genesis of karst landforms by applying the new conceptand comparing the modelled results with the actualsituation. Many authors (Eraso 1986; Huntoon 1995;Bakalowicz 2005; Supper et al. 2008) think that the current

mathematical models have given unsatisfactory results.So far this concept has touched only on some of the

most important fields of karstology and opened the spacefor different views on this subject. At this point, thepresented concept is just a hypothesis that needs towithstand the test of time. This subject matter might bebetter elaborated on by including more researchers thatwould utilize numerous facts from their research fields.

Unlike many other scientific disciplines, karstologycannot boast a great success in the twentieth century. It isbelieved that the establishment of new conceptual modelwould speed up the development of this subject area and

improve the principles and methods for solving a numberof practical issues such as: modelling of the developmentof subsurface karst network and groundwater circulation,simplifying detection of aquifers, as well as deposits ofbauxite, oil and gas within carbonate rocks.

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A New View on Karst Genesis by Radulovic M. Milan

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