Author's personal copy Morphogenesis of hypogenic caves

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Morphogenesis of hypogenic caves

Alexander Klimchouk ⁎Ukrainian Institute of Speleology and Karstology, 4 Prospect Vernadskogo, Simferopol 95007, Ukraine

a b s t r a c ta r t i c l e i n f o

Article history:Received 1 November 2007Received in revised form 28 June 2008Accepted 18 September 2008Available online 4 October 2008

Keywords:SpeleogenesisCave morphologyHypogenic cavesHypogenic speleogenesisHypogenic karst

Hypogenic speleogenesis is the formation of solution-enlarged permeability structures by waters ascendingto a cave-forming zone from below in leaky confined conditions, where deeper groundwaters in regional orintermediate flow systems interact with shallower and more local groundwater flow systems. This is incontrast to more familiar epigenic speleogenesis which is dominated by shallow groundwater systemsreceiving recharge from the overlying or immediately adjacent surface.Hypogenic caves are identified in various geological and tectonic settings, formed by different dissolutionalmechanisms operating in various lithologies. Despite these variations, resultant caves demonstrate aremarkable similarity in patterns and meso-morphology, which strongly suggests that the hydrogeologicsettings were broadly identical in their formation. Hypogenic caves commonly demonstrate a characteristicsuite of cave morphologies resulting from rising flow across the cave-forming zone with distinct buoyancy-dissolution components. In addition to hydrogeological criteria (hydrostratigraphic position, recharge–discharge configuration and flow pattern viewed from the perspective of the evolution of a regionalgroundwater flow system), morphogenetic analysis is the primary tool in identifying hypogenic caves.Cave patterns resulting from ascending transverse speleogenesis are strongly guided by the permeabilitystructure in a cave formation. They are also influenced by the discordance of permeability structure in theadjacent beds and by the overall hydrostratigraphic arrangement. Three-dimensional mazes with multiplestoreys, or complex 3-D cave systems are most common, although single isolated chambers, passages orcrude clusters of a few intersecting passages may occur where fracturing is scarce and laterallydiscontinuous. Large rising shafts and collapse sinkholes over large voids, associated with deep hydrothermalsystems, are also known.Hypogenic caves include many of the largest, by integrated length and by volume, documented caves in theworld. More importantly, hypogenic speleogenesis is much more widespread than it was previouslypresumed. Growing recognition of hypogenic speleogenesis and improved understanding of its peculiarcharacteristics has an immense importance to both karst science and applied fields as it promises to answermany questions about karst porosity (especially as deep-seated settings are concerned) which remainedpoorly addressed within the traditional epigenetic karst paradigm.

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1. Introduction: the hydrogeologic background ofhypogenic speleogenesis

Three basic genetic settings are broadly recognized now fordissolution caves (Klimchouk et al., 2000; Ford, 2006; Ford andWilliams, 2007): 1) syngenetic/eogenetic (coastal and oceanic), inyoung rocks of high matrix porosity and permeability; 2) hypogenic,predominantly confined, where water enters the soluble formationfrom below, and 3) hypergenic (epigenic), unconfined, where water isrecharged from the overlying surface.

Whereas epigenic karst has historically received the most atten-tion and is well studied in both regional and theoretical aspects,

hypogenic karst is still at the beginning of proper recognition andunderstanding.

Following the recent suggestion of Ford (2006), hypogenicspeleogenesis is defined here as the formation of solution-enlargedpermeability structures by water that recharges the soluble formationfrom below, independent of recharge from the overlying or immedi-ately adjacent surface. It occurs mainly in leaky confined conditions.Where hypogenic caves are shifted to the shallower, unconfinedsituation due to uplift and denudation but continue receivingupwelling recharge from deep systems, this is still hypogenicdevelopment although unconfined. Unconfined hypogene develop-ment can be considered as an extinction stage of hypogenespeleogenesis. The term “hypogenic” here does not necessarily meanthe occurrence at great depth but, rather, refers to the origin of thecave-forming fluids from depth.

The upward groundwater movement can be driven by hydraulicgradients, or other sources of energy. Upward, cross-formational

Geomorphology 106 (2009) 100–117

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hydraulic communication that dominates in hypogenic speleogenesisallows deeper groundwaters in regional or intermediate flow systemsto interact with shallower and more local groundwater flow systems.This is in contrast to epigenic speleogenesis which is dominated byshallow groundwater systems receiving recharge from the overlyingor immediately adjacent surface.

Hypogenic and epigenic karst systems are regularly associatedwith different types, patterns and segments of flow systems,characterized by distinct hydrokinetic, chemical and thermal condi-tions. Epigenic karst systems are predominantly local systems, and/orparts of recharge segments of intermediate and regional systems.Hypogenic karst is associated with discharge regimes of regional orintermediate flow systems which establish in areas of potentiometriclows and breaches of major confinement.

Confined settings are the principal hydrogeologic environment forhypogenic speleogenesis. Transverse hydraulic communication acrosslithological and porosity system boundaries commonly coincide withmajor contrasts inwater chemistry, gas composition and temperature.It brings different geochemical milieus and flow systems in interactioncausing various disequilibrium and reaction dissolution mechanismsto operate. Hypogenic speleogenesis occurs in both carbonates andevaporites, but also in some clastic rocks with soluble ingredients suchas conglomerates.

The definition of hypogenic speleogenesis given above is largelyhydrogeological: it refers to the source of fluid recharge to the cave-forming zone, and type of flow system. This contrasts to previousviews of hypogenic speleogenesis as a specific geochemical phenom-enon. Hypogenic caves were viewed previously as those formed byhydrothermal waters or by waters containing hydrogen sulfide (e.g.Ford and Williams, 1989; Worthington and Ford, 1995; Hill, 2000).Palmer (1991) defined hypogenic caves as those formed by acids ofdeep-seated origin, or epigenic acids rejuvenated by deep seatedprocesses. Later on, he presented the definition in a broader form:hypogenic caves are formed by water in which the aggressiveness hasbeen produced at depth beneath the surface, independent of surfaceor soil CO2 or other near surface acid sources (Palmer, 2000a). Thesedefinitions stress acidity (thus disregard non-acidic dissolution) andrely on the source of aggressiveness. However, the latter is a transientattribute of water, which can be produced at depth, or acquiredwithina given soluble formation (due to mixing or redox reactions, forinstance), or be kept from distant recharge areas such as in case ofartesian waters in a clastic aquifer entering evaporites from below. Inhypogenic settings a number of dissolution mechanisms can operate(Palmer, 1995; Klimchouk, 2007), either in combination or sequen-tially. The primary role of the hydrogeologic backgrounds inhypogenic speleogenesis is strongly corroborated by the remarkablesimilarity in morphological features of hypogenic caves formed bydifferent dissolutional processes in different lithologies, by the overallregularities in the hydrostratigraphic occurrence of such caves and bybasinal evolution analysis.

Within a multi-aquifer setting, the common hydrostratigraphicarrangement in stratified sedimentary successions, soluble units arecommonly vertically conterminous with non-soluble formations ofinitially higher permeability. Due to their low matrix permeability,soluble units serve as separating beds (aquitards) prior speleogenesisand during its early stage. As a rule, hypogenic caves evolve tofacilitate cross-formational hydraulic communication between later-ally transmissible more pervious beds, across soluble aquitards. Theoverall result is the conversion of the soluble unit into an aquifer thatcan be more prominent than the adjoining non-soluble aquifer units.

Flow from the underlying aquifer (“feeding formation”) to theoverlying aquifer (“receiving formation”) through the dissolvingaquitard (“cave formation”) is determined by the conductivity of theleast permeable member. This suppresses the positive flow-dissolu-tion feedback and speleogenetic competition between alternativeflowpaths, the main mechanism of unconfined (epigenic) speleogen-

esis, and promotes the enlargement of fracture networks into apervasive conduit system, creating maze-like caves. As forced-flowregimes in confined settings are commonly sluggish and water withlesser density enters the cave-forming zone from below, buoyancy-driven flow pattern powered by either solute or thermal densitydifferences is widely operative in hypogenic speleogenesis producingupward-pointed dissolution.

Initial porosity structures can vary not only between majorlithological varieties, but between individual beds within a solubleformation. As the vertical hydraulic connectivity between the strati-form porosity structures is often initially imperfect, ascending flowcan acquire lateral component within particular beds that encloselaterally connected porosity systems. This is why resultant hypogeniccaves are commonly multi-storey.

The inherent trend of basinal evolution during uplift is that deeper,confined, sections are being brought to the epigenic realm due todenudational lowering and erosional entrenchment. Hypogenic cavespass through transitional conditions of initial breaching and draining,and get fossilized in the vadose zone. They can be partly inherited andconsiderably reworked by the newly established unconfined flowpatterns. Where observed caves are in the transitional stages andoverprint is obvious, it is most tempting to relate their origin to thecontemporary epigenic conditions. This, along with the historicdominance of the epigenic paradigm in karst science, caused thathypogenic origin had been overlooked for many caves.

One of the main characteristics of hypogenic speleogenesis is thelack of genetic relationship with groundwater recharge from theoverlying or immediately adjacent surface. It may not bemanifested atthe surface at all, receiving some expression only during later stages ofuplift and denudation. In many instances, hypogenic speleogenesis islargely climate-independent.

In identifying hypogenic caves, the primary criteria are morpho-logical (patterns and meso-morphology) and hydrogeological (hydro-stratigraphic position and recharge/flow pattern viewed from theperspective of the evolution of a regional groundwater flow system).This paper discusses themorphology of hypogenic caves and describesa characteristic suite of cave morphs resulting from upwelling flowacross the cave-forming zone, which can be used as diagnostic featurefor hypogenic speleogenesis.

The generalization that follows is based on detailed field studiesand cursory observations by the author in many caves from differentregions, settings and lithologies. These caves include: maze caves inthe Neogene gypsum in Western Ukraine (Optimistychna, Ozerna,Zoloushka, Kristal'na, Mlynki, Slavka, Atlantida, Dzurinska, etc.); WindCave and Jewel Cave in Lower Carboniferous Madison limestones(South Dakota, USA); Carlsbad Cavern, Lechuguilla Cave, Dry Cave,Endless Cave, Spider Cave, and Yellow Jacket Cave in Permian reef andbackreef limestones in the Guadalupe Mountains (NewMexico, USA);Deep Cave, Caverns of Sonora, Amazing Maze Cave and Robber BaronCave in Cretaceous limestones (Texas, USA); Mystery Cave inOrdovician limestones (Minnesota, USA); Blowing Hole Cave inEocene limestone (Florida, USA); maze caves in dolomites of theNeoproterozoic Vasante Group (Minas Gerais, Brazil); Toca da BoaVista and Toca da Barriguda maze caves in carbonates of thePrecambrian Una Group (Campo Formoso, Brasil); Pál-völgyi Caveand Ferenc-hegy Cave in Eocene limestones in the Buda Hills(Hungary); Fuchslabyrinth (Germany) and Moestrof (Luxemburg)caves in Triassic Muschelkalk limestones; Knock Fell Caverns inPermian limestones (UK); Estremera Cave in Neogene gypsum in theMadrid Basin (Spain); Coffee Cave in Permian gypsum in the RoswellBasin, (New Mexico, USA); Kungurskaya Cave in Permian gypsum inthe fore-Ural region (Russia); numerous isolated single-passage orsmall maze-cluster caves in Neogene limestones in the Pricherno-morsky Basin (south Ukraine). Other important examples derivedfrom published papers include: established or suspected hypogeniccaves in Jurassic and Paleogene limestones in Italy (Galdenzi and

101A. Klimchouk / Geomorphology 106 (2009) 100–117


Menichetti, 1995); Eocene limestones in Hungary (Takács-Bolner andKraus,1989; Dublyansky,1995, 2000); Lower Ordovician limestones inthe Angaro-Lensky artesian basin, Siberia, Russia (Botovskaya Cave;Filippov, 2000); Paleogene gypsum in the Paris Basin, France (DenisParisis Cave; Beluche et al., 1996); Jurassic limestones in Alps andPrealps, France (Audra et al., 2002, 2007); Permian gypsum in theSouth Harz, Germany (Kempe, 1996); Cretaceous limestones anddolomites in Israel (Frumkin and Fischhendler, 2005; Frumkin andGvirtzman, 2006); late Archaean carbonates in the Transvaal Basin inSouth Africa (Sterkfontein Cave; Martini et al., 2003), Ordovicianlimestone in Missouri, USA (Brod, 1964); Paleozoic limestones ineastern Australia (Osborne, 2001a,b).

2. Cave patterns

Hypogenic caves display variable, often complex patterns stronglyguided by the original (pre-speleogenetic) permeability structure in acave formation. They are also influenced by the discordance of

permeability structure in the adjacent beds and by the overallhydrostratigraphic arrangement (recharge–discharge configurations).

Branchwork caves, with conduits converging as tributaries in thedownstream direction, the most common pattern for epigenicspeleogenesis, never form in hypogenic settings. This reflects thefundamental difference between the mechanisms of epigenic speleo-genesis, largely competitive, and of hypogenic speleogenesis, inwhichthe competition between alternative flowpaths is subdued. This is alsodue to the fact that epigenic speleogenesis is associated with lateralflow, whereas hypogenic speleogenesis is driven by predominantlyupwelling cross-formational flow (“transverse” speleogenesis, Klim-chouk, 2003a).

Elementary patterns typical for hypogenic caves are networkmazes, spongework mazes, irregular chambers, isolated passages orcrude clusters of passages, and rising shafts. They often combine to formcomposite patterns and complex 3-D structures. A variant of suchcomplex patterns is distinguished as a ramiform (ramifying) pattern,which Palmer (1991, 2000a) described as “caves composed by

Fig. 1. Examples of network maze patterns: A through C = Gypsum caves in Neogene gypsum, western Ukraine: A = Ozerna Cave (117 km; map courtesy of the Ternopol Speleo Club);B = Zoloushka Cave (92 km; map courtesy of the Chernivtsy Speleo Club); C = Mlynki Cave (28 km; map courtesy of the Chortkiv Speleo Club). Limestone caves: D = FuchslabyrinthCave in Triassic Muschelkalk limestone, Germany (6.4 km; fromMüller et al., 1994); E = Botovskaya Cave in Ordovician limestone, Siberia, Russia (60.8 km;map courtesy of the IrkutskSpeleo Club); G = Amazing Maze Cave in Cretaceous limestone, Texas, USA (9 km; map courtesy of the Texas Speleological Survey, Elliott and Veni, 1994).

102 A. Klimchouk / Geomorphology 106 (2009) 100–117


Fig. 2. Examples of systematic (A = fragment of Lechuguilla Cave survey) and polygonal (B = Yellow Jacket Cave in the Yates Formation) network patterns in the same region,Guadalupe Mountains, USA. Yellow Jacket: simplified map from the original survey drawn by D. Belski, courtesy of the Pecos Valley Grotto. Lechuguilla: a fragment from the originalsurvey drawn by P. Bosted, courtesy of the US National Park Service.

Fig. 3. Distribution of point feeders (red dots; sub-vertical conduits connecting trunk passages) through the network of master passages in maze caves: A = Ozerna Cave, westernUkraine (from Klimchouk, 1990); B = Coffee Cave, NewMexico, USA (mapped and sketched by K.Stafford). Lower level passages locally form maze clusters that connect to the masterlevel through sub-vertical point feeders. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)



irregular rooms and galleries in a three-dimensional array withbranches, that extend outward from the central portions.”

Network maze patterns are most common for hypogenic caves.Passages are strongly controlled by vertical fractures and formmore orless uniform networks, which may display either systematic orpolygonal patterns, depending on the nature of the fracture networks.Systematic, often rectilinear, patterns are most common, reflectingtectonic influence on the formation of fracture networks (Figs. 1, 2A,and 3). Polygonal patterns are guided by discontinuities of syndeposi-tional or diagenetic origin. Examples include predominantly poly-gonal networks in the upper storey of some western Ukrainiangypsum mazes, guided by early diagenetic structures and a polygonalnetwork of Yellow Jacket Cave in the McKittrick Hills in the GuadalupeMountains, New Mexico, USA, guided by tepee-type syndepositionaldiscontinuities (Fig. 2B). Fracture and cave networks displayingdifferent patterns may be present within a single area or at variousstories/parts of a single cave, especially when confined to differentrock units.

When aggressive recharge from below is uniformly distributed,passages that hold similar positions in the system in relation to theflowpaths' arrangement (guided by the same set of fractures andoccurring within a single cave series or at the same storey) arecommonly uniform in size and morphology. Larger volumes may bedissolved where aggressive recharge from below is concentrated byvirtue of hydraulic properties and the porosity structure of the feedingformation. A common feature of network mazes is high passagedensity (Klimchouk, 2003b).

Spongework maze patterns are less typical than networks. Highlyirregular passages develop through enlargement and coalescing ofvuggy-type initial porosity in those horizons of the cave formationthat have no major fractures but interconnected pores and vugs.Clusters or levels of spongework-type cavities are commonly combinedwith other patterns in adjacent horizons to form complex cavestructures. An enlarged version of spongework, locally called boneyard,is represented in parts of some caves of the Guadalupe Mountains. Inmany hypogenic caves, it seems that diffusely distributed risingbuoyancy currents play a significant role in spongework development.

Irregular chambers can be isolated cavities, or parts of compositepatterns. In hypogenic settings they form in two situations: 1) bybuoyant dissolution at the bottom of the cave formation, commonlyevaporites, where a major aquifer immediately underlies it; 2) wherethe recharge from below is localized and flow in the cave formation isguided by prominent fractures. In the latter case, the chamberdevelopment is commonly induced by intersection of the cross-cut

vertical flow path with a lateral flow-conducting horizon (a stratiformpermeability system) that bursts dissolution through mixing mechan-isms. Irregular chambers in hypogenic karst can attain very largedimensions, such as directly documented cavities in evaporates ofsouthern Harz, Germany (cavities of the “schlotten” type; Fig. 4;Kempe, 1996), the Big Room in Carlsbad Cavern, or the indirectlydocumented (via drilling) hydrothermal cavity in the Archean andProterozoic marbles in southern Bulgaria with a maximum verticaldimension of 1340 m and an estimated volume of 237.6 million m3

(Sebev, 1970; Dublyansky, 2000), probably the largest known,although not accessible, cave chamber on Earth. It is likely thathypogenic mega-sinkholes associated with hydrothermal systems,such as Sistema El Zacatón inMexico (Gary and Sharp, 2006) or obruks(local name in Turkey for cenote-like sinkholes) in Konya Basin,Turkey (Bayari, 2007, personal communication) are collapse featuresover giant chambers. Lesser but instructive examples of hypogenicisolated chambers are described by Frumkin and Fischhendler (2005)from the central mountain range of Israel.

Isolated passages or crude clusters of passages also form in twosituations: 1) in a manner similar to chambers, by buoyant dissolutionat the bottom of the cave formation, where there is some initial linearguidance (by fractures or other kinds of weaknesses) but little or noforced flow across the formation; 2) by forced or mixed flow across asoluble bed, where fracturing is scarce. In the former case some bigirregular passage-like cavities may form, often associated withchambers, exemplified again by some “schlottens” in the South Harz(Kempe, 1996). In the latter case isolated slot-like passages or crudeclusters of passages form, such as those intercepted by mines in theNeogene limestones in the southern Ukraine (Klimchouk, 2000a,2007).

Rising shafts are outlets of deep hypogenic systems and commonlyhydrothermal. A type example is the 392-m deep Pozzo del Merronear Rome, Italy, presumably formed by rising thermal water chargedwith CO2 and H2S (Gary et al., 2003). It shows the morphology of arising shaft, in contrast with roughly cylindrical morphology, withwalls diverging from each other toward the bottom, of shafts of the ElZacatón and obruk type, where hydrothermal cavities at depth aresupposed to open to the surface through collapse.

Consideration of hypogenic cave patterns only in plan view can bemisleading, giving a false impression of seemingly two-dimensionalstructures in the case of laterally extensive network or spongeworkmazes. Further confusion arises from the fact that in many relicthypogenic mazes sediment fill obscures the “root” morphology at thepassage floors, and that minor bottom features are rarely documented

Fig. 4. Occurrence and development of hypogenic caves at the base of a sulfate formation due to buoyancy-driven dissolution, example from South Harz, Germany (from Kempe,1996). Aggressive water in the basal limestone aquifer rises up and attacks the gypsum from below because of buoyancy. Dissolved load returns into the aquifer with descendinglimbs of natural convection cells and is removed with the regional flow.

Fig. 5. A = Plan of Wind Cave, Black Hills, South Dakota, USA (courtesy of Wind Cave National Park); B = A diagram illustrating the concept of the lifting origin of the Black Hills mazecaves and the stair-case effect of recharge–discharge offset (redrawn from Ford, 1989); C = Regionalization of Optymistychna Cave, western Ukraine, a 214+ km long maze, accordingto its multi-storey structure. Recharge–discharge arrangement during cave development is shown by arrows in plan and profile (D) views; D = Diagrammatic representation of thehydrostratigraphy and the stair-case effect of the recharge–discharge offset. The thickness of the gypsum bed (18 m) is exaggerated (From Klimchouk et al., 1995).



while surveying large maze caves even when they are recognizable.First recognized in the Western Ukrainian gypsum mazes andsubsequently found in many maze caves around the world, both inlimestone and gypsum, are numerous feeding conduits at lower levels,scattered throughout maze patterns at the master storey (Fig. 3). Withthese feeders and their lower conduits, even largely horizontallaterally extensive maze patterns become complex three-dimensionalstructures.

Complex 3-D cave structures may develop within a rather thinformation (e.g. two- to four-storey mazes in the western Ukraineconfined within the 16–20 m thick gypsum formation) or extendthrough a vertical range of several hundred meters (e.g. Monte Cuccosystem, Central Italy: 930 m; Lechuguilla Cave in the GuadalupeMountains, New Mexico, USA: 490 m). They often display a staircasearrangement of storeys within a system, with cave areas at differentstoreys shifted relative to each other (Fig. 5), or have feeders at thelower level randomly or systematically distributed throughout thesingle master passage network (Fig. 3). The lateral shift of storeyscontrolled by stratiform fracture networks is because areas ofpreferential recharge from below often do not coincide with areas ofdischarge from a confined system so that general pattern of upwellingflow becomes staircase-like. Some vertically extensive caves in theGuadalupe Mountains have prominent feeders as large isolatedsteeply ascending passages or clusters of rift-like passages connectingto some master levels, and prominent outlet segments rising from thebulk of passages and rooms (Fig. 6). These structures are composed ofnetwork and spongework mazes at various levels connected throughrising vertical conduits, coalescing with large chambers and passages.Other examples include Monte Cucco system in Italy, complex bush-like upward-branching structures of hydrothermal caves in the BudaHills, Hungary, composed by rising sequences of chambers and largespherical cupolas (Dublyansky, 2000), and network maze clusters atthe base of the Joachim Dolomite in eastern Missouri, USA, withascending staircase limbs of vertical pits and sub-horizontal passages(an outlet component; Brod, 1964).

Multi-storeymazes are “layered” variants of complex 3-D patterns.In a typical system, lower storeys or individual rising conduits arerecharge elements to a cave system. Master storeys develop atintermediate elevations where there are laterally connected fracturesystems. Upper storeys serve as outflow structures (“outflow mazes”of Ford, 1989). Small patches of maze or lateral extensions of highcupola structures may develop at higher or highest elevations withoutbearing outflow functions (“adventitious”mazes of Ford), especially insystems where buoyancy flow plays a role.

Occurrence of storeys in three-dimensional mazes is guided by thedistribution of initial porosity, which is commonly (although notalways) stratiform. Storeys may be horizontal or inclined, stratiform or

discordant to bedding. Storeys in ascending hypogenic systems formsimultaneously within a complex transverse flow path, in contrast toepigenetic caves where storeys reflect progressive lowering of thewater table in response to the evolution of local river valleys, henceupper storeys being older than lower.

The distinctions between hypogenic (confined) and epigenic(unconfined) speleogenesis can be illustrated by the analysis ofmorphometric parameters of typical cave patterns. Klimchouk(2003b) compared two representative samples of typical cave systemsformed in these two settings. The sample that represents unconfinedspeleogenesis consists of solely limestone caves, characteristicallydisplaying branchwork patterns. Gypsum caves of this type tend to belinear rather than dendritic. The sample that represents hypogenicconfined speleogenesis consists of both limestone and gypsum cavesthat have network maze patterns. Passage network density (the ratioof the cave length to the area of the cave field, km/km2) is one order ofmagnitude greater in confined settings than in unconfined (average167.3 km/km2 versus 16.6 km/km2). Similarly, an order of magnitudedifference is observed in cave porosity (a fraction of the volume of acave block, occupied by mapped cavities; 5.0% versus 0.4%). Thisillustrates that storage in maturely karstified confined aquifers isgenerally much greater than in unconfined aquifers. Average arealcoverage (a fraction of the area of the cave field occupied by passagesin a plan view) is about 5 times greater in confined settings than inunconfined (29.7% versus 6.4%). This means that conduit permeabilityin confined aquifers is appreciably easier to target with drilling thanthe widely spaced conduits in unconfined aquifers. However, as a rule,these high characteristics of confined karst porosity and permeabilityare not uniformly distributed laterally in regional scales as confinedspeleogenesis is largely clustered.

The fundamental cause for these differences in conduit porositybetween hypogenic and epigenic settings is demonstrated to be aspecific hydrogeologic mechanism inherent in confined transversespeleogenesis (restricted input/output) which suppresses positiveflow-dissolution feedback and speleogenetic competition in fissurenetworks (Klimchouk, 2000a, 2003a, 2007). This mechanism accountsfor the development of more pervasive channeling and maze patternsin confined settings where appropriate structural prerequisites exist.In contrast, the positive flow-dissolution feedback and competitionbetween alternative flowpaths dominates in unconfined settings toform widely spaced dendritic cave patterns.

In summary, the spatial structure of hypogenic caves is controlledmainly by the distribution of initial permeability structures across thecave formation and adjacent formations, interaction of differentpermeability structures at various levels and the overall recharge–discharge configuration. Geochemical interaction of flow systemsguided by transverse and lateral permeability pathways also may play

Fig. 6. Profile of Lechuguilla Cave, NM, USA, by Lechuguilla Cave Project, courtesy of US National Park Service. The cave is currently surveyed at 193.4 km in length and 490m in depth.This is an example of a complex 3-D structure, in which some prominent inlet (feeding) and outlet components are well recognizable.



a significant role in determining resultant patterns. Buoyancy effectsin free convection and mixed systems can be also important increating complex cave structures.

3. The maze caves controversy

The most common (although not the only) pattern for hypogenictransverse speleogenesis is a network maze. Network mazes, oftenwith several superimposed storeys, constitute entire caves or parts ofcomplex cave structures.

The formation of maze cave patterns has been specificallyaddressed in the karst literature for many years. Researchers thatpreviously attributed the origin of maze caves to artesian conditions(e.g. Howard, 1964; White, 1969; Ford, 1971; Huntoon, 2000) ordisregarded this possibility (Palmer, 1975, 1991, 2000b), implied the:“classical” concept of lateral artesian flow through a soluble unit.Palmer examined the hydraulic–kinetic conditions within a simpleloop in which water diverges into two branches that rejoin down-stream, and showed that these branches cannot develop at compar-able rates except at very high discharge to flow length (Q/L) ratios.Such conditions are not characteristic of lateral artesian flow, so heconcluded that slow groundwater flow near chemical equilibrium,typical of confined aquifers, is least likely to produce maze caves(Palmer, 1975; 1991, 2000a).

White (1969) described the type of a “sandwich aquifer,” where athin carbonate unit is overlain and underlain by insoluble strata. Henoted that network caves are characteristic for this situation andpointed out that such patterns form due to the lack of concentratedrecharge from overlying beds.

Palmer (1975) specifically addressed the problem of maze patternsand suggested two main settings favorable for their development:

1) High-discharge or high-gradient flow during floods in the vicinityof constrictions in the main stream passages (floodwater mazes)and,

2) Diffuse recharge to a carbonate unit through a permeable butinsoluble caprock such as quartz sandstone.

Later he added the cases of sustained high gradients, such asbeneath dams, and of mixing zones where the groundwater aggres-siveness is locally boosted, and generalized that the formation of mazecaves requires high Q/L ratios (Palmer, 2002).

Ford (1989) for the Black Hills caves and Klimchouk (Klimchoukand Rogozhnikov, 1982; Klimchouk, 1990, 1992; 2000b) for thewestern Ukrainian caves suggested the model of maze developmentunder confined conditions by dispersed ascending recharge from theunderlying formation. Klimchouk (2000a, 2003a, 2007) generalizedthat this is the most common mechanism for confined speleogenesis.

An interesting suggestion of yet another mechanism of mazedevelopment is due to “phantomisation” (rock-ghost weathering) byslow flow through fractures and dissolution of cement in thesurrounding matrix at depth and subsequent erosional removal ofthe impure residue in the vadose zone (Vergari and Quinif, 1997;Audra et al., 2007). As little is known about caves assigned to form bythis mechanism, it is not discussed here.

Floodwater high gradient origin is a feasible mechanism forproducing small mazes proximal to obstructions that occur alongwell-defined stream passages conducting highly variable flow(Palmer, 1975, 2001), or larger mazes in the epiphreatic zone ofhigh-gradient alpine cave systems subject to quick and high rises ofwater table (Audra et al., 2007). However, in relatively low-gradientenvironments (such as in cratons), it is less likely to create large mazeclusters linked to rather small streams, such as in Skull Cave, NewYork, USA, often referred to as an example of floodwater development(Palmer, 2001). An alternative possibility is that clusters of hypogenictransverse mazes, inherited from the confined stage, are encounteredby invasion stream passages during the subsequent unconfined stage.

Another frequently cited example of a floodwater maze is 21-km longMystery Cave in Minnesota, USA, which is thought to form by thesubterranean meander cutoff of a small river. The cave does functionin this way at the present geomorphic stage, but recent examination ofthe cave revealed numerous morphologic features that stronglysuggest hypogenic transverse origin of the cave (see next section fordiscussion of hypogenic morphology). Meander cutoff flow hasproduced considerable morphological overprint and fluvial sedimen-tation in certain passages but it did not erase hypogenic speleogenseven in those central flow routes.

The floodwater model is often applied to explain mazes near riversin somewhat static conditions (static “backflood mazes”). Althoughthis might contribute to enlargement of already existing caves, itseems unlikely that mazes can originate in such situations because

Fig. 7. Conceptual models of maze caves development: A, B = by diffuse recharge frombelow (from Palmer, 2000b), C = by upward flow (from Ford and Williams, 1989).



uniform early enlargement of initial porosity cannot be expected withside recharge and sluggish flow conditions, as shown with regard tolateral artesian flow (Palmer, 1975, 1991). Palmer's high Q/L ratiocondition for maze development is not met in this situation.Furthermore, no maze caves referred to as being formed by backfloodwaters from the nearby river is demonstrated to exhibit decrease inpassage size or other regular changes in morphology in the directionaway from the side recharge boundary, as would be expected if thisorigin was the case. Floodwaters from the nearby river can contributeto maze cave development where considerable conduit permeabilityis already available during river entrenchment, but it can be a self-standing speleogenetic mechanism only where there is intense openjointing.

The mechanism of diffuse recharge through a permeable butinsoluble caprock, proposed by Palmer (1975) and widely used toexplain maze patterns, requires additional discussion. It contains animportant idea about the governing role of an adjacent porousformation for the amount of flow to fissures in a soluble unit (alsoexpressed by White, 1969). This is the mechanism of restricted inputthat suppresses the positive flow-dissolution feedback and hencespeleogenetic competition. However, the hydrogeological conceptualmodel that implies maze origin in unconfined settings by downwardrecharge from the overlying permeable caprock (Fig. 7A–B) has someproblems to be widely applicable. The hydrogeologic situationdepicted represents certain evolutionary stages of breaching thecaprock and the cave-hosting unit by denudation/erosion, and impliesthat it used to be a stratified multi-aquifer system, a common case inmany sedimentary basins experiencing uplift and denudation. Themodel ignores the fact that in this hydrostratigraphic setting flow in

the low-permeability bed (initially — limestone) would be predomi-nantly vertical, cross-formational, with descending flow withintopographic/piezometric highs and ascending flow from underlyingaquifers beneath valleys incising into the caprock. Most maze caves,for which this origin was suggested, are concentrated around rivervalleys or other prominent topographic lows (Palmer, 2000b, referredto diminished thickness of the caprock due to erosion), which impliesthat ascending flow across the cave unit had been operative belowvalleys before their entrenchment. Hence, such caves are fullycompatible with the model of ascending (recharge from below)transverse speleogenesis.

From the perspective of basinal flow zones of ascending cross-formational flow in multi-aquifer systems (beneath incising valleys)are more important in supporting speleogenesis than zones ofdescending flow (beneath watersheds) due to more vigorous circula-tion and a number of dissolution mechanisms that can be involved. Itis indicative that a hydrostratigraphic setting, largely similar to thatdepicted by Palmer (1975, 2000b), was used to suggest the uprisingdevelopment formaze caves (Ford andWilliams,1989; Fig. 7C). This is,in fact, one of the basic settings discussed in this paper in the contextof confined transverse speleogenesis. Ford and Williams (2007) alsopointed out that the Palmer's model can explain only single-storeymazes directly beneath the sandstone cover. It cannot explaindevelopment of multi-storey mazes and mazes lying without directcontact with the bottom of the caprock, both being the most commoncases of maze caves structure and occurrence.

Morphologically, there is no unambiguous evidence reported formaze caves that would suggest a descending flow pattern during theirformation. Instead, at least in some of the caves referred to in various

Fig. 8. The morphologic suite of rising flow, diagnostic of confined transverse origin of caves. The geometry of a cave segment, the relative scale of features and hydrostratigraphy onthis diagram is directly representative for Ozerna Cave inwestern Ukraine. However, the diagram is generic and elastic; it can be stretched vertically, and a complexity can be added toaccount for multiple storeys. The arrangement of the forms will repeat itself on each storey, and functional relationships between the forms will remain the same. Hypogene cavesmay consist of a few elementary segments or combine hundreds and thousands of them within a single system.

Fig. 9. Feeders: A through J = point feeders; K through N = fissure- and rift-like feeders in passage floors. A = Ozerna Cave, western Ukraine (Miocene gypsum); B = Jubilejna Cave, westernUkraine (Miocene gypsum); C = Fuchslabyrinth Cave, Germany (Triassic Muschelkalk limestone); = Carlsbad Cavern, Guadalupe Mountains, NM, USA (Permian limestone); E = ZoloushkaCave, western Ukraine (Miocene gypsum); F and G = Toca da Boa Vista Cave, Brazil (Precambrian limestone and dolomite); H and I = Optymistychna Cave, western Ukraine (MioceneGypsum); J = Endless Cave, NM, USA(Permian limestone); K = Aneva Cave, Israel (Cretaceous limestone); L = Mlynki Cave, western Ukraine (Miocene gypsum); M = Knock Fell Caverns,Northern Pennines, UK (Carboniferous limestones); N = Ozerna Cave, western Ukraine (Miocene gypsum). Photo K by A. Frumkin, other photos by A. Klimchouk.



works to be formed according to this model, unambiguous evidencefor the rising flow pattern has been recently recognized (themorphologic suite of rising flow; see Section 4.1). Eventually, thosemaze caves where descending origin could be potentially supposed(where there is a permeable caprock currently exposed to the surface),are in all major respects similar to the caves where this origin can bedefinitely ruled out, e.g. beneath low-permeable cover, and wheretheir confined transverse origin has been unequivocally established bybulk evidence.

Maze cave origin is frequently attributed to hydrothermalspeleogenesis, the tendency reinforced by the paper by Bakalowiczet al. (1987) which suggested a hydrothermal origin for the Black Hillsmazes. Other known examples of network mazes for which hydro-thermal dissolutional mechanism is well established are caves in theBuda Hills in Hungary. However, an emphasis on hydrothermaldissolution should not obscure the fact that these caves are attributedto a confined flow system and rising cross-formational flow, and thatmaze caves are known to form by a number of dissolutionalmechanisms.

Frequent association of maze caves and hydrothermal systems canbe easily explained by considering that deep basinal flow is commonlyheated. Where structural and hydrodynamic conditions allow upwardcross-formational flow, this familiarly creates high-gradient thermalanomalies that favor hydrothermal dissolution. However, the origin ofmaze patterns should be attributed not to hydrothermal dissolution(nor to sulfuric acid dissolution, as some other works suggests) but tohydraulic conditions that favor disruption of discharge-dissolutionfeedback mechanism. A number of dissolutional mechanisms canoperate in hypogenic transverse speleogenesis creating largely similarmorphologies.

The broad evolutionary approach to speleogenesis implies thatcaves may inherit prior development through changing settings(Klimchouk and Ford, 2000). Hence, the problem of cave originrequires specifying the mechanisms that were operative, and thefeatures produced, during each of themain stages. The skeletal outlineof a cave pattern is perhaps the most definite feature that can beattributed to certain recharge modes and flow systems (Palmer, 1991).As confined settings commonly pass into unconfined ones, phreaticthrough vadose, each subsequent setting may contribute substantiallyto cave development, sometimes adding a significant volume to acave. In this sense, bothmechanisms questioned abovewith respect tothe origin of maze caves, floodwater (backflooding) dissolution anddissolution by recharge from overlying permeable caprock, maycertainly contribute to cave development, being operative during therespective transitional stages. However, the rapidly growing numberof evidences from regions around the world (see an overview inKlimchouk, 2007) leads the author to believe that most known mazecaves were formed in confined conditions, as the product of ascendinghypogenic transverse speleogenesis.

4. Cave meso-morphology

As with macro-morphological features (cave patterns), the meso-morphology of caves is another most important characteristic of cavesindicating their origin. This is simply because a cave is primarily aform, produced by interaction between groundwater and its environ-ment. The analysis of spatial and temporal relationships of differentcave morphologies in the context of the regional geomorphic andhydrogeologic evolution is the most powerful tool for inferring caveorigin.

As discussed in Section 2, hypogenic caves may have variablepatterns, controlled mainly by local geological and structural condi-tions (which also determine the mode of recharge from below, e.g.dispersed or localized). Despite this variability, and also regardless ofparticular dissolution mechanisms involved, meso-morphologicalfeatures of hypogenic caves exhibit remarkable similarity betweencaves and commonly comprise a specific suite of forms.

4.1. Morphologic suite of rising flow

Some medium-scale morphological features of ascending hypo-genic transverse caves have specific hydrologic functions and usuallyoccur in a characteristic suite of forms; therefore they are particularlyindicative of themode of cave origin. Their occurrence in a suitemakesthe interpretation of their hydrologic function and origin especiallyunequivocal. Such regular combination of forms, called here themorphologic suite of rising flow (MSRF), was first recognized inwestern Ukrainian gypsum mazes firmly established as ascendinghypogenic caves, and subsequently found in many maze and complex3-D caves around the world both in carbonates and gypsum. Some ofthe caves, where MSRF has been recognized, were previouslyattributed to hydrothermal, sulfuric acid or gypsum speleogenesis,or (maze caves) were viewed as developed by backflooding, byrecharge through a permeable caprock, or not clearly interpretedgenetically. The recognition of MSRF in such a great variety of caves,which have been previously seen as genetically different, suggests acommon origin and is the strongest argument in favor of the dominantrole of hydrogeological factors in speleogenesis, i.e. the type andregime of groundwater flow and the modes of recharge and discharge.

The morphologic suite of rising flow consists of three majorcomponents: 1) feeders (inlets), 2) transitional wall and ceilingfeatures, and 3) outlet features (Fig. 8).

4.1.1. FeedersOriginal feeders are basal input points to hypogenic transverse

systems, the lowermost components, vertical or sub-vertical conduits,through which fluids rise from the source aquifer. Such conduits arecommonly separate but sometimes they form small networks at thelowermost storey of a system, which bear the feeding function relativeto the upper storey. Feeders join master passages located at the nextupper storey and commonly scatter rather uniformly through theirnetworks (Fig. 3). Many feeders are point features; they may join thepassage from the end (Fig. 9 A), from a side (Fig. 9, B through J), or arescattered through the passage floor. Where master networks occur atthe base of a soluble bed, they can receive recharge throughout theentire length of fissures to guide passage development, withoutmorphologically distinct feeders. Feeders also can be rift-like featuresat the floor of master passages which extend down to the contact withthe underlying aquifer bed (Fig. 9, K through N).

Master passages (in multi-storey mazes) are passages thatconstitute laterally extensive networks within certain horizons in asoluble unit. They receive dispersed recharge from numerous subvertical feeding channels and reflect the lateral component of flowdue to discordance in initial porosity structure between differenthorizons or due to a major confinement above. In some complex 3-Dstructures there can be several storeys of lateral development in thesystem. In that case, feeders of an upper storey are the continuation ofoutlet features of the adjacent lower storey. Hence, the lower storeysfunction to recharge the upper storeys. Sizes of feeders vary greatly,from small conduits (tubes, rift-like fissures, etc.) on the order of tens

Fig. 10. Transitional features: rising wall channels above feeders (D, I, L, M and N), ceiling channels (A, B, C, D, J and K), rising sets of coalescing cupolas and upward-convex arches(E and F) and ceiling pendants between braided ceiling channels (G and H). A = Mystery Cave, MN, USA (Ordovician limestone); B and C = Jubilejna Cave, western Ukraine (Miocenegypsum); D = Carlsbad Cavern, Guadalupe Mountains, NM, USA (Permian limestone); E = Caverns of Sonora, TX, USA (Cretaceous limestone); F = Spider Cave, Guadalupe Mountains,NM, USA (Permian limestone); G = Krystalna Cave, western Ukraine (Miocene gypsum); I = Jubilejna Cave, western Ukraine (Miocene gypsum); J and K = Dzhurinskaja Cave, westernUkraine (Miocene gypsum); L = Coffee Cave, NM, USA (Permian gypsum); M and N = Mystery Cave, MN, USA (Ordovician limestone). Photos by A. Klimchouk. Arrows on solid linesgive a scale (approx. 1 m if not specified). Dashed lines indicate inferred rising buoyancy currents.



of centimeters to features many meters in the cross-section. Thevertical extent of feeders also varies greatly: from less than a meter tomany tens and even a few hundred meters, such as prominent feedersin Carlsbad Cavern (the Lake of Clouds and the Nicholson Pit) andLechuguilla Cave (Rift and Sulfur Shores) in the GuadalupeMountains,New Mexico. Feeders observable in currently shallow-lying relictmulti-storey maze caves in dolomites of the Neoproterosoic VazanteGroup, Minas Gerais, Brasil, extend down for more than two hundredsmeters, as evidenced by their interceptions at different levels by zincmine. In many instances, dimensions of feeder conduits are smaller intheir lower parts and they often have ear-shaped orifices (Fig. 9). Thisis due to buoyancy effects, shielding of walls in the lower parts fromdissolution by more saturated water in sinking limbs of freeconvection cells, and mixing effects, enhanced dissolution at theorifices due to mixing of waters of different chemistry.

Feeders are often obscured by the presence of sediment fill, but stillcan be identified in many cases by the presence of rising wall channelsabove them (if feeders connect a passage from a side, beneath ahanging wall), or misinterpreted as “swallowing” or entrenchmentforms rather than forms that transmitted rising flow.

4.1.2. Transitional wall and ceiling featuresThese features include: rising wall channels, rising sets of

coalesced ceiling cupolas or upward-convex arches, ceiling channels(half-tubes), and separate ceiling cupolas. They are commonlyarranged in continuous series, ultimately connecting feeders tooutlets, indicating rising flow patterns and a considerable role ofbuoyant effects (upward-focussed dissolution by buoyant currents —rising limbs of free convection cells).

Rising wall channels (Fig. 10, D, I, L, M, K; see also Fig. 9, B, C, D) arefound above feeders, continued through ceiling half-tubes to outletcupolas and domepits (Fig.10, B, C and D). Rising sets of ceiling cupolasor series of upward-convex arches are also common for passages orrooms connecting different storeys in a cave system (Fig. 10, E, F).

Cupolas on the ceiling are often arranged in linear series comprisinga kind of a channel (Fig. 10D; Fig. 11C) but they can occur separately.In many cases where bottom features are observable, prominentcupolas or complexdomeswithnumerous smaller cupolasmatch in theplan view to particular feeders or groups of feeders at the floor, clearlysuggesting convection origin of ceiling features. This origin for cupolashad been well recognized for hydrothermal caves (Müller and Sarvary,1977; Dublyansky, 1980; Lauritzen and Lundberg, 2000) but largelysimilar features at all scales are common for other types of hypogeniccaves (sulfuric acid, “normal” limestone caves, caves in gypsum). Manycupolas have guiding fractures at their apexes but others show nosuch guidance. Cupolas alone are not exclusive to hypogenic speleo-genesis; they may form in phreatic caves in unconfined aquifers(reflecting the confinement of water within a passage itself), but theiroccurrence in a suite together with other features as described here isclearly indicative of hypogenic speleogenesis and buoyant dissolutioneffects. Extensive discussion of cupolas has been recently provided byOsborne (2004).

Ceiling channels, also often called half-tubes, although werecommonly interpreted as paragenetic features formed when sedimentfill directs phreatic dissolution upward (Renault, 1968; Lauritzen andLundberg, 2000), are very typical for hypogenic caves which have neverbeen filled with sediments to the ceiling level (Fig. 10, B, C, D, J, K).Instead, their relationships with feeders (through rising wall channels),and outlets in hypogenic caves, and rising patterns from the former to

the latter, clearly suggest an origin due to buoyancy effects (Fig. 8). Inlarge passages or rooms where multiple feeders are present, severalceiling channelsmay braid in a close proximity, leaving ceiling pendantsin between. Particularlygood examplesof suchpendants canbe found insomegypsumcaves in thewesternUkraine (Fig.10G), in USA at CarlsbadCavern, NewMexico and in Caverns of Sonora, Texas, and in Toca da BoaVista Cave in Brazil (Fig. 10H). The vertical relief between pendants andadjacent channels can be as great as several meters, and such pendantsare often well prepared to break down when a cave is drained andbuoyant support is lost.

4.1.3. Outlet featuresThese are cupolas and domepits (vertical tubes) that rise from the

ceiling of passages and rooms at a certain storey and connect to the nextupper storey, or ultimately to the discharge boundary — the bottom ofthe overlying formation, a prominent bedding plane or the landsurface. The ultimate outlets served as discharge paths in a confinedtransverse system. The ascending formation of outlet cupolas anddomepits is suggested by their smoothed, curving walls, and bycontinuous morphology from connecting rising ceiling/wall features(Fig. 11, E, F, G, I, K). In many caves, the bottom of the overlyingaquifer bed (“receiving unit”) is exposed at the apex of an outletcupola (Fig. 11, A, B, C), sometimes with a gaping contact suggestingoutflow via the bedding plane (Fig. 11C). Outlets that break into thenext upper cave storey, or to the ultimate discharge boundary are“successful” outlets, whereas blind-terminated cupolas can beregarded as “underdeveloped” outlets. Closely spaced individualoutlet cupolas in passages lying not far below the upper aquifer maymerge to open the upper contact through a broader area along apassage, the ultimate case being where the upper contact is openedat the ceiling along the entire length of a passage.

Individual outlets can vary greatly in sizes, from less than a meterto many meters in cross-section and from less than a meter to tens ofmeters in the vertical extent. Complex outlets from large systems mayhave composite morphology and rise for tens of meters from the maincave level (the entrance series of Lechuguilla Cave and the SpiritWorld above the Big Room in Carlsbad Cavern are good examples).

4.2. Subaerial and other alternative possibilities for the origin of wall andceiling features

Some individual morphologies that compose the above describedsuite were previously interpreted in different ways. See Ford andWilliams (1989, 2007) and Lauritzen and Lundberg (2000) foroverviews of cave meso-morphology and Osborne (2004) for discus-sion of cupolas.

Cupolas (ceiling pockets) commonly occur in unconfined phreaticcaves, reflecting the confinement of water within a passage itself. Suchcupolas normally have simple forms, are not connected by rising wallchannels or ceiling half-tubes, or are not located just above a feeder;hence buoyant convection is not a contributing factor. Anotherinterpretation is that cupolas form in subaerial conditions by moistair convection driven by the heat from a pool of thermal water in achamber that is closed to outside air flow (Cigna and Forti, 1986).Acidic vapor (especially when H2S is involved) is condensed ontocooler cave walls. Szunyogh (1989) showed that spherical pockets canbe shaped in this way. Dreybrodt (2003), Dreybrodt et al. (2005) andLismonde (2003) discussed complications with condensation pro-cesses arising from heat release at the wall surface, which slows down

Fig.11.Outlets in cupolas and domepits, viewed frombelow. A = Zoloushka Cave,westernUkraine (Miocene gypsum); B = Caverns of Sonora, TX, USA (Cretaceous limestone); C = FuchslabyrinthCave, Germany (Triassic Muschelkalk limestone); D = Slavka Cave, western Ukraine (Miocene gypsum); E = Mystery Cave, MN, USA (Ordovician limestone); F = Optymistychna Cave, westernUkraine (Miocene gypsum); G = Carlsbad Cavern, Guadalupe Mountains, NM, USA (Permian limestone); H = Optymistychna Cave, western Ukraine (Miocene gypsum); I = Carlsbad Cavern,GuadalupeMountains, NM,USA (Permian limestone); J =WindCave, SD,USA (Carboniferous limestone); K =DryCave, GuadalupeMountains, NM,USA (Permian limestone); L =CarlsbadCavern,GuadalupeMountains,NM,USA(Permian limestone);M=AmazingMazeCave, TX,USA (Cretaceous limestone);N=Cavernsof Sonora, TX,USA(Cretaceous limestone). Photo J byA.Palmer, otherphotos by A. Klimchouk. Arrows on solid lines give a scale (approx. 1 m if not specified). Dashed lines indicate inferred rising buoyancy currents.



or terminates condensation. A sufficient gradient has to bemaintainedto enable continuing condensation. When this condition is met (e.g.above warm lakes in caves located close to the surface), thedevelopment of cupolas by condensation–corrosion (especially sphe-rical and semi-spherical ones) is a sound possibility. It is likely that thisprocess re-shapes original cupola-like forms created in confined/phreatic conditions. The condensation–corrosion mechanism, how-ever, does not serve to explain cupolas when they occur through allparts of extensive 3-D systems with vertical ranges of several hundredmeters, including areas quite far from where warm lakes at watertable could be presumed (e.g. Monte Cucco in central Italy, CarlsbadCavern and Lechuguilla Cave in the Guadalupe Mountains, USA).

There are some other arguments why the origin of cupolas bycondensation–corrosion should not be applied too broadly. Cupolasare common in hypogenic caves for which neither thermal nor sulfuricacid processes are applicable, such as hypogenic caves in gypsum.Among caves whose origin involved hydrothermal or/and sulfuricprocesses, cupolas are common also in those where no signs of watertable effects are recognizable, such as maze caves composed bypassages arranged in inclined storeys. In 3-D cave systems, cupola/domepit complexes often extend upward from a base passage orchamber for tens of meters and are terminated at, or interrupted by,differently oriented storeys of maze passages with which the cupola/domepit complexes show clear functional relationships. Their devel-opment due to condensation–corrosion seems to be highly unlikely insuch situations.

Traditional interpretation of ceiling half-tubes (ceiling channels) isthat they are paragenetic features formed when sediment choke ofpassages directs phreatic dissolution upward (Renault, 1968; Ford andWilliams,1989; Lauritzen and Lundberg, 2000). This is an obvious casein many epigenic caves. However, half-tubes are commonly observedin caves which have never been filled by sediments, such as in mosthypogenic caves. Their incompatibility with the paragenetic model isespecially evident in multi-storey and complex 3-D caves where half-tubes occur at different levels and are connected by rising forms flowbelow and connect to cupolas/domepits at higher ceiling elevations.

Pendants are residual pillars of rock between channels cut intoceiling. Theyare traditionally interpreted as remnants of bedding planeanastomoses (when themain body of passages had entrenched down)or as pillars between closely-spaced paragenetic ceiling channels. Thisfits well to observations in many epigenetic caves. However, bothexplanations are not applicable to many hypogenic caves wherebroadly braiding ceiling channels (creating pendants in betweenthem) are not related to bedding planes and fit best to the model ofbuoyancy currents rising from multiple feeders at the bottom.

Rising wall and ceiling channels are sometimes explained as trailscurved by degassing bubbles in phreatic thermal CO2-H2S systems(Audra et al., 2002). De Waele and Forti (2006) presented goodexamples of small bubble trail channels developed over “cave cloud”speleothems in hypogenic caves encountered by a mine in Sardinia.However, Palmer and Palmer (2000) noted that the maximum depthat which degassing to form bubbles can take place is limited by a few

Fig. 12. Bedrock partitions between closely-spaced passages in maze caves. A = Parallel slot-like passages opened to the walls of Marble Canyon, Arizona, USA. Top to bottom height isabout 65 m (from Huntoon, 2000); B = Slot-like passages with a thin partition in Wind Cave, SD, US (photo by A. Palmer); C and D = Thin partitions between passages in Slavka Cave,western Ukraine (photo by A. Klimchouk); E and G = Thin partitions between passages in Zoloushka Cave, western Ukraine (photos by B. Ridush and V. Kisselev). The pillars werethinned by dissolution at thewater table during a late stage of cave development; F = Thin partition between parallel passages in the Lower Cave section of Carlsbad Cavern (photo byA. Klimchouk).



meters below water table at commonly observed concentrations ofthese gases. Rising channels are wide-spread in hypogenic caves in alllevels within vertically extended caves. Furthermore, similar risingchannels are common in gypsum caves and caves where degassing ofrising water and enhanced condensation–corrosion could not takeplace.

Wide-spread occurrence of the above features in the characteristicsuite of forms (which also includes feeders), in a variety of caves thatshare the hypogenic origin in the hydrogeological sense, stronglysuggests their interrelated origin as described in Section 4.1 anddepicted in Fig. 8. In hypogenic transverse systems, local convectioncells can develop from even small density gradients (either thermal orsolute) in mature caves under confined conditions of sluggish risingforced flow and homogeneous hydraulic heads. Less dense and moreaggressive water tends to occupy the uppermost position in theavailable space geometry, producing upward-directed imprints suchas rising wall channels, ceiling half-tubes and cupolas. Buoyancycurrents begin from feeders—points fromwherewater enters a cave ora particular storey. Buoyant dissolution morphologies comprise acontinuous series, well recognizable in caves where the originalmorphology was not much disrupted or obscured by later water tableand vadose development, breakdown processes, or sedimentation.The morphologic suite of rising flow is best represented in limestonecaves where thermal waters were involved, and in gypsum caveswhere the gypsum strata are underlain by an aquifer with relativelylow solute load.

4.3. Dead ends, abrupt changes in morphology and partitions

Some morphologic features in caves, such as blind terminations ofpassages (dead ends), abrupt changes in size and morphology, andvarious kinds of bedrock partitions (vertical or horizontal) werealways regarded as odd and puzzling by researchers accustomed to“lateral” speleogenetic thinking. They are difficult to explain withinthe conventional speleogenetic concepts of caves formed in epigenicsettings, by lateral flow or by dissolution at the water table. Thesefeatures are sometimes considered as attributive to sulfuric acid

speleogenesis (e.g. Hill, 2003, 2006, Hose and Macaladi, 2006) but infact these are very common formost hypogenic caves regardless of thedissolution chemistry involved and host rock composition. Thesefeatures are perfectly consistent with ascending transverse speleo-genesis; lateral changes simply indicate largely independent risingdevelopment of numerous transverse segments (flow paths), andvertical changes indicate variations in initial porosity structures acrossthe vertical section.

Blind terminations of passages are inherent elements in almost allmaze caves (see cave maps on Fig. 1) and complex 3-D caves. In mostcases they are “dead ends” only from the perspective of lateralflowbut inthe transverse flow scheme they are open to both recharge (feeders frombelow) and discharge (outlets to above). The transverse speleogenesismechanism allows even a single, laterally isolated fracture to enlarge to apassable size by vertical flow through its entire length, but the passagewill remain blind-terminated (pinching out) laterally at both ends. Suchpassages are encountered by mines in many regions.

Partitions are thin separations between adjacent passages orchambers made up by bedrock or various kinds of planar resistantstructures exhumed by dissolution, such as lithified fill of fractures orfaults and paleokarstic bodies. They are common in many denselypacked maze caves, where bedrock separations between passages arecommonly thin (Fig. 12). In the Western Ukrainian mazes, bedrockseparations (“pillars”) between adjacent passages may be as thin asless than a meter (Fig. 12, C though G). Sometimes they are only a fewcentimeters thick so that a “window” can be broken by a punch. Whenwater table overprint was locally noticeable on transitional stages,thin partitions can be easily truncated by dissolution at thewater table(Fig. 12, E, G).

Another type of partition is represented by projections of lithifiedfracture fill exposed by dissolution. They may largely or completelypartition rather large passages (Fig. 13). Common in many hypogeniccaves (e.g. chert fill in limestones), in some gypsum mazes of thewestern Ukraine such partitions are composed by friable carbonatematerial and are only a few centimeters thick. The fact that theyremain intact, and passage morphology remains uniform on bothsides of such partitions, indicates sluggish flow within a mature cave

Fig.13. Partitions of carbonate fracture fill, only 1–10 cm thick, completely (left photo) or partially (right photo) partitioning large passages in Zoloushka Cave, western Ukraine. PhotoA by B. Ridush, photo B by A. Klimchouk.



system and an overall transverse flow pattern. Horizontal partitionsby more resistant beds in a stratified sequence may create multi-storey cave systems, where passages of different storeys are closelyspaced in a vertical cross-section (e.g. Endless and Dry caves in theGuadalupe Mountains, New Mexico, USA; Archeri Cave in the MinorCaucasus, Armenia; Coffee Cave in the Roswell Basin, New Mexico,USA, Stafford et al., 2008). Osborne (2007) described partitions ofvarious kinds in Australian caves and recognized that caves containingvertical and sub-vertical partitions are likely to be formed by perascensum speleogenetic mechanisms.

5. Conclusions

Hypogenic speleogenesis creates solution porosity which distribu-tion and patterns, in area and cross-section, are quite distinct fromporosity created by epigenic speleogenesis. Hypogenic speleogenesisserves to enhance cross-formational communication and to convergeflow to such zones by opening migration paths across solubleformations and the confining units (through fracturing and collapsingin response to growing cave porosity below). Therefore, it plays animportant role in (re)organization of regional flow systems.

Cave patterns resulting from ascending transverse speleogenesisare strongly guided by the permeability structure in a cave-hostingformation. They are also influenced by the discordance of permeabilitystructure in the adjacent beds and by the overall hydrostratigraphicarrangement (recharge–discharge configurations). Three-dimensionalmazes with multiple storeys, or complex cave systems are mostcommon, although single isolated chambers, passages or crudeclusters of a few intersecting passages may occur where fracturing isscarce and laterally non-uniform. Large rising shafts and collapsesinkholes over large voids, associated with deep hydrothermalsystems, are also known. Elementary patterns often combine toform composite patterns and complex 3-D structures.

Hypogenic caves are identified in various geological and tectonicsettings, and in various lithologies. Despite these variations, resultantcaves demonstrate a remarkable similarity in meso-morphology,which strongly suggests that the hydrogeologic settings were broadlyidentical in their formation. Presence of the characteristic morpholo-gic suites of rising flow with buoyancy components is one of the mostdecisive criteria to identify hypogenic speleogenesis.

The characteristic features of ascending hypogenic cave systemsare numerous blind terminations of passages in the lateral dimensionand abrupt variations in passage cross-sections. Lateral variationsindicate largely independent rising development of numeroustransverse clusters (flow paths), and vertical changes indicatevariations in initial porosity structures between lithological units.

Natural convection mechanisms (buoyancy-driven, upwardpointed dissolution), powered either by thermal or solute differences,are widely operative in hypogenic caves, contributing significantly tocharacteristic morphologies mentioned above and producing upward-directed flow markings. Directional markings produced by vigorousflow regimes and lateral flow, e.g. scallops, are generally absent inhypogenic caves, although they may be present locally whenconsiderable epigenic overprint occurs during the subsequentunconfined stage, e.g. by intercepted streams or backflooding. Watertable markings, such as horizontal notches, may develop if therespective conditions are stable enough.

Hypogenic caves include many of the largest, by integrated lengthand by volume, documented caves in the world. More importantly,hypogenic speleogenesis is much more widespread than it waspreviously presumed. Growing recognition of hypogenic speleogen-esis and improved understanding of its peculiar characteristics has animmense importance to both karst science and applied fields as itpromises to answermany questions about karst porosity (especially asdeep-seated settings are concerned) which remained poorlyaddressed within the traditional epigenetic karst paradigm.

Acknowledgements

The bulk of this paper was prepared during my time as a visitingscientist with the National Cave and Karst Research Institute in U.S.A.in 2006–2007. The work was partially supported by the project “Voidevolution in soluble rocks: Development and validation of numericalmodels by field evidence” funded by the Deutsche Forschungsge-meinschaft (German Research Foundation). The reviewers and thejournal editors, Dr. Harvey and Dr. De Waele, are thanked for theirconstructive comments on the manuscript and helpful suggestionsthat improved the final paper.

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