RESEARCH FRONTIERS IN SPELEOGENESIS. D OMINANT PROCESSES, HYDROGEOLOGICAL CONDITIONS AND RESULTING CAVE PATTERNS NOVA PODROJA SPELEOGENETSKIH RAZISKAV: POVEZAVA MED HIDROGEOLO KIMI RAZMERAMI, PREVLADUJOIMI PROCESI IN TIPI JAM Philippe AUDRA 1 & Arthur N. P ALMER 2 1 University of Nice Sophia-Antipolis, CNRS, IRD, Observatoire de la Cote dâ€™Azur, Geoazur UMR 7329 & Polytech Nice â€“ Sophia, 930 route des Colles, 06903 Sophia-Antipolis, Nice, France, e-mail: email@example.com 2 State University of New Y ork, Earth Sciences Department, Oneonta, NY 13820, USA, e-mail: firstname.lastname@example.org Received/Prejeto: 26.04.2015 COBISS: 1.02 ACTA CARSOLOGICA 44/3, 315, POSTOJNA 2015 Abstract UDC 551.435.84 Philippe Audra & Arthur N. Palmer: Research frontiers in speleogenesis. Dominant processes, hydrogeological condi tions and resulting cave patterns Speleogenesis is the development of well-organized cave sys tems by uids moving through ssures of a soluble rock. Epi genic caves induced by biogenic CO 2 soil production are domi nant, whereas hypogenic caves resulting from uprising deep ow not directly connected to adjacent recharge areas appear to be more frequent than previously considered. e concep tual models of epigenic cave development moved from early models, through the â€œfour-states modelâ€ involving fracture inuence to explain deep loops, to the digital models demon strating the adjustment of the main ow to the water table. e relationships with base level are complex and cave levels must be determined from the elevation of the vadose-phreatic tran sitions. Since ooding in the epiphreatic zone may be impor tant, the top of the loops in the epiphreatic zone can be found signicantly high above the base level. e term Paragenesis is used to describe the upward development of conduits as their lower parts ll with sediments. is process oen records a general baselevel rise. Sediment inux is responsible for the regulation of long proles by paragenesis and contributes to the evolution of proles from looping to water table caves. Dating methods allow identication of the timing of cave level evo lution. e term Ghost-rock karstication is used to describe a 2-phase process of speleogenesis, with a rst phase of partial solution of rock along fractures in low gradient conditions leaving a porous matrix, the ghost-rock, then a second phase of mechanical removing of the ghost-rock mainly by turbu lent ow in high gradient conditions opening the passages and forming maze caves. e rst weathering phase can be related either to epigenic inltration or to hypogenic upow, especially in marginal areas of sedimentary basins. e vertical pattern of epigenic caves is mainly controlled by timing, geological struc ture, types of ow and base-level changes. We dene several Izvleek UDK 551.435.84 Philippe Audra & Arthur N. Palmer: Nova podroja speleo genetskih raziskav: Povezava med hidrogeolokimi razmera mi, prevladujoimi procesi in tipi jam Speleogeneza je razvoj dobro (samo)organiziranih jamskih sis temov, ko podzemna voda vzdol toka raztaplja stene razpok. Najbolj poznane so epigene jame v karbonatih, kjer je poglavit ni vir kemine agresivnosti pedogeni CO 2 . Bolj pogoste, kot se je v preteklosti domnevalo, so hipogene jame, ki nastanejo z dviganjem globokega toka in niso neposredno povezane z lokalnim napajalnim obmojem. Prvotni konceptualni modeli razvoja epigenih jam so se preko modela tirih stanj, ki spe leogenezo pojasnjuje s frekvenco prevodnih razpok, razvili do raunalnikih modelov, ki pojasnijo prilagoditev glavnega toka freatini povrini. Povezava jamskih sistemov s poloajem erozijske baze ni enostavna, saj moramo pri interpretaciji upotevati viino prehoda iz freatine v vadozno cono. Zaradi visokih poplav v epifreatini coni so lahko temena jamskih zavojev visoko nad erozijsko bazo. Termin parageneza se upo rablja za opis razvoja kanalov od spodaj navzgor, ko se spod nji deli zapolnijo s sedimenti. Ta proces pogosto belei sploen dvig erozijske baze. Vdor sedimentov je tudi razlog za uravna vanje dolgih prolov s paragenezo in prispeva k prehodu jam z zavoji v navpini ravnini v jame uravnane z vodnim nivojem. Razline datacijske metode omogoajo doloanje asovnega razvoja jamskih nivojev. Speleogeneza lahko poteka tudi v dveh fazah; v prvi fazi voda ob nizkem gradientu raztopi topen del kamninske matrice (angleko Ghost rock weathering), v drugi fazi pa ob visokem gradientu turbulentni tok mehansko odnese preostali del matrice, pri emer praviloma nastane labirintni tip jam. Prva faza je lahko povezana z epigeno inltracijo ali s hipogenim dotokom predvsem na mejnih obmojih sediment nih bazenov. Vertikalna geometrija epigenih jam je pogojena s asovnim okvirom, geoloko strukturo, vrsto toka in spre membo erozijske baze. Razvoj mladih (juvenilnih) geometri jskih vzorcev nad nivojem neprepustnih plasti, je povezan
ACTA CARSOLOGICA 44/3â€“ 2015 316 P HILIPPE A UDRA & ARTHUR N. P ALMER cave types as (1) juvenile , where they are perched above un derlying aquicludes ; (2) looping , where recharge varies greatly with time, to produce epiphreatic loops; (3) water-table caves where ow is regulated by a semi-pervious cover; and (4) caves in the equilibrium stage where ow is transmitted without sig nicant ooding. Successive base-level drops caused by valley entrenchment make cave levels, whereas baselevel rise is de ned in the frame of the Per ascensum Model of Speleogenesis (PAMS), where deep passages are ooded and drain through vauclusian springs. e PAMS can be active aer any type of baselevel rise (transgression, uvial aggradation, tectonic sub sidence) and explains most of the deep phreatic cave systems except for hypogenic. e term Hypogenic speleogenesis is used to describe cave de velopment by deep upow independent of adjacent recharge areas. Due to its deep origin, water frequently has a high CO 2 H 2 S concentration and a thermal anomaly, but not systemati cally. Numerous dissolution processes can be involved in hy pogenic speleogenesis, which oen include deep-seated acidic sources of CO 2 and H 2 S, â€œhydrothermalâ€ cooling, mixing cor rosion, Sulfuric Acid Speleogenesis (SAS), etc . SAS particularly involves the condensation-corrosion processes, resulting in the fast expansion of caves above the water table, i.e. in an atmo spheric environment. e hydrogeological setting of hypogenic speleogenesis is based on the Regional Gravity Flow concept, which shows at the basin scales the sites of convergences and upows where dissolution focuses. Each part of a basin (mar ginal, internal, deep zone) has specic conditions. e coastal basin is a sub-type. In deformed strata, ow is more complex according to the geological structure. However, upow and hy pogenic speleogenesis concentrate in structural highs (buried anticlines) and zones of major disruption (faults, overthrusts). In disrupted basins, the geothermal gradient â€œpumpsâ€ the me teoric water at depth, making loops of dierent depths and characteristics. Volcanism and magmatism also produce deep hypogenic loops with â€œhyperkarstâ€ characteristics due to a combination of deep-seated CO 2 , H 2 S, thermalism, and micro bial activity. In phreatic conditions, the resulting cave patterns can include geodes, 2 â€“ 3D caves, and giant ascending shas. Along the water table, SAS with thermal air convection induces powerful condensation-corrosion and the development of up wardly dendritic caves, isolated chambers, water table sulfuri cacid caves. In the vadose zone, â€œsmokingâ€ shas evolve under the inuence of geothermal gradients producing air convectio nand condensation-corrosion. Likely future directions for research will probably involve analytical and modeling methods, especially using isotopes, dating, chemical simulations, and eld investigations focused on the relationships between processes and resulting mor phologies. Keywords: Speleogenesis, Epigenic cave, Base level rise, Cave level, Cave pattern, Epiphreatic cave, Flooded karst, Juvenile cave pattern, Looping cave, Mature rough Caves, Per as censum Model of Speleogenesis, Vauclusian cave, Water-table cave, Paragenesis, Ghost-rock karstication, Hypogenic Cave, Sulfuric Acid Speleogenesis (SAS), Condensation-corrosion, Regional Gravity Flow, hyperkarst, geode caves, 2 â€“ 3D caves, Giant ascending shas, Upwardly dendritic caves, Isolated chambers, Water table sulfuric caves, Smoking shas. s hitrimi tektonskimi dvigi in vrezovanji erozijske baze. V pogojih omejenega odtoka ob spremenljivem napajanju zaradi poplavljanja epifreatine cone nastajajo zavoji v navpini rav nini (angl. loops). Jame vodnega nivoja nastajajo na podrojih, kjer je kras pokrit z delno prepustnimi plastmi oz. kjer je spe leogeneza uravnoteena z najvejimi poplavami. Spreminjanje erozijske baze ob vrezovanju dolin se odraa v jamskih nivojih, medtem ko dviganje erozijske baze diktira razvoj jam od spodaj navzgor (Speleogeneza Per ecensum, PAMS) in nastanek izvi rov voklukega tipa. PAMS se lahko aktivira ob razlinih vrstah dviga erozijske baze (zaradi transgresije, renega naplavljanja, tektonskega ugrezanja) in pojasnjuje nastanek veine globokih freatinih jamskih sistemov, razen hipogenih. Izraz hipogena speleogeneza se uporablja za opis razvoja jam zaradi dvigan ja globokega regionalnega toka. Zaradi izvora iz globin ima voda pogosto visoko koncentracijo CO 2 â€“ H 2 S in temperaturno anomalijo. Pri hipogeni speleogenezi lahko sodelujejo tevilni procesi raztapljanja, ki so povezani z globokimi viri CO 2 in H 2 S, "hidrotermalnim" ohlajanjem, korozijo meanice, speleo genezo veplene kisline (Sulphuric Acid Speleogenesis, SAS), itd. Zlasti SAS vkljuuje kondenzacijsko-korozijske procese, zaradi esar prihaja do hitrega nastanka jam nad vodno gla dino v atmosferskem okolju. Hidrogeoloke razmere pri hipo geni speleogenezi so povezane z regionalnim gravitacijskim tokom, kjer je korozija najmoneja na obmoju stekanja in dvigovanja vodnih tokov. Vsak del poreja (obrobni, notranji, globoka cona) ima posebne pogoje. Eden od podtipov je tudi obalno obmoje. V deformiranih slojih je tok bolj zapleten in strukturno pogojen, pri emer sta vodni tok in hipogena spe leogeneza praviloma vezana na strukturne vrhove (prekrite an tiklinale) in na obmoja vejih strukturnih prekinitev (prelomi, narivi). V prekinjenih bazenih geotermalni gradient "rpa" me teorske vode v globine, kar povzroa zanke na razlinih globi nah in z razlinimi znailnostmi. Vulkani zem in magmatizem tudi povzroata globoke hipogene zanke s "hiperkrakimi" znailnostmi, ki nastajajo zaradi kombinacije globokih virov CO 2 , H 2 S, termalnih procesov in mikrobioloke aktivnosti. Geometrijski vzorci jam v freatinih pogojih lahko vkljuujejo geode, 2 â€“ 3D jame in navzgor razvijajoa se brezna izjemnih razsenosti. Nad vodno gladino se zaradi termalne konvekcije in kondenzacijske korozije ob prisotnosti veplove kisline raz vijajo razlini geometrijski vzorci jam; dvigajoe se razvejane jame, izolirane dvorane in jame vodnega nivoja nastale z de lovanjem veplene kisline. V vadozni coni nastajajo tudi par na brezna, ko se na obmojih termalnih vodonosnikov topel vlaen zrak dviga, ohlaja in kondenzira vzdol razpok in jih na ta nain iri v brezna. V prihodnosti bodo raziskave speleo geneze verjetno temeljile na analitinih in modelskih pristopih, izotopskih, datacijskih in geokeminih metodah ter terenskih raziskavah, ki se bodo osredotoala na odnose med procesi in posledino morfologijo. Kljune besede: speleogeneza, epigena jama, dvig erozijske baze, jamski nivo, geometrijski vzorci jam, epifreatina jama, poplavljen kras, juvenilni geometrijski vzorec, jama z za voji, zrele tunelske jame, model speleogeneze Per ascensum, vokluka jama, jama vodnega nivoja, parageneza, GhostRock zakrasevanje, hipogena jama, speleogeneza veplove kisline, kondenzna korozija, regionalni gravitacijski tok, hiperkras, geodske jame, 2 â€“ 3D jame, brezna izjemnih razsenosti, navzgor razvejane jame, izolirane dvorane, veplene jame na nivoju podtalnice, "parna" brezna.
ACTA CARSOLOGICA 44/3 â€“ 2015 317 RESEARCH FRONTIERS IN SPELEOGENESIS. D OMINANT PROCESSES, HYDROGEOLOGICAL CONDITIONS AND ... Speleogenesis is the development of well-organized cave systems by water owing through initial ssures of a soluble rock. Not all ssures will enlarge homogene ously through dissolution. e specicity of cave systems in karst relies on the selective process of ssure enlarge ment that progressively produces human-size passages, which are well-organized and where ow concentrates with high discharge. Most cave systems are formed by dissolution of meteoric water inltrating from the land surface. Ag gressiveness toward carbonate rocks is caused mainly by carbon dioxide absorbed from the air and soil. Such caves are called epigenic caves. Beside epigenic caves, a minority of explored cave systems owes its origin to deep upow, the so-called hypogenic caves. Aggressiveness toward evaporate rocks does not depend on CO 2 . Such caves were only recently recognized as a widespread phenomenon. ey generally contain features and pat terns that show evidence for up-owing of deep water, oen with high CO 2 H 2 S concentration and with a ther mal component. e rst conceptual models of cave patterns were based on eld experience. ey recently evolved thanks to the application of analytical then numerical modeling inputting hydraulics and chemical constraints and also thanks to the extension of eld knowledge. Assessing the role of the fractures, of the geological framework, of the recharge and ow regime, modeling allows demonstrat ing the demonstrating the patterns of ow and the re sulting caves in a variety of specied conditions. One of the main processes controlling the develop ment of epigenic caves is the evolution of the baselevel position, not only while dropping in successive steps as valleys entrench, but also while rising. Flow regime, and especially ooding have an important role in cave devel opment in the epiphreatic zone, which is ooded during high water. Recently, ghost-rock karstication has been recognized as an important process of cave development in certain conditions. In addition to the classical tools used in Earth Sciences such as sedimentology, hydrol ogy, geomorphology, etc., the recent development of dat ing methods oers new highlights for understanding not only the chronological frame, but also the relationships between cave development stages and the correspond ing environmental states. e epigenic cave patterns pat terns depend on the combination of the combination of the main parameters (geology, recharge, timing) acting during the development of the cave systems. Hypogenic caves result from the combination of complex dissolution processes oen involving deep-seat ed acidic sources, sometimes originating from the mantle or from volcanic activity, but also from â€œnormalâ€ rising meteoric water. e importance of regional ow was re cently stressed to explain the distribution and character istics of rising ows at the basin scale. Flow convergence, upow and outow loci are responsible for focusing hy pogenic speleogenesis, allowing the development of cave systems in specic settings, which are controlled by both topography and hydrogeology. e pattern of hypo genic caves shows a development in phreatic conditions, sometimes at great depth, and also close to the water ta ble. Recent investigations stressed the role of thermal air convections making condensation-corrosion, which can produce caves in short time spans due to extreme acidic conditions, especially in presence of sulfuric acid. In this paper, we present epigenic and hypogenic speleogenesis. For each, the main controlling parameters and speleogenesis processes are discussed, resulting in specic patterns of cave systems. EPIGENIC SPELEOGENESIS e epigenic caves are formed by water that acquires its solutional capacity from surface conditions. In particular, the ability of water to dissolve carbonate rocks is derived from carbon dioxide absorbed from the atmosphere and especially the soil. Organic acids may contribute to the solutional potential, although their role is not so well understood. Fresh inltrating water is, of course, capa ble of dissolving evaporite rocks. On a world-wide basis, epigenic caves probably account for at least 80% of known caves. While passing through limestone or other type of soluble rock, water from diuse inputs converges and generally emerges at discrete springs located at the bot tom of a valley. e type of ow determines the distribu tion between vertical zones of karst, as well as the pro les of cave passages (Fig. 1). e vadose zone contains conduits with free-surface streams similar to those on the surface. In the phreatic zone, closed-conduit ow takes place along gentle gradients. In between is the epiphreatic zone, which is ooded during high water and drained during low water, and thus contains both types of ow. As water ows through the initial ssures in a karst massif, it dissolves the surrounding rock and gradually produces well-organized cave systems. Such cave systems are controlled by passive parameters (litho logic and tectonic) and by boundary conditions (type of recharge, topographic gradient, base-level position, etc.). Cave patterns depend mainly on geologic structure, type of recharge, and changes in base level. e vertical development of epigenic karst is in timately associated with the geomorphic evolution of I NTRODUCTION
ACTA CARSOLOGICA 44/3â€“ 2015 318 the surrounding landscape. Cave proles and levels of development reect the local base level and its changes through time. ese cave features tend to be preserved far longer than correlative surface features, which are more susceptible to weathering and erosion. As a result, cave morphology oers abundant clues that are helpful in reconstructing the regional geomorphic history (Ford & Williams 2007). Concepts and modeling of cave origin, especially concerning vertical development ere have been many attempts to construct various models of karst and cave development (Palmer et al. 1999). Conceptual models are based on eld observa tions and the qualitative application of scientic princi ples. Analytical models rely on quantitative application of the guiding principles, mainly hydraulics and chemi cal kinetics. Digital models are constructed by nite-dif ference or nite-element analysis, in which the aquifer is considered to be composed of many small pieces, and the appropriate analytical equations are applied to each by computer soware to simulate karst development with time. Conceptual models One of the earliest controversies in karst involved the na ture of its groundwater ow. Grund (1903) viewed it as similar to any other kind of groundwater in porous mate rial, with a discrete water table. Katzer (1909) and Martel (1921) disagreed, citing evidence from caves that sub surface water in karst follows interconnected conduits, as though in a plumbing system, with no discrete water table. Today, most hydrologists recognize the merits of both models. Soon aerward there were debates as to where cave development took place relative to the water table. Davis (1930) and Bretz (1942) proposed that caves form deep beneath the water table, when groundwater owpaths are likely to remain stable for long time peri ods. Swinnerton (1932) contended that caves are more likely to form where groundwater ow is most vigor ous, i.e., at and just below the water table (Fig. 2). is origin can account for the low-gradient proles of many Fig. 1: Idealized cross section through a typical epigenic karst system. Recharge takes place through sinking streams, dolines, and the epikarst to form the tributaries of a branching cave system. V adose passages (formed above the water table) include shas and canyons. At the water table, groundwater follows a relatively gentle gradient to springs in nearby valleys. Most phreatic passages are tubular and form at or just below the water table, although many contain vertical loops. During oods (especially in caves fed by rapid runo), the phreatic passages may be unable to transmit all the incoming water, so complex looping overow routes form in the epiphreatic zone (zone of water-table uctuation). Large phreatic passages form when the erosional base level remains at one elevation for a long time, as when erosional benches are formed. As the base level drops and the surface river erodes downward, phreatic passages tend to drain through diversion routes, but the old phreatic passages give evidence of the former base level. P HILIPPE A UDRA & ARTHUR N. P ALMER
ACTA CARSOLOGICA 44/3 â€“ 2015 319 phreatic passages. He considered the zone of water-table uctuation to be most favorable for cave origin. Ford (1971), and Ford and Ewers (1978) proposed a model based on groundwater ow elds and the spatial density of ssures (i.e., fractures and partings). ey en visioned a four-state model with the following sequence (Fig. 2): (1) Where ssure frequency is low, only a few ssures are available to transmit enough water to form caves. In this case, there is a tendency for only a few phreatic loops to form â€“ or perhaps just a single loop. (2) Where ssure frequency is greater, multiple loops devel op that are shallower than in case 1. (3) Where ssures are still more numerous, both phreatic loops and watertable segments are able to develop. (4) Where ssure fre quency is so great that phreatic loops cannot form at all, cave passages develop almost entirely along the water ta ble. ere is a tendency for these states to follow one an other with time, as ssures become more numerous with time, owing to pressure release. In this situation, caves may evolve from deep systems with few phreatic loops to shallow systems dominated with water-table passages where ssures are numerous. For an illuminating discus sion on the topic, both pro and con, see Ford (2014). Worthington (2004, 2005) questioned the validity of the Ford-Ewers model by noting the development of sub-horizontal caves as much as 100 m below the water table. He also showed statistically that depth of phreatic cave development is proportional to the overall length of ow paths and angle of the stratal dip. Analytical models Palmer (1991) combined hydraulics with chemical kinet ics to explain cave patterns: (1) Early cave enlargement rates depend on the ratio of discharge to ow length (Q/L). Dierences in this ratio account for the varied growth rate among the competing ow paths. (2) Along any path, enlargement rates increase with discharge, but only up to a certain limit. From then on, greater dis charge aects the enlargement rate only slightly. (3) Only a few paths reach cave size, while others stagnate with lit tle further growth. Branchwork caves with relatively few passages are formed. (4) Maze caves form where Q/L is large along many alternate routes. Epigenic mazes form by recharge through adjacent permeable but insoluble rock (small L and/or uniform Q ), or where oodwaters with large discharge are ponded behind constrictions so that water is injected under pressure into all ssures in the adjacent bedrock. Digital models (computer simulation) Using nite-dierence modeling, Dreybrodt (1990, 1996) and Palmer (1991) independently determined the break through time ( t b ) needed for a ssure to reach its maxi mum growth rate. ey both showed that t b decreases with Fig. 2: Le: the water-table cave hypothesis proposed by Swinner ton (1932). Right: e four-state model of Ford and Ewers (1978). Depending on ssure frequency, various types of caves evolve: low ssure frequency (state 1) pro duces bathyphreatic caves. With increasing ssure frequency the number of phreatic loops increas es (states 2 and 3). High ssure frequency (state 4) results in wa ter-table caves. Extremely low or extremely high ssure frequency does not allow evolution of caves (state 0 and 5, not shown here). e four states do not necessar ily follow one another (aer Ford 1999). RESEARCH FRONTIERS IN SPELEOGENESIS. D OMINANT PROCESSES, HYDROGEOLOGICAL CONDITIONS AND ...
ACTA CARSOLOGICA 44/3â€“ 2015 320 the cube of the initial ssure width, and with roughly the 4/3 power of the hydraulic gradient and the /3 power of the ow distance. us the most favorable paths for cave development are (by far) the widest initial openings, and (less importantly) the steepest hydraulic gradients and shortest ow distances. Breakthrough time also decreases at higher CO 2 partial pressures and lower water tempera tures (Palmer 1991). Breakthrough times on the order of 10 5 yrs are typical in epigenic caves. Gabrovek (2000) showed that mixing of waters of varied CO 2 content can decrease the breakthrough time, but that large dierences in CO 2 concentration are necessary. Dreybrodt and others (2005) summarized the pre vious 15 years of karst modeling. Among their many conclusions are: (1) e main solution conduits tend to concentrate at or near the water table (Fig. 3). e pres ence of relatively wide fractures in the initial system can lead to phreatic loops, but only if they develop prior to those at the water table. (2) Depth of penetration of solu tion conduits below the water table is aected very little by aquifer thickness, because the water at depth is mainly saturated with dissolved carbonate minerals. ese last advances established that main drains devel op along the water table or at shallow depth, with possible loops of various depths if prominent fractures are present. Within this general context, passive parameters (geology) and boundary conditions (discharge, base-level position ) will play a key role to determine the cave pattern. Some of the most important processes acting in epigenic speleogenesis Complex relationships between cave levels and base level As a surface stream erodes its valley deeper, karst springs that emerge in the valley tend to shi to progressively lower elevations. e cave passages that feed the springs also migrate downward, following the drop in the wa ter table. erefore, in most caves the highest passages are the oldest and the lowest are younger. A cave level consists of one or more passages that are conned to a narrow vertical range. is term should apply only to passages that correlate with present or former base levels. Where structural or stratigraphic features are responsible for a narrow vertical range, the term tier or story is more appropriate. If a stream valley deepens rapidly, cave streams tend to shi to lower routes so frequently that their passages do not remain active long enough to reach a large size. When there is a pause or slowing of entrenchment, the valley bottom broadens into a oodplain, and cave pas sages at that level have time to acquire large cross sec tions. ese passages constitute a true cave level. e level may be represented by a single passage, but the interpretation is greatly supported if several passages in the same cave, or in adjacent caves, all have similar el evations. It is not the average elevation of a passage that is important, but instead the elevation of the present (or former) vadose-phreatic transition. Fig. 3: Simulation of vertical proles of cave development (Drey brodt et al. 2005). Dissolutional widening is most active in a re stricted region below the water table, where maximum ow oc curs. Gradually, by headward erosion, the water table becomes almost horizontal, whereas diuse ow in the phreatic zone simultaneously decreases. In this network, the initial aperture width a 0 = 0.009 cm and the dissolved solute entering the system is at 90 % saturation. e blue line shows a surface recharge of 400 mm/y. ere is no ow across the le and bottom bound aries; the river at base level provides a constant-head boundary (right). Flow rate is shown in color (increasing from blue to red); ssure aperture is shown in grey. Successive runs at t = 0, 5, 10, and 20 ky. Eventually a low-gradient water table concentrates most of the ow toward the spring at the base level. P HILIPPE A UDRA & ARTHUR N. P ALMER
ACTA CARSOLOGICA 44/3 â€“ 2015 321 Fluvial base levels change irregularly, oen with pe riods of valley lling, according to changes in sea level, climate, and rates of upli of the land. Glacial advanc es and retreats have a large eect, as do adjustments in the pattern of surface rivers. Some cave levels correlate with river terraces (Davies 1960; Droppa 1966; White & White 2001), which suggests cave origin at or just below the water table. But in recent decades a growing number of researchers have found that the relationship of cave passages to the water table is more complex. e presence of a vadose passage shows that, while it was forming, the oor of its outlet valley must have been at a lower elevation than the passage. Vadose pas sages cannot qualify as true cave levels because they form along the descending paths of gravitational water, which are independent of one another. A poorly soluble bed may cause perching of vadose passages at a common elevation, but this is a structural phenomenon that does not relate to the position of base level. A valid interpretation of cave levels, in the geomor phic sense, requires the recognition of vadose-phreatic transition zones (see Fig. 1). e hydraulic gradient in a water-lled cave passage is so low, except during ma jor oods, that these transition zones are only slightly higher in elevation than the local spring. Although caves enlarge fastest during oods, oodwater dissolution ex tends over a wide vertical range, with no single domi nant elevation, and this process can blur the distinction between cave levels. Epiphreatic cave development and signicance of vadose-phreatic transition e depths of phreatic loops do not invariably diminish in progressively lower passage levels. In Mammoth Cave, Kentucky, USA, the deepest known phreatic loop (21 m) is in the lowest and most recent of the major passages (Palmer 1987). is is the result of thick-bedded, promi nently jointed strata at that elevation. Many epiphreatic passages, which form above the low-ow water table under hydraulic pressure, also have irregular proles with high-amplitude loops. Audra (1994) emphasized the inuence of the epiphreatic (oodwater) zone for speleogenesis of passages of appar ent phreatic origin, and Huselmann et al. (2003) sub sequently rened the model, explaining speleogenesis of Brenschacht (Switzerland) on the basis of oodwater uctuations. Water chemistry measurements as well as direct observations in caves have shown that oodwa ters are much more corrosive and erosive than low wa ters (Palmer 2007), and that, for instance, scallops size reveals the high oodwater ow velocity (Lauritzen et al. 1983). We thus can ascertain that oodwater eects are very important in speleogenesis and mainly results in looping tubes developing in the epiphreatic zone (see be low â€œLooping cavesâ€). In such caves, high-level passages with large vertical loops are not necessarily the oldest. e widespread eect of rising base level: phreatic li and paragenesis Rises in base level may disrupt the tendency for passages in a cave to become progressively younger with depth. Some possible causes include rising sea level, decreasing stream ow, and glacial depression of the Earth's. Most of these eects are relatively short-lived, on the order of thousands or tens of thousands of years, but some have endured for millions of years. Even brief episodes can have long-lasting eects. A base-level rise is usually accompanied by sedi ment lling in valleys to that new level. In most cases, the sediment is deposited by the river itself (alluviation), although glacial or marine deposits may also be respon sible. Cave passages below this level become ooded. ey may eventually become sediment-lled, especially if their ow is feeble or is diverted into formerly aban doned passages at a higher level. Paragenesis is the upward dissolution of the ceiling in a water-lled cave passage because of sediment accumula tion on the underlying oor (Renault 1970; Farrant 2004; Farrant & Smart 2011; Pasini 2009; Lauritzen & Lauritsen 1995). e sediment shields the oor from aggressive wa ter, leaving only the upper surfaces of the passage exposed to dissolution (Fig. 4). As the ceiling dissolves upward, more sediment accumulates on the oor to maintain the Fig. 4: Paragenetic ceiling channel, Cameli Aven, France (Photo: J.-Y. B igot). It developed by water owing on top of a sediment lling, here later removed by erosion. e presence of generalized sediment lling associated to wall and ceiling channel make pos sible to distinguish such paragenetic feature from rising half tubes and condensation-corrosion channels, which are generally devoid of any sediment. RESEARCH FRONTIERS IN SPELEOGENESIS. D OMINANT PROCESSES, HYDROGEOLOGICAL CONDITIONS AND ...
ACTA CARSOLOGICA 44/3â€“ 2015 322 equilibrium among erosion, deposition, and water veloc ity. Upward migration stops when the tube reaches the water table. Later, as the water table drops, the passage is either abandoned or its sediment is excavated by the cave stream. Paragenesis is oen caused by oodwaters, which are highly aggressive, sediment-laden, and abrasive. e paragenesis process was previously oen con sidered only local in scale, resulting in passage collapse or various kinds of discrete plugging. Recent studies (Farrant & Smart 2011; Mocochain et al. 2009) found that paragenesis is generally widespread and may result from base-level rise that is either regional (e.g., from gla ciation) or widespread (e.g., from transgression). e inux of sediments by sinking streams contrib utes to paragenesis by plugging the lower parts of the loops resulting in regularized long proles. Consequent ly, sediment inux and paragenesis also contribute to the evolution toward the watertable pattern. e development of dating methods Because the vertical arrangement of an epigenic cave sys tem has such a close relation to the regional geomorphic history, dating of cave passages can be of great benet to reconstructing and interpreting that history. e main diculty is that the caves themselves are voids that can not be dated directly. However, dating of deposits in the caves can show the minimum ages of cave origin. Chemi cal deposits (speleothems) are easiest to date, but their ages generally have little direct bearing on cave origin. Most useful are dates on detrital sediment, because the sediment was presumably carried in by the same water that participated in the last phase of passage enlarge ment. Paleomagnetic measurements of cave sediments have been successfully used for geomorphic interpretations. However, the method relies on determining the positions of polarity reversals, and continuity of the sediment se quence is required to assure that there are no time gaps (Schmidt 1982; Sasowsky 2005). Measurement of the cos mogenic radionuclides 26 Al and 10 Be in quartz sediment is most appropriate, because it gives a continuous range of numerical dates, with no gaps; and its useful range ex tends to about 5 million years, which is sucient to cover the entire genetic history of most caves (Granger & Fabel 2005). In Mammoth Cave, Kentucky, USA, dates of pas sages at various levels have been obtained from 26 Al/ 10 Be ratios in detrital sediment from the last 3.5 Ma (Granger et al. 2001; Audra & Palmer 2013). Evolution of the cave has been controlled by the regional Ohio River system, which in turn has been aected by continental glaciation. Major alterations of the river pattern resulted in episodes of incision and aggradation that controlled the succes sive stages of cave development and sedimentation. Using K-Ar dating of illite originating from hydrothermal neo formation, Osborne et al. (2006) shown that part of Jeno lan Caves (Australia), formed in the Carboniferous Period (342 to 335 Ma). is is by far the oldest absolute age of a cave deposit found in a currently open cave. Similarly, Ar-Ar dating of K-bearing neoformation minerals such as jarosite and alunite were successfully used to date sulfuricacid caves millions of years old sulfuric caves (see section Special focus on sulfuric acid speleogenesis (SAS) and condensation-corrosion). Ghost-rock karstication Ghost-rock karstication is a process that was rst iden tied in Belgium (Vergari 1997), then Italy, Switzerland and France and which is in fact present in many areas (Dubois et al. 2004, and references herein). It diers from the traditional single-stage process of karst devel opment that proceeds in a total removal of rock along initial ssures. e rst stage of ghost-rock karstication is characterised by chemical dissolution and removal of the most soluble crystals of a grain-heterogeneous rock, i.e. the calcite grains in impure limestone, or the smallest micritic crystals vs. sparite in pure limestone. It requires low hydrodynamic energy and creates a ghost-rock fea ture, mimicking the fracture pattern lled with the resid ual weathering products, the ghost. It has a highly porous texture where initial rock features are preserved, such a calcite dikes and fossils. e ghost-rock karstication can develop at considerable depth below base level, provid ing some fresh ow is able to maintain the aggressivity and to transport the solutes during a long-lasting time, several orders of magnitude longer than in the classical epigenic speleogenesis. e second stage is characterised by mechanical erosion of the remaining undissolved par ticles of the ghost. It requires high hydrodynamic energy, i.e. a base level drop aer upliing and/or valley incision, and it is only then that open galleries are created, mostly by mechanical erosion and transport of the ne grains. It results in cave systems adjusted to the fracture pattern, such as mazes. However, branching patterns are also present aer ghost-rock karstication, since ow driven by gravity may select only some of the ghost directions corresponding to the ow lines. Passage morphologies are very similar to those of phreatic passages, with round proles, bridges, dead ends, etc. e ghost-rock karstication process has been poorly recognized up to now. Firstly, because the second phase of mechanical removing of the ghost erases the ini tial evidence, which may remain only in dead ends and in areas protected from turbulent ow (Fig. 5), and sec ondly, because the resulting morphologies (maze pattern with phreatic-like passages) were oen misinterpreted as phreatic in origin. P HILIPPE A UDRA & ARTHUR N. P ALMER
ACTA CARSOLOGICA 44/3 â€“ 2015 323 Originally, the rock-ghost karstication has been attributed to epigenic processes, i.e. a kind of weathering slowly developing downward from the surface (Q uinif 2010, and references therein). Such an origin is evident in many places, and it now appears that entire regions of ghost-rock karstication have been clearly aected by hypogenic ows (Bruxelles & Wienin 2009). In France, most of the marginal areas of basins at the contact of cra tons were subject to hypogenic processes by deep upow expelled from the basin central deep zones (peripheries of Central Massif, Vosges Mountain...). Such low regions are now characterized by the association of mineralized sulde ores and caves of predominantly maze pattern. ese caves result from the removal of the ghost rock fol lowing the rearrangement of the karst drainage aer the recent uplis. If the process is now well understood at the scale of the ghost (Q uinif 2010) a more general con ceptual model at the regional scale will probably soon in tegrate the ghost-rock karstication with both epigenic and hypogenic processes. GUIDANCE ON VERTICAL CAVE PATTERNS e geological structure, the type of ow, and the steps of base-level changes play a key role in organizing the verti cal pattern of epigenic caves. us, vertical cave pattern is mainly controlled by time, by the position of the aquifer (perched vs. dammed), by recharge type (regular vs. ir regular), and by base-levels changes (lowering vs. rising) (Audra 2001; Palmer & Audra 2004; Audra & Palmer 2013). e juvenile pattern: a time-dependent pattern e juvenile pattern prevails when soluble rocks are rst exposed by upli and removal of any impermeable cover. During cave inception, the mixing of saturated ground water with aggressive water forms points of recharge that allows corrosion to take place along a steep water table. Because of sparse fracturing, the water table can be steep and located high above uvial base level (Fig. 6). Initial phreatic paths are later entrenched as the wa ter table drops and the ow becomes vadose. Vadose Fig. 5: Le: dierent types of ghost rocks. e unweathered rock (1) is cut by numerous discontinuities (joints and bedding planes), which are partly used by the ghost-rock karstication. e ghost-rock occurs as weathered masses (2), as pockets and corridors (3), and as pseudo-endokarst with a hard rock ceiling, which pregures the future cave passages (4). Upper right (1a, b, c): â€œnormalâ€ karst evolution by complete removing of rock and branching pattern development. Lower right (2a, b, c): ghost-rock karstication. In the early stage of low-gradient, slow ow produces the residual matrix but cannot export it (A). Aer a base-level drop resulting in a steep hydraulic head, turbulent ow appears and exports the ne particles by headward erosion (B). Gradually, the fracture frame is revealed and a maze appears in lieu of the initial ghost (C). e remote parts, away from the turbulent ow, remain lled with the ghost (B rux elles et al. 2009). Fig. 6: Juvenile pattern: in the rst stage of karstication, when high gradient is present, a cave system develops a steep prole with the greatest hydraulic gradient. RESEARCH FRONTIERS IN SPELEOGENESIS. D OMINANT PROCESSES, HYDROGEOLOGICAL CONDITIONS AND ...
ACTA CARSOLOGICA 44/3â€“ 2015 324 enlargement, as shas, meanders, and canyons, oen obscures the initial phreatic paths, which are sometimes preserved as ceiling channels. e juvenile pattern oen corresponds to the initial phase for most vertical cave passages (shas). ese are common in young, rapidly developing karsts (Fig. 7) such as: in karst subjected to intense dissolution, aer rap id upli and great rainfall: Nakanai Mountains, Papua New Guinea; in evaporite rocks where sinkholes deliver water to through-caves directly connected to resurgences: G broulaz in gypsum, France; caves of the Mount Sedom in halite, Israel (Frumkin 1998); in limestone exposed for the rst time by glacial erosion, which has removed the impermeable cover, Grand Marchet, France. Perched caves and their geological control Where the aquifer is perched above base level on an underlying aquiclude, there is no signicant phreatic cave development (Fig. 8). Shas and canyons converge to form conduits at the aquiclude top and feed springs along hillslopes. Major springs drain into the heads of pocket valleys. Mechanical erosion, aided by detrital sediment, quickly enlarges the main routes by entrench ing the underlying aquiclude, especially if it is so ma terial such as marl. Such mechanical erosion of the im pervious basement is responsible for the development of one the second-largest underground chamber of the planet (Sarawak Chamber in Gunung Mulu). Large gal leries develop by collapse of the limestone ceiling and may ll with boulders. Base-level lowering promotes headward retreat of the spring but does not noticeably aect the cave pattern. Dammed karst and its control by base-level position In "dammed karst" the karst aquifer extends below the spring outlet, which is determined by a uvial or struc tural base level. In turn, the spring position determines the water-table elevation inside the karst. e main drain becomes established at the water table, at the end of the Fig. 7: Examples of juvenile cave patterns, displaying a straight long prole, where a sinkhole feeds through-caves directly con nected to the resurgence. Flow is vadose, and no signicant phre atic zone is present (prole with no vertical exaggeration). 1. Muruk System (Nakanai Mountains, Papua New Guinea). Intense rainfall (> 10 m/y) gener ates surface runo on clay covers, which feeds numerous sinkholes. Huge cave systems develop into recently uplied so Miocene limestone. High-discharge rivers ow through large galleries and canyons (Audra et al. 2011). 2. Gebroulaz Cave ( V anoise, France). Glacial meltwater sinks rapidly into a gypsum body and ows through it in a 350 m-long gallery with a gentle gradient (Audra & Hobla 1996). 3. Grand Marchet Sinkhole ( V anoise, France). A small stream sinks into a marble body. e pas sages develop along metamorphic schistosity and lithologic contacts (Audra & Palmer 2013). P HILIPPE A UDRA & ARTHUR N. P ALMER
ACTA CARSOLOGICA 44/3 â€“ 2015 325 passage with the lowest elevation. Major passages either follow the water table or contain shallow phreatic loops. Looping caves in the epiphreatic zone formed by irregular recharge During high ow, water rises in phreatic li tubes and emerges at overow springs, e.g., Castleguard Cave, Canada (Ford 1983). During low ow the water emerges at springs at lower elevations. (Fig. 9). In the Siebenheng ste Cave System (Switzerland), Huselmann (2002) stud ied the shiing from gravitational downcutting forming canyons to tubular passages formed by closed-conduit ow (Fig. 10, 11). is study allows reconstruction of the vadose-phreatic transition zones corresponding to former water-table positions. He demonstrated that these transi tion elevations decrease toward the springs and record the top of the epiphreatic zone, i.e. the highest position of the water table. Consequently, this study shows that high-amplitude looping passages form in the epiphreatic zone and are enlarged by aggressive high ows. Passage amplitudes can exceed 200 m. Fig. 8: Perched caves: a vadose system develops at the contact of the underlying aquiclude. e mechanical erosion of so under lying material enlarges galleries, sometimes forming huge cham bers. Collapses of the limestone ceiling have partly lled the gal lery with boulders. Fig. 9: Le â€“ An epiphreatic cave: irregular recharge causes backooding; drains develop throughout the epiphreatic zone, with looping proles resulting from the inuence of structural openings. Right â€“ A water-table cave: recharge through a poorly permeable cover is dif fuse and regular, so the water-table level remains stable with time and the drain develops at the water table. Fig. 10: e Hlloch System (Alps, Switzerland) is a maze of active and ancient epiphreatic passages. Such tubes ood over more than 200 m depth during high water (Photo: courtesy U. Widmer). RESEARCH FRONTIERS IN SPELEOGENESIS. D OMINANT PROCESSES, HYDROGEOLOGICAL CONDITIONS AND ...
ACTA CARSOLOGICA 44/3â€“ 2015 326 is observation shows that cave levels eectively record the base-level positions, but the cave-level alti tudes can be noticeably above that of the corresponding base level. In addition, the amplitude of the loops de pends on the vertical amplitude of the epiphreatic zone, and thus on the height and suddenness of ooding. Con sequently, epiphreatic speleogenesis is due to an irregu lar discharge, either from storms or from glacial or snow melt), or as the result of concentrated surface runo into dolines (Audra 1994). Water-table caves: regulated recharge or low head losses in mature through caves When a karst is overlain by a thick semi-permeable cover that acts as a lter, there is little uctuation in recharge. e regulated seepage induces the regularization of the transfer (Fig. 9). In contrast to bare karst, ooding and the development of an epiphreatic zone are very limited. e main drains concentrate at the water table where the water ow is continuous. Cave systems display low-gra dient passages with extensive pools. Similar long proles are characteristic of throughcaves fed by extensive impermeable catchment areas providing coarse sediments and where base level re mains stable over extensive periods. When such caves reach the equilibrium state, their passage size is large enough to allow the transfer of all stages of ow, includ ing seasonal peaks. Such through-caves are frequent on stable platforms such as in Brazil and in monsoonal Fig. 11: Cross section of the southern part of B renschacht, with indications of the speleogenetic phases (aer Huselmann et al. 2003; Huselmann & Granger 2005). Loops with amplitudes as high as 150 m are visible, and mazes are due to ooding and draining pro cesses. e successive lowering of base level produces several speleogenetic levels. Passages are braided because the amplitude of epiphre atic loops is higher than the altitude dierence between each speleogenetic phase. It results in a complex maze connecting active and fossil passages, altogether developing several dozen kilometers of passages. Fig. 12: Saint Paul Cave (Palawan Island, Philippines) is a 24 km-long cave system. e resurgence opens along the shoreline and the cave extends inland several kilometers. e main drain consists of a large water-table conduit in which the inuence of tides is felt as much as 6 km inside the cave (Photo: courtesy E. Procopio). Fig. 13: Prole of Saint Paul Cave (Palawan Island, Philippines). e cave drains a polje at low altitude through a 6 km long passage along the water table and discharges to the sea (Piccini & Iandelli 2011). P HILIPPE A UDRA & ARTHUR N. P ALMER
ACTA CARSOLOGICA 44/3 â€“ 2015 327 southeast Asia (Laos, Vietnam, China, ailand, Phil ippines; Figs. 12, 13). Development of cave levels resulting from base-level lowering In dammed settings the inner organization of karst drainage strongly depends on the base-level position. Any change in base level aects the position of the water table itself and induces a reorganization of the drainage. If uvial base-level drops stepwise, successively lower phreatic passages develop (Fig. 15). Pauses in base-level lowering produce cave levels that correlate with river ter races, e.g., at Mammoth Cave, USA (Granger et al. 2001). Vadose shas and canyons extend the vadose zone down to the new water table. Former conduits and springs are abandoned and partly lled with oodwater sediments and speleothems. Perched in the vadose zone, old phreat ic conduits are cut by new â€œinvasionâ€ shas and canyons that feed active conduits. Most of the large cave systems in the world corre spond to this type of system with integrated cave levels recording successive base level lowering, respectively: Mammoth Cave, USA; Siebenhengeste, Switzerland; Clearwater Cave, Malaysia; Dent de Crolles, France (Fig. 14). Fig. 14: e Dent de Crolles system, France, contains 57 km of passages over almost 700 m of depth, below a surface less than 1.5 km 2 in area. e cave consists of vadose shas and canyons originating from the plateau surface, and which connect to four distinct semihorizontal levels. e three highest levels are perched fossil galleries within the limestone mass; the lowest one, at the contact with the underlying aquiclude, is active. RESEARCH FRONTIERS IN SPELEOGENESIS. D OMINANT PROCESSES, HYDROGEOLOGICAL CONDITIONS AND ...
ACTA CARSOLOGICA 44/3â€“ 2015 328 et al. 1995). ey are now abandoned and resemble vertical shas. Rises in base level cause ooding of conduits. Some become sediment-lled, but the main ow lines remain active (Fig. 15). New ascending routes, or reactivation of relict conduits, may form phreatic lis (chimney-shas) and vauclusian springs such as Fontaine de Vaucluse, France. In Mediterranean karsts, the Messinian Salin ity Crisis induced rst a deepening of the karst systems, then a ooding aer the Pliocene transgression, and nally a reorganization of the drains aer this base level rise (Audra et al. 2004, 2009a). is is the reason why deep phreatic cave systems are so frequent around the Mediterranean basin (Fig. 16, 17). Currently, some cave systems remain ooded and others have been partly or entirely drained aer Pleisto cene re-entrenchment of the valleys. In partly exhumed canyons, the lower part of the karst has remained ooded since the beginning of the Pliocene, and they discharge Base-level rise, vauclusian caves and the Per Ascensum Model of Speleogenesis (PAMS) (Audra et al. 2009a) Per ascensum is used here for cave originating from a deep loops in which meteoric water rises. Such ow can be inuenced by local recharge. It strictly diers from hypogenic speleogenesis, where rising water is not di rectly connected to nearby recharge areas (see 2.1), and where hydrology is generally characterized by its regu larity. e depth that phreatic loops can extend below the water table is a matter of debate (Audra & Palm er 2013; Ford 2014). Some water-lled passages have been dived through their springs to depths of several hundred meters, at the limit of present-day techniques, and yet they still continue downward out of sight (Ex ley 1994). Most are located along major faults. Li tubes up to 100 m in relief have been mapped in caves in Sarawak, Malaysia (Farrant & Smart 2011; Farrant Fig. 15: Inuence of base-level change on cave pattern. Le: cave levels. A base-level drop causes the lowering of the karst drainage. e old drains are abandoned and remain perched. Right: Flooded cave. A base-level rise oods the deep part of the cave system. e main deep passages remain active; the water rises through a phreatic li (chimney-sha) and discharges at a vauclusian spring. Fig. 16: Per ascensum model of speleogenesis (PAMS) during the Messinian-Pliocene cycle around the Mediterranean (Audra et al. 2004, 2009a). Le. During the Messinian, the Mediterranean drying-up caused the entrenchment of canyons (1) and the deepening of karst drainage (2). Center. Pliocene base level rise occurred in two steps â€“ by marine ingress (dark gray 3), then by uvial aggradation (light gray 4). Deep drainage uses phreatic lis to emerge as vauclusian springs, recording successive positions of the base level. If the Messinian canyon is located below the current base level, it remains buried; the karst remains ooded and discharges by a vauclusian spring (Fontaine de V aucluse type). Right. If the Messinian canyon is located above the current base level, the canyon is eventually ex humed and the karst is drained. e current drainage reuses the deep Messinian drain (5); the Pliocene phreatic lis are abandoned as fossil â€œchimney-shasâ€. P HILIPPE A UDRA & ARTHUR N. P ALMER
ACTA CARSOLOGICA 44/3 â€“ 2015 329 Fig. 17: Deep-phreatic cave systems in Mediterranean France. All cave systems are connected to the Mediterranean or to the Pliocene rias (â€œooded valleysâ€). 01 53 04 56 0 75 km 7 6 5 4 3 43 44 NGulfofLionM E D I T E R R A N E A N S E A G u l f l l o f L i o i i n Rhne Va lleyCAMUS, 2003 AUDRA, 2007 Rivers Messinian incision Messinian shoreline Pliocene shoreline Messinian alluvial fans Large karst areasLower Provences e n n e v CCausses Ar dche V aucluse Perpignan Narbonne Montpellier Nmes Al s A vignon Marseille T oulon Nice Mont limar Deep phreatic karst systems (relative depth reached by scuba diving) -75/-100m -100/-150m > -150m Port Miou Fontaine de Va ucluse Lez Goul de la T annerie Font Estramar 163 -248 223 220 308 130 Goul du Pont -240 200 Mas de Banal Fig. 18: Per ascensum model of speleogenesis, caused by eustatism, valley lling, and tectonic, respectively. Le: Podtraov jeskyn, Moravian karst, Czech Republic, a 140-m high chimney-sha, the lowest part of which is ooded below the B eroukna valley (B ruthans & Zeman 2003). It may show a record of the base-level rise of the hydrologic network aer pre-B adenian entrenchment. Center: e Puits des B ans and the Gillardes Spring, French Alps. e basin ll (glacial, lacustrine, and uvio-glacial) has blocked the Gillardes Spring. In high water, the Puits des B ans, a 300m-high chimney-sha, oods and overows. Right: Lagoa Misteriosa, B razil, a 200-m deep ascending sha, a window in a karst aquifer ooded aer continental subsidence of the Pantanal region (survey by G. Menezes). A hydrothermal feeder also points at least partly toward a hypogenic origin. RESEARCH FRONTIERS IN SPELEOGENESIS. D OMINANT PROCESSES, HYDROGEOLOGICAL CONDITIONS AND ...
ACTA CARSOLOGICA 44/3â€“ 2015 330 as vauclusian springs (Fontaine de Vaucluse type) (Fig. 16, center). In the entirely exhumed canyons, the karst is now drained and the chimney-shas are fossil (Fig. 16, right). Other causes of base-level rise (marine transgres sion, uvial aggradation, continental subsidence), that are less signicant in amplitude have the same eect on per ascensum speleogenesis (Fig. 18). e deep karst is ooded and phreatic lis connect to vauclusian springs. Consequently, there should be a global genetic mod el for most deep-phreatic systems. Some of them have a hypogenic origin (part 2 of this paper). However, most of them could correspond to a base-level rise inducing the per ascensum speleogenesis, which rst ooded the karst and then allowed the development of phreatic lis, â€œchimney-shasâ€, and of vauclusian springs. HYPOGENIC SPELEOGENESIS A constantly increasing number of caves are being clas sied as hypogene caves since their clear distinction oc curred only a few years ago (Klimchouk 2007, 2009). HYPOGENIC SPELEOGENESIS DEFINITION e denition of hypogenic speleogenesis has been strongly debated during the recent decade (for detailed discussion, see Dublyansky 2014, and references therein). Actually, it was dicult to consolidate under a unique category caves as dierent as thermal, sulfuric, cold me teoric, mineralized with ore deposits, breccia pipes and sagging structures, huge horizontal or 3D mazes, giant ascending shas, etc . Successive attempts focused on geochemistry, stressing the deep-seated origin of aggres sivity (Palmer 1991, 2000), or on hydrogeology, pointing out the rising direction of upwelling (Klimchouk 1992, 2007), and on geomorphology, trying to classify caves patterns according to ow type and structural setting (Audra et al. 2009). e greatest support for hypogenic speleogenesis is given by Klimchouk (2007, and earlier papers) as the for mation of solutionally-enlarged permeability structures by the upwelling groundwater ow, independent of recharge from the overlying or immediately adjacent surface . e upwelling is focused along main regional fault lines al lowing cross-formational ow. As a consequence the following trends are generally observed for hypogenic speleogenesis: From a geochemical point of view, caves are gen erated by upwelling water in which the aggressiveness has been produced at depth beneath the surface, independent of surface or soil CO 2 or other near-surface acid sources (Palmer 2000). Such aggressiveness involves deep-seated sources of CO 2 , of suldes, and/or thermal processes (see below). Accordingly, water chemistry oen shows signif icant higher concentrations in suldes, sulfates, or dis solved carbon dioxide, together with metals and trace el ements. Due to consumption of oxygen at shallow depth below the recharge areas, deep upwelling waters are gen erally depleted in oxygen, i.e., reducing, allowing trans port of metallic species which otherwise are not mobile and precipitate in oxidized conditions. Incidentally, this is the genesis of â€œhydrothermalâ€ ores, which represent in karst environment some of the largest concentrations such as MVT deposits. Deep water contribution is the most frequent case. However some variants involve cold meteoric freshwa ter, such as artesian waters rising across evaporites where aggressiveness is not produced at depth and do no rely on any acids (Klimchouk 2013). Water that rises rapidly from considerable depth can remain relatively warm compared to the local sur face groundwater, within a range of a couple of degrees above the average temperature up to sometimes almost 100 C. Again, if thermalism is frequent or at least visible with slight temperature anomalies, some cases cases in volve normalf cold temperature, such as Frasassi Caves (Galdenzi & Menichetti 1995). Due to the considerable distance between recharge and discharge areas oen reaching dozen of kilometers or more, groundwater ow is driven by regional hydrau lic gradients, rather than by local shallow gradients, with permeability mostly provided by porosity and ssures, rather than conduits such as those in epigenic karst. As a consequence, outows are generally not, or only weakly, inuenced by rain events or seasonal climatic cycles, dis playing regular ow rates with low velocity. Mineraliza tion is much more stable, and no solid load is observed. Deep upwelling water is rarely of a pure deep ori gin. Cross-formational rising ow tends to mix with suc cessive types of water encountered during the upwelling along faults, each dened aquifer having its specic char acteristic. e uppermost aquifers are inuenced more by meteoric recharge, up to surcial aquifers of â€œpureâ€ meteoric fresh water. Accordingly, deep water mixes with P HILIPPE A UDRA & ARTHUR N. P ALMER
ACTA CARSOLOGICA 44/3 â€“ 2015 331 various amounts of shallower waters, which are less min eralized, colder, and more oxygenated. Solid load from typical epigenic karst aquifer can also be incorporated at shallower depth. Between â€œpureâ€ deep and shallow water end-members, all intermediate types of mixing can oc cur, and the outow may be more or less inuenced by the surface following recharge events, with decreases in temperature and mineralization, discharge, turbidity and bacteriological peaks, whereas long recessions allow the return of low discharge of less inuenced deep water. Due to the typically long distances of their rising water, hypogenic caves tend to have no relation to karst features formed by surcial processes. ey can, how ever, help to form surface features by collapse into hy pogenic voids. Hypogenic caves are oen located where typical surcial karst features are absent, in low cave den sity areas, simply because of the speleogenesis having a deep-seated origin that focuses toward the surface along discrete fractures. Except for their deep-rooted source, however, their relationship with the surface is strongly dependent to topography, since discharge areas tend to be located in topographic lows (e.g., valleys) where groundwater ow converges and wells up. Accordingly, these convergences produce a positive feedback that may partly explain the presence and development of pied mont and plain valleys. And since discharge areas, where hypogenic speleogenesis is focused, are commonly locat ed in areas of low topography, hypogenic caves oen rep resent excellent proxies for base-level records and highly valued milestones for the reconstruction of landscape evolution over long time spans. Hypogenic speleogenesis, i.e. mainly due to deep water upwelling, must not be confused with meteoric deep phreatic loops, such as vauclusian systems. Epigen ic cave systems may have deep phreatic loops, reaching sometimes several hundreds of meters. Vaucluse spring, in France, reaches at least 300 m deep (see â€œbase level riseâ€ in this paper). In such vauclusian epigenic systems, water is simply owing along deep loops before discharg ing. Consequently, the speleogenetic processes are typi cally epigenic. Springs are highly inuenced by irregular recharge with a large range of discharges and unstable mineralization. HYPOGENIC SOLUTIONAL PROCESSES Many solutional processes from upwelling hypogenic ow account for speleogenesis (Klimchouk 2007, 2013a; Palmer 2000, 2013). Among the following, one process is dominant but generally combines with one or more sec ondary processes. Dissolution of evaporites, in fact simple dissocia tion, which occurs when meteoric undersaturated water enters from below. Such a process accounts for the giant maze caves in gypsum in Ukraine and elsewhere (Klim chouk 2000a; Vigna et al. 2010). Carbonic acid solution due to deep-seated sources of CO 2 such as volcanism, mantle, and metamorphism (Gary & Sharp 2009). Increasing solubility of calcite along ascend ing owpaths due to water cooling, which is frequently called â€œhydrothermal speleogenesisâ€. Sulfuric acid solution. Deep sulfate beds are rst converted to H 2 S in presence of hydrocarbons (petro leum, methane) or organic carbon from marls. Deep H 2 S-rich upwelling water mixes with shallow meteoric oxygenated water, producing sulfuric acid. is process commonly called â€œsulfuric acid speleogenesisâ€ (SAS), oc curs at shallow depth in the phreatic zone but especially above the water table due to H 2 S degassing in cave at mosphere (see below). It accounts for some of the largest hypogenic caves in the world, such as Lechuguilla Cave and Carlsbad Cavern in New-Mexico (Hill 1987; Palmer & Palmer 2000a; Polyak et al. 1998). Hydrosulfuric acid solution. is process may occur at depth around reduction zones. However, since simultaneous oxidation of hydrocarbons leads calcite to supersaturation and H 2 S is a weak acid, such a process remains marginal (Palmer 2013). - â€œMixing corrosionâ€ of waters of contrasting chem istry. Such a process may occur along upwelling ow paths when given water mixes with the encountered water body, including shallow meteoric waters. Mixing corrosion is probably the most active process a depth, when dierent solutions are converging. At depth, mix ing of waters of contrasting H 2 S content can produce un dersaturated solutions (Palmer 1991). is process must take place in a closed system to prevent H 2 S from oxidiz ing, as well as loss of any CO 2 generated by the dissolu tion reaction. Common-ion eect with mixing of sulfates and carbonates producing dedolomitization. Other process may occur at depth involving or ganic acids at high temperature conditions, and silica solution in alkaline, high temperature and high pressure conditions. S PECIAL FOCUS ON SULFURIC ACID SPELEOGENESIS SAS AND CONDENSATION CORROSION e voids in SAS caves are mostly formed above the water table by waters which dissolving capacity is enhanced by abiotic and/or biotic oxidation of H 2 S deriving from a deep source (Galdenzi & Maruoka 2003). H 2 S can be generated by volcanic activity, reduction of sulphates such as gypsum or anhydrite, in the presence of hydrocarbons (petroleum, methane) or organic carbon from marls, and is brought to RESEARCH FRONTIERS IN SPELEOGENESIS. D OMINANT PROCESSES, HYDROGEOLOGICAL CONDITIONS AND ...
ACTA CARSOLOGICA 44/3â€“ 2015 332 the surface through deep tectonic structures. e origin of the sulphur can usually be ascertained using its stable iso tope signature and that of its possible sources (Onac et al. 2011). e oxidation of H 2 S produces sulphuric acid that reacts instantaneously with the carbonate host rock pro ducing replacement gypsum and carbon dioxide. CO 2 can dissolve in water again and increase its aggressiveness even more. Also the local oxidation of sulphides, such as pyrite, oen present in carbonate sequences, can generate sulphu ric acid, boosting rock dissolution (Tisato et al. 2012; De Waele et al. 2016). Sulphuric acid also reacts with other minerals such as clays and can cause the formation of a typical suite of sulphates including jarosite, alunite, halloysite, basa luminite, etc. Some of these minerals, especially those containing potassium such as alunite and jarosite, can be dated with radiogenic methods (Polyak et al. 1998). Also gypsum can be dated using the U/ method (San na et al. 2012; Piccini et al. 2015). e ages of minero genesis exactly corresponds to the most recent phase of cave formation, when the SAS process was active. us SAS byproducts oer a unique opportunity to date the speleogenesis phases, on the contrary to other classical methods where dating is made on the cave lling, which postdates the cave itself. Sulphuric acid caves are thus oen intimately re lated to the contact zone between the water level, from which H 2 S rises, and the air. Enlargement of the voids mainly happens by condensation-corrosion processes in a highly acidic environment (Audra et al. 2007). Con densation is greatly enhanced in the presence of thermal dierences between the upwelling waters and the cave walls and atmosphere, even in low thermal environment where the temperature contrast reaches only a few de grees (Gzquez et al. 2015; Sarbu & Lascu 1997). Disso lution of carbonate rock in these conditions is extremely fast compared to normal epigenic caves and can cause the formation of sizeable cavities in probably only a few thousands of years. HYDROGEOLOGICAL SETTING e principle of hypogenic speleogenesis relies on re charge at distance or at depth, slow transfer at depth, and focused discharge in topographic lows, typically along faults allowing fast and easier cross-formational upwelling. Flow transfer in the upper portion of basins is described by the â€œRegional Gravity Flowâ€ concept (Tth 1970, 2009), adapted to hypogene speleogenesis by Klimchouk (2013b, c). Topographic highs act as re charge areas, whereas topographic lows act as discharge areas (Figs. 19, 22). At shallow depth, local loops of meteoric water are dominant. For deep aquifers with in gravity-driven meteoric ow, most of the marginal recharge is discharged in marginal areas, whereas the internal area of the basin displays mainly vertical ow, with very low contribution of lateral ow from basin margins (Fig. 19). In conned aquifer systems, lateral ow is dominant in more pervious units, whereas verti cal ow is dominant in less pervious units. Klimchouk (2013b, c) distinguished several hydrogeological settings with dierent regularities of localization of upwelling ows and hypogenic speleogenesis. ese include large cratonic artesian basins (divided in marginal areas, in ternal areas, and deep zones), coastal aquifer systems in large carbonate platforms, fold/thrust regions, young disrupted intramontain basins, and areas of young vol canism. We follow his hydrogeological typology in the next paragraphs. Marginal basin areas In marginal areas of the sedimentary basins, meteoric water descending from recharge areas mixes with deep basinal water and wells up along topographic lows (typ ically river valleys), forming a belt of hypogenic spe leogenesis with large sinkholes corresponding to deep breccia pipes and sagging zones (Fig. 20). Numerous examples are present in USA, for example the Roswell basin feeding the Pecos River (Land 2003), and the Prairie aquifer in the Western Canadian Basin (Wright 1984). Most of the German caves â€“excluding epigenic alpine cavesin Iberg-Harz, Swabian Alb and Franco nian Alb seem to be related to a similar hydrogeological context (Kempe 2014a, b). In the NW European coal basin stretching between Northern France and Belgian Hainault, such breccia pipes were rst described in XIX th century and called â€œgeological orguesâ€ (organs), â€œDives shasâ€ (Renault 1970) or â€œcransâ€ ( Q uinif 1994). ey are buried under upper cretaceous strata, some Fig. 19: Regional hydrodynamics of a basin showing decrease in communication and a progressive shi from lateral to ascending ow with depth. B oxes distinguish marginal areas (A), internal areas (B) and deep zones (C). Hypogenic speleogenesis occurs in zones of convergence, where mixing occurs (Klimchouk 2013c, aer Vsevolozhskiy 2007). P HILIPPE A UDRA & ARTHUR N. P ALMER
ACTA CARSOLOGICA 44/3 â€“ 2015 333 reaching a depth of 1200 m, and contain dinosaurs. In the Crimean Piedmont deep upow toward sedimen tary cover is focused along a suture zone (Fig. 21). Internal basin areas In the upper hydrogeological storeys of internal areas of the basins, the layered aquifer/aquitard system dis plays topography-controlled ow pattern (Klimchouk 2013c). In aquitards, vertical ow is predominant with downward cross-formation communication below topo graphic highs and upward communication below valleys (Fig. 22). Large valleys can induce upward ow with ver tical relief up to 1.5 km. In aquifers, lateral ow is directed from topographic highs to lows. Because of relief undu lations, areas of recharge with downward ow alternate with discharge areas with upward ow. In Podolia plain, Ukraine, meteoric water owing in a limestone aquifer ows upward through gypsum strata making the largest gypsum maze caves in the world. Basin deep zone In deep basinal zones, ow is predominantly vertical along blocks and faults where deep uids from the base ment and upper mantle are rising and mix with upper stratiform aquifers (Klimchouk 2013b). Flow and pres sure are highly irregular. Physical-chemical parameters evolve along upwelling owpaths causing selective solu tion and precipitation. ermal and geochemical anoma lies thus develop in the upper storeys (Fig. 23). Coastal basins Coastal basins represent a variant where sea water plays the part of the conned basin internal area. Upward ow in the coastal and submarine discharge zone can be driven by both Fig. 20: Cave development along the margin of low-permeability igneous-metamorphic rocks, B lack Hills, South Dakota (Palmer & Palmer 2009). A = direct recharge from the surface; B = deep groundwater ow through Precambrian rocks; C = inltration through thin sandstone; D = recharge from deep within the aqui fer (questionable); E = discharge to springs through conning beds. ere is evidence that inltration through the sandstone (C) achieves low PCO 2 by closed-system dissolution at the top of the limestone and regains aggressiveness where it mixes with deeper water from A and perhaps B , e.g., at Wind and Jewel Caves. Fig. 21. In the Crimean Mountains, deep ow upwelling along faults of the suture zone mixes with lateral ow in stratied aquifers of the B lack Sea basin margin, where hypogenic karstication takes place (Klimchouk 2013c). Fig. 22: Alternation of recharge areas with downward ow and discharge areas with upward ow fed by convergence of lat eral ow in aquifers and cross formational upward ow across aquitards. Hypogenic speleogenesis focuses in convergence zones where undersaturated water enters the aquifer from below (Klim chouk 2013b). RESEARCH FRONTIERS IN SPELEOGENESIS. D OMINANT PROCESSES, HYDROGEOLOGICAL CONDITIONS AND ...
ACTA CARSOLOGICA 44/3â€“ 2015 334 topography-induced head and density gradients where freshwater aquifers extend below saline water aquifers and the seawater, plus geothermal heating of seawater en tering the platform at deep levels (Fig. 24). In Florida and Y ucatn, thermal and chemical anomalies are observed in the aquifer, together with â€œcenotesâ€ revealing solution down to considerable depth (> 200 m), rising springs with deep components, and huge seaoor â€œdolinesâ€ occurring oshore at depths up to 600 m (Cunningham & Walker 2009; omas 2010; Klimchouk 2013c). Deformed strata In deformed strata, as in marginal areas of the sedimenta ry basins, meteoric water descending from recharge areas Fig. 23: In deep zones, ow partly originating from the base ment or mantle is mainly vertical along faults, producing strong physical-chemical contrasts where dissolution occurs (Klimchouk 2013b) Fig. 24: In coastal basins conned by seawater, such as Florida, topographic head, density gradients and thermal heating pro duce upow and dissolution in convergence and mixing zones, revealed by cenotes and o-shore depressions (Klimchouk 2013c, aer Krause & Rundolf 1989 and Spechler 1994) Fig. 25: Upward ow of groundwater from a deep sandstone aquifer into overlying carbonate rocks along an anticline, where many s sure caves developed, eastern Missouri, USA (B rod 1964). Fig. 26: e B ighorn River cut through the Sheep Mountain anti cline allowing upow of a deep aquifer discharging sulfuric water at Kane Caves, Wyoming, USA (Palmer & Palmer 2009, aer Egemeir 1981). P HILIPPE A UDRA & ARTHUR N. P ALMER
ACTA CARSOLOGICA 44/3 â€“ 2015 335 Fig. 27: e thermal aquifer now discharging at 80 C at the foot of the Azrou anticline, Algeria, formerly discharged through caves now perched along the faulted anticline aer valley incision (Collignon 1990). mixes with deep basin water and wells up along faults or highs, allowing groundwater to discharge at topographic lows. ese topographic lows lows are valleys cutting through the geological structures, where valleys act as windows in the aquifer. Many valleys are incised by the in ux of deep aquifer water, which has risen along brecciated zones (Camus & Bruxelles 2013). Upow uses anticlines such as the classical examples of Eastern Missouri, USA, recharged from underlying sandstone strata (Fig. 25) and Kane Caves in Wyoming, USA (Fig. 26). Faults that inter sect anticlines provide more ecient routes for upward ow, as in Azrou Mountain, Algeria, where old perched fossil caves reect the current sulfuric outow at 80 C Fig. 28: e Aix-lesB ains thermal springs and caves discharge water from a deep Jurassic synclinal upowing along the overthrusted anticline where it acquires it sulfuric component at the contact of Triassic evaporites before mixing with shallow cold meteoric water (Geology aer Gallino 2007, in Hobla et al. 2010). Fig. 29: Hypogenic speleogenesis (le star) focused above the overthrust allowing deep upow, mainly from Jurassic aquifer. Current thermal spring ows at Groux-les-B ains (right star) (Geology aer Guyonnet-B enaize 2011). RESEARCH FRONTIERS IN SPELEOGENESIS. D OMINANT PROCESSES, HYDROGEOLOGICAL CONDITIONS AND ...
ACTA CARSOLOGICA 44/3â€“ 2015 336 (Fig. 27). Similarly, thermal springs are due to upow across overthrusting anticline in French Northern Alps (Fig. 28) or along overthrust in French Provence (Fig. 29). An interesting analogue is shown by geophysical interpre tation of the deep oshore Niger Delta, Nigeria, where hy drocarbons migrate along faulted anticlines making belts with depressions in the sea oor (Fig. 30). Such settings could include the early phases of some hypogenic karst connected to hydrocarbon basins, such as the Guadalupe Mountains above the Delaware basin. Disrupted basins In disrupted basins, there is a strong gradient (topo graphic, hydraulic, and thermal) between displaced blocks. In the example from Budapest, Hungary, upper blocks act as recharge areas (Fig. 31). e cold meteoric water feeds both a shallow meteoric loop and deeper loops toward the basin depths. e high geothermal gradient warms up the deep water that fast rises along extensional faults. is geothermal gradient acting on water density seems to be responsible not only for the hydrothermal upow but also for the meteoric water downow by a â€œpumping eectâ€, i.e. with the replace ment of upowing hot water by downowing cold me teoric water. Possibly without such a thermal pumping eect at the origin of the deep loops, the meteoric wa ter would have entirely focused along the water table and would have discharged at the foot of the hill as Fig. 30: e deep oshore Niger Delta, Nigeria, displays buried faulted anticlines where hydro carbons are upowing, making depressions at the sea bottom (Nosike 2009). Fig. 31: Model of the B uda ther mal karst, Hungary. Deep hot and mineralized water ows up from basin depth along normal faults feeding warm springs along the Danube, whereas mixing with the shallower meteoric com ponent feeds lukewarm and less mineralized springs at the contact of the B uda hills with the devel opment of active cave networks. Upper dry cave levels record the past position of the Danube base level (Erss 2010). P HILIPPE A UDRA & ARTHUR N. P ALMER
ACTA CARSOLOGICA 44/3 â€“ 2015 337 â€œnormalâ€ epigenic karst springs without a signicant deep component. Since the shallow cold meteoric loop is restricted below the uplied blocks, dierent kinds of thermal springs occur: direct thermal outow along basin faults makes hot thermal spots along the Danube River, whereas faults between the basin and the upli ed blocks allow mixing of the hot deep upow and the cold meteoric downow, resulting in lukewarm springs and cave development at the foot of the hill. e fossil perched cave tiers record the position of the base level and the gradual incision of the Danube in the Pannon ian plain (Szanyi et al. 2012). Volcanic and magmatic intrusions areas Volcanic and magmatic intrusions are a source of deep acids and heat. e geothermal gradient acts as a driv ing force for deep upows that can fast rise to the surface using tectonic disruptions and attracts complementary downward recharge from remote areas. e presence of concentrated acids (from CO 2 , H 2 S) boosts the dissolu tion. Microbial activity using sulfo-oxidant pathways strongly participates in the rock solution. is combi nation of geothermal gradient, deep-seated acids, major fractures, and chemoautotrophic microbial activity is so intense that it can be considered â€œhyperkarstâ€ (Audra et al. 2009b, c). e main illustrative features are giant ascending shas, which are among the deepest of the world, with known depth reaching 500 m. Such hypogen ic origin accounts for the genesis of numerous oversized karst features, evolving from solution-enlarged fractures, eventually giving collapse shas, and collapse shas over large chambers developed at depth: Examples include Pozzo del Merro, near the Latium volcanic eld, and the famous Tivoli travertines (Cara mana 2002); Hranica Propast, Czech Rep., is a phreatic sha deeper than 450 m which owes its origin to active deep-rooted faults of the European plate releasing deep CO 2 (Gerl et al. 2011). El Zacaton is a large collapse sha in Tamaulipas, Mexico, derived from solution along fractures (Gary 2010) (Fig. 32). Collapse shas above large chambers at depth in the Northern slope of the Caucasus Mountains around Elbruz volcanic area (Klimchouk 2013b); huge collapse dolines called â€œObruksâ€ in Taurus, Turkey (Bayari et al. 2009). Fig. 32: Hypogenic speleogenesis of the Sistema Zacatn karst area. Groundwater owpaths to Sistema Zacatn show enrichment of CO 2 and H 2 S from the Pleistocene volcanic complex to the east. e main recharge area is located to the west in the Sierra de Tamau lipas. A fracture zone focused initial owpaths. Heat source from volcanic activity drives convection in the groundwater system (Gary 2010). e combination of acids (H 2 S, CO 2 ), of thermalism, and of microbial activity is boosting dissolution along fractures acting as upow paths. Giant ascending shas are made by enlargement of fractures and by upward stoping of large chamber at depth that even tually open to the surface. is kind of â€œhyperkarstâ€ generates the deepest phreatic shas of the Planet. Fig. 33: Conceptual model of most typical hypogenic cave types, according to geological structure and type of ow (Audra 2007, 2009b, c). RESEARCH FRONTIERS IN SPELEOGENESIS. D OMINANT PROCESSES, HYDROGEOLOGICAL CONDITIONS AND ...
ACTA CARSOLOGICA 44/3â€“ 2015 338 TYPES OF HYPOGENIC CAVES A synthesis based on eld experience and literature takes into account geological structure, hydrology, morphol ogy of caves at dierent scales (wall features, passage morphology, and cave pattern), mineralogy, deposits, etc. e geologic framwork and speleogenetic processes can be combined into a conceptual model of cave pat tern, integrating most typical hypogenic caves (Fig. 33) (Audra 2007, 2009b, c). Patterns are subdivided into two main types: deep phreatic systems generally developed in conned aquifers by transverse speleogenesis (sensu Klimchouk 2000b), and cave systems developed above the water table, where condensation-corrosion plays a paramount role. Hypogenic cave pattern in phreatic conditions Isolated geodes In deep zones, vertical upow and mixing of deep u ids of dierent types of water allows complex dissolution and deposition processes. Dissolution voids are poorly integrated and some are huge. Large crystals (calcite, gypsum, etc.) are deposited in slightly saturated water, together with diverse minerals (such as metallic suldes) (Fig. 34). 3D multistorey maze caves e rising hypogenic ow alternately follows joints and bedding planes, producing a 3D maze cave, in a stair case pattern. Generally, the cave displays a main trunk where hypogenic ow was rising, surrounded by 3D mazes, smaller in size (Fig. 35). Monte Cucco Cave sys tem, Italy, with more than 900 m depth is the deepest known cave of this type in the world. e sulfuric water was rising toward the top of the anticline, where imper vious covers are breached, allowing the discharge of the karst aquifer. Some cave entrances record past base level positions (Galdenzi & Menichetti 1995). In the Black Hills, South Dakota, USA, Jewel and Wind Caves rank among the largest maze caves of the world (Fig. 36). eir genesis is complex, involving sev eral early phases (Palmer & Palmer 2000b). e pattern Fig. 34: Geode lined with calcite spar. Cave in Ardche, France (Photo: P. Deconinck). Fig. 35: Pigette Cave, Groux-les-B ains, France, is a 3D maze de veloped in the shallow phreatic zone. Rising ow enlarged alter natively bedding planes and vertical joints, producing a staircaselike pattern. Fig. 36: Wind Cave is a 230 km long 3D maze cave developed along fractures in inclined strata (Map courtesy: R. Horrocks). P HILIPPE A UDRA & ARTHUR N. P ALMER
ACTA CARSOLOGICA 44/3 â€“ 2015 339 resulting from the main speleogenetic phase is a dense network of enlarged discontinuities, similar to the pre vious examples. However, an origin by rising water is strongly debated. e limited stratigraphic range of the caves, and also their location almost entirely beneath thin overlying sandstone strata, argue for an origin by mixing between groundwater from the limestone out crops and diuse inltration from above. (Palmer et al. 2016). Caves are concentrated along zones of Carbonif erous paleokarst and early diagenetic alteration. 2D maze caves If a signicant permeability dierence exists between two strata, and if no signicant fracture is present, a 2D maze cave can develop in the more permeable bed and below the less impermeable ceiling (Fig. 37). e 2D maze cave is a subtype of 3D maze cave; some parts of 3D mazes lo cally develop as 2D mazes, when a less permeable stratum is present on the ceiling. e passages are horizontal or inclined, according to the dip. e Denis Parisis system in the central part of Paris basin is horizontal. In Monte Fig. 37: e cave of Saint-Sbastien (Groux-les-B ains, Alpes-de-Haute-Provence) is a 2D maze. It is an inclined planar maze conned below a marly ceiling. Dip is oriented toward the SE, the thermal water rose toward the NW (toward le on the sketch). Fig. 38: Giant ascending shas, the deepest such features in the world, are the result of â€œhyperkarstâ€ processes around volcanic or active deep-rooted faults combining thermalism, degassing, and microbial activity. Pozzo del Merro, Italy, is more than 450 m deep including the collapse doline (Caramana 2002); Zacaton sha, Mexico, survey using a 3D scanner (Gary 2000); Hranica propast, Czech Rep., fo cuses thermalism and C0 2 degassing from deep-rooted fault (http://www.boinc.sk). Pozzo del Merro and Hranica propast are enlarged fractures, whereas Zacaton eventually evolved as a collapse sha. RESEARCH FRONTIERS IN SPELEOGENESIS. D OMINANT PROCESSES, HYDROGEOLOGICAL CONDITIONS AND ...
ACTA CARSOLOGICA 44/3â€“ 2015 340 10 m Cucco, the Faggeto Tondo develops below the inclined marly cover. Giant ascending shas In active tectonic areas, the combination of rising warm water, CO 2 and H 2 S outgassing, and microbial activity makes â€œhyperkarstâ€ along major fault lines, producing the deepest ascending shafts of the world (Fig. 38). Hypogenic cave pattern along or above the water table At shallow depth along the water table, degassing, sulde oxidation and mixing with oxygenated meteoric water produces strong dissolution. is corrosive process also propagates through the cave atmosphere above the water Fig. 40. e Champignons Cave, Provence, France, is an isolated chamber. ermal hypogenic ow degassed CO 2 at shallow depth. ermal convection enhanced condensation-corrosion to develop a large isolated chamber more than 50 m wide, which tends toward a hemispherical shape. Simultaneously, massive calcite deposits occurred in the lake, which was supersaturated with calcite because of CO 2 degassing (Audra et al. 2002). Fig. 41: A water-table sulfuric acid cave. Headward evolution by condensation-corrosion along the water table, supplied by major sulfu ric upwelling along a fracture. Simultaneously, hydrothermal conditions li the hot air so that condensation-corrosion occurs, bells and chimneys develop, and some nally break through to the surface. e white arrows indicate the direction of cave development; inspired by V illa Luz Cave, Mexico (Audra et al. 2009b, c). Fig. 39: Stork-puszta, Hungary. e â€œmodelâ€ of hydrothermal cave, stacked spheres made by convective processes of sulfuric condensation-corrosion (Ford & Williams 2007). P HILIPPE A UDRA & ARTHUR N. P ALMER
ACTA CARSOLOGICA 44/3 â€“ 2015 341 eling has shown that such volume can develop in a rather short time span, about 10 000 years (Lismonde 2003). From examples in Israel, Frumkin and Fischhendler (2005) assign the origin of isolated chambers to phreatic convection. Water table sulfuric caves Above the water table, sulfuric vapors and thermal convec tions produce strong condensation-corrosion and replace ment gypsum crusts (Egemeier 1981). e main drain develops headwards from springs (Fig. 41). Due to the sulfuric corrosion the long prole displays a tiny gradient (Fig. 42). Minor changes in base level cause the ow to mi grate laterally forming incipient mazes (Audra 2007, 2009b, c). Condensation domes develop upward and may breach the surface (Fig. 41). e most demonstrative water table sulfuric cave are Cueva de Villa Luz (Hose and Pisarowicz 1999; Hose et al. 2000), Mexico; Chat Cave, France (De Waele et al. 2016); Kane Caves, Wyoming, USA (Egemeier 1981; Engel et al. 2004), Acqua Fitusa, Italy (De Waele et al. table by thermal air convection carrying aggressive drop lets. Upwardly dendritic caves CO 2 and H 2 S degassing enhance aggressivity. By conden sation-corrosion, cupolas develop upward as a dendritic pattern of stacked spheres (Audra et al. 2007). e devel opment of two neighboring spheres will be divergent, to ward the greatest potential heat transfer, because the rock in between the two spheres has less transfer potential and remains warm (Szunyogh 1990), giving the bush-like structure, as found in the Stork-puszta Cave, Hungary (Fig. 39). Above thermal water, condensation occurs at the ceiling which is cooler. Isolated chambers When strong degassing occurs, upwardly dendritic spheres enlarge and merge, eventually producing large isolated chambers (Fig. 40) (Audra et al. 2002). Taking into account moderate thermal gradient and pCO 2 , mod Fig. 42: Grotte du Chat, France. e longitudinal prole (top) shows a very low gradient (< 0.65%) made by sulfuric ow along the water table. A dead-end closes the passage upstream to the last sulfuric slot. e plan view (below) shows mazes, developed successively aer lateral sliding of the ow toward the west, following very small base-level drops. e largest chambers in the center developed by integration of the neighboring mazes around the main sulfuric slots (Audra et al. 2009b, c; De Waele et al. 2016). RESEARCH FRONTIERS IN SPELEOGENESIS. D OMINANT PROCESSES, HYDROGEOLOGICAL CONDITIONS AND ...
ACTA CARSOLOGICA 44/3â€“ 2015 342 C ONCLUSION e understanding of speleogenesis has increased con siderably in the past few decades, and this has helped to explain the pattern of caves, in both epigenic and hypo genic context. Regarding epigenic caves, the vertical pat tern of cave levels may be related not only to successive base level lowering, but also to base level rises or alterna tions of both. e Per Ascensum Model of Speleogenesis (PAMS) explains many deep phreatic systems discharg ing through vauclusian springs, not only around the Mediterranean aected by the Messinian Salinity Crisis, but also in any place where base level rise le its im print, especially in coastal areas which have undergone Fig. 43: V apor sha, Spain, is a smoking sha made by warm rising air producing condensation-corrosion along a fracture while cooling. e cave air is warmed by the thermal aquifer lo cated below (survey Cuatro Picos, Cartagena). 2016), Kraushhle and Bad Deutsch Altenburg Caves in Austria (Plan et al. 2012; De Waele et al. 2016). In case of major base level lowering, successive horizontal cave levels develop: Frasassi Cave, Italy (Galdenzi & Menichetti 1995; Galdenzi & Maruoka 2003). Smoking shas in the vadose zone Above thermal aquifers, the rock is signicantly heated by the geothermal gradient. In winter the atmosphere of open shas is unstable: the cold air sinks inside the sha and expels outside the warm air of the sha mak ing it condense, giving the impression that the sha is smoking. e rising warm air ow follows ceiling chan nels where condensation-corrosion focuses while cool ing. Eventually, it produces condensation ceiling cupolas and channels, which could leading to their misinterpre tation as phreatic in origin (Vapeur Sha, France; Nasser Schacht, Austria; Fumarollas and Vapor Shas, Spain). ese shas are generally guided by mechanical fractures (Fig. 43); the hypogenic role through thermal gradient and air convection is indirect and limited to the etching of the wall features. e diversity of hypogenic caves is now placed in a global model, explaining main types of patterns, de pending on the geological structure, the groundwater recharge, and the speleogenetic processes. In addition to hypogenic caves developed at depth by mixing corrosion and rising ow, some hypogenic caves are developing in the atmosphere at -or abovethe water table, mainly by condensation-corrosion, and also corrosion by sulfuric and carbonic acids. P HILIPPE A UDRA & ARTHUR N. P ALMER
ACTA CARSOLOGICA 44/3 â€“ 2015 343 regressions and transgressions, as well as glaciated areas, and subsident regions. Irregular recharge rates cause ooding in the epiphreatic zone, which results in looping caves, where the top of the loops indicates the maximal relief of the epiphreatic zone, which can be signicantly above the base level. Water table caves are generally lim ited to places where a semi-impervious cover can regu late discharge and limit ooding, or in stable areas where evolution toward equilibrium allows the regularization of the long prole, especially by paragenetic processes re sulting from sediment inux. e juvenile pattern oers a view of the initial stage of cave development. Ghost-rock karstication must be investigated more thoroughly, since it is probably a widespread process pro ducing maze patterns, especially in low relief areas such as plains and plateaus. e origin of ghost weathering is still under debate. Originally considered epigenic, it now appears to be more frequently hypogenic. Regarding hypogenic speleogenesis, the concept of Regional Gravity Flow oers a framework explaining the location and the conditions of uprising ows that produce discrete caves in discharge areas. Cave patterns display 3D networks developed in the phreatic zone. e giant ascending shas, which result from â€œhyperkarstâ€ process combining deep-seated acids (CO 2 + H 2 S), ther malism and microbial activity, are generally formed in active regions of volcanism or deep-rooted faults. Cave development along the water table points toward pro cesses involving Sulfuric acid speleogenesis (SAS), where thermal air convection and condensation-corrosion play a key role, a process that has oen been underestimated up to now. Likely future directions for research will rely in creasingly on analytical and modeling methods. e complex processes of hypogenic dissolution by ow mix ing at depth remains to be investigated. Isotopic assess ment will greatly enhance the understanding of enrich ment/depletion processes and the origin of uids. Dating will not only give insights into the chronology of speleo genesis, but will greatly help to correlate cave evolution phases to their corresponding environmental conditions. And eld investigation will remain an important ap proach, especially in testing the eect of processes with a permanent balance between theoretical processes and resulting morphologies, in terms of cave features and patterns, as well as their scale. A CKNOWLEDGEMENTS e two anonymous reviewers have helped to improve this paper a lot. REFERENCES Audra, P., 1994: Karsts Alpins, Gense de Grands R seaux Souterrains. Exemples: le Tennengebirge (Au triche), lâ€™Ile de Crmieu, la Chartreuse et le Vercors (France).Karstologia Mmoires, 5, pp. 280. Audra, P., 2001: L'organisation verticale des rseaux karstiques non conns. Contrle de la structure et du niveau de base. XI e Congrs national suisse de splologie, Genve , 125. Audra, P., 2007: Karst et splogense pignes, hypognes, recherches appliques et valorisation .Habilitation esis, University of Nice Sophia-Antipolis, pp. 278. Audra, P. & Hobla, F. 1996: La traverse du Gbroulaz en Vanoise. Morphologie dâ€™une cavit gypseuse de haute montagne. In: International Congress, Alpine caves: alpine karst systems and their envi ronmental context, Asiago 1992 , 49. Audra, P. & Palmer, A.N. 2013: e vertical dimension of karst. Controls of vertical cave pattern.In: Shroder J. (ed. in chief), Frumkin, A. (ed.) Treatise on Geo morphology, vol. 6 (Karst Geomorphology). Aca demic Press, 186, San Diego, CA. Audra, P., Bigot, J.-Y . & Mocochain, L., 2002: Hypogenic caves in Provence (France). Specic features and sediments.Acta Carsologica, 3, 33. RESEARCH FRONTIERS IN SPELEOGENESIS. D OMINANT PROCESSES, HYDROGEOLOGICAL CONDITIONS AND ...
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