Research frontiers in speleogenesis. Dominant processes, hydrogeological conditions and resulting cave patterns

Povzetek

Speleogenesis is the development of well-organized cave systems by fluids moving through fissures of a soluble rock. Epigenic caves induced by biogenic CO2 soil production are dominant, whereas hypogenic caves resulting from uprising deep flow not directly connected to adjacent recharge areas appear to be more frequent than previously considered. The conceptual models of epigenic cave development moved from early models, through the “four-states model” involving fracture influence to explain deep loops, to the digital models demonstrating the adjustment of the main flow to the water table. The relationships with base level are complex and cave levels must be determined from the elevation of the vadose-phreatic transitions. Since flooding in the epiphreatic zone may be important, the top of the loops in the epiphreatic zone can be found significantly high above the base level. The term Paragenesis is used to describe the upward development of conduits as their lower parts fill with sediments. This process often records a general baselevel rise. Sediment influx is responsible for the regulation of long profiles by paragenesis and contributes to the evolution of profiles from looping to water table caves. Dating methods allow identification of the timing of cave level evolution. The term Ghost-rock karstification is used to describe a 2-phase process of speleogenesis, with a first 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 turbulent flow in high gradient conditions opening the passages and forming maze caves. The first weathering phase can be related either to epigenic infiltration or to hypogenic upflow, especially in marginal areas of sedimentary basins. The vertical pattern of epigenic caves is mainly controlled by timing, geological structure, types of flow and base-level changes. We define several cave types as (1) juvenile, where they are perched above underlying aquicludes; (2) looping, where recharge varies greatly with time, to produce epiphreatic loops; (3) water-table caves where flow is regulated by a semi-pervious cover; and (4) caves in the equilibrium stage where flow is transmitted without significant flooding. Successive base-level drops caused by valley entrenchment make cave levels, whereas baselevel rise is defined in the frame of the Per ascensum Model of Speleogenesis (PAMS), where deep passages are flooded and drain through vauclusian springs. The PAMS can be active after any type of baselevel rise (transgression, fluvial aggradation, tectonic subsidence) and explains most of the deep phreatic cave systems except for hypogenic.

The term Hypogenic speleogenesis is used to describe cave development by deep upflow independent of adjacent recharge areas. Due to its deep origin, water frequently has a high CO2-H2S concentration and a thermal anomaly, but not systematically. Numerous dissolution processes can be involved in hypogenic speleogenesis, which often include deep-seated acidic sources of CO2 and H2S, “hydrothermal” cooling, mixing corrosion, 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 atmospheric environment. The hydrogeological setting of hypogenic speleogenesis is based on the Regional Gravity Flow concept, which shows at the basin scales the sites of convergences and upflows where dissolution focuses. Each part of a basin (marginal, internal, deep zone) has specific conditions. The coastal basin is a sub-type. In deformed strata, flow is more complex according to the geological structure. However, upflow and hypogenic speleogenesis concentrate in structural highs (buried anticlines) and zones of major disruption (faults, overthrusts). In disrupted basins, the geothermal gradient “pumps” the meteoric water at depth, making loops of different depths and characteristics. Volcanism and magmatism also produce deep hypogenic loops with “hyperkarst” characteristics due to a combination of deep-seated CO2, H2S, thermalism, and microbial activity. In phreatic conditions, the resulting cave patterns
can include geodes, 2–3D caves, and giant ascending shafts. Along the water table, SAS with thermal air convection induces powerful condensation-corrosion and the development of upwardly dendritic caves, isolated chambers, water table sulfuricacid caves. In the vadose zone, “smoking” shafts evolve under the influence of geothermal gradients producing air convectionand condensation-corrosion.
Likely future directions for research will probably involve analytical and modeling methods, especially using isotopes, dating, chemical simulations, and field investigations focused on the relationships between processes and resulting morphologies.

Nova področja speleogenetskih raziskav: Povezava med hidrogeološkimi razmerami, prevladujočimi procesi in tipi jam
Speleogeneza je razvoj dobro (samo)organiziranih jamskih sistemov, ko podzemna voda vzdolž toka raztaplja stene razpok. Najbolj poznane so epigene jame v karbonatih, kjer je poglavitni vir kemične agresivnosti pedogeni CO2. Bolj pogoste, kot se je v preteklosti domnevalo, so hipogene jame, ki nastanejo z dviganjem globokega toka in niso neposredno povezane z lokalnim napajalnim območjem. Prvotni konceptualni modeli razvoja epigenih jam so se preko modela štirih stanj, ki speleogenezo pojasnjuje s frekvenco prevodnih razpok, razvili do računalniških modelov, ki pojasnijo prilagoditev glavnega toka freatični površini. Povezava jamskih sistemov s položajem erozijske baze ni enostavna, saj moramo pri interpretaciji upoštevati višino prehoda iz freatične v vadozno cono. Zaradi visokih poplav v epifreatični coni so lahko temena jamskih zavojev visoko nad erozijsko bazo. Termin parageneza se uporablja za opis razvoja kanalov od spodaj navzgor, ko se spodnji deli zapolnijo s sedimenti. Ta proces pogosto beleži splošen dvig erozijske baze. Vdor sedimentov je tudi razlog za uravnavanje dolgih profilov s paragenezo in prispeva k prehodu jam z zavoji v navpični ravnini v jame uravnane z vodnim nivojem. Različne datacijske metode omogočajo določanje časovnega razvoja jamskih nivojev. Speleogeneza lahko poteka tudi v dveh fazah; v prvi fazi voda ob nizkem gradientu raztopi topen del kamninske matrice (angleško Ghost rock weathering), v drugi fazi pa ob visokem gradientu turbulentni tok mehansko odnese preostali del matrice, pri čemer praviloma nastane labirintni tip jam. Prva faza je lahko povezana z epigeno infiltracijo ali s hipogenim dotokom predvsem na mejnih območjih sedimentnih bazenov. Vertikalna geometrija epigenih jam je pogojena s časovnim okvirom, geološko strukturo, vrsto toka in spremembo erozijske baze. Razvoj mladih (juvenilnih) geometrijskih vzorcev nad nivojem neprepustnih plasti, je povezan s hitrimi tektonskimi dvigi in vrezovanji erozijske baze. V pogojih omejenega odtoka ob spremenljivem napajanju zaradi poplavljanja epifreatične cone nastajajo zavoji v navpični ravnini (angl. loops). Jame vodnega nivoja nastajajo na področjih, kjer je kras pokrit z delno prepustnimi plastmi oz. kjer je speleogeneza uravnotežena z največjimi poplavami. Spreminjanje erozijske baze ob vrezovanju dolin se odraža v jamskih nivojih, medtem ko dviganje erozijske baze diktira razvoj jam od spodaj navzgor (Speleogeneza Per ecensum, PAMS) in nastanek izvirov vokluškega tipa. PAMS se lahko aktivira ob različnih vrstah dviga erozijske baze (zaradi transgresije, rečnega naplavljanja, tektonskega ugrezanja) in pojasnjuje nastanek večine globokih freatičnih jamskih sistemov, razen hipogenih. Izraz hipogena speleogeneza se uporablja za opis razvoja jam zaradi dviganja globokega regionalnega toka. Zaradi izvora iz globin ima voda pogosto visoko koncentracijo CO2–H2S in temperaturno anomalijo. Pri hipogeni speleogenezi lahko sodelujejo številni procesi raztapljanja, ki so povezani z globokimi viri CO2 in H2S, "hidrotermalnim" ohlajanjem, korozijo mešanice, speleogenezo žveplene kisline (Sulphuric Acid Speleogenesis, SAS), itd. Zlasti SAS vključuje kondenzacijsko-korozijske procese, zaradi česar prihaja do hitrega nastanka jam nad vodno gladino v atmosferskem okolju. Hidrogeološke razmere pri hipogeni speleogenezi so povezane z regionalnim gravitacijskim tokom, kjer je korozija najmočnejša na območju stekanja in dvigovanja vodnih tokov. Vsak del porečja (obrobni, notranji, globoka cona) ima posebne pogoje. Eden od podtipov je tudi obalno območje. V deformiranih slojih je tok bolj zapleten in strukturno pogojen, pri čemer sta vodni tok in hipogena speleogeneza praviloma vezana na strukturne vrhove (prekrite antiklinale) in na območja večjih strukturnih prekinitev (prelomi, narivi). V prekinjenih bazenih geotermalni gradient "črpa" meteorske vode v globine, kar povzroča zanke na različnih globinah in z različnimi značilnostmi. Vulkanizem in magmatizem tudi povzročata globoke hipogene zanke s "hiperkraškimi" značilnostmi, ki nastajajo zaradi kombinacije globokih virov CO2, H2S, termalnih procesov in mikrobiološke aktivnosti. Geometrijski vzorci jam v freatičnih pogojih lahko vključujejo geode, 2–3D jame in navzgor razvijajoča se brezna izjemnih razsežnosti. Nad vodno gladino se zaradi termalne konvekcije in kondenzacijske korozije ob prisotnosti žveplove kisline razvijajo različni geometrijski vzorci jam; dvigajoče se razvejane jame, izolirane dvorane in jame vodnega nivoja nastale z delovanjem žveplene kisline. V vadozni coni nastajajo tudi »parna« brezna, ko se na območjih termalnih vodonosnikov topel vlažen zrak dviga, ohlaja in kondenzira vzdolž razpok in jih na ta način širi v brezna. V prihodnosti bodo raziskave speleogeneze verjetno temeljile na analitičnih in modelskih pristopih, izotopskih, datacijskih in geokemičnih metodah ter terenskih raziskavah, ki se bodo osredotočala na odnose med procesi in posledično morfologijo.