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De Waele J.,University of Bologna | Audra P.,University of Nice Sophia Antipolis | Madonia G.,University of Palermo | Vattano M.,University of Palermo | And 5 more authors.
Geomorphology | Year: 2016

Caves formed by rising sulfuric waters have been described from all over the world in a wide variety of climate settings, from arid regions to mid-latitude and alpine areas. H2S is generally formed at depth by reduction of sulfates in the presence of hydrocarbons and is transported in solution through the deep aquifers. In tectonically disturbed areas major fractures eventually allow these H2S-bearing fluids to rise to the surface where oxidation processes can become active producing sulfuric acid. This extremely strong acid reacts with the carbonate bedrock creating caves, some of which are among the largest and most spectacular in the world. Production of sulfuric acid mostly occurs at or close to the water table but also in subaerial conditions in moisture films and droplets in the cave environment. These caves are generated at or immediately above the water table, where condensation-corrosion processes are dominant, creating a set of characteristic meso- and micromorphologies. Due to their close connection to the base level, these caves can also precisely record past hydrological and geomorphological settings. Certain authigenic cave minerals, produced during the sulfuric acid speleogenesis (SAS) phase, allow determination of the exact timing of speleogenesis. This paper deals with the morphological, geochemical and mineralogical description of four very typical sulfuric acid water table caves in Europe: the Grotte du Chat in the southern French Alps, the Acqua Fitusa Cave in Sicily (Italy), and the Bad Deutsch Altenburg and Kraushöhle caves in Austria. © 2015 Elsevier B.V.

Vattano M.,University of Palermo | Audra P.,University of Nice Sophia Antipolis | Bigot J.-Y.,Association Francaise de Karstologie AFK | De Waele J.,University of Bologna | And 2 more authors.
Rendiconti Online Societa Geologica Italiana | Year: 2012

Hypogenic caves are generated by water recharging from below independently of seepage from the overlying or immediately adjacent surface. These waters are often thermal and enriched in dissolved gases, the most common of which are CO2 and H2S. Hypogenic caves can be thermal caves, sulphuric acid caves, basal injection caves. They differ from epigenic caves in many ways, such as: speleogenetic mechanisms, morphological features, chemical deposits, and lack of alluvial sediments (KLIMCHOUK, 2007; KLIMCHOUK & FORD, 2009; PALMER, 2011). Several studies were conducted to evaluate the hypogenic origin of a large number of caves (AUDRA et alii, 2010; KLIMCHOUK & FORD, 2009; STAFFORD et alii, 2009). A significant contribution was given by the work of Klimchouk (2007) that systematically provided instruments and models to better understand and well define the hypogenic karst processes and landforms. Detailed studies on hypogenic caves were carried out in Italy since the 90s in different karst systems, especially in the Central and Southern Appenines. These studies mainly concerned chemical deposits related to ascending water and microbiological action (GALDENZI & MENICHETTI, 1995; GALDENZI, 1997; PICCINI, 2000; GALDENZI & MARUOKA, 2003, FORTI & MOCCHIUTTI, 2004; GALDENZI, 2012). In this paper, we present the first results of researches conducted in Acqua Fitusa cave that was believed to be an epigenic cave until today. Acqua Fitusa cave is located in Central Sicily, along the north-eastern scarp of a N-S anticline, westward vergent, forming the Mt. La Montagnola. The cave formed in the Upper Cretaceous Rudist breccias member of the Crisanti Fm., composed of conglomerates and reworked calcarenites with rudist fragments and benthic foraminifers (CATALANO et alii, 2011). The cave consists at least of three stories of subhorizontal conduits, displaying a total length of 700 m, and a vertical range of 25 m. It represents a clear example of inactive water-table sulphuric acid cave, produced mainly by H2S degassing in the cave atmosphere. Despite the small size, Acqua Fitusa cave is very interesting for the abundance and variety of forms and deposits related to rising waters and air flow. A ∼ 7 m deep inactive thermo-sulphuric discharge slot intersects the floor of some passages for several meters (Fig. 1). Different morphologies of small and large sizes, generated by condensation-corrosion processes, can be observed along the ceiling and walls: ceiling cupolas and large wall convection niches occur in the largest rooms of the cave; deep wall convection niches, in places forming notches, incise cave walls at different heights; condensation-corrosion channels similar to ceiling-half tubes carve the roof of some passages; replacements pockets due to corrosion-substitution processes are widespread; boxwork due to differential condensation-corrosion were observed in the upper parts of the conduits. Sulphuric notches with flat roof, linked to lateral corrosion of the thermal water table, carve the cave walls at different heights recording past stages of base-level lowering. (Figure presented) Gypsum deposits have been found in many parts of the cave. Replacement gypsum crusts are common in many passages; the gypsum is located in large vertical fissures along the walls, it can partially cover wall convection notches, or replacement pockets (Fig. 2). A gypsum body of about 50 cm of thickness was found on the floor of the biggest room in correspondence of which small ceiling cupolas are associated on the roof. Finally, centimetric euhedral gypsum crystals grew inside mud sediments. (Figure presented) At present the cave is inactive with the thermal spring occurring 300 m north and at a lower altitude than the cave. These H2S-rich waters are indicated as chlorine-sulphate alkaline, and have a temperature of about 25°C (CARAPEZZA et alii, 1977). (Figure presented) The origin of the Acqua Fitusa cave is due to corrosion processes of carbonate rocks with replacement of gypsum by H2S-rich thermal water (Fig. 3). In particular, the enlargement of voids and formation of the main morphologies are due to H2S degassing in the cave atmosphere, oxidation of sulphides and thermal convection that produce strong condensation-corrosion processes above the watertable, according to the origin of sulphuric acid caves (FORD & WILLIAMS, 2007; AUDRA et alii, 2010). © Società Geologica Italiana, Roma 2012.

Audra P.,University of Nice Sophia Antipolis | D'Antoni-Nobecourt J.-C.,Association Francaise de Karstologie AFK | Bigot J.-Y.,Association Francaise de Karstologie AFK
Bulletin de la Societe Geologique de France | Year: 2010

Hypogenic caves develop by recharge from below, not directly influenced by seepage from the overlying land surface. Several processes of speleogenesis are combined, involving CO2 or H2S produced at depth. If the recharge from depth remains uniform, the growth of selected fissures is prevented, giving rise to maze cave systems with an upward development trend, which is defined as "transverse speleogenesis" [Klimchouk, 2003]. Hypogenic caves are much fewer than epigenic caves (i.e. developed downwards by meteoric water with aggressivity derived from soil). In France, as in the rest of the world, hypogenic caves were poorly recognized until recently because of their lower frequency, subsequent epigenic imprint often hiding the true origin, and the absence of a global conceptual model. However, about a hundred of hypogenic caves have been identified recently in France. The extreme diversity of hypogenic cave patterns and features is due to the variety of geological and topographic settings and types of flow. Thermal caves are a sub-set of hypogenic caves. Active thermal caves are few and small (Mas d'En Caraman, Vallon du Salut). Often, thermal influences only occur as point thermal infeeders into epigenic caves (Mescla, Estramar). In addition to the higher temperature, they may be characterized by CO2 (Madeleine) or H2S degassing, by warm water flowing in ceiling channels, or by manganese deposits. The Giant Phreatic Shafts locate along regional active faultlines. They combine all characteristics (thermal, CO2, H2S), due to the fast rising of deep water. The Salins Spring has been explored by scuba diving down to -70 m. Such a hyperkarstification is responsible for the development of the deepest phreatic shafts of the world: pozzo del Merro, Italy (-392 m). Inactive hypogenic caves may be recognized by their specific mineralization or by the presence of large calcite spar. Metallic deposits are due to the rising of deep waters that are warm, aggressive, and low in oxidation potential. Mixing with meteoric water generates Mississippi Valley Type (MVT) sulfidic ores. Iron deposits as massive bodies (Lagnes) or onto microbial media (Iboussières, Malacoste) making specific faciès, such as "black tubes", iron flakes, and iron pool fingers. Other frequent minerals are Mn oxides and Pb sulfur. In such low thermal conditions, calcite deposits occur as large spar in géodes or as passage linings. Other inactive hypogenic caves may also be recognized by characteristic patterns, such as mazes. The relatively constant recharge into confined karst aquifers suppresses fissure competition, so they enlarge at similar rates, producing a maze pattern. In horizontal beds, mazes extend centrifugally around the upwelling feeder. The juxtaposition of multiple discrete vertical feeders produces extended horizontal mazes. In gently tilted structures, 2D mazes extend below aquitards, or along bedding or more porous beds (Saint-Sébastien). In thick folded limestone the rising hypogenic flow alternatively follows joints and bedding planes, producing a 3D maze cave in a staircase pattern (Pigette). Isolated chambers are large cupola-like chambers fed by thermal slots. Thermal convection of air in a CO 2-rich atmosphere causes condensation-corrosion that quickly produces voids above the water table (Champignons Cave). Sulfuric acid caves with replacement gypsum are produced by H2S degassing in the cave atmosphere. H2S oxidizes to H2SO4, which corrodes the carbonate rock and replaces it with gypsum. The strongest corrosion occurs above the water table, where sulfide degassing and thermal convection produce strong condensation-corrosion. Caves develop headward from springs and from thermo-sulfuric slots upward (Chevalley-Serpents System). The low-gradient main drains record base-level positions and even the slightest stages of water-table lowering (Chat Cave). Hypogenic speleogenesis provides better understanding of the distribution of karst voids responsible for subsidence hazards and the emplacement of minerals and hydrocarbons.

Audra P.,University of Nice Sophia Antipolis | Gazquez F.,University of Cambridge | Rull F.,University of Valladolid | Bigot J.-Y.,Association francaise de karstologie AFK | Camus H.,CENOTE
Geomorphology | Year: 2015

The oxidation of hydrocarbons and sulfide sources (H2S, pyrite) produces sulfuric acid that strongly reacts with bedrock, causing limestone dissolution and complex interactions with other minerals from the bedrock or from cave fillings, mainly clays. This type of cave development, known as Sulfuric Acid Speleogenesis (SAS), is a subcategory of hypogene speleogenesis, where aggressive water rises from depth. It also produces uncommon minerals, mainly sulfates, the typical byproducts of SAS. Baume Galinière is located in Southern France, in the Vaucluse spring watershed. This small maze cave displays characteristic SAS features such as corrosion notches, calcite geodes, iron crusts, and various sulfate minerals. Sulfur isotopes of SAS byproducts (jarosite and gypsum) clearly show they derive from pyrite oxidation. Using XRD and micro-Raman spectroscopy, thirteen minerals were identified, including elemental sulfur, calcite, quartz, pyrite, goethite, gypsum, and fibroferrite, plus all of the six members of the jarosite subgroup (jarosite, argentojarosite, ammoniojarosite, hydroniumjarosite, natrojarosite, plumbojarosite). The Baume Galinière deposits are the first documented cave occurrence of argentojarosite and the second known occurrence of plumbojarosite, hydronium jarosite, ammoniojarosite, and fibroferrite. In the Vaucluse watershed, there were numerous upwellings of deep water along major faults, located at the contact of the karstic aquifer and the overlying impervious covers. The mixing of deep and meteoric waters at shallow depths caused pyrite depositions in numerous caves, including Baume Galinière. Sulfuric Acid Speleogenesis occurred later after base-level drop, when the cave was under shallow phreatic conditions then in the vadose zone, with oxidation of pyrites generating sulfuric acid. Attenuated oxidation is still occurring through condensation of moisture from incoming air. Baume Galinière Cave records the position of the semi-impervious paleo-cover and documents its retreat in relationship to valley incision caused by uplift and tilting of the Vaucluse block during the Neogene. © 2015 Elsevier B.V.

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