In memory of the regretted late Professor Alfredo Bini, our brother in friendship.
1Generally, speleologists and geomorphologists use karst morphologies and micromorphologies to speculate on the conditions of formation of karst cavities (Bögli, 1980; Ford and Williams, 2007; Ford and Cullingford, 1976; Renault, 1970; Salomon, 2006; White, 1989). These causal relationships between shape and processes are essentially based on analogies. As it is not an experimental science, the conclusions made can only be regarded as hypotheses and models from which you cannot draw certainties. Only a few microforms, such as scallops, have been recreated in the laboratory using analog or physical models (Curl, 1966, 1974) but even in this case, we must be cautious. Indeed, it is not because one phenomenon can result in the formation of some morphologies, that it is its only possible cause.
2Since the middle 2000s, new speleogenesis processes were highlighted: it is the ghostrock karstification (Dubois et al., 2014a; Kaufmann, 2000; Quinif, 2010a; Quinif and Bruxelles, 2011; Quinif and Maire, 2010; Quinif et al., 2014; Vergari, 1996). Unlike conventional theories of karstification which result in the direct creation of a macroscopic voids, the formation of caves by ghostrock karstification is a two-stage process: (i) at first the rock is weathered in-situ and only the dissolved elements are drained away, leaving in place a material called residual alterite composed of the undissolved components; (ii) in a subsequent evolution, the alterite is eroded, leaving macroscopic voids. In this context, a variety of karst morphologies were observed (Dubois et al., 2011), which requires us to completely reconsider the origin and genesis of these kind of forms (e.g., networks structuration, galleries shapes, walls microforms, a.s.o.).
3The first part of the manuscript makes a brief synthesis on the evolution of the geomorphology followed by the presentation of the two karstification processes: karstification by total removal and ghostrock karstification. The second part classifies and details the karst morphologies from the network to the microform scale. The third part presents the different morphologies observed in the context of ghostrock karstification. The last part highlights the implications of this new classification of karst morphologies in the study of caves.
4The literature on karst morphology is abundant. It would be pointless to make a comprehensive study in this manuscript. Only a few main stages of the evolution of this field will be discussed here. One of the founders of the micromorphology in caves is undoubtedly Bretz (1942) in his famous publication: “Vadose and phreatic features of limestone caves”. In this article, the author follows Davis (1930)’s concept of a two-stage speleogenesis: (i) deep phreatic speleogenesis with creation of macroscopic voids, and (ii) transition to vadose zone with the underground rivers flow and the filling of the cavity by speleothems. To this theory, Bretz adds a third stage: the sedimentation within the gallery. He studied the conditions of occurrence for each stage and the resulting karst forms. In French literature, authors such as De Joly (1946), Gèze (1973), Trombe (1952), Chevalier (1944), specifically describe some types of forms, however without making a synthesis of all the karst features. Gèze (1973) wrote the outline of a dictionary based only on the descriptive aspect of the forms. Renault and Bini are providing a much more comprehensive study of karst morphologies in relation with mechanical phenomena and sedimentology in speleogenesis (Bini, 1980; Renault, 1958, 1967, 1968, 1970; Renault and Caumartin, 1958).
5Following these researches, geomorphology has commonly been used to find the geological conditions and genetic phenomena responsible for the formation of cavities: phreatic or vadose zone, torrential or still water, chemical and physical effects, etc. Among all this literature, Slabe (1955) has proposed a very comprehensive thesis on the subject. On another hand, many microforms were studied individually, or as a synthetic aspect, in connection with some genetic mechanisms (Allen, 1972; Bini, 1978, 1979, 2007; Bini and Cappa, 1978; Bögli, 1964; Corbel, 1962; Curl, 1966; Ewers, 1966; Quinif, 1973; Viehmann, 1959, 1973). Some microforms, such as scallops, have also been studied combining observations and experimentation (Curl, 1974; Lismonde and Lagmani, 1987).
6Since the 2010s, new observations question the previously established links between microforms and setting conditions. It now appears that different phenomena may lead to the creation of similar forms (Dubois et al., 2011, 2014a).
7Historically, the formation of caves is considered as a one stage process including both chemical and mechanical erosion. When calcium carbonate comes into contact with water, dissolution occurs when the solution is undersaturated with respect to calcite. In the traditional karstification theory, the water flow drains the dissolved component of the rock but also erodes the not yet dissolved particles of the rock (Bretz, 1942; Bögli, 1980; Davis, 1930; Ford and Williams, 2007; Ford and Cullingford, 1976; Jakucs, 1977; Jennings, 1971; Renault, 1967; 1968; Salomon, 2006; White, 1989). This way, no material remains as a product of the weathering and the karstification by total removal leads to the creation of open voids. The environmental conditions of this karstification are (Quinif, 1998) (fig. 1): (1) the presence of more permeable zones in the rock such as opened joints or previous galleries, to let the stream flow; (2) water chemically aggressive to dissolve the calcium carbonate; (3) water with sufficient energy to remove both dissolved and undissolved particles.
Fig. 1 - The two processes of karstification (after Dubois et al., 2014a).
Fig. 1 - Les deux processus de karstification (d’après Dubois et al., 2014a).
A: Karstification by total removal requires a considerable amount of chemical energy to dissolve the calcium carbonate as well as a considerable amount of hydrodynamic energy to remove both the dissolved species and undissolved particles. Water must be able to flow through the rock for karstification to occur and preferential pathways are provided by opened joints or emptied galleries resulting from previous karstogenesis; B: ghostrock karstification requires a considerable amount of chemical energy to dissolve the calcium carbonate but only a small amount of hydrodynamic energy to remove the dissolved species while leaving the undissolved particles in place. Preferential pathways are provided by opened joints or more transmissive zones within the rock. The undissolved particles will be removed by the flowing water if the hydrodynamic energy increases sufficiently.
A : La karstification par enlèvement total nécessite une quantité considérable d'énergie chimique pour dissoudre le carbonate de calcium ainsi qu'une quantité considérable d'énergie hydrodynamique pour éliminer à la fois les espèces dissoutes et les particules non dissoutes. L'eau doit pouvoir s'écouler à travers la roche pour que la karstification se produise et des voies préférentielles sont fournies par des joints ouverts ou des galeries vidées résultant d'une karstogenèse antérieure ; B : La karstification par fantômisation nécessite une quantité considérable d'énergie chimique pour dissoudre le carbonate de calcium, mais seulement une petite quantité d'énergie hydrodynamique pour éliminer les espèces dissoutes tout en laissant les particules non dissoutes en place. Les voies préférentielles sont fournies par des joints ouverts ou des zones plus transmissives au sein de la roche. Les particules non dissoutes seront éliminées par écoulement de type fluviatile si l'énergie hydrodynamique augmente suffisamment.
8Unlike the theory of karstification by total removal, the ghostrock karstification is a two stages process separating the chemical dissolution of limestone from the mechanical erosion of the insoluble or less soluble substances (Dubois et al., 2014a; Quinif, 2010b; Quinif et al., 2014). When the water dissolves the carbonates of the rock, only the dissolved components are drained away with the water. The undissolved particles (e.g., sparitic part of the rock as fossils, filling of tension gashes and insoluble minerals such clays minerals) remain in place within the rock matrix constituting the residual alterite: a material related to the parent rock but whose properties have been altered by the weathering. The environmental conditions characterizing this karstification are (fig. 1): (i) the presence of permeable pathways in the rock such as opened joints (Quinif et al., 1997) or more porous zones (Dubois et al., 2011) to let the stream flow; (ii) water chemically aggressive to dissolve the calcium carbonate; (iii) water with low hydrodynamism; the flow must be sufficient to renew the acid solution but not too intense as to erode the alterite (Quinif, 1998). Later in the subsequent geological evolution of the region, if these conditions change and the hydrodynamic energy become sufficient, the mechanical erosion of the undissolved particles is possible leading to the creation of an opened karst. These changes can be due to the lowering of the water table by creation of a relief (Bini et al., 2012) or by anthropic effect (Quinif and Maire, 2010) or to the seasonal variation of the water table (Dandurand et al., 2014). When emptied this karst can evolve like any other classical karst, by mechanical and chemical erosion, speleothems development, aggradation, oversteepening, and other processes.
9The material resulting from the weathering is called alterite. It can be an isalterite or an alloterite depending on whether the material keeps or not the general structure of the rock. In this manuscript, the studied materials are essentially isalterite as defined by Delvigne (1998): “a weathering product with slight or no change in rock volume and remnant rock structure”. Therefore, the terminology alterite is used to refer to isalterite. This material, comprising the less soluble elements of the rock, is very different from its parent. The weathering affects the rock matrix in several ways: as the calcium carbonate is dissolved there is a change in the overall composition of the material and its structure, while removal of the dissolved species creates a secondary porosity within the matrix, altering the hydrological properties of the rock and its mechanical strength. However, the alterite keeps the original structure of the parent rock and insoluble elements remain distinguishable (e.g., fossils, cherts, calcite veining).
10The ghostrocks are all filled with their alterite and the overall structure of the parent rock is preserved. If ghostrock features form a network, the entire area is described as a ghostrock karst network. A typology is proposed based on the status of the ghostrock: (i) a ghostrock is active if the weathering phenomena are still occurring; (ii) a ghostrock is inactive if no more weathering phenomena are occurring; (iii) a ghostrock is fossilized if it is sealed by impermeable sediments and is, in this case, designated as a ghost-paleokarst (Bates, 1980; Jackson, 1977); (iv) a ghostrock emptied of its alterite is called open or emptied ghostrock.
11The ghostrock karst network may ramify through the bedrock (fig. 2) and like most geological objects; it is hardly possible to visualize a ghostrock in its whole. For this, geophysical methods must be use to visualize the ghostrocks and distinguish between voids and ghostrocks. The observable features are the traces of the ghostrock on intersecting planes. According to the section plane, the visible morphologies differ. A nomenclature of ghostrock features traces is established to highlight their characteristic geometries. In a fresh unweathered limestone (A), one finds wide weathered zones (B) which are situated beneath the overburden and are often affected by compaction due to their dimensions which draws down the overlying cover and weathered corridors (C) which are vertical features to more ten meters height and ghost-endokarst which appear in the form of sections of pseudo-galleries (D) or vast volumes totally included in the rock mass (E). Often, the limestone is covered by a transgressive overburden (F).
Fig. 2 - Schematic diagram showing typical ghostrock features (modified from Kaufmann, 2000).
Fig. 2 - Schéma montrant les caractéristiques typiques d’un massif fantômisé (modifié à partir de Kaufmann, 2000).
A: Fresh unweathered limestone; B: Large weathered features; C: Weathered corridors; D: Ghost-endokarst; E: Weathered galleries; F: Overburden.
A : Calcaire frais non altéré ; B : Grands volumes altérés ; C : Couloirs altérés ; D : Pseudoendokarst (fantômes internes); E : Fantômes en galeries; F : Formations de couverture.
12The more representative occurrences of ghostrock karstification are in the Carboniferous limestones of southern Belgium (from Tournai to Soignies, Hainaut). This area has been wildly studied in the literature (Dubois et al., 2014b; Havron et al., 2007; Kaufmann, 2000; Kaufmann et al., 1999; Quinif, 2010a; Quinif and Maire, 2010; Quinif et al., 2012, 2014; Vergari, 1996, 1998). This is a very competent limestone highly fossiliferous composed of 80% to 95% calcite. The strata are monocline and the bedrock has suffered several tectonic stages causing the creation of several conjugate families of subvertical joints. The basement is unconformably overlain by Cenozoic overburden. These limestones are exploited by numerous quarries which constitute exceptional places to observe karst phenomena.
13The ghostrock features spread vertically along the initial fractures of the rock. They are mainly weathered corridors under the overburden (fig. 3A) or within the rock (fig. 3B) as well as ghostrock endokarst (fig. 3C). As the weathering follows the different families of fractures, it forms a 3D maze network of karst.
Fig. 3 - Examples of ghostrock features in the Carboniferous limestone around Soignies in southern Belgium (after Dubois et al., 2014a).
Fig. 3 - Exemples de formations fantômes dans le calcaire carbonifère autour de Soignies dans le sud de la Belgique (d’après Dubois et al., 2014a).
A: Large weathered zone situated at the top of the limestone. As the alterite has undergone compaction the overburden has been drawn down into the weathered corridor; B: Weathered corridor still containing its residual alterite; C: Ghost-endokarst completely included in the rock.
A : Grande zone altérée située au sommet du calcaire. Au fur et à mesure que l'altérite a subi un compactage, les formations de couverture ont été entraînées dans le couloir altéré; B : Couloir altéré contenant encore son altérite résiduelle ; C : Pseudoendokarst complètement inclus dans la roche.
14The ghostrock features are filled with the undissolved components of the limestone: the insoluble particles (e.g., cherts, clays, quartz) and the not yet dissolved CaCO3 constituents (micritic calcite, sparitic calcite crystals, fossils). Several sets of analyses were performed on samples taken in stratigraphic continuity to represent a variation in the intensity of weathering ranging from fresh to completely weathered rock: Tournaisian siliceous limestone from Tournai (Kaufmann et al., 1999), limestone with cherts and encrinites limestone from Soignies (Dubois et al., 2014b; Havron et al., 1999), Jurassic limestone with cherts from Charente (Dandurand et al., 2014). Whatever the lithology, the results show the following outcomes (fig. 4): (i) the dissolution and drainage of part or all of the CaCO3; (ii) an increase in porosity; (iii) a weakening of the mechanical properties which makes the material friable. There is also possibility of (iv) a gravity compaction of the alterite and/or, (v) a slight illuviation of minerals (clays, organic matters, etc.) into the alterite (Dubois et al., 2014).
Fig. 4 - Thin-sections of the Carboniferous encrinites limestone from Soignies analysed under cathodoluminescence microscopy.
Fig. 4 - Lames minces du calcaire carbonifère à encrinite de Soignies analysées en microscopie à cathodoluminescence.
A: Fresh rock mainly composed by micro-crystalline calcite, carbonated fossils and dolomite. No porosity is visible on the thin section; B: completely weathered rock made of fragments of fossils, amorphous black minerals and voids. The calcareous matrix has been dissolved leading to the creation of porosity. Exogenous minerals have sweep in the alterite. From Dubois et al., 2014a.
A : Roche fraîche composée principalement de calcite microcristalline, de fossiles carbonatés et de dolomie. Aucune porosité n'est visible sur la lame mince ; B : roche complètement altérée constituée de fragments de fossiles, de minéraux noirs amorphes et de vides. La matrice calcaire s'est dissoute entraînant la création de porosité. Les minéraux exogènes ont balayé l'altérite. D’après Dubois et al., 2014a.
15This part gathers and summarizes the main families of underground karst features. The classification made is based purely on morphological description of the forms starting with cave patterns, then cavity shapes and finally parietal microforms.
16The network structures presented here are classified by increasing complexity. They are named and described in the terminology of Palmer (1991) (tab. 1, fig. 5). Longitudinally caves can be organized according to different patterns: horizontal, inclined, staggered and spread on multiple levels. Caves can also be abyssal and plunge quickly at great depths.
Tab. 1 - Presentation and description of the cave patterns (Palmer, 1991).
Tab. 1 - Présentation et description des réseaux de grottes (Palmer, 1991).
A: Single gallery; B: Branchwork; C: Network; D: Anastomotic; E: Spongework; F: Ramiform.
A : Galerie unique ; B : Ramifié ; C : Réseau ; D : En anastomose ; E : En éponge ; F : Ramiforme.
Fig. 5 - Common cave patterns (modified from Palmer, 1991).
Fig. 5 - Modèles de grottes communs (modifié à partir de Palmer, 1991).
The sketches can illustrate both plan and cross-section views. A: Single gallery: Vilaine Source Cave, Namur; B: Branchwork: Rouffignac Cave, Dordogne, France (from Martel in Gèze, 1965); C: Network: Breathing Cave, Virginia, USA (from Deike, 1960); D: Anastomotic: Hölloch Cave, Schwyz, Switzerland (from Bögli, 1970); E: Ramiform and Spongework: Carlsbad Cavern, New Mexico, USA (Bretz, 1949).
Les croquis peuvent illustrer des vues en plan et en coupe. A : Galerie unique : Grotte de la Source Vilaine, Namur ; B : Réseau en arborescence : Grotte de Rouffignac, Dordogne, France (de Martel dans Gèze, 1965) ; C : Réseau : Breathing Cave, Virginie, USA (d'après Deike, 1960) ; D : Anastomosé : Grotte de Hölloch, Schwyz, Suisse (d'après Bögli, 1970) ; E : Ramiforme et en épong : Carlsbad Cavern, Nouveau-Mexique, États-Unis (Bretz, 1949).
17The shapes of the cavities are classified according to their main extension (horizontal or/and vertical) and to their dimensions.
18Sections of karst which preferentially extend horizontally (fig. 6-7). These can be galleries, passages, or other horizontal or slightly inclined conduits. Their sections are often linked to the structural conditions (nature and geometry of fracturing, morphology of the more porous area, etc.). Their opening can vary from centimeters to several tens of meters while their extension is up to some kilometers. The morphologies of the horizontal features essentially depend on their development and on the parietal features that shaped them (cf. § 3.3.).
Fig. 6 - Common sections of horizontal features.
Fig. 6 - Sections courantes de formes horizontales.
A: Kavuaya Cave, Bas-Congo, RDC (Quinif, 1985); B: Chalet Cave, Aywaille, Belgium; C, E: Luweng Banteng, Gunung Sewu, Java (Quinif, 1984); D: Damous Fedjoudj, Algeria (Quinif, 1977); G, H, I: Fréyr Cave, Dinant, Belgium (Quinif, 1982); J: Lorette Cave, Rochefort, Belgium (Vandycke and Quinif, 2001).
A : Grotte de Kavuaya, Bas-Congo, RDC (Quinif, 1985) ; B : Grotte du Chalet, Aywaille, Belgique ; C, E : Luweng Banteng, Gunung Sewu, Java (Quinif, 1984) ; D : Damous Fedjoudj, Algérie (Quinif, 1977) ; G, H, I : Grotte de Fréyr, Dinant, Belgique (Quinif, 1982) ; J : Grotte de Lorette, Rochefort, Belgique (Vandycke et Quinif, 2001).
Fig. 7 - Some types of horizontal features.
Fig. 7 - Types de formes horizontales.
A: Cave of “Les Echelles” (Massif de la Chartreuse, Rhône-Alpes, France); B: Cave of “Père Noël” (Massif de Han-sur-Lesse); C: Cave of Han-sur-Lesse; D: Cave of Trabuc, Gard, France.
A : Grotte des « Echelles » (Massif de la Chartreuse, Rhône-Alpes, France) ; B : Grotte du « Père Noël » (Massif de Han-sur-Lesse) ; C : Grotte de Han-sur-Lesse ; D : Grotte de Trabuc, Gard, France.
19There are sections of karst which preferentially extend vertically (fig. 8-9). These can be pits, shafts, corridors or other vertical conduits. These features can be connected to other karst features. In this case, different resulting morphologies can develop such as dome pits or stepped conduits. They can also be isolated and directly opened from the surface like the « avens ». Pits aperture can vary from centimeters to several tens of meters and can reach several hundred meters deep.
Fig. 8 - Examples of vertical features.
Fig. 8 - Exemples de formes verticales.
A: Avens in Sidi Rgheiss Mountain, Algeria (Quinif, 1975); B: Aven of the “Bloc coïncé” in Rawil Mountain, Bern, Switzerland (Topography by Quinif and Maboge); C: Part of the cave Trou des Nûtons, Namur, Belgium (Quinif, 1978); D: Aven Armand, Languedoc-Roussillon, France (Martel, 1894).
A : Avens dans la montagne du Sidi Rgheiss, Algérie (Quinif, 1975) ; B : Aven du « Bloc coïncé », Rawil, Berne, Suisse (Topographie de Quinif et Maboge) ; C : Partie de la grotte du Trou des Nûtons, Namur, Belgique (Quinif, 1978) ; D : Aven Armand, Languedoc-Roussillon, France (Martel, 1894).
Fig. 9 - Examples of vertical feature.
Fig. 9 - Exemples de formes verticales.
A: “Trou Bernard”, Namur, Belgium. This pit is a step-in a down going gallery; B: Aven in the high Alps, Switzerland; C: Aven in Debagh Mountain (Algeria).
A : « Trou Bernard », Namur, Belgique. Ce puits est une galerie descendante en marches d’escalier; B : Aven dans les hautes Alpes, Suisse; C : Aven au Djebel Debagh (Algérie).
20Their dimensions are larger than the dimensions of the conduits and which are connected to it (fig. 10). Rooms can extend up to extensions of hundreds of meters long and tens of meters height.
Fig. 10 - “Dôme” Room, cave of Han-sur-Lesse, Rochefort, Belgium (Quinif and Bastin, 1984).
Fig. 10 - Salle « Dôme », grotte de Han-sur-Lesse, Rochefort, Belgique (Quinif et Bastin, 1984).
G.N. Geographic North. M.N. Magnetic North. S. Sumps.
G.N. Nord géographique. M.N. Nord magnétique. S. Puisards.
21Solutional sculpturing are morphologies visible on the walls, the ceiling and the base of cavities (White, 1988) (fig. 11). They may have a positive or negative relief and their dimensions are variable, but are often centimeter to meter scale. Solutional sculpturing classification based on morphological description has been established by Gèze (1973). This section describes common microforms (Jackson, 1997; Viala, 2000) and introduces the genetic causes usually associated: ceiling pocket, domes, notches, projections, stone laces, anastomosis, pendants, potholes, overdeepenings, scallops, stone laces, vertical flutes, meander niches (tab. 2).
Fig. 11 - Solutional sculpturing.
Fig. 11 - Microformes de dissolution.
A: Ceiling pocket - Cave of “Père Euchère”, Vaucluse, France; B: Differential dissolution features - Père Noël Cave, Han-sur-Lesse, Namur, Belgium; C: Anastomosis – “Vilaine Source” Cave, Namur, Belgium; D: Pendants – “Père Noël” Cave, Han-sur-Lesse, Namur, Belgium; E: Overdeepening and potholes - The “Barrique” Hole, Saint Martin du Puy, Aquitaine, France; F: Scallops, “Nou Maulin” Cave, Namur, Belgium.
A : Coupole - Grotte du « Père Euchère », Vaucluse, France ; B : Microformes de dissolution différentielle - Grotte du Père Noël, Han-sur-Lesse, Namur, Belgique ; C : Anastomoses – Grotte « Vilaine Source », Namur, Belgique ; D : Pendants de voûte – Grotte du « Père Noël », Han-sur-Lesse, Namur, Belgique ; E : Surcreusement et marmite - Le trou de la « Barrique », Saint Martin du Puy, Aquitaine, France ; F : Coups de gouge, Grotte du « Nou Maulin », Namur, Belgique.
Tab. 2 - Description and commonly accepted origins of microforms.
Tab. 2 - Description et origines communément acceptées des microformes.
22Karst networks from karstification by total removal get structured by the water flows. At first, the system is unstructured. Then but progressively, the opening of the conduits enable the flow to intensify and structure the network as a dissipative system (Prigogine, 2017). These are branchwork network. On the contrary, ghostrock karst systems are not structured by the water flow as no open duct is created during the first stage. Therefore, the structure and location of the network only depends on the initial permeability of the rock and of the acid source. Detail these two parameters are extensively detailed in the following paragraphs (§ 4.1.1. and 4.1.2.).
23Mangin (1975) describes karst aquifers as the combination of highly hierarchized permeable zones (main drains) and annex systems greatly capacitive but less permeable. This description closely fits to karsts from total removal, but this section will show that this description can also match ghostrock karst networks.
24The underground water is the main agent for rock weathering and for the drainage of the dissolved species. Therefore, the first condition to create a karst is a permeable pathway through the soluble rock. Several geological configurations may lead to this permeability: (i) open cracks; (ii) rock area with higher porosity; (iii) cavities resulting from previous karstogenesis. To enable efficient fluid circulation within the rock, these permeable zones should be sufficiently interconnected.
25Open cracks (joints, bedding planes, faults, etc.) are mostly inherited from past or/and current tectonic and only the cracks opened during the ghostrock karstification gets weathered. An example of permeability given by open cracks is ghostrock features of the Carboniferous limestone of Soignies (Hainaut, Belgium). During the Mesozoic, the limestone basement was affected by tectonic extension leading to the creation of 3 sets of open subvertical fractures: N60°E, N100°E, and N150°E (Quinif et al., 1997; Vandycke and Quinif, 1999). All of these fractures show weathering. These ghostrocks are then mainly structured as a regular network of vertical corridors (fig. 12-13).
Fig. 12 - Fracture Network in Clypot Quarry, Soignies, Hainaut (Belgium).
Fig. 12 - Réseau de fractures dans la carrière de Clypot, Soignies, Hainaut (Belgique).
A: Photo of the South wall of the quarry. It shows the density of large corridors in Thiarmont Formation and narrower features but as dense in Soignies Formation; B: Map of the karst features (Quinif and Quinif, 2002). 1: Corridors. 2: Ghostrocks recognized in section. 3: Features attributed to caves. 4: Caves resulting from the removing of alterite and replacement by river. 5: Lapiaz. 6: Blocks.
A : Photo du mur sud de la carrière. Il montre la densité de grands corridors dans la Formation de Thiarmont et des formes plus étroites mais aussi denses dans la Formation de Soignies ; B : Carte des formes karstiques (Quinif et Quinif, 2002). 1 : Couloirs. 2 : fantômes de roche reconnus en coupe. 3 : Formes attribuées aux grottes. 4 : Grottes résultant de l'enlèvement de l'altérite et remplacement par l’action d’une rivière. 5 : Lapiaz. 6 : Blocs.
Fig. 13 - Crinoidal limestone of Soignies, Hainaut, Belgium.
Fig. 13 - Calcaire crinoïdique de Soignies, Hainaut, Belgique.
The limestone is visible in Clypot Quarry, Soignies. It is a compact and non-porous limestone. The ghostrock features are located around the opened joints (A); B: The microscopic analyses of the fresh rock (PPL) show lots of fossils highly cemented by calcitic matrix; C: The microscopic analyses of the weathered rock (PPL) reveal voids (brown spots) between the fossils. The matrix has been dissolved.
Le calcaire est visible dans la carrière du Clypot, à Soignies. C'est un calcaire compact et non poreux. Les éléments de fantôme de roche sont situés autour des joints ouverts (A) ; B : Les analyses microscopiques de la roche fraîche (PPL) montrent de nombreux fossiles fortement cimentés par la matrice calcitique ; C : Les analyses microscopiques de la roche altérée (PPL) révèlent des vides (taches brunes) entre les fossiles. La matrice a été dissoute.
26When sedimentation and/or diagenesis are not homogeneous, the rock can present more porous area that can be organized along strata, lenses, etc., these areas are preferential flow paths. This is the case with the Oligocene bioclastic Limestone of Frontenac (Aquitaine, France) that presents a high initial porosity. Indeed, this limestone is slightly indurated due to low diagenetic process and very heterogeneous due to high lateral facies changes (Dubois et al., 2011). In this area, ghostrock features have been observed in quarries (fig. 14), they have globally horizontal extensions, connected to the surface through vertical cracks. The karst networks are structured as horizontal ghost-galleries linked by vertical ghost-pipes.
Fig. 14 - Bioclastic limestone of Bordeaux, Aquitaine, France (Dubois et al., 2011).
Fig. 14 - Calcaire bioclastique de Bordeaux, Aquitaine, France (Dubois et al., 2011).
A: The limestone is visible in Piquepoche Quarry, Frontenac. The darker area is the ghostrock features that have developed from an initial vertical fracture and then through a horizontal porous stratum; B: Rock sample of this limestone. Its texture shows fossils cemented to each other by calcite and a macroscopic porosity; C: The microscopic analyses of the fresh rock (XPL) expose the fossils, a calcitic matrix and some voids (black area); D: The microscopic analyses of the weathered rock (XPL) reveal big holes between the elements of the rock and the matrix; the fossils are riddled by holes.
A : Le calcaire est visible dans la carrière Piquepoche, Frontenac. La zone la plus sombre correspond aux éléments de fantôme de roche qui se sont développés à partir d'une fracture verticale initiale, puis à travers une strate poreuse horizontale ; B : Échantillon de ce calcaire. Sa texture montre des fossiles cimentés les uns aux autres par la calcite et une porosité macroscopique ; C : Les analyses microscopiques de la roche fraîche (XPL) mettent à nu les fossiles, une matrice calcitique et quelques vides (zone noire) ; D : Les analyses microscopiques de la roche altérée (XPL) révèlent de gros trous entre les éléments de la roche et la matrice ; les fossiles sont criblés de trous.
27In some cases, inherit karst systems have suffered a ghostrock karstification. It should be noted that all these systems are not within the conditions of ghostrock karstification. Indeed, the specificity of the ghostrock process is the resulting material left in-situ and the water flowing with little hydrodynamic energy. Thus, if an emptied gallery is in a system with high difference in elevation between the input and the output, the conditions for ghostrock karstification will not be met. By contrast, if the difference in elevation is low or if the gallery is filled by sediments, ghostrock karstification is conceivable. This is the case of the keyhole galleries in Gauthier-Wincqz Quarry, Soignies (Quinif et al., 1993). These features comprise 3 filling materials (fig. 15): stratified silt deposits at the top, a thin fluvial deposit with highly weathered quartz pebbles in the center and the residual alterite resulting from ghostrock karstification at the bottom. The presence of fluvial deposit indicates that the gallery had once been opened and that pebbles were carried and deposited there by an energetic water flow. However, the pebbles are now highly weathered and they could not have been transported in this state by the stream. Therefore, it appears that both weathering of the pebbles and weathering of the underlying limestone occurred after the filling of the gallery (Dubois et al., 2014a). The organization of this kind of network depends on the geometry of the initial karst system.
Fig. 15 - A ghostrock feature shaped as a keyhole at Gauthier-Wincqz Quarry in Soignies, Hainaut, Belgium.
Fig. 15 - Un fantôme de roche en forme de trou de serrure à la carrière Gauthier-Wincqz à Soignies, Hainaut, Belgique.
A: Photo from Quinif et al. (1993); B: Sketch of the feature. 1. Fresh unweathered limestone; 2. Stratified silt deposits; 3. Thin fluvial deposit with highly weathered quartz pebbles; 4. Residual alterite resulting from ghostrock karstification.
A : Photo issue de Quinif et al. (1993) ; B : Croquis de la forme. 1. Calcaire frais non altéré ; 2. Dépôts de limon stratifiés ; 3. Mince dépôt fluviatile à galets de quartz fortement altérés; 4. Altérite résiduelle résultant de la karstification de type fantôme de roche.
28The acid required for the dissolution of the limestone may have various origins: (i) pedological CO2; (ii) H2SO4 from sulphide oxidation; (iii) hydrothermal CO2; (iv) bacterial activity.
29(i) When meteoric waters infiltrate in the soil horizons, they become acidified by percolating through the organic matter which is rich in carbonic acid. This is probably the most common acid source for limestone weathering. In this case, the karst features develop from the top of the rock through the rock via joints or porous zones. It is not necessary to have a total dissolution of the carbonate at the top of the feature to get weathering in depth. Indeed, dissolution kinetics (Roques and Ek, 1973) shows a differential rate of dissolution according to the size of the particles. In reality the elements with high surface/volume ratio are more vulnerable to dissolution. Cave patterns from this process are mostly the combination of linear structures such as corridors, shafts, galleries.
30(ii) Sulphide minerals such as pyrite in the parent rock are very exposed to oxidation which produces sulfuric acid. This is a very corrosive acid that accelerate the dissolution of limestone. This source of acid has been related to the formation of ghostrock-feature in southern Belgium (Havron et al., 2007). As the acid source is dispersed through the rock, the structure of the karst feature is expending from the center of the weathering zone through the permeable pathways to form a star-shaped feature. Both hydrothermal and bacterial activity has been proved as potential source of acidity in the creation of karst in contexts identified as karstification by total removal. It is therefore likely that these sources of acidity are also involved in the process of ghost rock karstification. At present, hydrothermal or bacterial origin has not yet been proven in ghostrock karstification
31(iii) When they rise from the depth, hydrothermal waters are hot and can have high CO2 concentration (Audra et al., 2007; Bruxelles and Wienen, 2009; Egemeier, 1981; Klimchouk, 2007). In this case, the dissolution will spread from bottom to top but also in other directions according to the convection cells.
32(iv) Bacterial oxidation also has a role in the weathering of limestone (Barton et al., 2001; Northup and al., 2000). The most common reactions are redox reactions leading for example to the formation of sulphate from sulphide. However, these processes remain to be discovered in ghostrock karstification, even if the observations of bacteria have been made in Azé Cave (Burgundy, France) which is also known to be an emptied ghostrock (Baele et al., 2011; Papier et al., 2011). The structure of these features is star-shaped from the centrum of the weathering.
33From these considerations, it results that ghostrock karst systems can be located at any depth in the basement. As water flows slowly during the weathering, there is only little energy that is dissipated and the ghostrock systems is not hierarchized by the flow. It is solely organized according to the more permeable pathways. Their structure is then often labyrinthine with dead-end galleries (fig. 16). When the geological conditions change so as the height difference between input and output of the system increases (e.g., deepening of the valleys, uplift), groundwater potential increases. If the ghostrock feature is not covered (for example if it is intersected by the relief), it becomes a resurgence and water flows out of the alterite and evacuate it by regressive mechanical erosion (Bini et al., 2012; Dubois et al., 2011; Quinif and Maire, 2010). This phenomenon is also known as piping or sapping. When this erosion reaches the surface at the upstream of the system, a connected loss-resurgence system is established. The initial unique aquifer is then divided by karst networks.
34In this erosion stage, all the alterite is not evacuated, the emptied voids are often structured with the water flow and become drains while the other conducts remain clogged with the alterite and constitute annex systems as defined by Mangin (1975).
Fig. 16 - Examples of labyrinthine cave networks in which the residual alterite has been observed (Dubois et al., 2014a).
Fig. 16 - Exemples de réseaux de grottes labyrinthiques dans lesquels l'altérite résiduelle a été observée (Dubois et al., 2014a).
A: Trabuc Cave in Languedoc (France) is located at the contact between the Hettangian dolostone and Sinemurian limestone. This network comprises 10 km of galleries with difference in level of 200 m (Bruxelles, 1998); B: Fayt Cave in Ardennes (Belgium) is located in Devonian limestone. This network comprises 2 km of galleries with difference in level of 50 m; C: Felix-Mazauric network of Bramabiau Cave in Languedoc, France, is located in the Grands Causses. This network comprises 11 km of galleries with difference in level of 100 m (Bruxelles and Bruxelles, 2002).
A : La grotte de Trabuc en Languedoc (France) est située au contact de la dolomie hettangienne et du calcaire sinémurien. Ce réseau comprend 10 km de galeries avec un dénivelé de 200 m (Bruxelles, 1998) ; B : La grotte du Fayt en Ardenne (Belgique) est située dans le calcaire dévonien. Ce réseau comprend 2 km de galeries avec un dénivelé de 50 m ; C : Le réseau Félix-Mazauric de la Grotte de Bramabiau en Languedoc, France, est situé dans les Grands Causses. Ce réseau comprend 11 km de galeries avec un dénivelé de 100 m (Bruxelles et Bruxelles, 2002).
35Cavities from ghostrock karstification can be at three development stages: (i) filled with their alterite; (ii) emptied of their alterite but with no subsequent development of the cavity; (iii) emptied of their alterite with a subsequent hydrological development of the cavity that has modified or erased the inherited shape of the ghostrock.
36When the alterite is still in place, it is difficult to get a 3D view of the ghostrock. The morphology description is thus based on the 2D sections. For the cavities emptied of their alterite, it is not always easy to collect evidences for its ghostrock origin. The best is to find remaining alterite in some locations in the system. As for general karst morphologies, ghostrock cavities can have a horizontal or/and vertical extension. The cavities presented here constitute a non-exhaustive list of karst features where ghostrock karstification has been observed.
37Quentin Cave is a cave discovered in Nocarcentre Quarry (Soignies, Hainaut, Belgium). The dewatering of the quarry has caused the lowering of the water table. During 50 years of mining, regressive erosion of the alterite has excavated a network of galleries with a total length about 100 m (Quinif and Maire, 2010). This cave has developed on two subvertical fractures sets; therefore, the galleries have cross-sections higher than large. The wall morphologies present horizontal notches and ceiling pockets. The water stream responsible of the erosion of the alterite sprang directly from it (fig. 17). In two months, a new lower level of galleries has been opened by the river, with the drying of the upper galleries.
Fig. 17 - Quentin Cave in Nocarcentre Quarry, Soignies, Hainaut, Belgium (from Quinif and Maire, 2010).
Fig. 17 - Grotte Quentin dans la carrière de Nocarcentre, Soignies, Hainaut, Belgique (d'après Quinif et Maire, 2010).
A-B: Galleries of the cave; C: End of the gallery, the water spring out of the alterite; D: Map of the cave. 1. Rock slide. 2. River. 3. Fine sediments (reworked alterite). 4. Vault cupola. 5a. escarpment. 5b. Quarry wall.
A-B : Galeries de la grotte ; C : Au bout de la galerie, l'eau jaillit de l'altérite ; D : Plan de la grotte. 1. Pente. 2. Rivière. 3. Sédiments fins (altérite remaniée). 4. Coupole de voûte. 5a. Escarpement. 5b. Paroi de la carrière.
38La Fuie Cave, Chasseneuil (Charente, France), is a ghostrock cave where the alterite is still partly present. There are the seasonal variations of the water table level that cause its erosion (Dandurand and Maire, 2011). The slow erosion of the alterite along the walls is monitored. The weathering has formed ellipsoidal galleries (fig. 18). Numerous levels of beds of cherts are present in this limestone. They also suffered weathering, which has weakened their structure, making them brittle.
Fig. 18 - La Fuie Cave in Chasseneuil, Charente, France.
Fig. 18 - Grotte de la Fuie à Chasseneuil, Charente, France.
A-B: Galleries of the cave. Beds of weathered cherts are left in positive relief by the erosion of the alterite; C: Map of the cave (Dandurand, 2011; Dandurand and Maire, 2011). 1. Sinkhole; 2. Hopper – Fontis; 3. Channel; 4. Flow direction; 5. Collapse blocks; D: Slice in a weathered chert; a-slightly weathered zone; b-moderately weathered zone; c-highly weathered zone; E: Microscopic study of the weathered chert. On the perimeter there is a border of iron oxides.
A-B : Galeries de la grotte. Des lits de cherts altérés sont laissés en relief par l'érosion de l'altérite ; C : Plan de la grotte (Dandurand, 2011 ; Dandurand et Maire, 2011) ; 1. Doline ; 2. Trémie – Fontis ; 3. Chenal ; 4. Sens de l'écoulement ; 5. Blocs éboulés ; D : Lame dans un chert altéré ; une zone légèrement altérée ; b- Zone modérément altérée ; c- Zone fortement altérée ; E : Étude microscopique du chert altéré. Sur le périmètre, il y a une bordure d'oxydes de fer.
39The Barrique Hole, Saint Martin du Puy (Aquitaine, France), is a cave whose origins is a ghostrock network (Lans, 2014; Lans et al., 2006). It is a loss-resurgence karst system with 3 main losses and a resurgence linked by a principal meandering gallery (fig. 19). In enclosed places where mechanical erosion by the water flow was less pronounced, there is remaining alterite. The initial ghostrock was ellipsoidal shaped along the stratification. The initial permeability is constituted by higher porosity zones due to the low diagenetic process. This gallery makes the connection between resurgence, opened in the valley, and a swallow hole in the molassic overburden. After the partial erosion of the alterite, the gallery has evolved and was overdeepened by torrential stream with potholes (Dubois et al., 2011; Lans et al., 2006).
Fig. 19 - La Barrique Hole in Saint Martin du Puy, Aquitaine, France (Lans et al., 2006).
Fig. 19 - Trou de la Barrique à Saint Martin du Puy, Aquitaine, France (Lans et al., 2006).
A: Gallery in the cave; B: Map of the cave; 1. Major faults. 2. Areas with lot of speleothems.
A : Galerie dans la grotte. B : Plan de la grotte. 1. Failles majeures. 2. Zones avec beaucoup de spéléothèmes.
40Bucco della Volpe Cave, Monte Bisbino (Lago di Como, Italia), shows weathered strata and vertical joints. The western side of the lake is entirely composed of Moltrasio limestone from the Lower Lias. This is a well stratified dark grey limestone with flint nodules and interbedded with clays and marls. At present times, this cave constitutes the resurgence of the waters from Monte Bisbino karst fig. 20). Other cavities in the mountain (Grotta dell’Alpe Madrona and Zocca d’Ass) have revealed many ghostrocks. The altitude of the resurgence corresponds to the lowest level of ghostrock karstification (Tognini, 1999).
Fig. 20 - A cross-section through Monte Bisbino, Lombardy (Italy) showing the three main cave systems resulting from ghostrock karstification (Dubois et al., 2014a).
Fig. 20 - Coupe transversale du Monte Bisbino, Lombardie (Italie) montrant les trois principaux systèmes de grottes résultant de la karstification par fantômisation (Dubois et al., 2014a).
A: Bucco della Volpe; B: Grotta dell “Alpe Madrona”; C: Zocca d'Ass. The karst systems have developed above the limit of ghostrock weathering as depicted by the dotted line.
A : Bucco della Volpe ; B : Grotta dell « Alpe Madrona » ; C : Zocca d'Ass. Les systèmes karstiques se sont développés au-dessus de la limite d'altération de la fantômisation, comme illustré par la ligne pointillée.
41Pits, wells and shafts come from the emptying of vertical and subvertical ghostrock features that developed mainly on vertical joints. In some cases, these wells are intersected by the topography, and they are used to access the karst system. Vertical wells also make links between different gallery levels in the systems, but with no hydrogeological structure. In many cases, wells from ghosts-rock features are blind chimneys and end on squeezes at their top and bottom.
42In the area of Tournai (Hainaut, Belgium), the emptying of ghostrock features was activated by the artificial lowering of the water table due to the mining exploitation of quarries (Kaufmann and Quinif, 1999; Vergari et al., 1995). This led to the creation of collapse sinkholes such as the Gi Hole (fig. 21-22) (Quinif and Rorive, 1990). Wells are also to be found when cut in the quarries and their walls have benches following the bedding.
Fig. 21 - Collapse sinkholes, Gaurain-Ramecroix near Tournai (Hainaut, Belgium).
Fig. 21 - Gouffres d'effondrement, Gaurain-Ramecroix près de Tournai (Hainaut, Belgique).
Sinkhole opened by the lowering of the water table due to the mining of quarries and water exploitation (A and B). The limestone lies under a thick overburden.
Gouffre ouvert par l'abaissement de la surface piézométrique dû au creusement des carrières et à l'exploitation de l'eau (A et B). Le calcaire repose sous des formations de couverture.
Fig. 22 - Gi Hole in Gaurain-Ramecroix near Tournai (Hainaut, Belgium).
Fig. 22 - Trou Gi à Gaurain-Ramecroix près de Tournai (Hainaut, Belgique).
The cavity is part of an emptied ghost corridor. The void has been created by compaction of the alterite due to the lowering of the water table. A-B: Sinkhole in a field; C: The walls of the cave show lateral benches; D: Cross section of the pit. From the bottom to the top of the section, we have Carboniferous limestones, Cretaceous marls, Thanetian sands and soil.
La cavité fait partie d'un couloir fantôme vidé. Le vide a été créé par compactage de l'altérite dû à l'abaissement de la surface piézométrique. A-B : Gouffre dans un champ ; C : Les parois de la grotte présentent des banquettes latérales ; D : Coupe transversale du puits. Du bas vers le haut de la coupe, nous avons des calcaires carbonifères, des marnes crétacées, des sables thanétien et le sol.
43In the Valle Imagna (Alps, Italy), there are many caves presenting numerous dead-end chimneys (fig. 23A). Several outcrops near those caves, exhibit ghostrock features such as vertical corridors whose walls are carved of horizontal grooves and features from differential dissolution (fig. 23 B and C). These cavities are ghost-endokarst partially emptied.
Fig. 23 - Val Imagna, Alps, Italy.
Fig. 23 - Val Imagna, Alpes, Italie.
A: Cross-section of the Bus de la Siberia Cave. This cavity results from the emptying of a ghostrock network. It presents numerous blind pits; B: Outcrops of ghostrocks corridors; C: Side wall of a vertical ghost-corridor.
A : Coupe transversale de la grotte du Bus de la Siberia. Cette cavité résulte de la vidange d'un réseau de fantômes de roche. Il présente de nombreuses puits aveugles ; B : Affleurements de couloirs fantômes ; C : Paroi latérale d'un couloir fantôme vertical.
44In the Causses Plateaus (Midi-Pyrénées and Languedoc, France), occurrences of ghostrock karstification have been shown (Bruxelles, 2002; Bruxelles and Bruxelles, 2002). A lot of vertical pits (Abîme de Rabanel or Aven Armand) can also be interpreted like ghostrock (fig. 24). From a general point of view, all those cavities could be reinterpreted as ghostrock.
Fig. 24 - Trabuc Cave, Gard, France.
Fig. 24 - Grotte de Trabuc, Gard, France.
A: Map of the cave organized on 2 fracture sets; B: Great gallery on an initial ghostrock corridor; C: Junction between two great galleries; D: Cupolas resulting from the ghostrock process.
A : Plan de la grotte organisé sur 2 jeux de fractures ; B : Grande galerie sur un premier couloir fantôme ; C : Jonction entre deux grandes galeries ; D : Coupoles résultant de la fantômisation.
45In La Rochefoucauld karst system (Charente, France), ghostrocks morphologies consist in galleries locally enlarged to become rooms by the collapse of the roof. Fallen blocks and sandy-clayed sediments fill partially the voids resulting from the erosion of alterite (Dandurand, 2011; Dandurand and Maire, 2011; Dandurand et al., 2014).
46In Han-sur-Lesse Cave systems (Namur, Belgium), it has been demonstrated that the chambers were created by ghostrock karstification as remaining alterites were found on weathered strata at the top of the chambers (fig. 25). In a side network (Père Noël Cave), chambers come from the coalescence of emptied parallel ghostrock galleries whose separating walls have collapse. In the dell ‘Alpe Madrona Cave (Alps, Italy), chambers are the result of the collapse of overlapping and intersecting emptied ghost-galleries.
Fig. 25 - Weathered stratum at the top of the Antiparos Room, Han-sur-Lesse Cave, Namur (Belgium).
Fig. 25 - Strate altérée au sommet de la salle d’Antiparos, Grotte de Han-sur-Lesse, Namur (Belgique).
The alterite appears darker behind the hammer. The micritic cement of the fossils is dissolved highlighting the sparitic parts.
L'altérite apparaît plus foncée derrière le marteau. Le ciment micritique des fossiles est dissous mettant en évidence les parties sparitiques.
47Parietal microforms can be classified into two categories: (i) microforms ones that can form with the alterite still filling the feature; and (ii) microforms that may only be formed when the alterite is evacuated. The firsts are the actual ghost-microforms, with the shaping of the rock walls while in contact with the alterite. When the alterite is removed, these features are similar to that attributed to karstification by total removal (fig. 26). The second develop once the alterite has been removed and a river flows in the cavity.
Fig. 26 - Parietal microforms in Piquepoche Quarry, Frontenac (Aquitaine, France).
Fig. 26 - Microformes pariétales de la carrière Piquepoche, Frontenac (Aquitaine, France).
A: Ghostrock feature partially emptied of its alterite due to the quarrying, anastomosis and pendants are visible; B: Limestone block that previously was a wall of a ghostrock with anastomosis and pendants. Without the presence of alterite, it is not possible to determine the karst process that led to the creation these forms.
A : Forme issue de fantôme de roche partiellement vidée de son altérite en raison de l'extraction, des anastomoses et des pendentifs sont visibles ; B : Bloc de calcaire qui était auparavant un mur de fantôme de roche avec anastomoses et pendants. Sans la présence d'altérite, il n'est pas possible de déterminer le processus karstique qui a conduit à la création de ces formes.
48Ceiling pocket and domes. Ghostrock features can present ceiling pockets and domes on their walls in contact with the alterite (fig. 27A). These microforms can have diameters ranging between 10 cm to a few meters. As for karstification by total removal, they come from the lateral weathering of the bedrock from a central point. The shape is determined by the heterogeneity of the initial rock.
49Notches and projections. Ghostrock features can evolve laterally from an initial zone in the rock. According to the differences in the rock properties, the weathering can spread differentially in a non-homogeneous way creating notches and projections in the walls (fig. 27B).
50Differential dissolution features. The fragile features which appear in relief from a cave wall have been always interpreted as phreatic features, created with a very low flow. However, these stone laces are common in ghostrocks features (fig. 27C). Therefore, they cannot be used to determine the speleogenetic process. The stone laces can result from different causes leading to the differential dissolution of the rock: tension gashes filled with calcite, clayed or silica-rich strata, fossils, cherts, etc., depending on the lithology of the limestone. They are revealed after the emptying of the alterite.
51Anastomoses and pendants. Anastomoses are often associated to the karstification by total removal of the walls of the gallery above fluvial deposits, but they can also develop in ghostrock features where the filling material is the alterite (fig. 27D).
Fig. 27 - Parietal microforms observed in ghostrock features.
Fig. 27 - Microformes pariétales observées dans les fantômes de roche.
A: Ceiling pocket, Clypot Quarry, Soignies (Hainaut, Belgium); B: Notches and projections, Hainaut Quarry, Soignies (Hainaut, Belgium); C: Calcite vein highlighted by the differential dissolution, Milieu Quarry, Tournai (Hainaut, Belgium); D: Anastomosis and pendants, Piquepoche Quarry, Frontenac (Aquitaine, France).
A : Coupoles, Carrière du Clypot, Soignies (Hainaut, Belgique) ; B : Banquettes latérales, Carrière du Hainaut, Soignies (Hainaut, Belgique) ; C : Veine de calcite mise en évidence par la dissolution différentielle, Carrière du Milieu, Tournai (Hainaut, Belgique) ; D : Anastomose et pendants, Carrière Piquepoche, Frontenac (Aquitaine, France).
52Every feature previously classified as created by a river flow can affect a ghost-rock cavity when it has been emptied: e.g., potholes, overdeepenings, scallops, parietal clints, vertical flutes, meander niches. These features are evidently associated to fluvial erosion in a gallery, but they do not allow to make an interpretation on the process that had led to the creation of the cavity: total removal karst or emptied ghost-rock feature. We have to keep in mind that a parietal microform is a witness of only the last erosion process.
53Figure 28 shows a ghost-rock gallery emptied of its alterite (Dubois et al., 2011). Relicts of the alterite are preserved in the upper part of the gallery. Two hypotheses can be made to explain the evolution of the gallery. (i) A void developed at the top of the section from a joint along a weathered stratum (fig. 28B). The subsequent fluvial evolution shaped the fresh rock with overdeepening and benches. (ii) The whole cavity was entirely pre-shaped as a ghost -rock feature and the subsequent fluvial evolution resulted in the emptying of the alterite (fig. 28C). Even if the two hypotheses are possible, the first one is more probable. It is more likely that the weathering developed from a horizontal joint in the upper strata. Moreover, the upper part of the gallery shows a different morphology compared to the lower part. Indeed, the shape of the walls in the lower part shows characteristic morphologies that result from the erosion by a turbulent stream. It is likely that the bottom of the gallery was reshaped by fluvial flow.
Fig. 28 - The Barrique Hole, Saint Martin du Puy (Aquitaine, France).
Fig. 28 - Trou de la barrique, Saint Martin du Puy (Aquitaine, France).
A: Ghost-rock gallery emptied of its alterite with a subsequent hydrological development of the cavity (Dubois et al., 2011). Overdeepenings and potholes are visible on the walls and on the ground of the cavity. 1. Fresh rock; 2. Alterite; 3. Void. Two hypotheses of evolution: the void at the top of the section developed along a weathered stratum and a subsequent fluvial evolution shaped the overdeepening and benches (B); The whole cavity was entirely shaped by ghost-rock karstification and the free flow in open cavity resulted in the removal of the alterite (C).
A : Galerie issue de fantôme de roche, vidée de son altérite avec un surcreusement fluviatile ultérieur (Dubois et al., 2011). Un surcreusement et des marmites de géant sont visibles sur les parois et sur le sol de la cavité. 1. Roche fraîche ; 2. Altérite ; 3. Vide. Deux hypothèses d'évolution : le vide au sommet de la coupe s'est développé le long d'une strate altérée et une évolution fluviale postérieure a façonné le surcreusement et les banquettes (B) ; L'ensemble de la cavité a été entièrement façonné par la karstification de type fantôme et l'écoulement libre dans le fantôme de roche a entraîné l'élimination de l'altérite (C).
54An extensive literature discusses the relationship between cave patterns, cavity shapes, parietal microforms and their possible origin. Yet, these genetic conclusions are often drawn only regarding in situ observation and theoretical speculations without comparative experimental analysis. From these conclusions, scientists erroneously consider that this origin is the only one that can shape of this morphology, and when they encountered them in cave, they consequentially deduct the origin. However, we demonstrated in this paper that the origin of cave morphologies is rarely unique, except maybe for scallops and potholes.
55Ghostrock karst are geological objective features when the residual alterite is still unremoved. When microforms are observed in ghostrock in direct contact with the alterite, it then becomes obvious that they result from the in-situ weathering of the limestone. So, in the cases, the only origin possible for these morphologies is the ghostrock karstification. On the other hand, when microforms are found in open conduits, it is therefore no more possible to assume the origin based solely on their morphologies, because it can both come from: karstification by total removal, ghostrock karstification or subsequent evolution of a previous conduit. To deduce the origin of the conduit, it is then necessary to consider the global geological context as well as all morphologies apparent in the cave.
56This paper is divided in two main parts. The first one focuses on a descriptive presentation of the karst morphologies, from the cave pattern to the parietal microforms through cavity shapes. The second one places these morphologies in the ghostrock karstification context. It was demonstrated how ghostrock karstification preferentially leads to the creation of labyrinthine network and anastomotic patterns. The ghostrock conduits can develop both in horizontal and vertical extensions and can create rooms at the crossing of several features. Parietal microforms have specifically been observed in ghostrock features: ceiling pocket, dome, notch, projection, differential dissolution feature, anastomose and pendant. Finally, when emptied of the residual alterite, the feature can evolve as any cavity and can show parietal forms of fluvial flows: potholes, overdeepenings, scallops, parietal clints, vertical flutes, meander niches, etc.
57By analysing microforms in ghostrock features, it has been shown that karst morphologies can have multiple causes and that it is no more possible to deduce the origins of caves only thanks to the observation of the features. This requires us to revise interpretations of the origins of caves from the observed morphologies.
Corresponding author: +32 (0)65374603
yves.quinif@umons.ac.be (Y. Quinif)