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The evolution of hydraulic conductivity and flow patterns, controlled by simultaneous precipitation and dissolution in porous rocks, was examined in a series of laboratory experiments. Linear flow experiments were performed in columns of crushed calcareous sandstone by injecting different concentrations of HCl/H2SO4 mixtures at various flow rates. The effect of simultaneous calcium carbonate dissolution and gypsum precipitation was analyzed. Changes in head gradient, recorded at specific time intervals during the experiments, were used to calculate overall hydraulic conductivity of each column. The effluent acid was analyzed for Ca2+ and SO4 2_ concentrations in order to calculate porosity changes during the experiments. After each experiment, the rock sample was retrieved and sectioned in order to study the pore space geometry, micromorphology, and mineral concentrations. Arange of injected H+/SO4 2_ ratios and flow rates was identified which leads to oscillations in the effective hydraulic conductivity of the evolving carbonate rock samples. Because the dissolution of calcium carbonate is a mass transfer limited process, higher flow rates cause a more rapid dissolution of the porous medium; in such cases, with dissolution dominating, highly conductive flow wormholes were observed to develop. At slower flow rates, no wormhole formation was observed, but the porosity varied in different parts of the columns. Analysis of the sectioned parts of the column, after each experiment, showed that total porosity increased significantly by dissolution of carbonate mineral near the inlet of the column and decreased along the interior length of the column by gypsum precipitation. These findings are in qualitative accordance with conceptual understanding of such phenomena
Hypogenic speleogenesis can be identi?ed at different scales (basinal ?ow patterns at the regional scale, cave patterns at cave system scale, meso- and micromorphology in cave passages). We focus here on small scale features produced by both corrosion and deposition. In the phreatic zone, the corrosion features (speleogens) are a morphologic suite of rising ?ow forms, phreatic chimneys, bubble trails. At the water table are thermo-sulfuric discharge slots, notches with ?at roofs. Above a thermal water table the forms re?ect different types of condensation runoff: wall convection niches, wall niches, ceiling cupolas, ceiling spheres, channels, megascallops, domes, vents, wall partitions, weathered walls, boxwork, hieroglyphs, replacement pockets, corrosion tables, and features made by acid dripping, such as drip tubes, sulfuric karren and cups. Each type of feature is described and linked to its genetic process. Altogether, these features are used to identify the dominant processes of speleogenesis in hypogenic cave systems. Hypogenic caves were recognized early, especially where thermal or sulfuric processes were active (MARTEL, 1935; PRINCIPI, 1931). However SOCQUET (1801) was one of the earliest modern contributors to speleogenetic knowledge, and probably the ?rst to identify the role of sulfuric speleogenesis by condensation-corrosion due to thermal convection. More recent major contributions evidenced the role of sulfuric speleogenesis and hydrothermalism (e.g. DUBLYANSKY, 2000; EGEMEIER, 1981; FORTI, 1996; GALDENZI AND MENICHETTI, 1995; HILL, 1987; PALMER AND PALMER, 1989). However, most of these case-studies were often considered as “exotic”, regarding the “normal” (i.e. epigenic) speleogenesis. Only recently, KLIMCHOUK (2007) provided a global model, allowing the understanding of “hypogenic” speleogenesis and gathering the characteristics of hypogenic caves. Consequently, the number of caves where a hypogenic origin is recognized dramatically increased during the last years. The hypogenic origin can be recognized at the regional scale (deep-seated karst in basins), at the scale of an individual cave system because of distinctive features in its pattern, by studying the morphology of the cave conduits, or at the local scale of wall features made by corrosion processes (i.e. speleogens). Such type of features depict the characteristics of local cave development, and by extension the characteristics of speleogenesis. The description and interpretation of hypogenic speleogens is generally scattered in the literature. The aim of this paper is to gather the most important hypogenic speleogens, considered here as indicators, and used for the identi?cation and characterization of the hypogenic speleogenesis. Our knowledge is based on the compilation of about 350 caves from the literature, and the study of some of the most signi?cant caves (AUDRA, 2007; AUDRA et al., 2002, 2006). In this paper, we focus on the speleogens (i.e. wall- scale corrosion features) as indicators of hypogenic speleogenesis; we exclude here solution feature at larger scale such as conduits and cave systems and depositional features (sediments). Some of the features observed in the sulfuric caves are speci?cally caused by this strong acid. Some features are closely associated with hydrothermalism. Other features that are widespread in hypogene caves are created without sulfuric in?uence. The following typology mainly takes into account the type of runoff. In con?ned settings with slow phreatic ?ow, cave features are common to all types of hypogene processes, whether they are sulfuric or not (i.e. carbonic, hydrothermal…). In uncon?ned settings, condensation-corrosion processes take place above the water table. These aerial processes, enhanced by the oxidation of sul?des by the thermal convections, and by the microbial processes, result in a large variety of cave features. Some features are closely related to speci?c processes. Consequently, they are considered as valuable indicators of the sulfuric speleogenesis.
Cave sediments are commonly fine grained and lack macroscopic sedimentary structures. Only a detailed analysis of the micromorphological characteristics permits an accurate determination of the sedimentary dynamics of such cave deposits. Microscopic sorting, grading, clast orientation, lamination, intercalation, deformation structures, and porosity are some of the features used to identify microfacies such as lacustrine, slack water, debris flow, slumping, sheet wash, hyperconcentrated flows, and solifluction. In combination with micromorphological data derived from post-depositional diagenetic trend sand anthropogenic evidence, it is possible to reconstruct the evolution of a cave, and the climatic history and landscape volution of the area.
Thermal waters moving through soluble rock may create voids ranging in sizes from enlarged porosity and cavernosity to extensive two- and three-dimensional cave systems. Hydrothermal caves develop in a number of settings including deep seated phreatic, shallow phreatic (near-water table), and subaerial (above the thermal water table). Speleogenesis in eachsetting involves specific mechanisms, resulting in diverse features of cave macro-, meso-, and micromorphology. Mechanisms most characteristic of the hydrothermal speleogenesis are the free convection (in both subaqueous and subaerial conditions) and the condensation corrosion. This chapter describes the morphology of hydrothermal caves
Clay cortex from the contact zone between the host rock (chalk) and infilling deposits were examined in
paleokarst forms (pockets, pipes, and dolines of different age) from the Lublin–Volhynia chalk karst region. In light of the sedimentological and micromorphological analyses, it seems possible to work out a model as the basis for genetic and stratigraphic discussions. (1) Dolineswith the Paleogene orNeogene mineral infills are characterized by (a) homogeneous, residual type of massive clay gradually passing into the chalkmonolith, and at the sametime(b) relatively thickweathered zone. (2) Pipeswith glacigenic mineral infill fromthe Saalian Glacial are characterized by (a) sharp contact between host rock and clay, (b) narrow weathering zone of chalk, (c) diffuse nature of the contact zone between residual clay and mineral infill, and (d) contamination of clay by clastic material. (3) Pocketswith glacigenic mineral infill and traces of theWeichselian periglacial transformation are characterized by (a) strong contamination of chalk by quartz grains, (b) diffuse transition between clay and infill: fromclayey matrixwith single quartz grains (at the contactwith chalk) to clayey coatings and intergranular bridges (in the infill), (c) intensive weathering (cracking) of mineral grains in the infill.