Hypogene speleogenesis

MORP 00122 Treatise on Geomorphology. Vol. 6 Shroder, J., Jr., Frumkin, A. (Eds.) Academic Press, San Diego, CA MORP122 c0010 6.27 Hypogene Speleogenesis AB Klimchouk, Ministry of Education and Science and National Academy of Sciences of Ukraine, Simferopol, Ukraine r 2012 Elsevier Inc. All rights reserved. F Introduction Basic Concept and Definitions Hypogene Speleogenesis in the Framework of Hierarchical Flow Systems Evolution of Hydrogeologic Settings Dissolution Processes in Hypogene Speleogenesis Distribution of Hypogene Speleogenesis Hydrogeologic Control of Hypogene Speleogenesis Solution Porosity Patterns Produced by Hypogene Speleogenesis Mesomorphology Features of Hypogene Caves Hypogene Speleogenesis and Paleokarst Summary 2 3 4 4 7 10 10 12 16 17 19 19 d0030 d0035 d0045 ST R FI d0025 ER d0020 d0040 d0050 d0055 d0060 d0065 VI d0015 Hypogene (karst, caves, and speleogenesis) Refers to the origin by water that recharges the cavernous zone from below, independent of recharge from the overlying or immediately adjacent surface. Intrastratal karst Dissolution features that develop within rocks already buried by younger strata, where karstification is later than deposition of the cover rocks; a group of four types of karst in the evolutionary classification of karst types. Karst subsidence Lowering of the surface of the ground because of removal of support due to subterranean solution or collapse of caves. Phreatic zone The zone in the subsurface in which all the openings are completely filled with water (also called saturated zone). Vadose zone The zone between the land surface and the water table, in which water does not fill all the openings and moves by gravity and capillarity. Unconfined aquifer An aquifer that is not confined by overlying low-permeability material, limited above by a water table that is free to rise and fall. EL SE d0010 Breakthrough The moment in the early cave conduit development when dissolution has enlarged the fracture to the point where water can pass through the entire length of the fracture remaining considerably undersaturated. Confined aquifer An aquifer where groundwater is under pressure in a bed or stratum confined above and below by units of distinctly lower permeability. Epigene (karst, caves, and speleogenesis) Refers to the origin in the near-surface settings by downward recharge from the overlying or immediately adjacent surface. Evaporites Inorganic sedimentary rocks formed by chemical precipitation in a concentrated solution. Evolutionary classification of karst Classification that views types of karst as successive stages in geological/ hydrogeological evolution of a soluble formation, from deposition through burial to reemergence, between which the major boundary conditions, the overall circulation pattern, factors, and mechanisms of karst development change considerably. Gypsum A mineral composed of the hydrated calcium sulfate, CaSO4.2H2O. Also refers to a rock composed of the mineral gypsum. PR Glossary O O 6.27.1 6.27.2 6.27.3 6.27.4 6.27.5 6.27.6 6.27.7 6.27.8 6.27.9 6.27.10 6.27.11 References Klimchouk, A.B., 2012. Hypogene speleogenesis. In: Shroder, J., Jr., Frumkin, A. (Eds.), Treatise on Geomorphology. Academic Press, San Diego, CA, vol. 6, pp. xx–xx. [Please replace ‘xx’ by correct page number when available.] Treatise on Geomorphology, Volume 0 doi:10.1016/B978-0-08-088523-0.00122-2 1 MORP 00122 2 Hypogene Speleogenesis Abstract PR AU1 O O F Recognition of the wide occurrence, significance, and specific characteristics of hypogene speleogenesis during last two decades signifies a major paradigm shift in karst science, previously overwhelmingly dominated by epigene concepts and models. Hypogene karst is one of the fundamental categories of karst, at least of equal importance with more familiar epigenic karst. Hypogene and epigenic karst systems are regularly associated with different types, patterns, and segments of flow systems, which are characterized by distinct hydrokinetic, chemical, and thermal conditions. Hypogene speleogenesis is the formation of solution-enlarged permeability structures by water that recharges the cavernous zone from below, independent of recharge from the overlying or immediately adjacent surface. It develops mainly in leaky confined conditions, although it may continue through unconfined ones. Hydraulic communication along cross-formational flowpaths, across lithological boundaries, different porosity systems, and flow regimes allows deeper groundwaters in regional or intermediate flow systems to interact with shallower and more local systems, permitting a variety of dissolution mechanisms to operate. A specific hydrogeologic mechanism acting in hypogenic transverse speleogenesis (restricted input/output) suppresses the positive flow-dissolution feedback and speleogenetic competition seen in the epigenic development. Hypogenic caves occur in different soluble rocks in a wide range of geological and tectonic settings, basinal through orogenic. Overall patterns of cave systems are strongly guided by the spatial distribution of the initial (prespeleogenetic) permeability features and hydrostratigraphic barriers and interfaces within the soluble and adjacent units, by the mode of water input to, and output from, cave-forming zones and by the overall recharge–discharge configuration in the multiple aquifer system. Because of their transverse nature, hypogene caves have a clustered distribution in plan view, although initial clusters may merge laterally across considerable areas. Hypogene caves display remarkable similarity in their patterns and mesomorphology, strongly suggesting that the type of flow system is the primary control. The rapidly evolving understanding of hypogene speleogenesis has broad implications for many applied fields such as prospecting and characterization of hydrocarbon reservoirs, groundwater management, geological engineering, and mineral resources industries. 6.27.1 Introduction p0010 During the twentieth century, studies of karst systems had been mainly concerned with shallow, unconfined geologic settings, supposing that the karstification is ultimately related to the Earth’s surface and that dissolution is driven by downward water recharge. The respective karst systems are termed epigenic. Concepts and models in karst geomorphology and hydrogeology that deal with origin, functioning, and evolution of epigene karst overwhelmingly predominate. Recent advancements in karst and cave science, however, lead to the growing recognition of the fact that karst development driven by upward recharge to the soluble formation (hypogene speleogenesis) is not less important. The idea that some caves could form at depth by rising waters was introduced as early as in the mid-nineteenth century with regard to thermal systems. More significant attention has been given to hydrothermal speleogenesis since the midtwentieth century, particularly in Eastern Europe (Kunsky, 1957; Maksimovich, 1965; Dzulinski, 1976; Jakucs, 1977; Müller and Sárváry, 1977; Dublyansky, 1980; Dubljansky, 1990, 2000a). Later on, another chemical mechanism of cave development by rising waters was recognized, the oxidation of H2S and dissolution by sulfuric acid (Egemeier, 1973, 1981; Maslyn, 1979; Davis, 1980; Hill, 1981, 1987). By the end of the twentieth century, the term and concept of hypogene speleogenesis have been established, although largely limited to caves formed by the above-mentioned dissolution mechanisms (Ford and Williams, 1989; Palmer, 1991; Hill, 2000). A classical paper by Palmer (1991) on the origin of caves was a particularly important contribution to recognition and understanding of principal features of hypogene speleogenesis. R FI ER VI EL SE p0015 The recent past two decades have witnessed a growth in the number of in-depth studies of different kinds of hypogene speleogenesis in various places around the world, such as in the Western Ukraine (Klimchouk, 1992, 1996b, 2000b), Central Italy (Galdenzi and Menichetti, 1995; Galdenzi, 2009; Menichetti, 2009), Middle East (Frumkin and Fischhendler, 2005; Frumkin and Gvirtzman, 2006), Guadalupe Mountains and the Pecos River Basin, New Mexico and Texas, USA (Hose and Pisarowicz, 2000; Land et al., 2006; Stafford et al., 2009a), Australia (Osborne, 2001, 2007), and other regions. Also, it was recognized that cave development by various processes in deep-seated environments is much more common than previously thought (Palmer, 1995; Klimchouk, 2000a). At the same time, sedimentologists and industry geologists concerned with the origin of porosity began to realize limitations of the heavily used model of subaerial meteoric diagenesis in freshwater, which implied that deep-seated porosity in carbonates is related to unconformities and is mainly due to dissolution in paleo-vadose and paleo-phreatic freshwater zones (i.e., is paleokarst). They come to realize that deepburial diagenesis in the mesogenetic environment can contribute significantly to secondary dissolution porosity and permeability evolution in many carbonate hydrocarbon reservoirs (e.g., Mazzullo and Harris, 1992, 1994). Karst and cave scientists would treat this as karstification, or speleogenesis, although deep seated (Klimchouk, 2000a). Geologists studying the origin of carbonate-hosted ore deposits also often deal with deep-seated karst processes. The current burst of interest in hypogene speleogenesis in karst science has been largely related to the establishment of a hydrogeological rather than geochemical approach to its definition, which allowed the placement of hypogene speleogenesis into the systematic context of regional groundwater hydraulics (sensuTóth, 1995, 1999) and highlighted the p0020 ST s0010 p0025 AU2 p0030 MORP 00122 Hypogene Speleogenesis F O O p0055 EL SE VI p0045 ER FI R ST p0040 seated origin, or epigenic acids rejuvenated by deep-seated processes. Such caves have no relation to recharge through the overlying or adjacent surface. Later on, he broadened the definition: hypogenic caves are formed by water in which the aggressiveness has been produced at depth beneath the surface, independent of surface or soil CO2 or other nearsurface acid sources (Palmer, 2000). Substituting ‘acids’ for ‘aggressiveness’ is important, as aggressiveness in hypogene settings can also be supplied by nonacidic mechanisms. Speleogenetic studies in many regions, during the last two decades, have shown that caves that have no relations to recharge through the overlying surface form by a variety of geochemical mechanisms including those that do not rely on acids (such as dissolution of evaporites). At the same time, hypogene caves formed in different lithologies and by different dissolution processes demonstrate remarkable similarity in their patterns, morphologies, hydrostratigraphic occurrence, and current or inferred hydrogeologic functioning, which suggests their common hydrogeologic backgrounds. The modern hydrogeology recognizes the geologic role of groundwater flow systems (e.g., Sharp and Kyle, 1988; Tóth, 1995, 1999). Speleogenesis, one of the most expressive manifestations of the role of groundwater as a geologic agent, is a result of interaction between groundwater and its environment, determined by the various components and attributes of the two respective systems seeking equilibration. To cause speleogenetic development, dissolution effects of disequilibria have to accumulate over sufficiently long periods of time and/or to concentrate within relatively small rock volumes or areas. The systematic transport and distribution mechanism capable of supporting the required disequilibrium conditions is the groundwater flow system (Tóth, 1999), which suggests that the latter has to be a primary consideration. The present author adopts the hydrogeologic approach to hypogene speleogenesis, in which the latter is defined as the formation of solution-enlarged permeability structures by water that recharges the cavernous zone from below, independent of recharge from the overlying or immediately adjacent surface (Ford, 2006; Klimchouk, 2007, 2009b). The difference between the two approaches outlined above determines to a large extent the characteristics of speleogenetic environments and samples of caves to be considered as hypogene. For instance, the emphasis on aggressiveness (the geochemical approach) would imply that cave development in the mixing zone of unconfined karst aquifers (those receiving recharge from the overlying surface), such as by mixing of vadose and phreatic waters, or freshwater and marine water along the seacoast, would fall into the hypogene category, because the aggressiveness by mixing in the above situations is produced at depth below the surface and is not related to the surface. On the other hand, caves formed in gypsum strata by rising flow from an underlying aquifer, such as giant maze caves in the Western Ukraine or giant chamber caves (‘Schlotten’) in the South Hartz (Germany), would not classify as hypogene caves because the aggressiveness with regard to gypsum in these cases, although not related to near-surface acid sources, has not been gained at depth but kept because fresh groundwater infiltrated to the basal aquifers at distant recharge areas. Based on the geochemical approach, Palmer PR p0035 common hydrogeological genetic background and similarity of caves previously seen as unrelated, thus broadening the family of hypogenic caves (Klimchouk, 2007). The latter work has demonstrated that hypogene speleogenesis is not just an aberrant curious phenomena within the otherwise predominantly epigenic karst paradigm but also one of the fundamental categories of karst, at least of equal importance with epigene karst. The development of the hydrogeologic approach to hypogene speleogenesis was significantly promoted by acknowledgement of ideas of regional hydraulic continuity, well recognized in mainstream hydrogeology since the mid-twentieth century but largely overlooked in karst science until recently. The artesian paradigm, with its notions of confined flow through largely isolated aquifers, was replaced with the basin hydraulic paradigm, with its notions of a multiple aquifer system and significant leakage through strata (crossformational communication). The term ‘confined aquifer’ is not used anymore (and particularly in this chapter) in the sense of hydraulic isolation; a notion of semi-confinement is more appropriate as separating aquitards are commonly leaky at certain time and space scales. The recognition of the importance of cross-formational communication is particularly important for the understanding of hypogene speleogenesis (Klimchouk, 2007, 2009b). New developments have stimulated an intense ongoing reevaluation of the origin of many caves and previously accepted regional paleohydrogeological and karst evolution concepts according to the new understanding of hypogene speleogenesis, as well as further theoretical and modeling efforts. The recent advances in the topic were presented in two collections of papers (2009), ‘Advances in Hypogene Karst Studies’, published by the National Cave and Karst Research Institute (USA; Stafford et al., 2009b), and ‘Hypogene Speleogenesis and Karst Hydrogeology of Artesian Basins’, published by the Ukrainian Institute of Speleology and Karstology (Klimchouk and Ford, 2009). These volumes were the outcomes of the two major international theme conferences held in 2008 (a special symposium within the GSA-2008 program, Houston, USA) and 2009 (a conference in Chernivtsy, Ukraine). Speleogenesis is the creation and development of organized permeability structures that have evolved primarily through dissolution of a host rock (Klimchouk et al., 2000). It is the result of a complex interaction between geological, hydrogeological, and geochemical factors. These factors, as well as their interaction, are in some ways different for hypogene speleogenesis as compared to the better-studied epigene speleogenesis. Despite the recent rapid progress in studying hypogene speleogenesis, we are only at the beginning of its in-depth understanding. This chapter is an overview of principal regularities and factors of hypogene speleogenesis and features of respective solution porosity structures. 3 s0015 6.27.2 Basic Concept and Definitions p0050 The concept of hypogene speleogenesis does not necessarily mean cave development at great depth but, rather, refers to the origin of the cave-forming agency from depth. Palmer (1991) defined hypogenic caves as those formed by acids of deep- p0060 p0065 MORP 00122 Hypogene Speleogenesis p0070 The hydrogeological approach to hypogene speleogenesis allows placing it in the context of hierarchical organization of flow systems, characterized by hydraulic continuity and crossformational hydraulic communication (e.g., Pinneker, 1982; Sharp and Kyle, 1988; Shestopalov, 1981, 1989; Tóth, 1995, 1999). Principal continental karst-forming environments (epigene and hypogene) and resultant karst/speleogenetic styles are regularly associated with flow systems of different scales and their distinct regimes, dominated by distinct hydrokinetic, chemical, and thermal conditions (Klimchouk, 2007; Figure 1), which is a primary cause of genetic, functional, structural, and morphologic distinctions between epigenic and hypogene speleogenesis. Epigenic speleogenesis, directly related to the infiltration of meteoric water, is predominantly associated with local flow systems and/or recharge regimes of intermediate to regional flow systems. The recharge regime is characterized by highly variable input parameters, relatively high hydraulic heads, decreasing with depth, downward and divergent flow, chemically aggressive groundwaters with low TDS, oxidizing conditions, and negative anomalies of geothermal heat and gradient (Tóth, 1999). Hypogene speleogenesis (Figure 1) is associated with discharge regimes of regional or intermediate flow systems, either terminal or intervening, which establish in areas of potentiometric lows and breaches through major confinement. Discharge regimes are characterized by little variations of input parameters with low dependence on climate, relatively low hydraulic heads that decrease upward, resulting in converging and ascending flow, high TDS of groundwaters, chemical precipitation, accumulation of transported mineral matter, reducing conditions, and positive anomalies of geothermal heat and gradient (Tóth, 1999). Because of the vertical heterogeneity inherent in sedimentary sequences, an upwelling flow inherently implies a certain degree of hydrogeological confinement. Hypogene speleogenesis at depth occurs in leaky confined conditions. When hypogenic solution porosity structures are shifted to the shallower, unconfined situation due to uplift and denudation but their further development continues to be driven by upwelling flow from deeper systems, this is still hypogenic development although now unconfined. Unconfined hypogene development can be regarded as an extinction phase of hypogene speleogenesis. Exposed, unconfined karst aquifers are local systems characterized by recharge and discharge within the same outcrop. Epigenic cave systems in unconfined aquifers commonly p0095 p0085 p0090 EL SE VI ER p0080 FI AU3 R ST p0075 Hypogene Speleogenesis in the Framework of Hierarchical Flow Systems F 6.27.3 O s0020 contain phreatic parts, in which water flows under pressure in conduits being confined by the host rock itself. In discharge segments of epigenic phreatic systems, flow raises to surface outlets, but this is not a hypogene speleogenetic environment. Hypogene speleogenesis occurs where the whole aquifer or its substantial part is confined (not just water in individual passages) and where flow system of a larger scale contributes to the cave development in a given locality, entering the caveforming zone from below. More confusing situations, in terms of discriminating between phreatic epigene and hypogene environments, are those where the flow in a generally unconfined soluble aquifer rises through an area of interbedded varieties of the same soluble rock with different permeabilities, which creates local conditions of a leaky confined multiple aquifer system, and sometimes characteristic hypogene speleomorphs. If this situation is local, not dominating the whole aquifer or its large portion, it can be regarded as quasihypogene speleogenesis. However, in many cases, conditions often treated as bathyphreatic, where water infiltrated through an unconfined karst area flows to considerable depth (hundreds of meters) and then rises to the surface through the inhomogeneous sequence, may actually correspond to a multiple aquifer system and true hypogene speleogenesis. In the continental domain, meteoric recharge and topography-driven flow often dominate in regional groundwater systems, although nonmeteoric waters and other energy sources, such as sediment compaction and tectonic compression, can also contribute to variable extent. A topographydriven meteoric regime, with its own hierarchy of flow systems and interaction between them, is commonly perched above a compressional regime driven by tectonics. The interface between them migrates with geomorphic and tectonic developments, as well as with changes of the hydraulic function of soluble formations due to karstification. Zones of interaction between the two regimes are particularly favorable for hypogene speleogenesis. Hypogene speleogenesis is often a part of mixed flow systems, where gravity-driven flow interacts with flow driven by temperature or solute density gradients (Anderson and Kirkland, 1980; Klimchouk, 2007). Speleogenesis is a dynamic process capable of considerably changing primary porosity and permeability in soluble and overlying rocks. The overall hydrogeologic role of hypogene speleogenesis is the enhancement of vertical permeability and hydraulic communication across a sedimentary succession that contains soluble units. It can create zones of high vertical permeability along initially insignificant (in terms of regional or intermediate groundwater flow systems) cross-formational flow paths, or even without any initially guiding discontinuities, for example, through vertical stopping of breakdown above a hypogene chamber formed at the base of a soluble stratum. Thus, hypogene speleogenesis may give rise to new discharge limbs and contribute to segmenting laterally extensive ‘throughflow’ regions (Klimchouk, 2007). O (2007) placed the artesian cave development into the realm of epigenic speleogenesis, and cave development at the mixing zone in unconfined aquifers is placed into the hypogenic category. Within the hydrogeologic approach advocated by the present author (Klimchouk, 2007), the classifying of these environments is the opposite. The ongoing debates and live developments in studying hypogene speleogenesis will help to resolve this issue. PR 4 6.27.4 p0100 Evolution of Hydrogeologic Settings s0025 Speleogenesis and the aquifer evolution have to be viewed from the perspective of the overall geologic evolution. Evolutionary classification of karst types, elaborated by p0105 MORP 00122 Hypogene Speleogenesis 5 Recharge Recharge ES Local discharge l- Lo ca ca Lo + Eh Intermediate- HS HS e ++ΔT - CO2 Eh− r Inte Eh− Reg l l na gio Eh rites Organic carbon Igneous rocks CO2 Re − Evapo CO2 Igneous rocks Deep fluids Terrestrial heat flux Sedimentary rocks Recharge Eh+ Interm ediate HS m er HS Speleogenesis domains ER (b) VI ES Epigene speleogenesis qHS Quasy-hypogene speleogenesis HS Hypogene speleogenesis Epigene karst porosity Epigene cave systems (unspecified) EL SE Hypogene karst porosity Isolated chambers & ramiform caves Large rising conduits (shafts, etc.) Maze caves Isolated passages & rudimentary networks Vertical stoping structures (collapse shafts, breccia pipes, etc.): a b a - active; b - fossil Vertical sag structures Cavernous edging along fractures/conduits Breccia horizons f0010 AU23 AU24 CO2 H2 S ed ia Regional discharge te HS −ΔT HS Basinal fluids Region al Eh− CO2 +ΔT Legend Geologic features Soluble rocks: Limestone terrestrial heat flux Head level in major aquifers Flow systems: Local Intermediate Regional Salt Unsoluble rocks: Variable sedimentary rocks Faults Igneous rocks Hydrogeologic features Gypsum High permeability +ΔT CO2 Dolomite Low permeability HS Organic Sedimentary rocks carbon FI Sedimentary rocks HS ST Organic carbon Regional H2S R Eh − Int Eh+ Intermediate discharge PR Recharge Local discharge ES Regional/local discharge ES O O (a) Eh− ++ΔT iona ++ΔT H2S CO2 t dia me F HS Local qHS ed iat e- + Regional/local discharge l-phreatic ES m er Eh ES tic ea r ph Int Regional/intermediate discharge Local/intermediate discharge Specific flow directions Large to medium free convection systems, driven by density differences: a - thermal; b - solute load Color indicates temperature (blue - low, red - high) and salinity (blue - low, brown - high) of groundwater a b Intakes of deep fluids (a) and/or gases (b) +ΔT, −ΔT Temperature and gradient anomaly: positive, negative + − Eh , Eh Redox conditions: oxidizing, reducing a b c Spings: a - gravity; b-c - rising; blue - cold, red - hot and lukewarm Figure 1 A generalized sketch of hypogene speleogenetic situationsin various geologic settings and in the context of hierarchical flow systems: (a) in active orogenic setting and (b) in variously disturbed cratonic basinal settings. Some elements of the diagram were adopted from drawings of Kovács and Müller (1980), Tóth (1999), Klimchouk (2007), and Bayari et al. (2009). MORP 00122 6 Klimchouk (1996a) and Klimchouk and Ford (2000), provides a convenient framework to characterize speleogenetic environments (Figure 2). It distinguishes major stages in the geologic evolution of a soluble sequence as the types of karst, marked by distinct combinations of the diagenetic and structural prerequisites for groundwater flow and speleogenesis, flow regimes, recharge/discharge configurations, groundwater chemistry, and degree of inheritance from earlier conditions. They are (in the order as they potentially evolve): syngenetic/ eogenetic karst in freshly deposited rocks; deep-seated karst, which develops during the burial, particularly during mesogenesis (as soluble rocks reemerge to the surface due to uplift and erosion); subjacent karst, where the cover is locally breached by erosion; entrenched karst, in which valleys incise below the bottom of the karst aquifer and drain it, but where the soluble rocks are still covered by insoluble formations for the most part; and denuded karst, where the insoluble cover materials have been completely removed. If karst bypasses burial, or if the soluble rock is exposed after burial without having experienced any significant karstification during burial, it represents the open karst type. Deep-seated karst, subjacent karst, and entrenched karst represent the group of intrastratal karst types, whereas O O F denuded and open karst fall into the group of exposed karst types. Later on, karst may become mantled by a cover that develops contemporaneously with the karst, or reburied under younger rocks again to form paleokarst, and be reexposed again (exhumed karst). Although this classification does not directly specify the origin of caves, it characterizes dominant speleogenetic environments and their evolutionary changes. Karst types are viewed as stages of the hydrogeologic/geomorphic evolution, between which the major boundary conditions, the overall circulation pattern, and extrinsic factors and intrinsic mechanisms of karst development change considerably. The classification of karst types correlates well with the three major types of speleogenetic settings distinguished now (Klimchouk et al., 2000). Coastal and oceanic speleogenesis in diagenetically immature rocks falls into the syngenetic/eogenetic karst domain. Deep-seated karst is exclusively hypogenic. In subjacent karst, both hypogene and epigenic speleogenesis may operate, depending on the scale of the flow system, but hypogene speleogenesis still dominates. Entrenched and denuded karst types are overwhelmingly epigenic, with an inheritance of hypogenic features, which can be reworked by epigenic processes or get fossilized. In both karst types, PR Eogenetic stg. p0110 Hypogene Speleogenesis ST Syngenetic/eogenetic R Exposed karst ER Buried Limited or no inheritance intrastratal karst VI Uplift Telogenetic stage EL SE Subsidence Mesogenetic stage FI Mantled Entrenched Subjacent Exhumed Open Denuded Cover is older than karst Cover is contemporal with karst Deep-seated Cover is younger than karst Burial with no noticiable speleogenesis Karstifiable unit Underlying formation f0015 Figure 2 Evolutionary types of karst and speleogenetic environments. Reproduced from Klimchouk, A.B., Ford, D.C., 2000. Types of karst and evolution of hydrogeologic settings. In: Klimchouk, A., Ford, D.C., Palmer, A.N., Dreybrodt, W. (Eds.), Speleogenesis: Evolution of Karst Aquifers. National Speleological Society, Huntsville, pp. 45–53. p0115 MORP 00122 Hypogene Speleogenesis p0130 FI R AU4 Dissolution Processes in Hypogene Speleogenesis ER General geochemical characteristics of the discharge regimes of regional flow systems may seem not to favor speleogenesis. Also, there was a long-standing belief in the karst literature, based on the now-obsolete old simplistic notion of the artesian mechanism, which confined conditions generally offer limited dissolution potential for karstification as lateral artesian flow in aquifers containing soluble rocks would be saturated through the most of throughflow areas. However, the great importance of cross-formational hydraulic communication is almost universally accepted now in mainstream hydrogeology. It means leakage (recharge and discharge) across strata that were supposed to be ‘impermeable’ within the old artesian paradigm. The fundamental feature of hypogene speleogenesis is that it is driven mainly by transverse flow across boundaries between different formations, strata, and porosity systems, whereas the boundaries commonly coincide with major contrasts in water chemistry, gas composition, and temperature. This causes disequilibrium conditions and supports multiple dissolution mechanisms. Hypogene speleogenesis occurs where undersaturated fluids or hypogenic acids move from insoluble rocks into soluble ones, or where the aggressiveness is acquired or rejuvenated within the soluble formation in the course of VI p0125 6.27.5 EL SE s0030 p0135 AU5 PR O O F transverse flow across it because of mixing of different flow components, a drop in temperature, oxidation of sulfides, or other reactions. The variety of dissolution processes that can potentially operate in hypogene speleogenesis has been characterized by Palmer (1991, 1995, 2000, 2007) and Klimchouk (2000a, 2007). The most important processes are briefly overviewed below (Figure 3). Carbonic acid dissolution, which dominates epigenic carbonate speleogenesis, also operates as a hypogenic agent, though the origin of the acidity is different. It can be related to CO2 generated from igneous processes, by thermometamorphism of carbonates, or by thermal degradation and oxidation of deep-seated organic compounds by mineral oxidants. The first two origins are indicated by d13C values around or above zero % (vs. VPDB). Efflux of carbon dioxide of the deep origins into upper aquifers can be massive, pointwise, or disperse, depending on geologic/structural conditions. CO2 from oxidation of deep-seated organic compounds is common in the vicinity of hydrocarbon fields, where waters characteristically contain high CO2 concentrations (Kaveev, 1963). In several regions, the origin of CO2 by oxidation of methane is confirmed by very low (up to  32% vs. VPDB) values of d13C measured in CO2 itself or in calcite deposited with its participation (Gradziński et al., 2009; Klimchouk, 1994). CO2 is also a byproduct of several other reactions that take place in deep-seated environments, such as the reaction of calcite with sulfuric acid or other strong acids. Dissolution by cooling thermal water can occur along ascending flowpaths, even at constant CO2 levels. In a closed system, the solubility of calcite increases with decreasing temperature, so that more calcite can be dissolved. The effect increases with increasing CO2 partial pressure. This mechanism is commonly labeled as hydrothermal speleogenesis, occurring in high-gradient zones where ascending flow is localized along some highly permeable paths (Malinin, 1979; Dublyansky, 1980; Bakalowicz et al., 1987; Dubljansky, 1990, 2000; Ford and Williams, 1989; Palmer, 1991; Andre and Rajaram, 2005; see also Dubljansky’s Chapter in this volume). However, Palmer (1991, 2007) noted that dissolution by cooling of thermal water alone can produce substantial caves only under the most favorable conditions, such as at high thermal gradients sustained through long periods of time, and that most caves thought to be thermal actually owe their origin to the mixing of high-CO2 thermal water with low-CO2 water of shallower flow system. Hydrogen sulfide is another common hypogene source of acidity, abundant in deep meteoric groundwater and basinal brines. It is usually generated at depth by microbial or thermal reduction of sulfates at the presence of organic carbon (Machel, 1992). When dissolved in water, hydrogen sulfide forms a mild acid, but it can cause carbonate dissolution if this water flows elsewhere and mixes with water that has little or no H2S content (Palmer, 1991, 2007), or escapes from reducing zones as a gas and is reabsorbed in freshwater. H2S can also react with dissolved metals, such as iron, to produce sulfide ores and dissolve carbonate rocks. Despite the abundance of both major geogenic gases, CO2 and H2S in deep-seated fluids, these fluids are commonly saturated with respect to calcite due to the long residence time in deep-flow paths. Dissolution by water containing these ST p0120 however, still active but dying hypogene systems may operate. Open karst has exclusively epigenic speleogenesis. In deep-seated settings, speleogenetic development is generally slow, although it may encompass tens and even hundreds of millions of years. Regional tectonic disturbances and related changes in deep-flow systems may cause pulses of efflux of basinal fluids and geogenic gases into upper sequences and hence affect hypogene speleogenesis. When a cave-forming zone is shifted to a shallower position, cave development accelerates due to increasing leakage of deep flow to the surface and more vigorous mixing with shallower systems. The patterns of hypogene solution porosity structures become established during this stage. A more dramatic increase of flow and acceleration of cave growth occur when a major confining formation begins to breach by erosional entrenchment or upward propagation of stopping breakdown structures. Breaching of the major confinement, although commonly local, signifies the transition to the subjacent karst stage, during which most of the growth in hypogenic systems occurs. Flow pattern within cave systems reorganizes during this stage and focuses on few favorable paths according to the new configuration of recharge/discharge conditions, so that the patterns of hypogene caves can modify. Epigene processes start to contribute to karst development, further reworking hypogene features. With the onset of entrenched karst, unconfined conditions establish and epigenic karst processes began to dominate. They further modify or dramatically overprint hypogene features depending on climate and configuration of a local flow system. This process continues and becomes more diffused with the transition to denuded karst settings. However, hypogenic caves may pass the transient stages without major modification by the newly established flow patterns. 7 p0140 AU6 AU7 AU8 p0145 p0150 MORP 00122 Hypogene Speleogenesis B Calcite 10 Undersaturated 0 0 1 3 2 2 Mol added salt Concentration of CO2 or H2S in saturated solution, mmol l−1 (i) 0 1 2 3 4 5 6 Gyp. Dolomite Gypsum Gypsum 0 1 Added effect fo 2 cm3 l−1 NaCl 7 (ii) (iii) O Figure 3 Some mechanisms of renewal or enhancement of groundwater aggressiveness to soluble rocks. (i) Dissolution of carbonates by mixing. The graph shows saturation concentrations of calcite and dolomite as a function of CO2 or H2S concentration (mmol l1). The saturation concentration is given as volume of solid dissolved per liter (cm3 l1). Mixing of two solutions (A, B) produces a solution that is undersaturated (C) (ii) Gypsum solubility as a function of concentration of other salts. Magnesium chloride and nitrate have a particularly strong effect on gypsum solubility. (iii) The effects of dissolution in mixed carbonate/sulfate strata. The diagram shows solubilities of calcite, dolomite, and gypsum: (a) as individual minerals at 15 1C and PCO2 ¼ 0.01 atm; (b) in a mixture of calcite and gypsum (both become less soluble, although the effect on gypsum solubility is minor); (c) where water first encounters calcite and dolomite, and then gypsum (both dolomite and gypsum become more soluble as calcite is forced to precipitate). The shaded areas show the effect of 2 cm3 l1 of dissolved salt (NaCl). In (c), calcite is held at SI ¼ 0.1 to account for the residual supersaturation required to precipitate it. (i) Reproduced from Palmer, A.N., 2000. Hydrogeologic control of cave patterns. In: Klimchouk, A., Ford, D., Palmer, A., Dreybrodt, W. (Eds.), Speleogenesis: Evolution of Karst Aquifers. National Speleological Society, Huntsville, pp. 77–90. (ii) Reproduced from Shternina, E.B., 1949. Solubility of gypsum in water solutions of salts. lzvestija sectora fiz.-him. analiza IONH AN SSSR 17, pp. 203–206 (in Russian). (iii) Reproduced from Palmer, A.N., 2007. Cave Geology. Cave Books, Dayton, 454 pp. R ST PR f0020 NaCl 4 CO2 H2S NaNO3 6 c O A MgCl2 8 C 1 F 12 0.10 0.05 Mg(NO3)2 b Calcite, dolomite, + gypsum Calcite Supersaturated D C, g l−1 14 2 Dolomite Dolomite Max. precip (cm3 l−1) Concentration of CaCO3 in saturated solution cm3 l−1 0.15 a Calcite 16 Max. dissolution (cm3 l−1) Inidividual Calcite + minerals gypsum Calcite (at Sl=0.1) 8 p0155 p0160 EL SE VI ER FI hypogene gases is related to a drop in temperature (for CO2) or mixing (for both, CO2 or H2S) with water of shallower flow systems during the upwelling of the deep fluids. Where there is a low-permeability lithologic barrier that prevents direct mixing between the upwelling deep fluid and the shallow groundwater, the hyperfiltration effect can operate, limiting the migration of solute components, whereas allowing permeation of dissolved gases, as demonstrated for hypogene karst in the central Anatolia, Turkey (Bayari et al., 2009). As both CO2 and H2S are often present together in deep groundwater systems, and their proportion changes in the course of basinal evolution, their particular contribution in hypogene speleogenesis is difficult to assess. Mixing of waters of contrasting chemistry, particularly those differing in CO2 or H2S content or salinity, can renew or enhance solutional aggressiveness to carbonates due to the concavity of solubility curves (Laptev, 1939; Bögli, 1964; Wigley and Plummer, 1976; Palmer, 1991, 1995; see (Figure 3(i)), the effect widely referred to in the karst literature as ‘mixing corrosion’. Gases have a greater effect than salinity in controlling mixing dissolution (Palmer, 1991). Mixing corrosion is highly relevant for hypogene systems, where water that rises from depth encounters shallower meteoric water along paths of the cross-formational flow (Klimchouk, 2007). Dissolution by mixing is favored by a large difference in equilibrium gas content between two solutions, especially if one gas content is very low. The latter condition is more commonly met for H2S than for CO2. Dissolution by sulfuric acid, a very strong speleogenetic agent, occurs in shallower conditions where H2S-bearing waters rise to interact with oxygenated shallower groundwaters. The depth to which this process can take place is limited by the oxygen requirement (Palmer, 1991). It is recognized as the main speleogenetic process for certain large caves (e.g., caves in the Guadalupe reef complex in the USA and Frasassi Cave in Italy) and many smaller caves. Based on this, some researchers distinguish sulfuric acid karst/speleogenesis as a peculiar type (Hill, 2000; see Chapter 6.39 Sulfuric Acid Caves: Morphology and Evolution (00133)). Cave development by this process may occur at depth, where oxygenated water in shallow aquifers converges with upwelling regional H2S-bearing flow. The dissolution by sulfuric acid is most readily observable in caves that are now in unconfined settings but continue receiving rising H2S-bearing water from depth. Intense production of sulfuric acid near the water table and in the subaerial (above the water table) conditions causes pronounced speleogenic and mineralogical effects, which led some researchers to believe that most of the cave origin is due to subaerial sulfuric acid dissolution. However, in many instances, the water table and subaerial effects are overprinted on patterns and morphologies, suggestive of the development at considerable depth within a leaky confined aquifer. Moreover, in some regions, geochemical and p0165 MORP 00122 Hypogene Speleogenesis p0190 O O F p0195 p0200 EL SE p0185 VI ER p0180 AU9 FI R ST p0175 salinity of water. Figure 3(iii) shows the combined effects in typical groundwater conditions. Palmer has also indicated that in a simple carbonate–sulfate system, the volume of precipitated calcite is less than 30% of the combined total volume of dissolved gypsum and dolomite, which results in considerable solution porosity. The process is mediated by the relatively slow dissolution of dolomite, so that it is a viable mechanism for the development of dissolution paths in gypsum in deep-seated settings. There are an increasing number of arguments and evidence suggesting that more than one process could be involved in many cases, operating either in combination or sequentially in time. Either, carbonic acid or H2S dissolution, or both can operate in hydrothermal systems, which are essentially ascending transverse phenomena common in many regions. Mixing of contrasting waters is almost universally involved in hypogenic speleogenesis in carbonates, at least at some stages. The synergetic dissolution process in carbonate-/sulfate-mixed strata further illustrates the complexity. This, again, points to the benefits of the hydrogeologic approach to defining hypogene speleogenesis. Hypogene speleogenesis should not be related to a particular set of physical or chemical processes. Cross-formation flow and its interaction with the shallower environments are the main driving force for hypogenic speleogenesis, which can trigger and support many dissolution processes and integrate them into the cave-forming process. There may be other chemical conditions and reactions operative at depth, poorly known so far or still overlooked in karst science. Some of them were discussed in Klimchouk (2000a), for instance, low-TDS groundwater anomalies, commonly cross-formational, that are frequently documented at great depth, especially in the vicinity of hydrocarbon fields (Kartzev, 1984; Kolodij et al., 1991), or radiolysis of groundwater (Vovk, 1979), which can locally modify redox conditions, and therefore the speciation and the solubility of the compounds. It possibly accounts for the presence of oxygen in considerable amounts (up to 50 vol.% of dissolved gasses) in water at depths of 2–3 km and for related reactions involving oxidation of hydrocarbons, sulfides, and other compounds. The cursory review above shows that, contrary to the still existing views about limited potential for speleogenesis in deep-seated conditions, it is in fact quite substantial. The amount of dissolution is small in many processes. However, what has been mentioned by Palmer (2007) with regard to dissolution by mixing is also true for most of the other described mechanisms: ‘‘y the amount of dissolution is not as important as where it takes place. In many places, dissolution caused by mixing occurs well beneath the surface, where its effect is concentrated on forming caves and solutional pores, instead of on lowering the bedrock surface.’’ Also, in contrast to epigene settings where life cycle of caves is compatible to that of the landscape and is commonly measured by hundreds of thousands or a few millions years, hypogene speleogenesis can operate through time spans of tens and hundreds of millions years. PR p0170 paleohydrogeological evidences suggest that other processes, such as carbonic acid dissolution due to a drop of temperature or mixing of waters differing in H2S and CO2 content, could have taken part in the earlier development. The actual proportion of various deep-seated processes and the subaerial sulfuric acid dissolution remains unclear in most cases. Substantial sulfuric acid dissolution can also be caused by oxidation of iron sulfides such as pyrite and marcasite, where it is localized in ore bodies (Bottrell et al., 2000). Lowe (1992) and Lowe and Gunn (1997) suggested that oxidation of pyrite along certain horizons or bedding planes in carbonates (‘inception horizons’) may create preferential flowpaths that latter will be inherited by epigenic speleogenesis. Dissolution of evaporites is a simple dissociation of the ions and diffusional mass transport from the surface into the solution. The solubility of gypsum, the most common evaporite mineral, is one to three orders of magnitude greater than that of calcite (depending on partial pressure of carbon dioxide (PCO2) for the latter), and the solubility of salt is roughly 140 times greater than that of gypsum in pure water. In contrast to carbonates, solubilities of evaporates do not depend on dissolved gases and acidity. The solubility of gypsum increases significantly (up to 3–6 times) with the presence of other salts in the solution (Shternina, 1949; Figure 3(ii)). Gypsum dissolution can be rejuvenated by reduction of sulfates, which removes sulfate ions from the solution and allows more sulfates to dissolve, and by dedolomitization, which generates further dissolutional capacity with respect to gypsum because Ca2 þ is removed from solution and the sulfate ions react with Mg. Because mixed evaporite (gypsum/anhydrite and salt) and evaporate/carbonate formations are common in sedimentary successions, the above effects are highly relevant to hypogene speleogenesis in them. Dissolution in mixed carbonate/sulfate strata deserves further attention (see below). As evaporites dissolve at very high rates, and water in contact with them becomes saturated over short distances, epigenic speleogenesis in evaporate rocks is limited to flow paths that support high discharge/length ratio. Hypogene caves in gypsum are more common (and far more extensive). The requirements for them to develop include that aggressive water would be delivered through adjacent insoluble or carbonate aquifers, or through intervening permeable beds, or the aggressiveness should be constantly rejuvenated. Dissolution in mixed carbonate/sulfate strata is a kind of synergetic process, more complex than in each lithology alone (Stankevich, 1970; Back et al., 1983; Bischoff et al., 1994; Raines and Dewers, 1997). Palmer (2000, 2007) stressed on its importance for hypogene speleogenesis. This mechanism does not require an acid source. As water flows through carbonates, calcite is usually the first mineral to approach saturation. Dolomite dissolves more slowly. If gypsum is then encountered, its dissolution forces calcite to precipitate by the common-ion effect. This removes some calcium and bicarbonate from solution, reduces the saturation ratios of both dolomite and gypsum, and allows more of them to dissolve. Compared to the solubilities of gypsum and dolomite alone, up to 1.5 times more gypsum can dissolve, and up to 7 times more dolomite (Palmer, 2007). The effect increases with 9 AU10 p0205 MORP 00122 10 p0220 The nature of groundwater recharge is the most important factor in determining cave patterns (Palmer, 1991). This holds largely true for hypogenic caves, although with some ER Semi-arid VI Arid Denudation rate EL SE 500 1000 p0230 O t ars ound ic k gr n e g der Epi Un > > tion olu so dis iss ed Hypogene karst rfac u S Surface dissolution << Underground dissolution 0 1500 Precipitation (mm f0025 on luti Temperate and humid 2000 2500 Karst development Hydrogeologic Control of Hypogene Speleogenesis R 6.27.7 FI s0040 p0225 F The recent overview in Klimchouk (2007) and papers in Stafford et al. (2009) and Klimchouk and Ford (2009) demonstrate that hypogene speleogenesis is much more widespread in nature than previously thought. It operates in various geological and tectonic settings, active orogenic-folded regions through cratonic basins, at different depths (ranging from a few tens of meters to several kilometers), due to different dissolutional mechanisms operating in various lithologies. Hypogenic speleogenesis is largely independent of climate, but the epigenic overprint of hypogenic caves inherited by entrenched and denuded karst types strongly depends on climate. The overprint is particularly strong in regions of moderate and high runoff, from which most of karst knowledge historically originated, which contributed to the dominance of the epigenic karst paradigm. In arid and semi-arid regions, epigenic speleogenesis and surface karst morphologies are normally subdued due to limited infiltration and short supply of CO2 and organic acids, but hypogenic features are often abundant in the subsurface, resulting in a strong contrast in the degree of karstification between the surface and subsurface (Auler and Smart, 2003; Figure 4). Figure 4 serves to clarify another common misconception about hypogenic karst in the karst literature, namely that hypogenic karst is peculiar to arid climates and less common in humid regions. In reality, it is just better preserved and more readily recognizable in arid regions, but it is overall equally present, although overwhelmed by the epigenic development in shallow systems in humid karst regions. important peculiarities, imposed by the external controls of flow through a hypogene cave system through hydraulic properties of adjacent insoluble formations. Epigenic caves are formed by recharge through an overlying or adjacent karst surface. The karst surface, and hence the mode of recharge, evolves in coordination with the development of solution conduits (Bauer et al., 2005). The evolution is characterized by the strong flow/dissolution feedback mechanism, and by a switch from the hydraulic control of flow (by size of a conduit itself) to the catchment control (available recharge) when the conductivity of a growing conduit becomes larger than available recharge (Palmer, 1991). By contrast, in hypogene settings, recharge to a caveforming zone comes from below and is independent of the surface conditions. The mode of recharge is conservative as water comes from the underlying adjacent insoluble formation, although the amount and chemical characteristics of recharge may change in geologic timescales as the regional flow system changes. Discharge conditions are dynamic in the long term, as they change with denudation and erosion of the cover and with breaching of the major confinement. The breaching can be by external geomorphological processes, such as erosional entrenchment in the surface, or by the internal development of the hypogene cave system, such as the formation of an outlet through vertical stopping of a breakdown structure above a cavity. If there was no major geologic confinement, for example, the initial confined development was mainly due to the discordance of permeability structures at different levels within the soluble strata itself, changes in the discharge conditions are most dynamic and occur mainly through the internal development of a cave system. In a multiple aquifer system, soluble units are commonly vertically conterminous with insoluble or less soluble units of initially higher permeability. Variants are manifold in combinations and a scale. Typical examples include alternations of limestone and pervious clastic rocks, gypsum, and dolomite. Thick overall soluble formations can contain intercalations of more pervious, fractured beds, insoluble or less soluble. More ST p0215 Distribution of Hypogene Speleogenesis O p0210 AU11 6.27.6 PR s0035 Hypogene Speleogenesis 3000 yr−1) Figure 4 Schematic representation of the relative importance of hypogene and epigenic karst in shallow subsurface environments in different climatic settings. Modified from Auler, A.S., Smart, P.L., 2003. The influence of bedrock-derived acidity in the development of surface and underground karst: evidence from the Precambrian carbonates of semi-arid Northeastern Brasil. Earth Surface Processes and Landforms 28, 157–168. p0235 MORP 00122 Hypogene Speleogenesis 3000 ER 34 Low permeable dividing bed (a) f0030 Depth (m) VI EL SE Confined aquifer r 38 ifie qu 42 3100 re 46 se ac ffu 3200 45 3300 52 55 60 Unit 5 interface Unit 4 interface Unit 3 interface Unit 2 interface 64 3400 0 200 100 300 400 Unit 1 Di α4 Discharge to the upper unit po Fractured soluble formation Upward percolation Hydrostratigraphy Downward percolation Multi-storey fracture networks α3 α2 α1 α2 α1 Recharge from the lower unit Spread (m) Legend: 1: 2: (b) 56 3: −20 p0255 p0260 F O FI R ST p0250 O p0245 Prior to speleogenesis, cross-formational hydraulic communication across hydrostratigraphic barriers and interfaces occurs through regular cross-cut jointing, points of intersection of vertically adjacent permeability structures, prominent disruptions (conductive faults), and sedimentary windows (local areas where a barrier bed has a higher fabric-controlled permeability). The distribution of these features in the lower and the upper adjacent units and their connection with the permeability structures comprised by the soluble unit determines the mode of recharge and discharge to the cave-forming zone (Klimchouk, 2007, 2009b). Both recharge into and discharge out of the cave-forming zone can be diffuse or pointwise, which determines to a large extent the pattern of speleogenesis. The overall flow pattern in a multiple aquifer system, and recharge/discharge pattern for particular aquifers, is complex, influenced by the topography, geological structure, lithostratigraphy, prominent cross-cutting disruptions, and sedimentary windows (Figures 5(a) and 5(b)). As recharge to and discharge from the cave-forming zone occurs through underlying and overlying insoluble or less soluble units, flow rates are determined by the conductivity of the least permeable member. Thus, there is an external conservative hydraulic control on the amount of flow through the evolving conduits. Where conduits have evolved (i.e., after kinetic breakthrough) and the hydraulic gradient across the unit diminished, their further growth is not accelerated significantly as flow across the cave-forming unit is controlled, not by the hydraulic resistance of the conduit system, but by the permeability of the least permeable member, and by the boundary conditions of the system. This suppresses the positive flow-dissolution feedback and speleogenetic competition, the main mechanism acting in unconfined (epigenic) speleogenesis, and promotes more pervasive and uniform enlargement of initial permeability structures in the cave- PR p0240 permeable beds support predominantly lateral flow in the system (Figures 5(a) and 5(b)). Indurated soluble rocks often serve as separating beds (aquitards, or hydrostratigraphic barriers) prior to speleogenesis and during its early stage due to their low matrix permeability. Flow is predominantly vertical in them (transverse; Figure 5). A situation where the soluble unit is sandwiched between insoluble pervious rocks is one of the most favorable for hypogene speleogenesis. However, it is also common that soluble rock units or formations are separated from major aquifers by some beds of lower bulk permeability. Distinct integrated permeability structures (hydrostratigraphic units, or hydrofacies sensuEaton (2006)), relatively independent of each other, can be comprised by several lithostratigraphic units or be related to single beds. Contrasts and discordance between permeability structures in different hydrostratigraphic units are common, so that the vertical hydraulic connection between them can be limited even where units of relatively high lateral bulk permeability are immediately adjacent to each other (hydrostratigraphic interface). Leakage through the hydrostratigraphic interface occurs though points where permeable features of adjacent units vertically intersect (Figure 5(c)). Although both insoluble and soluble units can initially serve as hydrostratigraphic barriers in an aquifer system, their further behavior is different. The vertical permeability of the former remains conservative, while in the latter the conduit permeability develops in the course of speleogenesis, so that they lose their separating function. The same takes place with hydrostratigraphic interfaces within an inhomogeneous soluble succession. In this way, hypogene speleogenesis serves to facilitate cross-formational hydraulic communication across initially less pervious soluble beds or hydrostratigraphic interfaces. The net result is that multiple aquifer system achieves greater integration. 11 −18 −16 −14 −12 (c) Figure 5 Flow patterns in leaky-confined, multiple aquifer systems. (a) Topography-driven flow in undisturbed relatively shallow settings. (b) Flow generated by a pressure source at the domain’s base in the vicinity of a cross-cut conductive fault displacing aquifers (as modeled by Matthäi and Roberts, 1996; modified by Czauner et al., 2008).l, fluid pressure isobars (MPa); 2, fluid-flow vectors; 3, log permeability (m2). (c) Transverse flow across several hydrostratigraphic units and interfaces an denotes exchange terms between different permeability structures. (a) Reproduced from Shestopalov, V.M., 1981. Natural Resources of Underground Water of Platform Artesian Basins of the Ukraine. Naukova Dumka, Kiev, 195 pp. (in Russian). (c) Reproduced from Klimchouk, A.B., 2003a. Conceptualisation of speleogenesis in multi-storey artesian systems: a model of transverse speleogenesis. Speleogenesis and Evolution of Karst Aquifers 1(2), 18 pp. http://www.speleogenesis.info/archive/ publication.php?PubID ¼ 24&Type ¼ publication (accessed December 2010). p0265 MORP 00122 R FI ER Solution Porosity Patterns Produced by Hypogene Speleogenesis Solution porosity patterns in hypogenic speleogenesis are the result of the complex interaction of structural, hydraulic, and geochemical conditions, all varying in the course of geological evolution. The overall position of cave-forming zones is controlled by the three-dimensional (3D) distribution of soluble rocks, their position and hydraulic function within the hierarchic structure of flow systems, and the pattern of geochemical environments in a given flow configuration. Overall patterns of cave systems are strongly guided by patterns of the initial (prespeleogenetic) permeability features in a sedimentary sequence, that is, by the spatial distribution of the initial permeability structures and hydrostratigraphic interfaces within the soluble and adjacent units, by the mode of water input to, and output from, cave-forming zones and by the overall recharge–discharge configuration in the multiple aquifer system. Modes of recharge and discharge, again, depend on interaction between permeability structures within cave-forming zones and units that lie below and above them. The presence of cross-cutting permeability features, such as VI p0275 6.27.8 EL SE s0045 p0280 O F faults, can exert strong effects on cave patterns through their inflow, throughflow, and outflow controls. In contrast to epigene settings where initial effective permeability structures are exploited by speleogenesis in a very selective manner, hypogene speleogenesis tends to exploit most of them within caveforming zones, provided the aggressiveness is maintained. Geochemical interactions of flow components guided by transverse and lateral permeability pathways determine zones of pronounced speleogenetic development and influence the resultant patterns. General evolutionary factors, such as regional tectonic and geomorphic developments that control rates and architecture of flow and timing of speleogenesis, also affect cave patterns forming in hypogene settings. Hypogene caves demonstrate a variety of patterns, as classified and briefly described below. For more extended discussion the reader is referred to Palmer (1991, 2000, 2007), Ford and Williams (2007), Klimchouk (2000a, 2007, 2009a), and Audra et al. (2009b). Same patterns are known to form in different lithologies and by different dissolution mechanisms. Most of the patterns known to be produced by hypogene speleogenesis can also be formed locally in epigenic environments, although respective caves are not large or extensive. Conversely, branchwork patterns, the most common for epigenic speleogenesis, with conduits converging as tributaries in the downstream direction, never form in hypogenic settings. This reflects the fundamental difference between the mechanisms of epigenic speleogenesis, largely competitive, and of hypogenic speleogenesis, in which the competition between alternative flowpaths is subdued. The following elementary cave patterns are typical (although not necessarily exclusive) for hypogene speleogenesis: ST AU12 forming zone where recharge is diffuse (Klimchouk, 2000a, 2003a, 2007). Conditions and manifestations of this effect depend on different variables that have been studied by numerical modeling of speleogenesis in gypsum in artesian hypogenic settings (Birk, 2002; Birk et al., 2003, 2005; Rehrl et al., 2008). The presence of hydrostratigraphic interfaces within the soluble formation, as well as the restricted outflow, also favors pervasive conduit development. If recharge and discharge are though a single cross-cut fracture, such as a conductive fault, the conduit development is localized, although it can get dispersed below the upper insoluble barrier that restricts outflow despite of the growth of the conduit in the soluble unit. In hydrothermal speleogenesis, in carbonates, there is another specific mechanism, caused by the thermal coupling between the fluid and rock, which also suppresses speleogenetic competition (Andre and Rajaram, 2005). As forced-flow regimes in leaky confined settings are commonly sluggish and water with lesser density enters the cave-forming zone from below, free convection flow patterns powered by either solute or thermal density gradients are widely operative in hypogenic speleogenesis. Various morphological effects of buoyancy dissolution are recognized in hypogenic caves, among which upward-pointing dissolution morphs are most common (Klimchouk, 1997, 2000a, 2007, 2009a). Speleogenesis at the base of the soluble unit due to buoyancy dissolution may operate even without guiding disruptions and forced throughflow across the unit. In this way, large isolated cavities can form at the base of thick evaporites underlain by an aquifer that supplies fresh groundwater and takes away a solute load. More common, however, are mixed convection systems, where buoyancy dissolution effects are particularly pronounced during the mature stage of speleogenesis, where considerable conduit space has been created and vertical hydraulic gradients across the cave-forming zone are diminished. O p0270 Hypogene Speleogenesis PR 12 • • • • • • • cavernous edging along hypogene conduits; single passages or rudimentary networks of passages; network maze; spongework maze; irregular isolated chambers; rising, steeply inclined passages or shafts; and collapse shafts over large hypogenic voids. Cavernous edging (or selvedge) of hypogene conduits is a common feature of hypogene speleogenesis, less appreciated in cave science due to its traditional focus on larger voids. Zones of cavernous porosity comprised by closely spaced irregular small vugs and conduits (up to 2–5 cm in diameter) commonly border walls of transverse hypogene conduits, such as fracture-type passages or solution-enlarged fractures (‘underdeveloped passages’), up to depth of a meter. Size of voids diminishes with depth, and further into the rock they pinch out. Such cavernous edging is stratiform, or distributed by clusters along the fracture/conduit walls (Figure 6). The formation of this type of cavernousity is apparently related to the mixing corrosion effect, where pore water in more diffusely permeable beds or zones converges toward a large fracture that conducts flow from a deeper source. Single isolated passages or rudimentary networks of passages are probably the most common cave structures for hypogene speleogenesis. They form by transverse flow across a single soluble bed where guiding cross-cut fractures are single or have poor lateral connection. The passages are typically p0285 p0290 p0295 p0300 p0305 p0310 p0315 p0320 p0325 p0330 MORP 00122 Hypogene Speleogenesis 13 (a) slot-like, sometimes with rounded section in the middle part of the cross section, single or combining in small clusters of few intersecting passages according to the pattern of guiding fractures. Their notable characteristic is that they are dead end in every direction, if not encountered by erosion or mines. They can be observed directly when exposed in cliffs or encountered by underground mines. In the south of Ukraine, there are many examples of such relict caves developed in Neogene and Paleogene limestones, naturally exposed by erosion through the large artesian basin. An outstanding case is the area of the city of Odessa, where more than 100 such caves are documented, encountered by a huge (more than 1000 km of integrated passages) old limestone mine (Klimchouk, 2007; Pronin, 2009). Most of them are single passages, but some are rudimentary networks up to 1.3 km of integrated passages (Figure 7). A good example in gypsum is Denis Parisis Cave, also encountered by a mine in the Paris Basin. Network maze patterns are most common for hypogenic caves. Passages are strongly fracture-controlled and form more or less uniform networks, which may display either systematic or polygonal patterns, depending on the nature of the fracture networks. Systematic, often rectilinear, patterns are most common, reflecting tectonic influence on the formation of fracture networks (Figure 8). Polygonal patterns are guided by discontinuities of syndepositional or diagenetic origin. Networks displaying different patterns may be present within a single area or at various stories/parts of a single cave, especially where confined to different rock units (Figure 8(a)). Network maze caves of hypogene origin are known in limestones, dolomites, and gypsum, being particularly common in mixed limestone–dolomite–gypsum strata, and also in conglomerates. When aggressive recharge from below is uniformly distributed, passages that hold similar positions in the system in relation to the flowpaths’ arrangement (guided by the same set PR Figure 6 Zones of cavernous porosity bordering hypogene passage (a) and a solution-enlarged fracture (b) in the Paleocene limestones of the Piedmont cuesta ridge (Crimea, Ukraine). of fractures and occurring within a single cave series or at the same storey) are commonly uniform in size and morphology. Larger volumes may be dissolved where aggressive recharge from below is concentrated by virtue of hydraulic properties and the porosity structure of the feeding aquifer (Figure 8(a)). A common feature of network mazes is a very high passage density (Klimchouk, 2003). Spongework maze patterns are less common than networks. Highly irregular passages develop through enlargement and coalescing of pore- and vuggy-type initial porosity in those horizons of the cave-forming zone that have no major fractures but interconnected pores and vugs. Clusters or levels of spongework-type mazes are commonly combined with other patterns in adjacent horizons to form complex cave structures. An enlarged version of spongework, locally called boneyard, is represented in parts of some caves of the Guadalupe Mountains. Isolated chambers commonly occur at the bottoms of soluble formations but they are also frequent within particular beds in stratified carbonate sequences. They form in two situations: (1) by buoyant dissolution at the bottom of the a soluble formation, commonly evaporites, where a major aquifer immediately underlies it and (2) where the recharge from below is localized in large fractures or faults and aggressiveness is enhanced by mixing of a deep source of geogenic gases with a shallower-flow system, often restricted to a particular bed in a stratified sequence. Irregular chambers of hypogene origin can attain very large dimensions, such as directly documented cavities in evaporates of southern Harz, Germany (cavities of the ‘schlotten’ type; Kempe, 1996), the Big Room in Carlsbad Cavern, or a huge cavity encountered by boreholes in the Archean and Proterozoic marbles in southern Bulgaria with a maximum vertical dimension of 1340 m and an estimated volume of 237.6 million m3 (Dublyansky, 2000), probably the largest known, although not accessible, EL SE p0335 VI ER FI R ST f0035 (b) p0340 O O F 0.3 M AU13 p0345 p0350 MORP 00122 14 Hypogene Speleogenesis 20 m Mine Cave 40 m Mine (a) 2 3 Figure 7 Single isolated passages (a) and rudimentary networks (b) encountered by mines in the Neogene limestones beneath Odessa city, south Ukraine. More than 100 documented caves there range between a single-passage shown in (a) and the largest maze cluster (1292 m) shown in part not shown here.in (b). More commonly they are composed by a few intersecting passages. Photos show typical cross sections. These caves are fed through fractures at the bottom of passages, now obscured by sediment fill (photo 1 and 3), along which more prominent point-wise feeders can be locally developed (in the front of photo 2). Photo 1 shows a passage cut by a mine. Photos and cave maps by K Pronin. VI E R f0040 FI R ST PR O 1 O F (b) SE EL p0355 cave chamber on Earth. Lesser but instructive examples of hypogenic isolated chambers are described by Frumkin and Fischhendler (2005) from the central mountain range of Israel. Where large irregular chambers are complicated by blind branches extending outward, the pattern is termed ramiform (Palmer, 1991). Rising, steeply inclined passages or shafts are outlets of deep hypogene systems in which the ‘root’ structure remains unknown in most cases. A type example is the 392-m-deep Pozzo del Merro near Rome, Italy, presumably formed by rising thermal water charged with CO2 and H2S (Caramana, 2002). It shows the morphology of a rising shaft, in contrast with roughly cylindrical morphology, with walls diverging from each other toward the bottom, of shafts of the El Zacatón and obruk type, where hydrothermal cavities at depth are supposed to open to the surface through collapse. Collapse shafts over large hypogenic voids can form megasinkholes at the surface, locally termed ‘cenote’ (Mexico) and ‘obruks’ (Turkey). The type examples are Sistema El Zacatón in Mexico (several features with depth up to 329 m from the p0360 surface and B317 m below the water table; Gary and Sharp, 2006) and obruks in the Central Anatolia, Turkey (71 features with depths up to 125 m from the surface; Bayari et al., 2009). They are clearly collapse features over giant chambers formed by thermal fluids charged with geogenic CO2. In Sistema El Zacaton, methane is also measured at depth. Fossil features of this type are known as breccia pipes, collapse columns, or ‘geologic organs’, and also termed ‘vertical through structures’, a common byproduct of hypogene speleogenesis in a variety of geological settings (see Figure 1). They originate from collapse of large cavities, commonly at the base of soluble formations, and propagate upward by stopping across sedimentary successions that include soluble rocks. Such features may reach many hundred of meters in the vertical extent. They are not merely breakdown structures but complex hydrogeologic structures whose development depends on (and favors to) focused cross-formational groundwater circulation and continuing dissolution of intercepted soluble beds and infallen clasts (Huntoon, 1996; Klimchouk and Andrejchouk, 1996). MORP 00122 Hypogene Speleogenesis 15 Middle level (master passages) Upper level 0 10 20 30 m O F Newly discovered area O Lower level Lowest level (feeders) 100 m Points of connection with upper passages (feeders) Upper level (master passages) (a) Figure 8 Examples of network maze patterns in the Ozerna cave, a 131-km-long maze in the Neogene gypsum rocks (entrenched karst settings, Western Ukraine). (a) Map of the southern part of the cave showing a clustered distribution of passage networks and (b) large-scale fragment showing distribution of feeders at the lower level through the master passage network. The master network is at the middle level, shown in grey. Passages at the lower level can form continuous networks such as in the SW parts of (a) and (b), shown in red. More commonly, separate steeply rising conduits at the lower level are diffusely scattered through the area and join the master passages in many points (feeders). They are not shown in (a), but shown as blue circles in (b). Passages and rooms at the upper level (outlet segments of the system) are sparse, shown in green in (a). Maps by Ternopil Speleological Club ‘Podillya’. Composite 3D systems are comprised by various elementary patterns at different levels, such as irregular chambers, clusters of network or spongework mazes, and rising subvertical conduits and other morphs connecting them. Composite 3D systems include many of the world’s largest caves. Their organization reflects vertical heterogeneity in distribution of initial permeability structures that guided or impeded rising flow within a soluble formation, and interplay between structural, hydrogeological, and chemical factors. Composite 3D cave structures may develop within a rather thin formation and be laterally extended (e.g., two- to fourstory mazes in the western Ukraine confined within the 16–30-m-thick gypsum formation), or be vertically extended through a range of several hundred meters (e.g., Monte Cucco system, Central Italy: 930 m; Lechuguilla Cave in the Guadalupe Mountains, New Mexico, USA: 490 m). Multistory mazes are ‘layered’ variants of composite 3D systems. Many seemingly single-story mazes are in fact also 3D structures as they have feeders at the lower level randomly or systematically distributed, connected to a master passage network. In multistory mazes, stories (up to 5–6) are commonly comprised by stratiform networks differing in patterns, connected via rising conduits or ‘chimneys’, or cross-cutting rift passages. The stories can superpose each other within the same area or display a staircase arrangement within a system, p0375 EL SE p0370 VI ER p0365 FI R ST f0045 (b) PR 0 with cave areas at different stories shifted relative to each other (e.g., in the Wind Cave, South Dakota, USA). The lateral shift of storiesand outlet passages is because areas of preferential recharge from below commonly do not coincide with areas of discharge from a confined system, so that general pattern of upwelling flow becomes staircase-like. In a typical system, lower stories or individual rising conduits are recharge elements to a cave system (Figure 8). Master stories develop at intermediate elevations where fracture networks are well connected laterally. They provide for lateral distribution of upwelling flow to areas of preferential discharge. Upper stories serve as outflow structures (‘outflow mazes’ of Ford (1989)). Small patches of mazes, or blind lateral extensions of high domepit structures may develop at higher or highest elevations without bearing outflow functions (‘adventitious’ mazes of Ford), especially in systems where buoyancy flow plays a role. Storiesin ascending hypogenic systems form simultaneously within a complex transverse flow path, in contrast to epigenetic caves where storiesreflect progressive lowering of the water table in response to the evolution of local erosional base levels, hence upper stories being older than the lower. However, distinct horizontal storiessuperimposed onto otherwise ascending 3D pattern may develop at former water table levels during the extinction p0380 MORP 00122 6.27.9 Mesomorphology Features of Hypogene Caves O F Hypogene caves can be formed by a number of dissolution mechanisms, occur in various geological and structural conditions, and may have different patterns. Despite these variabilities, mesomorphological features of hypogene caves exhibit remarkable similarity, generally indicating sluggish flow conditions. Individual occurrences of some of these features, such as cupola-form solution pockets, also occur in epigenic caves where they form in the unconfined phreatic zone (especially in eogenetic environments; Mylroie and Mylroie, 2009) or subaerial conditions. Specific to hypogene speleogenesis is, however, that different morphologies commonly occur in spatially related groups where fluid-flow paths, including distinct buoyant convection flow components, can be traced from rising inlet conduits, through transitional wall and ceiling features (rising wall channels and ceiling halftubes), to outlet features (cupolas and domepits). This particular combination has been distinguished as the morphologic suite of rising flow (morphologic suite of rising flow (MSRF); Klimchouk, 2007, 2009a) and provides diagnostic evidence for hypogene speleogenesis (Figure 9). MSRF has been recognized in hundreds of hypogene caves across the globe. The diagram is generic and elastic; it can be stretched vertically, and complexity can be added to allow for multiple stories. The combination of the forms will repeat itself on each storey, and functional relationships between the forms will remain the same. Hypogene caves may consist of a few elementary segments like the one depicted in Figure 9, or FI R ST p0390 phase of hypogene speleogenesis, especially where sulfuric acid dissolution is involved. Some vertically extensive caves in the Guadalupe Mountains have prominent feeders as large isolated, steeply ascending passages, or clusters of rift-like passages connecting to some master levels of passages and chambers, and prominent outlet segments rising from the latter. The resultant 3D structures are composed of network and spongework mazes at various levels connected through rising vertical conduits, coalescing with large chambers and passages. Other prominent examples include the Monte Cucco system in Italy, complex bush-like upward-branching structures of hydrothermal caves in the Buda Hills, Hungary, composed by rising sequences of chambers and large spherical cupolas (Dubljansky, 2000b), and network maze clusters at the base of the Joachim Dolomite in eastern Missouri, USA, with ascending staircase limbs of vertical pits and subhorizontal passages (an outlet component; Brod, 1964). Composite 3D structures dominated by large irregular chambers, from which blind branches extending outward, are distinguished as ramiform patterns (Palmer, 1991). Because of the transverse nature of hypogene speleogenesis, caves and their parts tend to have a clustered distribution in plan view, although clusters may merge to extend over considerable areas (Figure 8). Laterally extensive multistory maze caves such as in the Western Ukraine, or in the Black Hills, South Dakota, or vertically extensive 3D structures such as in the Guadalupe Mountains, are in fact combinations of many clusters of passages representing relatively independent transverse-flow subsystems. O p0385 Hypogene Speleogenesis PR 16 ER Confining bed VI Rising chain of arches EL SE Rising wall channel Upper aquifer Cupola, domepit Rising chain of arches Ceiling half-tube Half-tube Master passage Free convection Connection to master passage feeder’s “shell” Forced convection Feeder Lower aquifer f0050 Figure 9 The morphologic suite of rising flow (MSRF), diagnostic of hypogenic transverse origin of caves. Modified from Klimchouk, A.B., 2003a. Conceptualisation of speleogenesis in multi-storey artesian systems: a model of transverse speleogenesis. Speleogenesis and Evolution of Karst Aquifers 1(2), 18 pp. http://www.speleogenesis.info/archive/publication.php?PubID ¼ 24&Type ¼ publication (accessed December 2010). s0050 p0395 MORP 00122 Hypogene Speleogenesis EL SE VI ER FI R p0415 p0420 F O Hypogene Speleogenesis and Paleokarst Within the conventional, predominantly epigenic, karst paradigm, instances of deep-seated karst have commonly been interpreted as paleo(epigenetic) karst because the possibility of karstification in deep environments without recharge from the immediately overlying or adjacent surface has been neglected for a long time. Paleokarst is not a particular type of karst but is, rather, a fossilized condition. Features become paleokarst as they get hydrologically decoupled from contemporary systems, in contrast to relict features that exist within contemporary systems but are removed from the environment in which they developed (Ford and Williams, 1989). True paleokarst is buried karst (see Figure 2), which is a complete infilling and burial of epigenic (including coastal/oceanic) karst by later materials such as transgressive marine sediments. Paleokarst horizons are reliably recognized where they underlie unambiguous stratigraphic unconformities related to subaerial exposure. With growing recognition of hypogenic speleogenesis, it becomes increasingly obvious that, in many cases (although certainly not in all), features previously interpreted as paleo(epigenetic) karst, including coastal/oceanic karst, can be better explained as active or relict hypogenic features. Review of the international literature, especially concerned with carbonate-hosted hydrocarbon reservoirs, reveals that, in many cases, because of their occurrence beneath distinct formational contacts, paleokarst features have been dubiously interpreted as evidence of subaerial exposure by a process of reciprocal reasoning. Other common cases of problematic paleokarst are stratiform breccia horizons that are commonly the ultimate result of hypogenic speleogenesis, namely – the collapse of laterally extensive, stratigraphically conformable maze caves. Hypogene speleogenesis tends to operate over long time spans, intermittently or being repeatedly suspended and reactivated. Hypogenic features may become relict but still remain within the contemporary systems, for example, in a system where original confinement was breached and the flow pattern reversed from upwelling to descending. Hypogenic features are not paleokarst unless their evolution is completely halted by the removal of the cave-forming units from the geological section and their substitution by stratiform breccia horizons, complete sealing by cementation (mineralization) p0425 s0055 p0430 O p0410 6.27.10 PR p0405 limestone caves where deep recharge by sulfidic deep waters continues during the unconfined development. In the latter case, distinct upward-oriented morphological effects of subaerial sulfuric acid dissolution (i.e., condensation corrosion above the water table) may also develop, such as air-convection cupolas, which are difficult to distinguish from speleogens formed under submerged conditions. Their distribution, however, is restricted to the zone immediately above the water table, whereas in composite 3D systems the elements of the MSRF are systematically distributed through various stories in large vertical ranges. For a more extended recent discussions of the morphology of hypogene caves, the reader is referred to Klimchouk (2007, 2009b), Palmer (2007), and Audra et al. (2009a, 2009b). p0435 ST p0400 combine hundreds and thousands of them within a single system. Among elementary morphs in this suite, inlet features are the most indicative of recharge from below and hence the hypogene origin for a cave. They are termed also vents, feeders, or risers in recent publications. Because of their diagnostic role, they are characterized in some detail below. Original feeders are basal input points to hypogenic cave systems, the lowermost components, vertical or subvertical conduits, through which fluids rise from the source aquifer. Such conduits are commonly separate but sometimes they form small networks at the lowermost storey of a system, which bear the feeding function relative to the upper storey (see Figure 8). Feeders join master passages located at the next upper storey and commonly scatter rather uniformly through their networks. Feeders are commonly point features (Figures 10(a)–10(k)); they may join the passage from the end, from a side, or are scattered through the passage floor. Where a passage occurs at the very base of a soluble bed, it can receive recharge throughout the entire length of a guiding fracture that serves as a linear rift-like feeder Figures 10(l)–10(o). In composite 3D caves, there can be several stories of lateral passage development in the system. Where stories are superimposed, the lower ones function to recharge the upper ones. In that case, feeders at the upper storey are continuations of outlet features at the adjacent lower storey. Sizes of feeders vary greatly, from small conduits or rifts on the order of tens of centimeters to features many meters in the cross section. The vertical extent of feeders also varies greatly: from less than a meter to many tens and even a few hundred meters. Feeders observable in currently shallow-lying relict multistory maze caves in dolomites of the Neoproterosoic Vazante Group, Minas Gerais, Brasil, extend down for more than 200 m, as evidenced by their interceptions at different levels by an underlying mine. In many instances, dimensions of feeder conduits are smaller in their lower parts and they often have ear-shaped orifices. This is due to buoyancy convection effects, shielding of walls in the lower parts from dissolution by more saturated water in sinking limbs of convection cells, and mixing effects, enhanced dissolution at the orifices due to mixing of waters of different chemistry. Interestingly enough, in hypogene caves that have not experienced much sediment influx from the surface during the epigenic stage, feeders tend to remain empty of sediments despite that master passages they join can be filled considerably by fine sediments. This is due to the fact that, in many cases, feeders continued to conduct rising water during late phases of hydrogeologic activity such as the water-table phase (Figure 10(e)). However, in relict hypogene caves that passed the transition long ago, feeders are commonly obscured by the sediment fill, but still can be identified by the presence of rising wall channels above them (if feeders connect a passage from a side, beneath a hanging wall). Where a water table is established at a given level within a former confined hypogenic cave system, significant lateral widening and merging of passages into large chambers may occur, with characteristic speleogens such as horizontal notches and corrosion tables (Klimchouk, 2007; Audra et al., 2009a). This is typical for caves in evaporites and limestones subjected to back-flooding from a nearby river, and for 17 p0440 p0445 MORP 00122 18 Hypogene Speleogenesis 0.5 m 1m 0.5 m (b) (c) (d) (g) (f) (i) (j) (k) (l) (m) (n) (o) Figure 10 Variability and similarity in the morphology of feeders. Point-wise feeders: (a) Robber Baron Cave, Texas, USA (Cretaceous limestones); (b) Ozerna Cave, Western Ukraine (Miocene gypsum); (c) Fuchslabyrinth Cave, Germany (Triassic Muschelkalk limestone); (d) Carlsbad Cavern, Guadalupe Mountains, NM, USA (Permian limestone); (e) Ordynskaya Cave, Fore-Urals, Russia, an active feeder in an underwater cave (Permian gypsum); (f) and (g) Toca da Boa Vista Cave, Brazil (Precambrian limestone and dolomite); (h) and (i) Hamilton and Troat caves, West Virginia, USA (Devonian Helderberg limestone); (j) Optymistychna Cave, Western Ukraine (Miocene gypsum); (k) Endless Cave, Guadalupe Mountains, NM, USA (Permian limestone); Fissure- and rift-like feeders in passage floors: (l) Natalina Cave, Black Sea region, south Ukraine (Neogene limestone); (m) Aneva Cave, Israel (Cretaceous limestone); (n) Mlynki Cave, western Ukraine (Miocene gypsum); (o) Knock Fell Caverns, Northern Pennines, UK (Carboniferous limestones. Photos (e) by P Sivinskykh, (h) and (i) by G Schindel, (l) by K Pronin, (m) by A Frumkin, and other photos by A Klimchouk. EL SE f0055 VI ER FI R (h) ST PR O (e) O F (a) or lithification of the fill material, or by closure of the contemporary hydrogeological cycle with a new marine transgression. However, establishing the paleo-status of hypogenic features and their distinction from epigenic paleokarst requires additional discussion. Many important hydrocarbon and mineral deposits are karst related, but commonly thought to be related to paleo(epigenic) karst. Better understanding of hypogene speleogenesis is of paramount importance for their proper genetic interpretation, which in turn is crucial for the development of p0450 MORP 00122 Hypogene Speleogenesis more adequate approaches to the prediction, prospecting, and exploitation of these resources. 10. May operate during long time spans (millions to hundreds of millions of years). s0060 6.27.11 Uncited references p0455 Recognition of the wide occurrence, significance, and specific characteristics of hypogene speleogenesis during last two decades signifies a major paradigm shift in karst science, previously overwhelmingly dominated by the epigene concepts and models. The more comprehensive approach to karst that emerges implies that speleogenesis should be viewed in timescales of the host formation life, in the context of regional groundwater flow systems and their evolution in response to diagenetic and tectonic processes, uplift, denudation, and geomorphic development. The rapidly evolving understanding of hypogene speleogenesis has broad implications for many applied fields such as prospecting and characterization of hydrocarbon reservoirs, groundwater management, geological engineering, mineral resources industries, and related activities. The list below is an attempt to summarize essential features and roles of hypogene speleogenesis. In brief, hypogene speleogenesis: p0470 p0490 p0495 p0500 p0505 VI p0485 EL SE p0480 ER FI R p0475 1. Occurs much wider than previously thought, in various geological settings and in different soluble lithologies. 2. Has no genetic relationship with groundwater recharge from the overlying or immediately adjacent surface, and may have no surface manifestation at all. 3. Develops by recharge from adjacent underlying formations mainly under leaky confined conditions, which determines specific speleogenetic mechanisms, imposed on by the external controls of flow in hypogene systems through hydraulic properties of adjacent insoluble formations. However, hypogene speleogenesis may continue to operate in unconfined conditions at the latest phases. 4. Involves various dissolution mechanisms at transitional geochemical environments and thresholds that commonly occur along interfaces between different flow regimes and cross-formational flowpaths. 5. Is controlled by the 3D distribution of soluble rocks, their position and hydraulic function within the hierarchic structure of flow systems and the pattern of geochemical environments in a given flow configuration. 6. Creates solution porosity structures that are commonly quite distinct from those formed by epigenic speleogenesis. They range from concordant structures controlled by individual strata to discordant cross-cutting structures within tens and hundreds of meters of sedimentary sequences. Combination of discordant and concordant elements is common. 7. Creates aerially extensive (although commonly clustered) or localized zones of high vertical permeability. 8. Serves to enhance cross-formational communication and converges flow to zones of high vertical permeability by opening migration paths across soluble formations and overlying nonkarstic aquitards. 9. Plays an important role in (re)organization of regional flow systems. p0510 AU14 p0515 References bib1 bib2 bib3 O O F Anderson, R.Y., Kirkland, D.W., 1980. Dissolution of salt deposits by brine density flow. Geology 8, 66–69. Andre, B.J., Rajaram, H., 2005. Dissolution of limestone fractures by cooling waters: early development of hypogene karst systems. Water Resources Research, 41. doi:10.1029/2004WR003331. Audra, Ph., Mocochain, L., Bigot, J.-Y., Nobecourt, J.-C., 2009a. Morphological indicators of speleogenesis: hypogenic speleogens. In: Klimchouk, A.B., Ford, D.C. (Eds.), Hypogene Speleogenesis and Karst Hydrogeology of Artesian Basins. Special Paper 1. Ukrainian Institute of Speleology and Karstology, Simferopol, pp. 17–22. 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Earth Surface Processes and Landforms 32, 2075–2103. Palmer, A.N., 1991. Origin and morphology of limestone caves. Geological Society of America Bulletin 103(1), 1–21. EL SE bib65 21 bib83 bib84 bib85 bib86 AU21 bib87 bib88 bib89 bib90 bib91 bib92 bib93 bib94 bib95 bib96 bib97 bib98 bib99 bib100 MORP 00122 22 Hypogene Speleogenesis Biographical Sketch EL SE VI ER FI R ST PR O O F Alexander Klimchouk is director of the Ukrainian Institute of Speleology and Karstology, senior scientist of the Institute of Geological Sciences of the National Academy of Sciences of Ukraine. Earned his MSc degree in geomorphology from the Kiev State University (1983) and PhD in hydrogeology from the Institute of Geological Sciences of the National Academy of Science of Ukraine (1998). His principal scientific interests lay in karst hydrogeology, karst geomorphology and speleogenesis. Authored more than 300 scientific papers and books on various aspects of karst and cave science, edited several major international books. He is an active cave explorer. Most of his research and cave exploration was done in various regions of the former Soviet Union (Western Ukraine, Crimea, Central Asia, Caucasus, and the Russian North), and also in Britain, Canada, China, Ethiopia, Germany, Italy, Slovenia, Spain, Turkey, USA and other countries. In the past, he served as a president of Kiev Speleological Club and vice-president of the National Association of Soviet Speleologists. A founder (in 1991) and past president of the Ukrainian Speleological Association (1992–98; 2003–05). A founder (in 2006) and a current director of the Ukrainian Institute of Speleology and Karstology double affiliated to the Ministry of Science and Education of Ukraine and the National Academy of Sciences. Serves on the Bureau of the International Union of Speleology (UIS) since 1992, past UIS vice-president (2001–05) and a senior vice-president (2005–09). Since 1994, he is a president of the UIS Commission on Karst Hydrogeology and Speleogenesis. Member of the IGU Karst Commission and corresponding member of the IAH Karst Commission. Honorary member of the National Speleological Society (USA) and the Ukrainian Speleological Association.