Alexander Klimchouk

Sulphate rocks as an arena for karst development

The rocks in which karst systems develop are most commonly composed of carbonate, sulphate and chloride minerals. The sulphate minerals are quite numerous (see Table 1), but only gypsum and anhydrite form extensive masses in sedimentary sequences. Other minerals, which represent sulphates of K, Mg and Na, normally occur as minor beds (0.1-5.0 m), or as inclusions associated with chloride rocks. However, some minerals precipitated in salt-generating basins, such as mirabilite and glauberite (typically formed in the Kara-Bogaz-Gol Gulf, salt lakes of Siberia and in China), form sequences up to 5-10 m thick where karst may develop. Due to the velY high solubility of Na -sulphates, karst processes and features occurring in these rocks resemble salt karst. Thus, the term sulphate karst, although not strictly correct, is used mainly to indicate karst developed in gypsum and anhydrite.

Gypsum karst of the world: a brief overview

Introduction On the glohal scale, surface outcrops of gypsiferous strata appear quite limited. This apparent scarcity can he explained hy the relatively low resistance of gypsum to denudation effects rather then it reflecting an actual limited occurrence of sulphate rocks. The extent of territories where sulphate rocks are present at the surface or at depth is great: Ford & Williams (1979) estimated that gypsum/anhydrite and/or salt deposits underlie 25% of the continental surface (approx. 60 million km2), while Maximovich (1962) calculated that the area of the continents underlain hy gypsum/anhydrite alone is ahout 7 million km2 As is demonstrated in Chapter 1.4and elsewhere in this volume, karst processes operate extensively in intrastratal settings, heneath various types of cover heds, where gypsum heds occur within at least the upper few hundred metres of the rock sequence. Taking this into account, gypsum karst appears to he a much more widely developed phenomenon than is commonly helieved. The largest areas of sulphate rocks are found in the Northern hemisphere, particularly in the United States, where they underlie 35 to 40% of the nations land area (Chapter 11.2),and in Russia and surrounding states, where Gorhunova (1977) estimated an extent of 5 million km2 in the former USSR. However, many other countries within the American continents, Europe and Asia host important, and commonly quite extensive, gypsum karst. Detailed characteristics of many of these are provided within the national reviews comprising later chapters of this volume. The aim of this chapter is to present a brief overview of the geographical distrihution of gypsum karst in the world, with particular reference to those areas that are not descrihed separately, either due to a real scarcity of data or hecause editors were unable to involve local experts. The general order of the reviews hegins in the Americas and proceeds towards the cast. The first hrief glohal reviews specifically dealing with gypsum karst were provided hy Maximovich (1955, 1962). Since then knowledge of gypsum karst, in terms of its morphological and hydrogeological peculiarities, development mechanisms and geographical distrihution, has increased dramatically. Recently the glohal distrihution of gypsum karst has heen considered hy Nicod (1992, 1993).

Mechanisms of karst breakdown formation in the gypsum karst of the fore-Ural region, Russia (from observations in the Kungurskaja cave).

89 The fore-Ural is a classical region of intrastratal gypsum karst. The intensive development of karst in the Permian gypsums and anhydrites causes numerous practical problems, the subsidence hazard being the most severe. Mechanisms of karst breakdown formation were studied in detail in the Kunguskaya Cave area. The cave and its setting are characteristic to the region and, being a site of detailed stationary studies for many years, the cave represents a convenient location for various karst and speleological investigations. Breakdown structures related to cavities of the Kungurskaya Cave type develop by two mechanisms: gravitational (sagging and fall-in of the ceilings of cavities) and filtrational/gravitational (crumbling and fall-in of the ceilings of vertical solution pipes, facilitated by percolation). The former implies upward stoping of the breakout roof and cessation of the process at some height above the floor of the cave due to complete infilling by fallen clasts. This mechanism cannot generate surface deformation where the overburden thickness exceeds a certain value. The latter mechanism implies that breakdown will almost inevitably express itself at the surface, most commonly as a sudden collapse, even where the thickness of the overburden is large. These mechanisms result in different appearance, distribution and further evolution of the respective surface forms , so that subsidence hazard assessment should be performed differently for these types of breakdown. The conclusions reached by this study are representative for the region , although some of them bear more general validity for intrastratal karst conditions. This study underlines the ultimate importance of speleological investigations to the understanding of karst breakdown mechanisms.

Breakdown development in cover beds, and landscape features induced by intrastratal gypsum karst

It is shown in Chapter 1.4 and elsewhere in Part II of this volume that intrastratal karst is by far the predominant gypsum karst type. Its development may hegin in deep-seated settings within rocks already huried hy younger strata, and it proceeds increasingly rapidly as uplift hrings gypsum sequences into progressively shallower positions. Such development commonly occurs under confined (artesian) hydrogeological conditions, that subsequently change to open comlitions (phreatic-water table-vadose). The general evolutionalY line of intrastratal karst is typified hy progressive emergence of a sequence into a shallower position, activation of groundwater circulation and development of cave systems within karst units, commencement of gravitational breakdown and its upward propagation through overlying heds, and development of a karst landscape. These processes and phenomena progress through the directed evolution of karst types as follows: deep-seated intrastratal karst (IK) :=} suhjacent IK :=} entrenched IK :=} denuded karst (see Chapter 1.4). One of the main characteristics of intrastratal karst is that it induces gravitational breakdown in cover beds. With the aid of processes other then simple breakdown, such effects may propagate upwards and may, or may not, reach the surface, depending upon the thickness and structure of the overhurden. A karst landscape evolves when such features reach the surface. This paper considers the conditions and mechanisms of such development.

Speleogenetic effects of interaction between deeply derived fracture-conduit flow and intrastratal matrix flow in hypogene karst settings

In carbonate rocks, especially in those with high primary porosity such as most Cenozoic carbonates, the interaction between deeply derived rising flow through sub-vertical fracture-controlled conduits and intrastratal matrix flow of shallower systems can invoke mixing corrosion and result in prominent speleogenetic effects. This paper outlines a conceptual model of such interaction and provides instructive field examples of relevant morphological effects from two different regions within the Prichernomorsky (north Black Sea) basin, where karst features are developed in lower Pliocene, Eocene and Paleocene limestones. In the Crimean fore-mountain region, extensive steep to vertical limestone scarps formed through recent exposure of hypogenic fracture-controlled conduits provide outstanding possibilities to directly examine details of the original karstic porosity. The morphological effects of the conduit/matrix interaction, documented in both caves and exposed scarps, include lateral widening of sub-vertical conduits within the interaction intervals (formation of lateral notches and niches) and the development of side bedding-parallel conduits, pockets and vuggy-spongework zones. Natural convection circulation, invoked by interaction of the two flow systems, spreads the morphological effects throughout the conduit space above the interaction interval. Where the interaction of the two flow systems is particularly strongly localized, such as along junctions of two vertical fracture sets, the resultant morphological effect can take the form of isolated chambers. The variety of speleogenetic features developed through the conduit/matrix interaction, can be broadly grouped into two categories: 1) variously shaped swells of the major fracture conduit itself (morphological features of its walls – niches and pockets), and 2) features of the vuggy-spongework halo surrounding the conduit. This halo includes clustered and stratiform cavities, spongework zones and lateral side conduits. The speleogenetic features due to conduit/matrix flow interaction, especially the halo forms, often demonstrate distinct asymmetry between opposite walls of the conduits. The prominent phenomenon of the vuggy-spongework halo around fracture-controlled conduits has important hydrogeological implications. A comparison of karst features in different regions and rock formations clearly shows that in spite of some distinctions imposed by local structural, sedimentological and paleo-hydrogeological peculiarities, hypogenic speleoforms in limestones of different age and of different degree of diagenetic maturity demonstrate remarkable similarities.

Unconfined versus confined speleogenetic settings: variations of solution porosity

Abstract: Received 10 November 2005; Revised 15 November 2005; Accepted 28 November 2005 International Journal of Speleology 35 (1) 19-24 Bologna (Italy) January 2006 Re-published from: Speleogenesis and Evolution of Karst Aquifers 1 (2), www.speleogenesis.info, 7 pages (ISSN 1814-294X). 1 Institute of Geological Sciences, Natl. Academy of Science, Ukraine - P.O.Box 136, Kiev-30 01030, Ukraine. E-mail: klim@speleogenesis.info INTRODUCTIONSpeleogenesis in confined settings generates cave morphologies that differ much from those formed in unconfined settings. Speleogenesis in unconfined settings tends to produce broadly spaced dendritic patterns of channels due to highly competing development (Fig. 1A). In contrast, caves originated under confined conditions tend to form two- or three-dimensional mazes with densely packed conduits (Fig. 1B). These caves form as the result of vertical hydraulic communication between “common” insoluble or less soluble porous/fissure aquifers across the soluble bed (“transverse speleogenesis”). There is a specific hydrogeologic mechanism inherent in artesian transverse speleogenesis (restricted input/output) that suppresses the positive flow-dissolution feedback and hence speleogenetic competition in fissure networks which accounts for the development of more pervasive channelling in confined settings. This results in maze patterns where appropriate structural prerequisites exist. This is the fundamental cause for the distinctions between cave morphologies evolving in unconfined and confined aquifers, and for eventual distinctions of karstic permeability, storage characteristics and flow system behaviour between the two types of aquifers (Klimchouk, 2000a, 2003).This paper aims to illustrate variations of solution (channel) porosity resulted from speleogenesis in unconfined and confined settings. This can be done by the analysis of morphometric parameters of typical cave patterns.

Gypsum karst in the western Ukraine: Hydrochemistry and solution rates

Gypsum karst in the western Ukraine underlies a large territory of more than 20,000 km2 and is represented by a range of stages (evolutionary types), from deep-seated through subjacent to entrenched. Correspondingly, hydrogeological settings of karst development, circulation patterns, and chemical characteristics of groundwaters differ substantially between the respective areas. Based on 1,800 analyses, the paper summarises hydrochemistry of the gypsum-hosting Miocene aquifer. Most of the sampling has been performed in conjunction with a regime study of gypsum-solution rates by means of standard tablets. This study included 53 tablet stations representing varying conditions of water-rock interaction, where 644 weight-loss measurements have been made during 1984–1992. The highest rates are characteristic of entrenched karst, although active dissolution there is localised along well-defined sinking streams with short underground courses, rare vertical-percolation paths, and the water table. Lower, but still quite substantial, rates are characteristic for subjacent and deep-seated (confined) karst. However, the overall dissolution removal is higher there, due to higher rates of flow through the gypsum and the larger area of rock/solvent contact. The results are generalised in order to derive the approximate solution rates that characterise major situations and that are suitable for modeling purposes.