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10 NCKRI Special Paper No.1 Figure 1. Epigenic and hypogenic karst in the context of basinal groundwater flow. Adopted and modified from Tóth (1999). The figure shows mainly gravity-driven flow in an idealized homogenous basin. In reality, most sedimentary sequences are highly heterogeneous, and gravity-driven flow interacts with other flow mechanisms. As speleogenesis is a dynamic process capable of considerably changing primary porosity and permeability, it can itself create zones of high permeability along initially insignificant (in terms of regional or intermediate groundwater flow systems) cross-formational flow paths, or even without any initially guiding disruptions. In soluble beds (which originally commonly serve as confining beds; see Section 3.2) this goes through one of the mechanisms of ascending transverse speleogenesis, and in overlying insoluble beds permeability enhancement occurs via fracturing and brecciation above cave porosity zones. Thus, hypogenic speleogenesis may give rise to new discharge zones and contribute to segmenting laterally extensive “throughflow” regions. The classification of regional hydrogeologic structures, introduced at the beginning of this section, can be presented as an evolutionary succession: artesian basins - disrupted basins - layered massifs - hydrogeologic massifs. This corresponds to the successive stages in the general tectonic and geomorphic evolution of sedimentary basins. Similarly, this evolutionary trend provides a framework to classify karst types and speleogenetic environments based on the evolutionary history of a soluble-rock aquifer (Klimchouk, 1996a; Klimchouk and Ford, 2000; Figure 2): from deposition and early emergence above sea level (syngenetic /eogenetic karst) through deep burial and re-emergence (the group of intrastratal karst types: deep-seated karst, subjacent karst, entrenched karst) to complete exposure (denuded karst). 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. Different types of karst, which concurrently represent the stages of karst development, are marked by distinct combinations of the structural prerequisites for groundwater flow and speleogenesis, flow regimes, recharge/discharge configurations, groundwater chemistry, and degree of inheritance from earlier conditions. 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 hydrogeologic/geomorphic evolution, between which the
KARST IN THE CONTEXT OF REGIONAL GROUNDWATER FLOW major boundary conditions, the overall circulation pattern, and extrinsic factors and intrinsic mechanisms of karst development appear to change considerably. The classification of karst types correlates well with the three major types of speleogenetic settings discussed above. Coastal and oceanic speleogenesis in diagenetically immature rocks falls into the syngenetic/eogenetic karst domain. Deep-seated karst is almost exclusively Figure 2. Evolutionary types of karst and speleogenetic environments (from Klimchouk and Ford, 2000). hypogenic. Subjacent karst can be induced by both, hypogenic and/or epigenic speleogenesis, depending on the scale of the flow system. Entrenched and denuded karst types are predominantly epigenic, with possible inheritance of hypogenic features, which are relicts in most of these cases. Open karst has exclusively epigenic speleogenesis. 11
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