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6 NCKRI Special Paper Series n.1 Though some uncertainties still remain in the scope of the related and overlapping concepts/terms (discussed in the next section), three basic genetic settings are broadly recognized now for caves (Ford and Williams, 1989; 2007; Klimchouk et al., 2000; Ford, 2006): 1) coastal and oceanic, in rocks of high matrix porosity and permeability; 2) hypogenic, predominantly confined, where water enters the soluble formation from below, and 3) hypergenic (epigenic), unconfined, where water is recharged from the overlying surface. Although coastal and oceanic settings are commonly characterized by unconfined circulation, they are treated separately because of the specific conditions for speleogenesis determined by the dissolution of porous, poorly indurated carbonates by mixing of waters of contrasting chemistry at the halocline. 1.2 Hypogenic, confined and deep-seated speleogenesis Hypogenic (or hypogene) caves are usually considered the opposite to the common epigenic caves formed by water recharged from the overlying or immediately adjacent surface due to carbonic acid dissolution. A more appropriate antonym to “hypogenic” is hypergenic (or hypergene); the term widely used in Eastern Europe to denote processes operating near the surface through the action of descending solutions. The term and concept of hypogenic speleogenesis has seen increasing use during the recent two decades, although still with some uncertainty in its meaning. Two approaches appear in recent works. Ford and Williams (1989) and Worthington and Ford (1995) defined hypogenic caves as those formed by hydrothermal waters or by waters containing hydrogen sulfide. Hill (2000a) tends to narrow the notion of hypogenic karst and speleogenesis to H2Srelated processes and forms. Palmer (1991) defined hypogenic caves more broadly, as those formed by acids of deep-seated origin, or epigenic acids rejuvenated by deepseated processes. Later on, Palmer (2000a), presented the definition in a slightly modified, even broader form: 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. This modification is important, as it formally allows us to include in the class of features formed by still surface-independent but non-acidic sources of aggressiveness (such as aggressiveness of water with respect to evaporites). Reference to acid sources seems to be confusing however, as it again tacitly implies that features formed by non-acid dissolution are not pertinent. Palmer's definition relies on the source of aggressiveness. The aggressiveness is a transient attribute of water, which can be delivered from depth or acquired within a given soluble formation (due to mixing or redox processes, for instance). It is suggested here that the definition of hypogenic speleogenesis should rather refer to the source of groundwater, as it is a medium of transport of aqueous and nonaqueous matter and energy, a reactive agent and a major component of the speleogenetic environment. Hypogenic speleogenesis is defined here, following the recent suggestion of Ford (2006), as “the formation of caves by water that recharges the soluble formation from below, driven by hydrostatic pressure or other sources of energy, independent of recharge from the overlying or immediately adjacent surface.” Hypogenic speleogenesis does not rely exclusively on certain dissolutional mechanisms; a number of dissolutional processes and sources of aggressiveness can be involved (see Section 3.6 below). Its main characteristic is the lack of genetic relationship with groundwater recharge from the overlying surface. In many instances, hypogenic speleogenesis is climate-independent. It may not be manifested at the surface at all (deep-seated karst). Hypogenic caves commonly come into interaction with the surface as relict features, largely decoupled from their formational environment, when ongoing uplift and denudation shift them into the shallow subsurface. The concept of hypogenic speleogenesis is closely related to the notion of artesian or confined speleogenesis. These terms refer to the important aquifer condition, where groundwater is under pressure in a bed or stratum confined by a less permeable rock or sediment above it. The criterion of non-relevance of hypogenic speleogenesis to overlying surface recharge and sources of aggressiveness implies substantial separation of groundwater circulation from the overlying surface, i.e. some degree of confinement or rising flow. Groundwater rises through soluble but initially poorly permeable or heterogeneous formations. Most hypogenic speleogenesis initially occurs under confined conditions, which accounts, as will be shown in the following sections, for its most essential features (see also Klimchouk, 2000a, 2003a, 2003b, 2004). However, there is an evolutionary trend for hypogenic karst systems to lose their confinement in the course of uplift and denudation, and due to their own expansion. Hypogenic development may continue in unconfined settings, but confined conditions are the most essential element of hypogenic speleogenesis. Other concepts that are relevant to hypogenic speleogenesis are intrastratal karst and deep-seated karst. Intrastratal karst is developed within rocks already buried by younger strata, where karstification is later than deposition of the cover rocks (Quinlan, 1978; Palmer and Palmer, 1989). This meaning does not relate to genesis but implies stratigraphic, although not necessarily hydrogeologic, separation of karst development from the surface by overlying non-soluble strata and emphasizes the
evolutionary aspect (i.e. karst is later than the cover). Intrastratal karst can be deep-seated, subjacent, entrenched or denuded (the latter represents the former intrastratal karst); this subdivision has an evolutionary meaning and relates karst settings to the depth of erosional entrenchment and the degree of denudational exposure (Klimchouk, 1996a; Klimchouk and Ford, 2000). Depending on the cover lithology and the depth of erosional entrenchment, intrastratal karst can develop in confined or unconfined BASIC CONCEPTS AND TERMINOLOGY conditions. Deep-seated karst is developed without any exposure of the soluble rock to the overlying surface, and is not related to it. It implies ongoing contemporary karstification, so it should be differentiated from buried karst, or paleokarst. Deep-seated karst is always intrastratal, although the opposite is not always true. Deep-seated karst is predominantly confined and hypogenic; this is discussed further in the next section. 7
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