Download PDF - Speleogenesis

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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