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NCKRI Special Paper No. 1
3.6 Dissolution processes in hypogenic
speleogenesis
A common view, that confined conditions offer
limited dissolution potential for karstification, is partially
based on the deeply-rooted but generally inadequate
simplistic concept of lateral flow through soluble
formations, viewed as aquifers in an artesian basin.
Alternatively, transverse hydraulic communication across
lithological and stratiform porosity boundaries, which
commonly coincide with major contrasts in water
chemistry, gas composition and temperature, is especially
potent in driving various disequilibrium and reactiondissolution
mechanisms.
Aggressiveness may represent an original
undersaturation of groundwater with respect to the solid
phase that is being entered, such as in the case of lowsulfate
waters from underlying carbonates or sandstones
entering a gypsum bed. Solubility of gypsum, quite high
compared to carbonates, is further enhanced by several
factors commonly operative in deep-seated environments:
The solubility of gypsum increases by up to 3-6 times in
the presence of various other dissolved salts, such as NaCl,
Mg(NO3)2, etc. (the foreign ion effect);
The reduction of dissolved sulfates removes sulfate ions
from the solution and allows more sulfates to dissolve;
Dedolomitization generates further dissolutional
capacity with respect to gypsum, because Ca 2+ is removed
from solution and the sulfate ions react with Mg.
These processes also affect the origin of porosity in
carbonates, so that interbedded evaporites and carbonates,
a common stratigraphic arrangement in many basins, are
particularly favorable for hypogenic speleogenesis
(Palmer, 1995, 2000a).
Carbonic acid dissolution, which dominates
overwhelmingly in 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 latter is
common in hydrocarbon fields, where waters
characteristically contain high CO2 concentrations
(Kaveev, 1963). Hydrogen sulfide is another common
hypogenic source of acidity where there are sufficient
sources of dissolved sulfate for reduction and where H2S
generated can escape from the reducing zones. However, it
is generally believed that creation of significant caves,
where these acids provide the dissolution mechanism,
depends mainly upon rejuvenation of aggressiveness by
mixing or cooling. These conditions are commonly met in
the ascending limbs of intermediate or regional flow
systems, when they interact with shallower flow systems.
Aggressiveness in hypogenic speleogenesis may also
reflect acquisition of new sources of acid within the
soluble formation itself, when groundwater flows
transversely across it, or can be due to a number of
mechanisms that rejuvenate the dissolutional capacity of
fluids, such as the cooling of water, mixing of
groundwaters of contrasting chemistry, and sulfate
reduction and dedolomitization as mentioned above.
In shallower conditions, where H2S-bearing waters rise
to interact with oxygenated meteoric groundwaters,
sulfuric acid dissolution can be a very strong speleogenetic
agent. 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,
1996; 2000a; 2003a). 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) suggested that
oxidation of pyrite along certain horizons or bedding
planes in carbonates (“inception horizons”) may create
preferential flowpaths that later will be inherited by
epigenic speleogenesis.
The increased solubility of calcite in cooling waters
can cause dissolution along ascending flowpaths. The latter
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 V., 1980; Dubljansky
Ju., 1990, 2000a; Ford and Williams, 1989; Palmer, 1991;
Andre and Rajaram, 2005). Solutional aggressiveness can
be renewed or enhanced by mixing of waters of contrasting
chemistry and dissolved gas content (Laptev, 1939; Bogli,
1964; Palmer, 1991), the effect widely cited in the karst
literature as “mixing corrosion”.
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. Cross-formation flow is the main mechanism
for hypogenic speleogenesis, which can integrate and
trigger many cave-generating processes. Either carbonic
acid or H2S dissolution can operate in hydrothermal
systems, which are essentially ascending transverse
phenomena common in many basins. Mixing of
contrasting waters is also commonly involved in
hypogenic speleogenesis, at least at some stages. This,
again, suggests that labeling types of karst and
speleogenesis by a single dissolutional mechanism is
misleading and should be avoided.