Download PDF - Speleogenesis

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NCKRI Special Paper No. 1
significant when initial fractures are substantially enlarged.
However, some recent studies demonstrate the pronounced
role of buoyancy in rather small aperture fractures (Dijk
and Berkowitz, 2000, 2002). In any case, it becomes more
obvious now that buoyancy dissolution is of tremendous
importance in hypogenic speleogenesis, which is strongly
suggested by both regional karst and cave morphogenetic
analyses (see next section and Section 4.3).
3.8 The role of free convection
Free convection (buoyancy) develops due to density
variations in groundwater caused by solute concentration
or temperature gradients, both being commonly
pronounced in confined flow systems. Dissolution always
tends to build up the solute concentration gradient that can
drive buoyancy, hence free convection effects can be
assumed to be inherently involved in speleogenesis,
although they are overridden by forced convection in most
unconfined systems. As forced-flow regimes in confined
settings are commonly sluggish, the buoyancy-generating
potential of dissolution itself gives rise to mixed
convection regimes in most confined karst systems. In
mixed-regime flow systems, speleogenesis commences in
zones of upward cross-formational communication
(Section 3.1), where the vertical hydraulic gradient and
buoyancy potential are co-linear and congruous. The
ascending transverse flow pattern is particularly favorable
for density gradients of both types to develop (solute
concentration and thermal) because less dense fluids enter
a cave-forming zone from below, so that buoyancy effects
play a particularly significant role in confined
speleogenesis (Klimchouk, 1997b, 2000a).
Large-scale (regional) convection cells driven by
density gradients are generally recognized to play a
substantial role in groundwater circulation in sedimentary
basins and adjacent massifs. The resultant flow patterns
favor dissolution through various geochemical
mechanisms. These aspects have been discussed in relation
to many karst regions around the world. The effects are
particularly pronounced in high-gradient zones of
hydrothermal systems and where fresh groundwater comes
in contact with evaporites from below.
Anderson and Kirkland (1980) provided a compelling
demonstration that dissolution due to free convection is the
main mechanism of ascending transverse speleogenesis in
the thick sulfate and salt succession of the Castile and
Salado formations in the Delaware Basin (western Texas
and southeastern New Mexico, USA), resulting in the
development of cavities at depth and cross-formational
collapse breccia (breccia “pipes”) and masses of
bioepigenetic calcite (“castiles” or “buttes”). Relatively
fresh water is supplied through the underlying shelf and
basin aquifers of the Delaware Mountain Group.
Dissolutional chambers develop upward from the base of
the Castile evaporites, with subsequent collapsing and
formation of breccia. In other cases, the evolving
transverse high permeability paths gave rise to replacement
of sulfates by bioepigenetic calcite and the formation of
“buttes” (Kirkland and Evans, 1980). The free convection
flow pattern establishes itself in such zones, with rising
fresh water and sinking dense brine components. The
descending brine ultimately flows out through the basal
aquifer to points of natural discharge.
A similar mechanism was shown to form vast
chambers (Schlotten) in the upper Permian gypsum in the
Sangerhausen and Mansfeld districts of Germany (Kempe,
1996). Schlotten are large voids, commonly isometric,
elongated along the major tectonic fissures. About 100
cavities of this type are known in the region, encountered
through the centuries in the course of mining operations at
depths up to 400 m at the base of the Zechstein gypsum.
Small cavities of this type, rising from the top of the
underlying bed and remaining seemingly isolated from any
integrated cave system, are commonly observed in gypsum
quarries in the western Ukraine.
Speleogenesis at the base of deep-seated evaporites,
driven by free convection from an underlying aquifer, is
responsible for initiation of vertical breakdown structures
called “breccia pipes,” “breccia chimneys,” “collapse
columns,” “geologic organ pipes,” or “vertical through
structures,” which are abundant in many deep-seated
evaporite karsts as illustrated by many studies from the
United States, Canada, China, Germany and Russia. They
have diameters ranging from tens to over 100 m and
propagate by upward dissolution and stoping from depths
as great as 1200 m (Klimchouk and Andrejchuk, 1996).
Anderson and Kirkland (1980) generalized that “If the
source of the dissolving water is artesian or otherwise
continuous, a flow cycle is developed in which the salt
itself supplies the density gradient that becomes the vehicle
of its own dissolution” (1980, p.66). It can be further
generalized that in confined settings the buoyancy
“vehicle” operates in all major karst lithologies and at
various scales, powered by solute concentration, thermal
gradients, or both.
Curl (1966) provided a theoretical analysis of conduit
enlargement by natural convection in a limestone aquifer,
depicting transitional conditions that determine the
prevalence of natural convection or forced flow regimes.
He found that, with sufficiently slow water circulation
(common in confined settings) convection caused by
density differences might be the primary flow mode for
limestone removal. This is made possible by even
extremely small compositional differences. It is apparent
that the effect becomes much stronger where evaporites are
underlain by aquifers containing relatively fresh waters,
and where thermal gradients are involved.