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