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ASCENDING HYPOGENIC SPELEOGENESIS
Particular morphological features resulting from free
convection are discussed for limestone (hydrothermal
caves), by Szunyogh, 1989; Bakalowicz et al. (1987),
Collignon (1983) and Dublyansky (2000b, 2000c); for
caves in gypsum by Kempe (1972, 1996), Klimchouk
(1996b, 1997b, 2000a, 2000b); and in salt by Frumkin
(1994, 2000). These features include cupolas, rising wall
channels, ceiling channels and pendants, flat ceilings,
inclined wall facets and keyhole cross-sections, although
some of these features can be produced by other processes.
However, when these forms occur in a characteristic suite,
in which particular forms are in paragenetic relationships
reflecting the operation of rising currents of dissolving
water continuously from “feeders” to “outlets,” they offer
the most compelling morphological evidence for uprising
flow patterns and the important role of free convection in
hypogene transverse speleogenesis (see Section 4.3). Such
suites are recognized in caves of many regions throughout
the world, developed in evaporites and in carbonates.
Two types of free convection flow patterns can be
recognized in hypogene speleogenesis (Klimchouk,
2000c). Closed loop patterns include rising limbs of less
dense water and return (sinking) limbs of denser water,
operating in the same pathway or segregated through
adjacent alternative pathways. The solute load outflows via
the same basal aquifer that supplies low-density water.
Open flow patterns develop in mixed convection regime
systems where there is an outflow pathway other than
through the recharging aquifer, commonly an upper aquifer
(above the soluble unit) and/or permeability structures that
provide discharge to the surface through a leaky confining
unit. Anderson and Kirkland (1980) described another
variant of an open flow pattern in the Delaware Basin.
Relatively fresh water is supplied laterally from the reef
aquifer to the dissolving point of the dissolution “wedge”
penetrating laterally deep through the evaporite succession.
The dense brine produced by dissolution is drained
downward into the lower aquifer, and ultimately flows out
through it. For this pattern to operate, high permeability
flow paths across the lower evaporite unit should have
already been established by the upward-progressing
dissolution (through the looped, or ascending open
pattern), so the “descending” open cycle and the
progression of a dissolution “wedge” probably come to
operate during later stages of karst development. Given
that buoyancy flow patterns are generally highly complex,
both types of patterns may operate in a particular karst
system.
It is commonly assumed that the buoyancy effects
become significant when initial fractures are substantially
enlarged. None of the numerical models studying the early
evolution of dissolution conduits from initial fractures
account for buoyancy. However, some recent studies
demonstrate the pronounced role of buoyancy in rather
small aperture fractures. Dijk and Berkowitz (2000, 2002)
applied nuclear magnetic resonance imaging to
quantitatively study the developing morphology, flow and
dissolution patterns in natural, rough-walled, watersaturated
halite fractures with 1-2 mm mean apertures.
They found pronounced effects of buoyancy, resulting in
vertical asymmetry during fracture evolution, with
preferential dissolution at higher elevations. In horizontal
fractures, the lower walls dissolve less rapidly than the
upper walls (by a factor of ~2 to 3). The buoyancy effects
in a vertical fracture with horizontal flow cause more rapid
dissolution at higher elevation. For vertical fractures with
upward flow, it is expected that the increasing saturation of
the solution with elevation will be disturbed by the sinking,
saturated solution. This will inhibit preferential dissolution
at the lower (inflow) region and enhance dissolution
farther downstream at higher elevations. Morphological
studies in hypogene caves provide strong evidence for this
effect.
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