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ASCENDING HYPOGENIC SPELEOGENESIS
Discharge from the cave formation is commonly
mediated by some kind of aquifer with diffuse
permeability (“receiving formation”; rows 1 and 2 in
Figure 11), but may also occur via localized zones of
enhanced permeability in the immediately overlying leaky
aquitard (rows 3 and 4). In gravity-driven flow systems,
zones of upward flow establish themselves when hydraulic
gradients across the separating beds, particularly across the
major confining formation, are maximized below river
valleys or other prominent topographic lows, and/or where
zones of enhanced vertical permeability across the leaky
aquitard facilitate discharge. The overall discharge from a
system can be diffuse (A1 in Figure 11), or localized in
single (B1, A3, B3, A4) or multiple (A2, B2) fault zones.
Zones of preferential recharge to the cave formation and
zones of overall discharge can be laterally shifted relative
to each other, resulting in a staircase effect in the
arrangement of a multi-story cave system, with offset of
upper stories toward the focuses of discharge. This is
exemplified by Jewel and Wind caves in the Black Hills
(Ford, 1989) and by the Optymistychna caves (Klimchouk
et al., 1995), as shown in Figures 12 and 13.
For cross-formational flow the least permeable units
dominate the system. Initially poorly permeable soluble
beds may develop dramatically increased permeability due
to transverse speleogenesis, although non-soluble beds do
not. Figure 11 (see also Figure 6) illustrates an important
feature of hypogenic, confined transverse speleogenesis,
distinct from epigenic settings; when conduits have
evolved (i.e. after kinetic breakthrough), the flow across
the cave formation is limited by the permeability of the
feeding and receiving formations and boundary conditions
of the respective intermediate or regional flow system.
This has important speleogenetic consequences, as it
suppresses speleogenetic competition in the developing
transverse system and favors the formation of pervasive
cave patterns where proper structural prerequisites exist
(Klimchouk, 2000a; 2003a; Birk et al., 2003; see Section
3.7).
Figure 10. Modes of recharge to a cave formation from a feeding formation, depending on juxtaposed permeability structures. Note that
similar relationships may occur between different beds in the cave formation, leading to complex 3-dimensional organization of ascending
cave systems. Key to legend: permeability styles: 1 = soluble rocks of low matrix permeability; 2 = poorly connected vug-type porosity – low
effective permeability; 3 = high matrix-vug permeability; 4 = high fracture permeability; 5 = insoluble rocks of low permeability; 6 = prominent
fractures and their planes; 7 = recharge to a cave formation; 8 = points of fracture intersections; 9 = lateral flow through aquifers; 10 =
density-driven dissolution.
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