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ASCENDING HYPOGENIC SPELEOGENESIS
3.7 Mechanisms of hypogenic transverse
speleogenesis
Field observations and numerous quantitative
modeling studies (summarized in Palmer, Palmer and
Sasowsky, 1999, and Dreybrodt, Gabrovšek and Romanov,
2005) suggest that speleogenesis in epigenic unconfined
settings tends to produce broadly dendritic patterns of
conduits due to highly competing development. Such
development occurs because the positive feedback
relationship between dissolution rate and discharge causes
accelerated growth of selective favorable paths. Discharge
increases with the growth of the conduit before and, more
dramatically, after breakthrough. Discharge through a
developing conduit is governed by the resistance of the
conduit itself, by its narrowest downgradient part until the
amount of available recharge begins to limit the flow.
Another factor favoring the formation of branchwork
patterns is that recharge in epigenic conditions is quickly
rearranged in response to the competitive speleogenetic
development, further concentrating flow through
successful conduits.
In hypogene confined settings several important
hydrogeologic and geochemical conditions account for the
specificity of speleogenetic mechanisms involved, making
them distinct from epigenic speleogenesis. The most
important is that both recharge and discharge occur
through adjacent insoluble formations or rocks of
considerably lower solubility than a given cave formation,
hence there is an external conservative hydraulic control
on the amount of flow through the system. The resistance
of the transverse conduit itself may control the amount of
flow only in the very early stages of its growth. Then
control is ultimately exerted by conductivity of the least
permeable immediately adjacent formation, either the
feeding or receiving one, or by the overall conductivity of
the major confining formation. This suppresses the positive
flow-dissolution feedback and hence speleogenetic
competition in fracture networks, favoring the
development of more pervasive conduit systems (maze
patterns) where appropriate structural prerequisites exist.
Palmer (2000b) has shown that discharge through the
adjacent diffusely permeable formation (the feeding
formation) is hardly affected by variations in width of the
fracture being fed. Discharge to the fracture depends on the
log of the fracture width, thus a ten-fold difference in
fracture width produces only a two-fold difference in
discharge. All fractures at the contact with the feeding
formation receive nearly the same amount of recharge and
grow at uniform rates.
Basic mechanisms of ascending transverse
speleogenesis in a gypsum bed sandwiched between two
non-soluble aquifers have been simulated by numerical
modeling (Birk, 2002; Birk et al, 2003). The model
combines a coupled continuum-pipe flow model,
representing both diffuse-flow and conduit-flow
components of karst aquifers, with a dissolution-transport
model calculating dissolution rates and corresponding
widening of karst conduits. Maze cave development is
favored by the presence of systematic heterogeneities in
vertical conductivity of a fracture network, which is shown
above (Section 3.5) to be a familiar case because of
discordance in the permeability structure between fracture
networks at various intervals of a soluble formation.
Hence, lateral components in speleogenetic development
within certain beds are favored by the limited vertical
connectivity of the adjacent fracture networks. In addition
to structural preferences, the variation of boundary
conditions in time, e.g. increasing hydraulic gradient
across the soluble unit due to river incision into the upper
confining bed, further influences the development of maze
patterns.
Andre and Rajaram (2005) investigated dissolution of
transverse conduits in hypogenic karst systems by rising
thermal waters, using a coupled numerical model of fluid
flow, heat transfer, and reactive transport. The key
dissolution mechanism considered was the increased
solubility of calcite along a cooling flow path. The
physical domain of the model was a 500-m long fracture,
with initial aperture of 0.05 mm and upward fluid flow at
constant gradient. They found that during the very early
stages of fracture growth, there is positive feedback
between flow, heat transfer and dissolution. The period of
relatively slow growth is followed by a short, abrupt period
of rapid growth (“maturation” of Andre and Rajaram, an
analogue to the “breakthrough” in the modeled
development of early epigenic speleogenesis). However,
soon after maturation, thermal coupling between the fluid
and rock leads to negative feedback and a decrease in
thermal gradient, especially near the entrance, resulting in
shifting the growth area farther up into the fracture and in
reduction of the overall fracture growth rate. They suggest
that this suppresses the selectivity in conduit development
in complex flow systems and allows alternative flow paths
in a fracture network to develop, thus resulting in mazelike
patterns.
These modeling attempts gave important insights into
mechanisms of hypogenic transverse speleogenesis.
However, they used highly simplifying assumptions for
domain geometry and boundary conditions. More realistic
fracture networks and more realistic positions of modeling
domains within a regional flow system should be further
studied, as well as effects of time-variant boundary
conditions.
All numerical models studying the early evolution of
dissolution conduits from initial fractures imply forced
convection flow, and none of them account for buoyancy.
It is commonly assumed that the buoyancy effects become
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