Download PDF - Speleogenesis

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NCKRI Special Paper No. 1
Later he added the cases of sustained high gradients,
such as beneath dams, and of mixing zones where the
groundwater aggressiveness is locally boosted, and
generalized that the formation of maze caves requires high
Q/L ratios (Palmer, 2002).
Ford (1989) for the Black Hills caves and Klimchouk
(Klimchouk and Rogozhnikov, 1982; Klimchouk, 1990,
1992; 1994) for the western Ukrainian caves suggested the
model of maze development under confined conditions by
dispersed ascending recharge from an underlying
formation. Klimchouk (2000a, 2003a) generalized that this
is the most common mechanism for confined
speleogenesis.
An interesting suggestion of yet another mechanism of
maze development is due to “phantomization” (rock-ghost
weathering) by slow flow through fractures and dissolution
of cement in the surrounding matrix at depth, followed by
subsequent erosional removal of the impure residue in the
vadose zone (Vergari and Quinif, 1997; Audra et al.,
2007a). As little is known about caves assigned to form by
this mechanism, it is not discussed here.
A floodwater high gradient origin is a feasible
mechanism for producing small mazes proximal to
obstructions along well-defined stream passages
conducting highly variable flow, or larger mazes in the
epiphreatic zone of high-gradient alpine cave systems
subject to quick and high rises of the water table (Audra et
al., 2007a). However, in relatively low-gradient
environments (cratons and low mountains), it is less likely
to create large maze clusters linked to rather small streams,
such as in Skull Cave, New York, USA, often referred to
as an example of floodwater development (Palmer, 2001;
the cave plan on his Figure 10). An alternative possibility
is that clusters of hypogenic transverse mazes, inherited
from the confined stage, are encountered by invasion
stream passages during the subsequent unconfined stage. A
photograph of a typical floodwater (supposedly) passage
on the cited figure shows a hole with a smooth edge in the
bedrock floor, which is a typical example of a feeder (riser)
in hypogenic transverse caves (see next section, photos F,
H and I on Plate 3). Another frequently cited example of a
floodwater maze is 21-km long Mystery Cave in
Minnesota, USA, which is thought to form by the
subterranean meander cutoff of a small river. The cave
does function in this way at the present geomorphic stage,
but recent examination of the cave by C. Alexander and
the author revealed numerous morphologic features that
strongly suggest a hypogenic transverse origin of the cave
(see next section for discussion of hypogenic morphology
and Section 4.5 for Mystery Cave). Meander cutoff flow
has produced considerable morphological overprint and
fluvial sedimentation in certain passages (Plate 14, upper
left photo) but it has not erased hypogenic speleogenesis
even in those central flow routes.
The floodwater model is often applied to explain
mazes near rivers in somewhat static conditions (static
“backflood mazes”). Although this might contribute to
enlargement of already existing caves, it seems unlikely
that mazes can originate in such situations because
uniform early growth of initial porosity cannot be expected
with side recharge and sluggish flow conditions, as shown
with regard to lateral artesian flow (Palmer, 1975, 1991).
Palmer's high Q/L ratio condition for maze development is
not met in this situation. Furthermore, no maze caves
referred to as being formed by backflood waters from the
nearby river exhibit decrease in passage size or other
regular changes in morphology in the direction away from
the side recharge boundary, as would be expected if this
origin were the case. Floodwaters from the nearby river
can contribute to maze cave development where
considerable conduit permeability is already available
during river entrenchment, but it can be a self-standing
speleogenetic mechanism only where there is intense open
jointing.
The mechanism of diffuse recharge through a
permeable but insoluble caprock, proposed by Palmer
(1975) and widely used to explain maze patterns, requires
additional discussion. It contains an important idea about
the governing role of an adjacent porous formation for the
amount of flow to fissures in a soluble unit (also expressed
by White, 1969). This is the mechanism of restricted
input/output that suppresses the positive flow-dissolution
feedback and hence speleogenetic competition, as
discussed in Section 3.7 in relation to confined settings and
upward flow. However, the hydrogeological conceptual
model that implies maze origin in unconfined settings by
downward recharge from the overlying permeable caprock
(Figure 18, A-B) has some problems if it is to be widely
applicable. The hydrogeologic situation depicted
represents certain evolutionary stages of breaching the
caprock and the cave-hosting unit by denudation/erosion,
and implies that it used to be a stratified multi-aquifer
system, a common case in many sedimentary basins
experiencing uplift and denudation. The model ignores the
fact that flow in the low-permeability bed in this
hydrostratigraphic setting (initially limestone) would be
predominantly vertical, cross-formational, with descending
flow within topographic/piezometric highs and ascending
flow from underlying aquifers beneath valleys incising into
the caprock. Most maze caves for which this origin was
suggested are concentrated around river valleys or other
prominent topographic lows (Palmer referred to
diminished thickness of the caprock due to erosion;
2000b), which implies that ascending flow across the cave
unit had been operative. Hence, such caves are fully
compatible with the model of ascending (recharge from
below) transverse speleogenesis.
From the perspective of basinal flow, as shown in
Section 3.1, zones of ascending cross-formational flow in