Download PDF - Speleogenesis

84
NCKRI Special Paper No. 1
Synopsis
Several general points can be derived from the above
regional overview.
1) Hypogenic speleogenesis is much more widespread
that previously assumed. It is identified in various
lithologies and geological and tectonic settings. Despite
these variations, the resultant caves demonstrate a
remarkable similarity in their patterns and mesomorphology,
which suggests that the hydrogeologic
settings were broadly identical in their formation. The
morphologic suites of rising flow with buoyancy
components are clearly identifiable in most of these caves.
2) In many areas more than one dissolutional process,
such as CO2-driven hydrothermal dissolution and H2S
(sulfuric acid) dissolution, is recognized to form hypogenic
caves. They operated either simultaneously or sequentially,
and it is often difficult to discriminate between their
respective speleogenetic effects.
3) The great majority of accessible hypogenic caves
are relict. Many of them bear signs of overprint by
epigenetic processes. However, many active (in the sense
of continued hypogenic development) caves are
documented, either directly or indirectly. Active hypogenic
caves are found in various current hydrogeologic
environments: at the water table, at depth in presently
unconfined aquifers (rising flow through them indicates
vertical head gradient from still deeper aquifers and the
hypogenic component), and in currently confined
conditions. In many cases these settings (with regard to
cave-hosting formations) occur proximal to each other in
the same region, suggesting a speleogenetic evolutionary
sequence in response to uplift and erosional
lowering/entrenchment.
4) Many maturely developed hypogenic caves with
type morphological characteristics are unambiguously
shown to develop under confined conditions. None of the
morphologically mature caves presently active at a water
table were unambiguously shown to form in the respective
contemporaneous settings. This suggests that confined
settings are the principal hydrogeologic environment for
hypogenic speleogenesis, which is in agreement with the
broad analysis of hydrogeological evolution and the
ascending transverse speleogenetic model. However,
hypogenic caves may experience substantial modification
under subsequent unconfined stages, especially when H2S
dissolution mechanisms are involved.
5) Whether or not water table/subaerial dissolution can
be a major mechanism in also creating features that occur
by hypogenenic processes remains an open debate and
requires more research.
4.6 Comparison of confined versus
unconfined conduit porosity
The distinctions between hypogenic (confined) and
epigenic (unconfined) speleogenesis can be illustrated by
the analysis of morphometric parameters of typical cave
patterns. Klimchouk (2003b) compared two representative
samples of typical cave systems formed in these two
settings. The sample that represents unconfined
speleogenesis consists solely of limestone caves,
characteristically displaying branchwork patterns. Gypsum
caves of this type tend to be less dendritic. The sample that
represents hypogenic-confined speleogenesis consists of
both limestone and gypsum caves that have network maze
patterns.
Passage network density (the ratio of the cave length
to the area of the cave field, km/km 2 ) is one order of
magnitude greater in confined settings than in unconfined
(average 167.3 km/km 2 versus 16.6 km/km 2 ). Similarly, an
order of magnitude difference is observed in cave porosity
(the fraction of the volume of a cave block occupied by
mapped cavities; 5.0% versus 0.4%). This illustrates that
storage in maturely karstified confined aquifers is
generally much greater than in unconfined aquifers.
Average areal coverage (a fraction of the area of the cave
field occupied by passages in a plan view) is about 5 times
greater in confined settings than in unconfined (29.7%
versus 6.4%). This means that conduit permeability in
confined aquifers is appreciably easier to target with
drilling than the widely spaced conduits in unconfined
aquifers.
The results in Tables 1 and 2 clearly demonstrate that
there are considerable differences between confined and
unconfined settings in the average characteristics of cave
patterns and porosity. The fundamental cause of this
difference in conduit porosity is demonstrated to be a
specific hydrogeologic mechanism inherent in confined
transverse speleogenesis (restricted input/output), which
suppresses positive flow-dissolution feedback and
speleogenetic competition in fissure networks (Klimchouk,
2000a, 2003a). This mechanism accounts for the
development of more pervasive channeling and maze
patterns in confined settings where appropriate structural
prerequisites exist. In contrast, the positive flowdissolution
feedback and competition between alternative
flowpaths dominates in unconfined settings to form widely
spaced dendritic cave patterns.
Table 2 shows no appreciable difference of parameters
between gypsum and limestone caves formed in confined
settings. However, there are noticeable differences
between parameters of particular caves even from the same
region (Table 1). For example, compare the characteristics
of Jewel and Wind caves, both occurring within the slopes
of the structural dome of the Black Hills, or characteristics
of the gypsum mazes in the western Ukraine.
There are two explanations for such differences. First,
one of the implications of the hypogenic transverse
speleogenetic model is that virtually all hydrogeologically