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