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30<br />
NCKRI Special Paper No. 1<br />
4.1 Criteria for distinguishing the hypogenic<br />
transverse origin for caves<br />
The following geologic, morphologic, sedimentologic<br />
and mineralogic criteria are, in certain combinations,<br />
indicative of hypogenic transverse speleogenesis:<br />
1. Presence of a source of recharge to the cave<br />
formation from below. It can be the immediately<br />
underlying aquifer, a laterally conductive bed within the<br />
aquifer system, or discrete vertical high-permeability paths<br />
conducting flow from still deeper aquifers. The common<br />
case is an insoluble porous or fractured bed, such as quartz<br />
sandstone or sand that serves as a regional aquifer and the<br />
source of water for transverse speleogenesis. It can also be<br />
a less soluble and more diffusely permeable material than<br />
the cave unit, such as oolitic limestone, densely fractured<br />
dolomite or marly limestone underlying gypsum or a less<br />
permeable limestone bed. To provide for dispersed and<br />
uniform recharge to the cave unit, the permeability<br />
structure of the source aquifer should be much more<br />
densely spaced than fissures in the soluble unit. Otherwise,<br />
discrete cavities would form in the cave unit, matching<br />
discrete paths of recharge from below.<br />
2. Presence of an overlying aquifer bed. It can occur<br />
immediately above the soluble unit, or be separated by a<br />
thin leaky aquitard. The overlying aquifer acts as a<br />
governor for outflow, and allows transverse speleogenesis<br />
in a soluble bed to occur through areas offset from major<br />
flowpaths or breaches that discharge water out of the<br />
confined system. In some cases there can be no overlying<br />
aquifer, with a confining formation lying immediately<br />
above the cave formation. The confining formation should<br />
be considerably leaky to favor transverse speleogenesis in<br />
the cave formation.<br />
3. Presence of a confining formation,commonly of<br />
regional extent and of low permeability. Transverse<br />
speleogenesis operates where the thickness of the<br />
confining strata is reduced due to erosional incision that<br />
induces considerable leakage, or where faulting or<br />
stratigraphic weaknesses allow discharge from the<br />
confined system to occur.<br />
4. The overall layout of hypogenic cave systems and<br />
the position of their entrances show no genetic relationship<br />
to modern landscapes. However, significant cave<br />
development is normally induced by, and converges<br />
toward, valleys incising into the upper confining<br />
formation. Where modern valleys have incised below the<br />
cave-hosting formation, caves tend to border them.<br />
Paleovalleys, often buried, that cross modern watersheds<br />
could have induced transverse speleogenesis beneath them<br />
so that hypogenic cave systems can be found in the internal<br />
parts of modern intervalley massifs.<br />
5. Cave patterns resulting from ascending transverse<br />
speleogenesis are strongly guided by the permeability<br />
structure in a cave formation. They are also influenced by<br />
the discordance of permeability structure in the adjacent<br />
formations and by the overall hydrostratigraphic<br />
arrangement (recharge-discharge configurations). Three-<br />
dimensional mazes with multiple stories or complex cave<br />
systems are common, although single isolated chambers,<br />
passages or crude clusters of a few intersecting passages<br />
may occur where fracturing is scarce (Section 4.2). Large<br />
rising shafts and collapse sinkholes, associated with deep<br />
hydrothermal systems, are also known.<br />
6. Stories in three-dimensional mazes are guided by<br />
the distribution of initial porosity, which is commonly (but<br />
not always) stratiform. They may be horizontal or inclined,<br />
stratiform or discordant to bedding. Stories in ascending<br />
hypogenic systems form simultaneously within a complex<br />
transverse flow path, in contrast to epigenetic caves where<br />
stories reflect progressive lowering of the water table in<br />
response to the evolution of local river valleys, hence<br />
upper stories being older than lower.<br />
7. When aggressive recharge from below is uniformly<br />
distributed, passages that hold similar positions in the<br />
system in relation to the flowpaths' arrangement (guided by<br />
the same set of fractures, or occurring within a single cave<br />
series or at the same story) are commonly uniform in size<br />
and morphology. A common feature of network mazes is<br />
high passage density. Spongework mazes may also occur<br />
where the initial porosity is represented by interconnected<br />
vugs. Larger volumes may be dissolved where aggressive<br />
recharge from below is concentrated by virtue of hydraulic<br />
properties of the feeding formation.<br />
8. The characteristic features of ascending hypogenic<br />
cave systems are numerous blind terminations of passages<br />
in the lateral dimension and abrupt variations in passage<br />
cross-sections. Lateral changes indicate largely<br />
independent rising development of numerous almost<br />
independent transverse clusters (flow paths), and vertical<br />
changes indicate variations in initial porosity structures<br />
between lithological units.<br />
9. The morphology of hypogenic caves, developed in<br />
varying lithologies by different dissolutional mechanisms,<br />
displays a very characteristic suite of similar forms<br />
indicating rising flow patterns during cave formation. This<br />
suite is the strongest diagnostic feature of hypogenic caves<br />
(Section 4.4) and consists of three major functional<br />
components: feeders (inlets), transitional wall and ceiling<br />
features, and outlet features.<br />
10. Natural convection mechanisms (buoyancy-driven,<br />
upward pointed dissolution), powered either by thermal or<br />
solute differences, are widely operative in hypogenic<br />
caves, contributing significantly to the characteristic<br />
morphologies mentioned above and producing upwarddirected<br />
flow markings (Section 4.4). Directional markings<br />
produced by vigorous flow regimes and lateral flow, e.g.<br />
scallops, are generally absent in hypogenic caves, although<br />
they may be present locally when considerable epigenic<br />
overprint occurs during the subsequent unconfined stage,<br />
e.g. by intercepted streams or backflooding. Water table<br />
markings, such as horizontal notches, may develop if the<br />
respective conditions are stable enough.<br />
11. Clastic cave sediments are represented mainly by<br />
fine-grained clays and silts. These can be partly or largely<br />
autochthonous (comprising insoluble residues), although<br />
they often include considerable allochthonous sediments