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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

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