Download PDF - Speleogenesis

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NCKRI Special Paper No. 1
4. Hypogenic cave features
Sedimentary basins and fold-and-thrust geological
structures that experienced a variable degree of uplift and
denudation, and contain carbonate and sulfate formations,
are widespread throughout cratonic and fold-and-thrust
regions. Most of them display features of ascending
hypogene transverse speleogenesis, both relict (in the
entrenched and drained sections) and currently operative
(in confined, deep-seated sections, but also in entrenched
settings). It is argued here that hypogene speleogenesis and
resultant caves are much more widespread than commonly
believed.
Identification and recognition of hypogenic caves is
hindered and often confused due to three main causes.
Firstly, as shown in Section 1.2, there is still considerable
uncertainty in the scope of the hypogenic speleogenesis
concept and in defining the term. Instead of restricting
hypogenic speleogenesis to specific dissolutional
mechanisms (hydrothermal or sulfuric acid), it is suggested
here that hypogenic speleogenesis should be defined from
the hydrogeological perspective, i.e. the formation of caves
by recharge from below in leaky confined conditions by a
number of dissolutional mechanisms that can be involved
in specific cases. This approach is strongly corroborated by
the remarkable similarity in morphological features
exhibited by hypogenic caves formed by different
dissolutional processes in different lithologies (Sections
4.2 and 4.4 below), by the overall regularities in the
hydrostratigraphic occurrence of such caves, and by basin
evolution analysis.
Secondly, the identification of hypogenic caves is
difficult because, by the nature of their origin, they lack
genetic relationships with the overlying or adjacent
surface. They become accessible for direct observations
when occasionally intercepted by denudational lowering,
erosional entrenchment or human activities. However, the
lack of genetic relationships with the surface may serve as
one of the diagnostic criteria for hypogenic caves.
Thirdly, hypogenic caves become accessible being
already relict, largely decoupled from their original caveforming
environment. Features created by epigenic
processes often overprint original hypogenic
morphologies, especially in wet climates, and sediment
accumulations tend to mask important diagnostic features,
especially in floors of caves.
Because of these difficulties, and also due to the still
overwhelming dominance of epigenic concepts and models
in karst science, many caves have been genetically
misinterpreted. Even more known caves with “odd”
patterns, which are not large or outstanding enough to
attract specific scientific attention, remain without clear
speleogenetic interpretation. Examples of regional
speleogenetic analyses well connected with regional
geological (hydrogeological and geomorphological)
evolution are still scarce.
Speleogenetic considerations should be based on the
broad evolutionary approach to karst development, as
described in Chapter 2 and suggested by the classification
of karst types presented in Figure 2. The inherent trend of
basinal evolution during uplift is that deeper, confined,
sections are being brought to the epigenic realm due to
denudation, so that hypogenic caves pass through
transitional conditions of initial breaching and draining,
and get fossilized in the vadose zone. Hypogenic caves
may pass the transient stages without major modification
by the newly established unconfined flow patterns, or they
may be considerably overprinted by epigenic processes.
Where observed caves are in the transitional stages and
overprint is obvious, it is most tempting to relate their
origin to the contemporary epigenic conditions.