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HYPOGENIC CAVE FEATURES
somewhat controversial, based on the “source of acidity”
definition of the term rather than on hydrogeological
criteria. Referring to the lack of deep ascending passages
that mark the vertical path of the acid source and the
absence of volcanic activity and hydrocarbon deposits in
the area to supply acidity from depth, they proposed that
the caves were “formed when oxic meteoric waters
penetrating from former recharge zones percolated
downwards towards the laterally flowing aquifer, coming
in contact with pyrite contained within the carbonates” (p.
165). For this dissolution mechanism to be a feasible
option, it is necessary to show that pyrite could occur in
considerably higher concentrations than commonly
observed in the carbonate rocks. More importantly, the
suggested cave-forming flow system is essentially epigenic
and unconfined, implying recharge from the surface and
lateral flow through the carbonates. Such a flow system is
unlikely to produce extensive maze caves that have
hypogenic characteristics and display no morphogenetic
relationships to the surface. A possible alternative option is
the hypogenic origin in the hydrogeological sense, with
recharge from a basal unit. Ascending feeders to the master
passages do not necessarily have to be prominent conduits
but could be smaller features scattered through the
network, as documented in many maze caves in other
regions. It is noted in the cited paper that the original
bedrock floor in these caves is almost always obscured by
breakdown or sedimentation, so that structures of recharge
from below may be present but not noticed. More detailed
morphological studies and a consideration of litho- and
hydrostratigraphy and the geomorphic evolution of the
basin are needed to decipher the origin of these remarkable
caves.
Figure 55. Toca da Boa Vista and Toca da Barriguda caves in
Brazil (from Auler and Smart, 2003).
Australia
Osborne (2001b) summarized that many caves in
Paleozoic limestones of eastern Australia have
morphological, hydrological, and mineralogical features
that suggest a hydrothermal or artesian origin. He pointed
to many features that conform to the above criteria of
ascending transverse speleogenesis, disregarding the
possibility of origin due to downward recharge through the
caprock, and concluded that they have developed by
upward recharge from basal aquifers rather than by sinking
meteoric waters. These caves are structurally guided, show
maze patterns (Figure 56), have “halls-and-narrows”
morphology (Osborne, 2001a), numerous dead-end
terminations, blades, partitions (Osborne, 2003), roof
pendants, cupolas and pockets. Previously such caves were
described by the term “nothephreatic” (Jennings, 1985),
with the meaning that they were excavated by diffuse flow
under phreatic conditions, which is a close approximation
to confined settings.
A remarkable example is Exit Cave in Tasmania
consisting of over 40 km of network passages through
which a major stream is captured underground. The
captured stream flows through only some of the passages
and was not responsible for the cave's development
(Osborne, 2001b).
Figure 56. Typical hypogenic caves of Eastern Australia (from
Osborne, 2001a). A = The Queenslander-Cathedral Cave System,
Chillagoe, Queensland; B = Ashford Cave, New South Wales.
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