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