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HYPOGENIC CAVE FEATURES
The evolutionary scenario for Sterkfontein suggested
by Martini et al. (2003) and slightly modified here in view
of speleogenetic clarification, is as follows. The hypogenic
transverse development of Sterkfontein could have started
in the early Miocene, about 18 mya, following continental
uplift and tilting of the African surface. The lack of cave
fillings older than Upper Pliocene is explained by the
presence of the protective confining cover during the
African peneplanation. Considering the age of the oldest
sections of the cave-hosted fossiliferous breccia, the
breaching of the confinement might date to the end of the
Miocene (~5 mya). The dating constraints for the
Australopithecus bones found in the younger member of
the breccia indicate that 3.3 mya the cave was already dewatered
at 20-25 m above the present water table. The
secular drop of the water table was irregular, comprising
temporary rises, as evidenced by re-solution of calcified
silt and breccia about 12 m above the present water table.
It is remarkable that water table conditions lasting more
than 3 My did not result in considerable modification of
cave morphology and development of “water table” levels
(see Figure 38-B).
North America
In North America, a hypogenic origin has been
recognized for a number of caves, including such
outstanding examples as the caves of the Black Hills and
Guadalupe Mountains, but the true extent and role of
hypogenic speleogenesis in this part of the world is still to
be properly acknowledged. It is far beyond the scope of
this work to provide a comprehensive and systematic
review and re-interpretation of all cases where a hypogenic
origin of caves was not previously recognized but can be
suspected. Instead, only some instructive cases are
Figure 39. Fissure-like caves and ascending pits in eastern Missouri (from Brod, 1964).
mentioned, most of which are familiar to the present author
through personal experience.
One of the earliest works that suggested a hypogenic
transverse origin of caves in North America is an excellent
study by Brod (1964; Figure 39) from eastern Missouri.
Rectilinear fissure caves and small maze clusters are,
developed along the bottom of the Ordovician Plattin
Limestone by recharge from basal sandstones. These caves
ascend to create a succession of pits and passages which
breach the upper beds of varying lithologies to eventually
provide focused discharge outlets for the artesian aquifer.
Outstanding examples of 3-dimensional (multi-story)
network mazes are Wind and Jewel caves, some of the
longest caves in the world, in the Mississippian Madison
limestone in the Black Hills, South Dakota, USA (Figures
13 and 40). There are many smaller caves of this type in
the area. Multiple stories in these mazes are stratiform,
dipping in accordance with the stratal dip. The origin of
the Black Hills caves is still debated (Palmer and Palmer,
2000b). Ford (1989) suggested the lifting maze model for
the Black Hills caves, which is essentially a hypogenic
transverse model. Bakalowicz et al. (1987) provided
evidence for a hydrothermal origin of these caves and
suggested that they were formed by thermal waters rising
from the basal aquifer into the Madison limestones. Palmer
and Palmer (2000b) suggest that mixing dissolution played
a role in the main cave-forming stage and stress the
importance of paleokarst zones in guiding cave
development. The patterns and morphology of the Black
Hills caves, however, display all the major features of
confined transverse speleogenesis, the model being fully
consistent with regional hydrogeologic settings and
evolution.
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