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HYPOGENIC CAVE FEATURES<br />

The evolutionary scenario for Sterkfontein suggested<br />

by Martini et al. (2003) and slightly modified here in view<br />

of speleogenetic clarification, is as follows. The hypogenic<br />

transverse development of Sterkfontein could have started<br />

in the early Miocene, about 18 mya, following continental<br />

uplift and tilting of the African surface. The lack of cave<br />

fillings older than Upper Pliocene is explained by the<br />

presence of the protective confining cover during the<br />

African peneplanation. Considering the age of the oldest<br />

sections of the cave-hosted fossiliferous breccia, the<br />

breaching of the confinement might date to the end of the<br />

Miocene (~5 mya). The dating constraints for the<br />

Australopithecus bones found in the younger member of<br />

the breccia indicate that 3.3 mya the cave was already dewatered<br />

at 20-25 m above the present water table. The<br />

secular drop of the water table was irregular, comprising<br />

temporary rises, as evidenced by re-solution of calcified<br />

silt and breccia about 12 m above the present water table.<br />

It is remarkable that water table conditions lasting more<br />

than 3 My did not result in considerable modification of<br />

cave morphology and development of “water table” levels<br />

(see Figure 38-B).<br />

North America<br />

In North America, a hypogenic origin has been<br />

recognized for a number of caves, including such<br />

outstanding examples as the caves of the Black Hills and<br />

Guadalupe Mountains, but the true extent and role of<br />

hypogenic speleogenesis in this part of the world is still to<br />

be properly acknowledged. It is far beyond the scope of<br />

this work to provide a comprehensive and systematic<br />

review and re-interpretation of all cases where a hypogenic<br />

origin of caves was not previously recognized but can be<br />

suspected. Instead, only some instructive cases are<br />

Figure 39. Fissure-like caves and ascending pits in eastern Missouri (from Brod, 1964).<br />

mentioned, most of which are familiar to the present author<br />

through personal experience.<br />

One of the earliest works that suggested a hypogenic<br />

transverse origin of caves in North America is an excellent<br />

study by Brod (1964; Figure 39) from eastern Missouri.<br />

Rectilinear fissure caves and small maze clusters are,<br />

developed along the bottom of the Ordovician Plattin<br />

Limestone by recharge from basal sandstones. These caves<br />

ascend to create a succession of pits and passages which<br />

breach the upper beds of varying lithologies to eventually<br />

provide focused discharge outlets for the artesian aquifer.<br />

Outstanding examples of 3-dimensional (multi-story)<br />

network mazes are Wind and Jewel caves, some of the<br />

longest caves in the world, in the Mississippian Madison<br />

limestone in the Black Hills, South Dakota, USA (Figures<br />

13 and 40). There are many smaller caves of this type in<br />

the area. Multiple stories in these mazes are stratiform,<br />

dipping in accordance with the stratal dip. The origin of<br />

the Black Hills caves is still debated (Palmer and Palmer,<br />

2000b). Ford (1989) suggested the lifting maze model for<br />

the Black Hills caves, which is essentially a hypogenic<br />

transverse model. Bakalowicz et al. (1987) provided<br />

evidence for a hydrothermal origin of these caves and<br />

suggested that they were formed by thermal waters rising<br />

from the basal aquifer into the Madison limestones. Palmer<br />

and Palmer (2000b) suggest that mixing dissolution played<br />

a role in the main cave-forming stage and stress the<br />

importance of paleokarst zones in guiding cave<br />

development. The patterns and morphology of the Black<br />

Hills caves, however, display all the major features of<br />

confined transverse speleogenesis, the model being fully<br />

consistent with regional hydrogeologic settings and<br />

evolution.<br />

69

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