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NCKRI Special Paper No. 1
long but rather simple post-confined evolution of a maze
cave. Sterkfontein Cave, together with the adjacent
Lincoln and Fault caves, total 5.23 km in length, and form
a complex, densely packed network of passages in cherty
dolomite over a restricted area of 250x250 m (Figure 38).
Twenty-five entrances open to the surface from near the
top of a hill, the result of intersection of the cave by
denudation lowering. The cave straddles the boundary
between the basal Oaktree Formation and the overlying
chert-rich Monte Christo Formation with distinct oolitic
beds at the contact. The strata dip about 30° to the
northwest and the cave layout shows the overall
stratigraphic control.
In the chert-poor Oaktree Formation (the greater part
of Sterkfontein), the passages are mainly of the fissure
type. They reach heights of 15 m while the widths are on
the order of a few meters. Large chambers can form by
dissolution of partitions separating swarms of tightly
spaced passages. Passages are often superimposed, adding
more complexity to the map. Martini et al. (2003) report
interesting observations of the original joints controlling
passage development on the chert ceilings, which is
unaffected by karst dissolution. The recorded width of
these cracks varies from fractions of a millimeter to one
centimeter. In the up-dip parts of the system where
passages extend to the base of the chert-rich Monte-Christo
Formation, they are broad and low, sandwiched between
chert intercalations. In the down-dip direction, passages
retain the same fissure-like morphology down to and
below the water table over a vertical range of about 50 m.
Caves extend to a greater depth elsewhere in the area, for
instance to 79 m below the water-table, as observed by
exploration after artificial de-watering of the aquifer
(Moen and Martini, 1996).
Sterkfontein is a typical cave of the karst of the
Transvaal basin, characterized by a deficiency of surface
karst morphology but numerous network caves (Martini et
al., 2003). Another notable example is Wonderfontein
Cave, a 9.4 km maze that is confined within an elevation
range of only 3-4 m (Kent et al., 1978). Martini et al.
(2003) point out that the restricted extent of the easily
penetrable passages, forming a dense network in plan
view, is a characteristic shared by the majority of caves of
the Transvaal Basin. This can be readily explained by the
cluster nature of hypogene transverse speleogenesis.
Thirty years ago, when little was known about
hypogenic speleogenesis, Martini and Kavalieris (1976)
placed labyrinthic joint-controlled caves of the region in
the hyperphreatic type, although specifics of this caveforming
mechanism were not really understood at that
time. Apparently, “hyperphreatic” conditions are
essentially confined conditions in cases of a stratified
formation with vertical heterogeneities in permeability. In
the recent work, Martini et al. (2003) guessed that these
caves may have hypogenic origins, forming where deep
water rose up and mixed with shallower flow systems
close to the surface. This model was suggested by
comparison with the caves of northern Namibia, where
there is evidence of such upwelling for some of them
(Martini and Marais, 1996). The confining cover in the
Sterkfontein area was comprised of low-permeability shale
and sandstone of the Ecca Formation (Permian), now
retreated to the south. This suggestion, as well as
characteristics of Sterkfontein described above, is fully
compatible with the hypogenic transverse model.
Figure 38. A = Simplified plan map of Sterkfontein Cave (gray, overlays and details omitted) and Lincoln-Fault Cave System (black). B =
Cross-section of the Sterkfontein Cave showing unroofing of breccia body and breaching of the cave by denudation (adapted from Martini et
al., 2003).