Geology Reference
In-Depth Information
(a)
Figure 6.10. (a) Development of the Pic Parana, southeastern Brazil, as an upfaulted hill (Barbier, 1957).
however, there are four main arguments, involving faulting, lithological control, cross-folding and
contrasts in fracture density.
First, some few bornhardts are of tectonic origin for they appear to be upfaulted blocks. In this
regard it is interesting to note that the local Aboriginal tribe, the Pitjandjara, believed that Ayers
Rock, a well-known bevelled bornhardt of Cambrian arkosic sandstone which stands in isolation
above the desert plains of central Australia (see Chapter 12)
,
simply rose out of a large flat sand
hill. There is a didactic philosophy based on myth and legend, and they sought no evidence, but
had they done so they would not have found faults delineating the arkosic bornhardt (Mountford,
1965).
The Pic Parana, on the other hand, is a horst residual located in southeastern Brazil, and delim-
ited by faults that have been recently active (Fig. 6.10a). Other bornhardts may well be of similar
type, but at the Pic Parana the crucial evidence (Barbier, 1957) was exposed in excavations related
to a hydroelectric scheme. Unless there are such exposures, it is difficult to demonstrate first that
the residual in question is delineated by faults, and second that the faults have recently been active.
Bearing in mind this proviso, the field evidence suggests that most bornhardts are not tectonic for
there is no e vidence that most of them are delineated b y large-displacement faults, and, even
in those e xamples cited as of tectonic origin, it is not e verywhere apparent w hether the f ault-
delineated scarps are tectonic or structural, whether they are fault or fault-line scarps. Few bornhardts
are demonstrably horsts.
This is not to suggest that faults play no part in bornhardt morphology, for they form zones of
weakness within residuals, for e xample the Pão de Açucar massif in the Rio de Janeiro area of
