Geology Reference
In-Depth Information
(a)
Figure 6.13. (a) An exposed stock of Donkahoek Granite intruding schist, central Namibia.
domical hills in the Sudeten Mountains of southern Poland (Migon, 1993). In the northern Flinders
Ranges, South Australia, Mt. Ward and the Armchair are pegmatitic, standing above the level of an
all slopes topography eroded in a complex of granite and gneiss.
In this context, however, it must be pointed out that much of the evidence that would permit the
significance of lithological contrasts to be assessed has disappeared. It is the character of the com-
partments of rock that w ere formerly located abo ve the present plains, vis-à-vis the sur viving
bornhardt masses, that is crucial; and these have been eroded. All that can be done is to point to
known compositional and textural variations within plutonic masses and to the possible geomor-
phological implications of such diversity.
It has been suggested that some of the bor nhardts of Mozambique (Holmes, 1918) are merely
projections or apophyses developed at the margins of the plutons, or that they are simply exposed
stocks, and similar claims have been made in respect of forms in western Arabia, Zimbabwe, and
Namibia (Fig. 6.13a). Again, several of the prominent domes in and around Mitchell's Nob, in the
Musgrave Ranges of northern South Australia, are e xposed granite stocks intr uded into gneiss
(Fig. 6.13b). Thus, some bornhardts are undoubtedly a manifestation of more resistant intr usive
masses. In most batholithic e xposures, however, there are indications of profound erosion of the
crystalline rocks, suggesting that the original outlines of the plutons have long since been lost. But
whatever the validity of these claims, lithological contrast is again involved.
Cross-folding may also cause the development of resistant compartments on which bornhardts are
based. An analo gue is pro vided b y w ell-known sedimentar y inselber gs from central Australia
(Twidale, 1978). The plains surrounding the Olgas and Ayers Rock are eroded in sediments (con-
glomerate and arkose respectively) that are physically contiguous with the strata exposed in the resid-
uals. The Olgas, Ayers Rock and Mt. Conner (a sandstone-capped mesa) are aligned E-W and the
alignment cuts across the fold str uctures evidenced in the three residuals. Some time after a later
Palaeozoic orogeny the re gions suffered N-S compression resulting in cross-folding in the Earl y
Palaeozoic strata in which the residuals are formed, and in the development of compressional cores
that are the present uplands. In similar f ashion, and as suggested b y Dale (1923), sheet fractures
delineate structural domes and basins, which argue either shearing or cross-folding (compression from
markedly different directions) and resulting in an egg-box type of structure at a regional scale (Vidal
Romaní, 1991). The zones of pronounced compression so developed could resist water penetration
and weathering, and form the bases for residual hills. Deep erosion, taking the land surface through
the upper tensional zone of antiforms and into the deeper compressional sectors, is implied.
