Democracia y representación
B. Latin American Public Opinion Project
The removal of irregularities of the bedrock pediment surface projecting above the general level of the subaerial pediment surface and the comminution of gibbers and finer material are achieved by a variety of processes. Granite gibbers and outcrops are usually very rotten . Thin sections reveal that the feldspar in the groundmass of the porphyritic granite and biotite and muscovite are selectively weathered to form clay minerals in the manner described earlier. The phenocrysts of feldspar and some of the quartz are added to the mantle as gibbers and coarse surface lamina respectively. Individual grains can often be removed from an outcrop by light finger pressure.
Granite gibbers, boulders, and outcrops also weather in both a cavernous manner and by flaking (Ollier 1969, p.186): tafoni and spalling are often found on different parts of the same boulder. Oxidation, case-hardening, and lichens appear to be associated with the development of tafoni. Basal tafoni (Jennings 1968) are common on the undersides of gibbers and flakes. Side tafoni are also developed. However, such processes do not appear to favour any particular aspect
in the manner suggested by Birot (1968, p. 88). The formation of ground-level platforms by flaking (Ollier 1969, p. 185) is found on exposed bedrock at the northern end of GA (just outside the area of detailed study).
Chemical weathering of gibbers, boulders, and outcrops is often concentrated at or just below soil level. This is presumably favoured by the retention of moisture alongside rock outcrops or gibbers long after other areas have become completely dry. Notching
(Mabbutt 1966) is common, and abandoned notches indicate that surface lowering has taken place. The development of side tafoni appears to be initiated at ground level. Varnishing of gibbers and outcrops is also common.
The trimming of exposed dikes often lags behing the grading of the remainder of the footslope. Narrow outcrops of disintegrating dikes
)
up to 0.50 m high are common. Some dikes are comparatively inert chemically (e.g. aplite), whereas others are very vulnerable to chemical weathering (e.g. rhyolite), but all are eventually trimmed to the general level of the subaerial pediment surface, leaving local
accumulations of angular gibber.
4
The comminution of gibbers by mechanical means is also important. Charred gibbers clearly testify to the occurrence of bushfires, and the explosion of campfire stones was a constant reminder of the efficacy
of this process. In addition, high summer temperatures may be adequate
for insolation weathering of chemically inert gibbers in the various ways indicated by Ollier (1963).
The erosion and transport of weathered material is done by
sheet erosion and linear erosion. Sheet erosion is particularly
effective at the onset of the wet season when the scant vegetation offers little protection: large bare patches are common in non-spinifex areas, while spinifex may offer only 10% canopy cover at this time of
the year. In contrast, at the height of a good wet season the vegetation
canopy cover is virtually 100% and the initiation and maintenance of sheetfloods is hindered.
Two sheetfloods were witnessed in February 1971 on GC and near
Turkey Creek. The onset of a sheetfiood is marked by the change from
rainsplash erosion (M.A.J. Williams 1969c) to the development of a thin sheet of water less than 0.5 cm deep which partly protects the surface from
further rainsplash erosion. The flow of water is discontinuous: it
disperses and converges around major clumps of vegetation such as
spinifex. The water is not particularly discoloured, and saltating
sand grains are visible. This stage is hereinafter termed sheetwash.
As the depth of flow increases the water becomes discoloured by its debris load, and the flow is visibly turbulent, and vegetation is less
important in determining the detailed course of the water. This
stage is hereinafter termed sheetfiood. Considerable amounts of
dead vegetable matter are carried on the surface of the flood and are often dammed up behind large spinifex tussocks, thereby diverting the flow
and increasing the turbulence. Examination of water samples shows that
coarse sand and finer material is transported. Later, as seen in the flood
near Turkey Creek, the footstream becomes unable to cope with the
large volumes of water draining into it and overflows its banks to merge with the sheetfiood. At this stage a distinction between sheetfiood and
streamflood can scarcely be made (figure 4.47). As the flood- loses )
its impetus and the depth of flow decreases, the vegetation once again determines the detailed course of the flow and runoff becomes concentrated
dumped on t h e s u b a e r i a l s u r f a c e , o f t e n f o r m i n g " d e l t a i c " f a n s b e t w e e n two l a r g e s p i h i f e x t u s s o c k s . T h e r e i s no d o u b t i n t h e w r i t e r ’ s mind t h a t t h e s h e e r f o r c e o f a s h e e t f l o o d , o f t e n m a k i n g i t h a z a r d o u s t o w a d e , i s q u i t e c a p a b l e o f a c t i v e l y e r o d i n g w e a t h e r e d r o c k , e s p e c i a l l y s i n c e l i g h t f i n g e r p r e s s u r e c a n d i s l o d g e i n d i v i d u a l g r a i n s . The c o l l a p s e o f t e r m i t e mounds d u r i n g a s h e e t f l o o d was a l s o w i t n e s s e d . The d i s t r i b u t i o n and c h a r a c t e r i s t i c s o f t h e c o a r s e s u r f a c e l a m i n a p o s e two' p r o b l e m s : ( a ) why i s i t so d i s t i n c t i v e f r o m t h e d e p o s i t s i m m e d i a t e l y b e l o w ? a n d (b) on w h a t t i m e s c a l e i s t h i s m a t e r i a l i n t r a n s i t ? The a n s w e r s t o t h e s e p r o b l e m s a r e r e l a t e d . D u r i n g t h e d r y s e a s o n t h e f i n e r m a t e r i a l b e l o w i s b a k e d h a r d by i n s o l a t i o n . S h e e t w a s h a t t h e o n s e t o f t h e w e t s e a s o n i s u n a b l e t o e r o d e o r t r a n s p o r t s i g n i f i c a n t a m o u n t s o f t h i s f i n e r m a t e r i a l . However, s m a l l g r a v e l an d s a n d p r o d u c e d by t h e c o m m i n u t i o n o f g i b b e r s an d o u t c r o p s on t h e b a c k i n g h i l l s l o p e and f o o t s l o p e i s n o t c o m p a c t e d a n d s h e e t w a s h i s a b l e t o t r a n s p o r t t h e s e f r a g m e n t s o v e r t h e i m p e r m e a b l e m a t e r i a l . The p r o p o r t i o n o f f i n e e a r t h s i n s u c h m a t e r i a l i s v e r y l o w . As s h e e t w a s h s u b s i d e s , t h e s e c o a r s e p a r t i c l e s a r e d e p o s i t e d on t h e s u r f a c e , an d l i t t l e f i n e e a r t h i s d e p o s i t e d w i t h t h em . T h i s a c c o u n t s f o r t h e d i s t i n c t i v e n e s s o f t h e c o a r s e s u r f a c e l a m i n a . The d o w n s l o p e d e c r e a s e i n t h e s i z e o f t h i s m a t e r i a l an d t h e i m p r o v e d s o r t i n g a r e c h a r a c t e r i s t i c o f t r a n s p o r t by w a t e r . The d o w n s l o p e d e c r e a s e i n t h e % o f t h e g r o u n d s u r f a c e c o v e r e d by t h i s l a m i n a may b e a s s o c i a t e d w i t h t h e t e n d e n c y f o r s h e e t w a s h t o d e v e l o p i n t o s h e e t f l o o d s a s d i s t a n c e f r om t h e n i c k l i n e i n c r e a s e s , an d t h e r e f o r e t o become more e f f e c t i v e i n t r a n s p o r t i n g t h i s c o a r s e s u r f a c e l a m i n a t o t h e f o o t s t r e a m . The s h o r t t i m e s c a l e a t w h i c h t h i s m a t e r i a l i s i n t r a n s i t i s d e d u c e d f r o m o b s e r v a t i o n s on s h e e t w a s h . The movement o f i n d i v i d u a l g r a i n s o v e r a d i s t a n c e o f s e v e r a l m e t r e s i n l e s s t h a n a m i n u t e was o b s e r v e d when t h e d e p t h o f f l o w was l e s s t h a n 0 . 5 cm . F u r t h e r m o r e , i m m e d i a t e l y p r i o r t o a s t o r m on GA t h e r e was no c o a r s e s u r f a c e l a m i n a on a c e r t a i n p a r t o f t h e f o o t s l o p e , b u t s h e e t w a s h d e v e l o p e d d u r i n g t h e s t o r m and l e f t a c o a r s e s u r f a c e l a m i n a w h i c h c o u l d b e t r a c e d 20 m a c r o s s t h e f o o t s l o p e f r o m t h e n i c k . S i n c e m a t e r i a l o f i n d i v i d u a l q u a r t z g r a i n s i z e c a n b e moved s u c h d i s t a n c e s i n a n i n d i v i d u a l s h e e t w a s h »
it is unlikely that it remains on the footslope surface long enough
to be further4reduced in size. Indeed the beds of footstreams are
covered with such material, and some channels are almost completely
choked with it. This coarse surface lamina must therefore be regarded
as in transit at the time scale of a few years.
Although this material is commonly fresh and unoxidised in appearance, the occasional occurrence of oxidised grains in this lamina may have one of' two origins: localised reworking of the transported
regolith; or the incorporation into this lamina of coarse material brought to the surface by ants (Williams 1968).
Sheetfloods may erode finer material from below this surface . lamina and deposit material in the form of lenses which lack the
characteristics of the coarse surface lamina. However, since transport by
sheetwash probably occurs much more frequently than by sheetflood, it is thought that movement by sheetwash is sufficient to explain the
characteristics and distribution of this coarse surface lamina. The transported regolith below this coarse surface lamina
is regarded as being
in situ
at the same time scale. However, observationshave shown that it may be reworked by sheetfloods. The great areal
variation in percentages of gravels, sands and fine earths is probably accounted for in part by the characteristics of sheetflood deposition, and in part by the incorporation into the mantle of weathered material
eroded from subaerial exposures of the bedrock surface. The general
decrease in mean particle size with distance from the niclcline on GA, G C , and GD (as shown by the correlation coefficients in linear and exponential regression) could be explained by sheetflood erosion
and transport during several small-scale cycles of regrading, with the tendency for coarser material to be dumped near the nickline and finer material at the end of the footslope.
It might therefore be expected that the trend surfaces of mean
particle size of the surface samples would correspond to the trend surfaces of the subaerial pediment surfaces, since the inclination of the latter
should control the direction of flow of sheetfloods. However, if we
compare these surfaces for GA (figures 4.34 and 4.16), GC (figures 4.35 and 4.20), and GD (figures 4.36 and 4.22) we find that the trends are not always in the same direction. This may be explained in part by the incorporation of additional debris into the mantle by weathering
the lower part of GA9 may be related to the swarm of dikes in that
vicinity). However, in other places this type of explanation does
not apply (e.g. the trend in mean particle size between transects 7 and 9 at Pompey’s Pillar is oblique to the trend of the subaerial pediment surface, but bedrock is not exposed in this vicinity).
The available evidence suggests that gibbers are rarely