%
Although the geomorphic setting of the granite pediments is very different from the area studied by Mabbutt, many observations concerning process and form suggest that mantle-controlled planation may also operate in the East Kimberleys. It involves alternate mantling and stripping of the footslope (i.e deposition and erosion respectively), less drastic reworking of the mantle, and small-scale regrading to lower levels. It must be stressed that the time-perspective of such planation must be seen against longer-term downwearing and backwearing elucidated earlier (4.4.3). Nevertheless, to the extent that mantle-controlled planation involves (a) surface lowering following selective weathering of irregularities of both the subaerial and bedrock pediment surfaces
and (b) subsoil notching of corestones at the nick, it can be said that mantle-controlled planation reinforces and is part of the longer- term evolution by downwearing and backwearing.
footslopes lie below either the major or the most recent Tertiary
weathering front, and that backwearing is also important. In this setting,
mantle-controlled planation is believed to operate in the following manner (see also figure 4.55):
STAGE I A footslope originating from compartment weathering and slope
retreat is deeply weathered. Stripping of the soil profiles and saprolite
reveals a surface of considerable irregularity. Stripping may be induced
by a change in climate, a fall in base-level, or both, and saprolite
will be selectively etched by sheet and linear erosion. However, the
whole weathering front is not necessarily exhumed, since there may be no way in which certain pockets of saprolite can be evacuated.
Nevertheless, the exhumed surface will be considerably more irregular than the former footslope surface, and transverse profiles could well
exhibit reversals of slope if sheet and linear erosion have exploited lithological weaknesses to evacuate saprolite in an approximately
longitudinal direction. In this way the footslope may be formed partly
in fresh rock (i.e. where the weathering front has been exhumed) and partly in saprolite (i.e. where there are pockets of saprolite which
cannot be evacuated). This is called surface ’A * .
*
STAGE II A change of climate or halt in the fall of base-level may
cause surface ’A ’ to be mantled by material derived mainly from the
backing hillslope, but also possibly from limited subaerial weathering of
surface 'A' itself. The depositional surface will be considerably
smoother than the buried surface: the mantle "fills in depressions and
blankets rises’ in the bedrock surface (Twidale 1976a, p. 278).
STAGE i n (a) any subaerial projections of bedrock surface ’A' are subjected to ground-level trimming in the manner described by Mabbutt
(1966). Evidence of this has been given, and is shown in figure 4.55
at point Y.
(b) in the mantle, any upward projections of surface ’A ’ will
eluviation has been given, and there is no reason why subsoil weathering
(Mabbutt 1966) should not occur. (This is shown at point TX ’ on figure
4.55.) Certainly there is abundant moisture in the mantle, even in
the middle of the dry season, and the sharp discontinuity between
transported regolith and the bedrock pediment surface should facilitate movement of water along the interface as suggested by Mabbutt (1966).
(c) concurrently, pedogenesis of transported regolith occurs. The degree of pedogenesis increases downslope, since areas near to the
nickline are particularly subject to small-scale regrading (see below). (d) concurrently, the weathering front will be lowered in between upward projections of surface ’A ’, but weathering will be most effective in attacking the projections.
STAGE IV Stripping recurs. The products of ground-level trimming and
subsoil weathering are evacuated during the formation of exhumed surface
' B ’. The development of the longitudinal component on the bedrock
pediment surface ('B’) may also be developed subaerially at such
times by sheetfloods and streamfloods. In the lower parts of the
footslope the direction of flow will be determined in part by the
slope of the footstream. If the stripping is shallow, then the form
of the exhumed surface will be less irregular than the earlier exhumed
surface (’A') produced in stage I . However, if the stripping of
saprolite is more complete,vit may exploit parts of the weathering front formed in stage H i d to reveal once again a surface as irregular as
(or even more irregular than) the exhumed surface ’A". In other words,
the landscape is back to stage I . Such vigorous stripping belongs to
the longer-term cycles of compartment weathering, dissection, and lowering (4.4.2.2): mantle-controlled planation is presented here as a shorter-term cyclical process which smooths (and thereby lowers in
a limited way) irregularities of an exhumed surface, and involves only partial stripping.
In the East Kimberleys it is argued that stripping in recent cycles of mantle-controlled planation has been shallow, since the bedrock pediment surface is considerably less irregular in overall
form than the weathering front. Certainly, no large-scale exhumation
of a weathering front to form a bedrock pediment surface seems to have
occurred recently in this region. This being the case, then it is
argued that shallow stripping has formed a surface which is planar overall but irregular in detail. When the mantle is partially stripped the
new bedrock pediment surface will be cut mainly in saprolite, but there will be some, irregularities: these will be remnants of upward projections of the former bedrock pediment surface and the former weathering front
(if the latter formed part of the bedrock surface) which have been selectively weathered (either subaerially or beneath the mantle) and then partially bevelled during stripping of the mantle.
STAGES II, III, and IV may be repeated in further cycles of mantling and stripping, and irregularities will be reduced further.
Evidence from soil profiles on basalt and sandstone footslopes will be given to suggest that less drastic reworking of the mantle may also occur at any time during stage III, and can probably be attributed to sheetfloods of exceptional magnitude. Reworking of this kind has been described by Fölster in south-western Nigeria and by Churchward
(1969) in south-western Australia. Fölster describes how changes in climate during the last 30,000 years have led to shallow stream incision accompanied by slope retreat or pedimentation within the soil profile. The reworking of the mantle which he describes is of the same magnitude as that found on basalt and sandstone pediments.
Small-scale regrading of the subaerial pediment surface by rills, sheetwash, and occasional sheetfloods is of a smaller magnitude, but nonetheless imuortant since it probably occurs each year in
response to wet-season conditions and will be repeated numerous times within the life-span of a single mantle. Overall the footslopes are graded to the footstreams, but small-scale regrading probably occurs most frequently near the nickline (this will be considered in greater detail in Chapter Five with reference to figure 5.39; see also
Mabbutt (1966)). As a result, mantle-controlled planation is less effective near the nick compared with the lower parts of the footslope where regrading is less frequent, less profound, and the mantle therefore more stable. This greater stability permits more effective mantle-
controlled planation (particularly by subsoil weathering and eluviation). Thus a closer correspondence between subaerial and bedrock pediment surfaces can be expected in the lower parts of the footslope compared with the upper parts: this has been confirmed by Mandelbaum’s d (4.5.2).
Mantle-controlled planation can therefore explain both the overall parallelism of the subaerial and bedrock pediment surfaces,
and the detailed irregularities and reversals of transverse slope on the latter. ‘ Provided that stripping is shallow, it can also explain why the bedrock pediment surface is less irregular in overall form than the weathering front. It should be noted that as successive cycles of mantling and stripping occur, upward projections of the weathering front may be exposed subaeriallv during stripping. Such a situation may well
occur in the future near the nick at GA4, in the lower parts of GA5, GA6, GD3, and in the middle parts of GDI, and has already occurred on GA9, GA10, and GB5 where relatively unweathered dikes project above the general level of the bedrock pediment surface.
Inspection of transverse profiles in Appendix II shows that the weathering front is generally separated from the bedrock pediment surface by a layer of saprolite of variable thickness.
This suggests either (a) that the most recent stripping did not expose any upward projections of the weathering front at all (except the dikes mentioned above) but etched a surface wholly within saprolite or
(b) that the most recent stripping did expose upward projections of the weathering front in some places, but that these have since been
weathered selectively. Certainly, weathering is dominant at present (4.4.2.2.1), but reinvigoration of the footstream and the development of a few tributaries on the footslopes could certainly give rise to stripping in the future.
Finally, it is clear that subsurface changes in the form
of transverse profiles of the weathering front in the East Kimberleys are very different from Nigeria where Thomas (1965b) suggests that
subsurface smoothing of the weathering front occurs most rapidly beneath divides. However, elsewhere (Thomas 1974, p. 96) he notes that
"interfluve weathering appears limited to areas of low or moderate relief in humid areas with a natural cover of evergreen forest", and agrees with Mabbutt (1961e) that "in regions subject to strong surface erosion across poorly vegetated slopes, deep profiles appear restricted to valley floors and other topographic lows" (Thomas 1974, p.96). However, seismic records show that the latter situation does not
obtain in the East Kimberleys either: the lack of a downslope increase in the depth of the weathering front may be explained in part by
overall lowering following compartment weathering; and the downslope increase in the degree of weathering mav be explained by the greater amounts and longer retention of moisture near the footstream. This
would also be facilitated by the greater stability of the mantle
(4.5.2) below which such weathering occurs. Since the degree of weathering
is greater near the footstream than near the nick, it might also be expected that the weathering front (which is separated from the bedrock pediment surface by a variable depth of saprolite) would be more
irregular in the lower parts of the footslope. Unfortunately the seismic data is inadequate to judge whether or not this is so, since it was not possible to obtain continuous seismic returns from the weathering front.