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In assessing the relevance of one major weathered Tertiary land surface to present-day pediments, two questions must be raised:

(a) at what level was the Tertiary land surface? (b) at what level was the Tertiary weathering front?

The level of the Tertiary land surface can only be determined with any certainty in the few places where weathering profiles are

preserved. Such profiles are found on the southern margins of the East Kimberleys and were discussed earlier (2.5.1). Elsewhere there are few indications of the position of the Tertiary land surface, although Paterson (1970) considers that the present-day sandstone plateau surfaces (e.g. on the Kimberley Block and in the Carr Boyd

Ranges) formed the Tertiary land surface, and some evidence of weathering has been found (Plumb and Veevers 1971).

However, the level of the Tertiary land surface in other areas,

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including the sites of the four granite cases, can only be inferred

in general terms from other evidence. It seems probable that the relative

relief on the subaerial surface in the Tertiary was less than the level of the

present subaerial surface. For example, the laterite caps preserved

on the bevelled crests of granite tors and residuals to the SW of the study area are clearly accordant with each other and dissection has

increased the relative relief. On a much bigger scale drainage

incision between the Miocene and Pleistocene has produced spectacularly dissected relief even in unweathered and resistant rocks (e.g. in

the Carr Boyd Ranges).

As a corollary it is important to note that the footslopes on the four granite cases did not exist during the Tertiary: to suggest

this would be to disregard the evidence for dissection to the SW which has a drier climate and lies farther from the coast.

Although laterite may be preserved on granite (e.g. in the SW) , it has been suggested by others that , despite the local development of

deep weathering profiles on granite, extensive laterite sheets are often not developed (Bleackley 1964, Thomas 1974, p. 244). However that might

apply here, it is reasonable to assume that the generally irregular weathering front which most probably developed on the granite would have promoted dissection and removal of the weathered surface once

the laterite, if any, was breached. It is not surprising therefore

that all traces of deep weathering have been removed from granitic areas, except in inland areas which are distant from the coast, are at lower elevations, and experience drier climates.

The probable position of the Tertiary weathering front must

now be considered. Unfortunately, even in the SW where granite weathering

profiles are preserved on laterite capped hills, the weathering front is

not exposed. The depth of weathering was therefore at least as much as

the height of the hills now (up to 30 m ), since the footslopes are

apparently cut within the pallid zone. Furthermore, we do not know the

form which the weathering front might have assumed: Thomas (1974, p. 91)

reports that they are often very irregular, but very even weathering fronts have occasionally been found (e.g. cyi the Hanson-Lander Plains in Central Australia (Mabbutt 1965a)).

In4 the absence of suitable exposures we must use indirect evidence to consider the four possible general positions of the Tertiary weathering front with respect to the granite cases:

(a) at the level of the present-day weathering front.

(b) at the level of the present-day bedrock pediment surface. (c) at the level of the present-day subaerial pediment surface. (d) above the present-day subaerial pediment surface.

As Mabbutt indicated, the relationship between the present-day pediment surface and the former weathering front will vary with respect to the vigour of stream incision (Mabbutt 1965a).

It is unlikely that the present-day weathering front on the hillslope and footslope is also the Tertiary weathering front. In view of the present climate and the fact that the saprolite is permanently damp it would be unreasonable to suggest that there has been no

alteration of the weathering front since the early to mid-Tertiary. Additional indirect evidence includes:

(a) if this i_s the Tertiary weathering front, then it would be expected that boulder mantles on backing nillslopes would not be renewed as weathering and removal proceeds

(Oberländer 1972). This is not the case here.

(b) present-day residuals would have been upward projections of the weathering front. Such features are usually determined by joint patterns, but there is no evidence on aerial photographs that hillfronts correspond to joint patterns.

(c) the height of certain residuals exceeds the maximum depth of weathering in a single cycle normally reported from tropical

areas (about 100 m — see Ollier 1969, p. 121). (However,

Ollier does record several exceptional cases where the depth of ' weathering exceeds 100 m ).

(d) if .this is_ the Tertiary weathering front then we would expect

a complex pattern in transverse profile. However, this is

not so, and even allowing that fairly even weathering fronts may be formed, as illustrated by Mabbutt (1965a), it is unlikely

that this would be found on every case. Furthermore,

the profiles of the weathering front correspond in general terms to the present-day subaerial surface and lie within a

few metres of it on all four cases: this would be unlikely

if this is also the Tertiary weathering front,

(e) if this is^ the Tertiary weathering front, then the bedrock

pediment surface is formed in saprolite dating from the Tertiary. However, it is unlikely that this could happen so close to the Tertiary weathering front (a discontinuity which could be exploited readily) on all four cases.

It is also unlikely that the Tertiary weathering front

coincides with the present-day bedrock pediment surface and its backing

hillslope. If this were the case, then the observed depth of saprolite

below the bedrock surface has been formed subsequent to the exhumation of the Tertiary weathering front to form the bedrock pediment surface.

This in itself is not unreasonable. However, the evidence of boulder

mantles, the lack of coincidence between major joints and hillfronts, and the heights of the residuals, (elaborated more fully as (a), (b), and (c) above) also argue against the Tertiary weathering front coinciding with the present-day bedrock pediment surface and its

backing hillslope. Furthermore, even allowing that fairly even weathering

fronts may form, and allowing for mantle-controlled planation, it is very unlikely that bedrock pediment profiles on all four cases would be so planar in overall form if they correspond to the position of the Tertiary weathering front.

Little need be said in connection with the hypothesis that the Tertiary weathering front coincides with the present-day subaerial

surface. In all soil profiles there is a distinct discontinuity at

the base of the solum which could not be explained by such an hypothesis, and the solum itself is clearly depositional.

The final possibility is that the Tertiary weathering front

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lay above the present-day subaerial surface over most of the East

Kimberleys, and that incision has reached below that level. In the absence

of exposures of the Tertiary weathering front it is impossible to disprove thi

hypothesis. However, quite apart from its being the only remaining

possibility (the other three having been shown to be improbable), this hypothesis is also compatible with the following observations on granite cases:

(a) the relative relief exceeds the maximum depth of weathering in a single cycle normally reported from tropical areas. (b) there is a general correspondence between the present-day

subaerial pediment surface and the weathering front. (c) hillfronts rarely coincide with major joints.

Furthermore, this hypothesis is reasonable in the light of other evidence from non-granitic areas:

(a) the vigour of stream incision, especially by the Ord River, as

noted by van Andel and Veevers (1967). The granite cases

are nearer to the coast, have greater elevation, and have wetter climates than the granite area to the SW where the removal of lateritic profiles is incomplete.

(b) at SD (Chapter 6) the Tertiary weathering front must have been above the present subaerial surface.

4.A.1.2 The Relationship if there were Several Periods of Tertiary Deep Weathering

The conclusion just reached in the previous section, namely that these granite pediments lie beneath the main front of Tertiary deep

weathering, may be an oversimplification. Instead there may have been

several weathered land surfaces, each of which emphasized compartmentation of the landscape developed under a previous weathered land surface.

In the absence of firm stratigraphic or morphological evidence from within the region or at its margins, this alternative line of

argument is very difficult to evaluate. Furthermore, this approach

which is preserved on the Sturt Plateau.

(b) there were several cycles of deep weathering: the most recent was cut and weathered coastward of the Sturt Plateau, and below the level of an earlier Tertiary surface which covered the East Kimberleys and which is preserved in part on the Sturt Plateau. This later surface was not developed on the

Sturt Plateau.

If the first possibility is correct, then the conclusions reached in 4.4.1.1 would only need to be modified slightly: i.e. these granite pediments lie beneath the weathering front of the most recent Tertiary land surface which was incised in the late Tertiary.

If the second possibility is correct, then it might be suggested that the present pattern of uplands and lowlands could have been achieved by cycles of compartment weathering and etchplanation, one or more

of which postdates the surface preserved on the Sturt Plateau. However, no stratigraphic of morphological evidence of a lower surface was

found in the field between the edges of the Sturt Plateau and the

pediments studied in detail. In the extreme SW of the East Kimberleys, Roberts, Halligan, and Playford(1968) also find no evidence of a

lower surface. It is not possible to disprove this hypothesis, and it must remain as a possibility.

Nevertheless, for the reasons given in 4.4.1.1 it must still be concluded that the present landscape lies below the most recent Tertiary weathering front even if that weathering front lies below the weathering front of the land surface now preserved on the Sturt Plateau. Otherwise it would be reasonable to expect evidence of multiple surfaces and cycles preserved on the flanks of hillslopes studied, some of which exceed the depth of weathering which can be envisaged in one cycle (e.g. Pomney’s Pillar standing over 140 m above the weathering front on the footslope). Such is not the case.