An Hypothesis of Lateral Planation

This hypothesis was summarised in 1.5.2.2.

4.4.2.1.1 Evidence for the hypothesis

(a) The only firm evidence of pedimentation by lateral planation is found on a basalt footslope near Spring Creek. A small stream has incised itself into the bedrock pediment surface and

stripped the saprolite from the weathering front to develop an apical rock sector of about 0.5 ha. However, it is argued that this planation postdates the formation of the bedrock surface (5.2.1.3).

4.4.2.1.2 Evidence against the hypothesis

(a) Streams flowing onto granite footslopes are very few in number. Most channels on tfie backing hillslope are very small

and follow tortuous and anastomosing paths around boulders: only rarely do they reach the nickline.

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(b) Channels reaching the nickline often dissipate within a few metres. If the channel form persists on the footslope, it is very rarely seen to be diverted laterally (e.g. the stream which flows in the nickline depression at GD), and where this occurs the stream does not plane bedrock: on the contrary, it erodes vertically in the nickline depression. Usually such streams flow transversely across the footslope and are confined to channels up to 2 m deep. They do not appear to deposit appreciable amounts of material at the nick, and there is no evidence of stream deposits diverting the flow.

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(c) Pediments extend in a horizontal direction for several hundreds of metres in places where no stream emerges from the backing hillslope.

(d) No fresh undercut faces are found on the hillslope.

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(e) Re-entrants are joint-guided and are not fan-shaped (as Mabbutt (1966) also noted).

(f) Embayments are characteristically blunt—headed rather than apical (as Mabbutt (1974) also notes): e.g. the embayment west and south-west of Alice Hill (figure 4.4).

(g) A small residual (e.g. at GB) can hardly nourish a stream

of sufficient power to remove itself, and there is no evidence of other streams swinging laterally against such residuals in the manner described by Rahn (1966).

(h) Detailed irregularities on the bedrock pediment surface are unexplained.

4.4.2.1.3 Evaluation

Lateral planation is only important in one very localised

situation on basalt, and postdates the formation of the pediment. It

does not appear to be important on granite footslopes. The complete

lack of evidence of the operation of this mechanism at present carries implication that it was unimportant in the actual initiation

of granite pediments.

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4.4.2.2 An Hypothesis of Compartment Weathering, Dissection, and Subsequent Lowering

This hypothesis was summarised in 1.5.2.4.4.

4.4.2.2.1 Evidence for the hypothesis

Although it is not possible to prove that this hypothesis is correct from the primary evidence given below (items (a) to (j) inclusive), it is relevant to note from the outset that such an hypothesis is compatible with our limited knowledge of the geomorphic and climatic history of the region:

(1) Dissection of a deeply weathered land surface is clearly evident to the south-west of the region, and for reasons given earlier it seems that deep weathering in association with laterite formation would have occurred where these case-studies are now found.

(2) Compartment weathering is compatible with laterite formation, evidence of which is preserved to the south and south-west. (3) Dissection may have been initiated by broad uplift in the late

Tertiary (Stephens 1971), which may have occurred in several

stages (Paterson 1970). Evidence of changes in base-level

is given by van Andel and Veevers (1967), and Whitaker (1974b). (4) Multiple weathered land surfaces in the West Kimberleys and the

Katherine-Darwin region suggest that renewed selective weathering may have occurred several times in the East Kimberleys,

and this is in accord with the limited evidence for climatic

change within the region itself. For example, changes

in climate during the Quaternary could well have affected the balance between planar and linear transport and erosion and

thus the stability of the pediment mantle.

However, this speculation concerning episodic lowering is merely the background against which the following primary evidence of

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the overall and detailed morphology of slopes must be considered:

(a) Although major joints rarely coincide with the nickline, there is often a close correspondence between the positions of

joints and streams, suggesting that joints may have guided dissection.

(b) Bedrock at the nickline is weathered and there is localised overdeepening of the weathering front at the nick on some transects (noted in 4.3.1.2), and corestones are being

exposed in the weathering profile of the nickline depression at GC.

(c) The maintenance of an angular nick is readily explained by this hypothesis (see 1.5.2.4.4).

(d) Evidence for periodic mantling and stripping may be afforded by the occurrrence of a few incised channels on the footslopes

(e.g. on BB and BC discussed in Chapter 5 ), although it appears that weathering of the footslope is more important at present on these granite cases.

(e) Irregularities and slope reversals on the bedrock pediment surface are easily understood in the context of etchplanation

and subsequent mantling. Etching could occur either within the

saprolite (to form the bedrock pediment surface), or could expose the weathering front which is subsequently modified by mantle-controlled planation (see 4.5).

(f) The characteristics and distribution of gibbers may indicate reworking of the mantle (see Chapters Five and Six).

Furthermore, the occurrence of granite gibbers on the footslope is unlikely unless there has been reworking.

(g) This hypothesis can explain the occurrence of a slope unit

of low angle between the steepest segment of the backing hillslope and the footslope at GC (i.e. on transects 3, 5, 6, 7, and

10) : it can have originated as a subsurface feature

similar in morphological role to the forms described by Twidale and Bourne (1975a, and 1975b).

(h) The lack of a significant downslope increase in the depth of the weathering front is in accord with a theory of episodic

weathering and stripping.

(i) The general parallelism between the subaerial and bedrock surfaces, and the fact that both are graded to footstreams, suggests that episodic etchplanation is related to stream incision.

(j) The absence of truncated profiles suggests that weathering is dominant at present.

4.4.2.2.2 Evidence against the hypothesis

(a) The lack of coincidence between major joints and the nickline may imply that residuals are not set back to resistant

structural compartments. Some writers (e.g. King 1966) have

seen this as critical evidence against the exhumation

hypothesis. Nevertheless, as Thomas (1974, p.211) has indicated,

this does not prove that scarp retreat has occurred. He cites

the work of Hurault (1967) and Wahrhaftig (1965) who show how domes can originate where savanna vegetation permits

powerful stripping, and outcrops "appear randomly and without obvious structural control" (Thomas 1974, p. 211).

(b) On some profiles the weathering front does not plunge steeply at the nickline (noted in 4.3.1.2), and seismic velocity does not decrease in the saprolite there.

(c) There is no seismic or geological evidence for a significantly greater degree of weathering at the nick than elsewhere on the footslope, which may be exüected if this hypothesis is correct .

(d) There is some evidence that hillslopes retreat (see later), and in particular it should be noted that there is considerable weathering of the hillslope close to the nickline to form

saprolite and emerging corestones.

(e) Apart from a slope unit of low angle at GC, there is little local evidence of episodic lowering of the footslope either at the nick or preserved on the backing hillslope.

(f) The evidence at GB strongly suggests the former existence of a larger residual comprising at least both present-day

residuals. Although the regional slope of the land is eastwards towards the Upper Panton River (figure 4.4), the present-day residual at GB has both its subaerial and bedrock pediment surfaces graded to nearby streams to the west, north, and east. If the footslope is simply an exhumed feature, then it might be expected that all the footslope would

be graded eastwards — even that part of the footslope to the west of the residual (a situation described by Savigear 1960, and Thomas 1967). Clearly, if the residual is an exhumed feature, then there has been considerable subsequent

regrading of the footslopes to small streams. Although this is compatible with episodic weathering and stripping, the apparent development of two residuals from a former residual may suggest slope retreat along a pediment pass. However, Twidale (1976a, p. 278) argues that such a case is also compatible with downwearing along joints.

(g) There is little evidence of new residuals emerging from deeply weathered saprolite, which can be expected in the light of work by others (e.g. Thomas 1962, Twidale and Bourne

1975a, figure 12).

(h) The development of broad blunt-headed embayments which do not appear to be joint-bound, and which do not contain basins of deep weathering (e.g. west and south-west of Alice Hill) are more difficult to account for under this hypothesis than pediment passes which often are related to joints. Also, the development of a very narrow ridge at the head of

this particular embayment appears to have formed by slope retreat of both flanks, rather than to be related to compartment

weathering.

(i) This hypothesis cannot explain the development of pediments on sandstone in the same region (Chapter Six), where the evidence for slope retreat is considerable, and the hypothesis of

compartment weathering is untenable on account of rock lithology and structure.

4.4.2.2.3 Evaluation

No known fact of climatic or geomorphic history argues conclusively against this hypothesis for the evolution of granite

pediments which can explain many observations on slope form and process. However, it cannot explain slope development on sandstone, and basalt slopes certainly appear to undergo significant slope retreat (Chapter Five). Also, certain aspects of slope development on granite are rather hard to reconcile with this hypothesis - in particular the evidence which suggests that slope retreat occurs (see below). If this hypothesis can be applied in total or in part to granite slopes, it must accommodate the fact that the present day slopes on these cases are developed below the level of the Tertiary weathering front (or

fronts) for reasons given earlier.