Pediment form and evolution in the East Kimberleys : granite, basalt, and sandstone case-studies
Creators: name: Whitaker, Christopher Robert
Publisher: name: The Australian National University
Year: 1976
Subjects: subject: Pediments (Geology) Australia Kimberley (W.A.)

description: Previous work on pediments has left several outstanding problems. In particular, little information is available on the form of the bedrock pediment surface or the role of lithology, and few writers have considered the pediment form in three dimensions. Four footslopes on each of granite, basalt, and sandstone were selected to study these problems. The subaerial surface comprising the backing hillslope, pedimentary nick, and the footslope are studied in terms of form, regolith, and process. The bedrock pediment surface and weathering front are studied by seismic refraction and digging. Pediments have been formed following drainage incision below the Tertiary weathering front. On granite a combined theory of compartment weathering, dissection, and subsequent lowering on the one hand, and slope retreat on the other hand can explain the initiation and continuing evolution of pediments. There is less evidence of downwearing on basalt and sandstone pediments where the principal mode of evolution appears to be by backwearing. Mantle-controlled planation is important, especially on granite and basalt pediments. Boulder-mantled slopes on granite and basalt are often inclined at angles considerably less than the angle of sliding friction. "Caprock control'1 of slope retreat occurs on basalt and sandstone, and slope decline occurs following the complete removal of the resistant strata. Nickline depressions do not appear to be related to a greater degree or depth of weathering at the nick than elsewhere. The subaerial pediment surface commonly has a weakly exponential form in transverse profile. The bedrock pediment surface is planar in overall form, but is more irregular than the subaerial pediment surface in detail. The correlation between the form of the bedrock surface and the subaerial surface is assessed visually by overlays, and statistically: subaerial form is not always a reliable indicator of bedrock form. A coarse surface lamina of loose sand-size material on the footslope is moved by sheetwash, and is in transit at the time scale of a few years. The density, size, and shape of gibbers are usually determined in part by proximity to outcrops on the footslope: they are rarely moved by sheetfloods. The mean particle size of surficial material on the footslope decreases downslope and is probably the product of numerous cycles of small-scale regrading. Soil profiles are skeletal on the backing hillslope and near the nick, but pedogenesis usually increases downslope. Accounts of sheetwash, sheetfloods, and a rockfall are given. It is shown that the detailed form and evolution of pediments on a given lithology may be quite distinctive, while at the same time such pediments retain quite a high perfection of form. It is possible to distinguish between "granitic" and "non-granitic" pediments: a causal relationship between the size of material in the mantle and the gradient of the slope is suggested to explain this. Morphogenesis in this dry savanna region is related to both pedimentation of arid and semi-arid regions, and to the formation of washplains in the savanna zones.