Engineers Australia

Distribution of ‘laterites’ and lateritic weathering profiles, Darling Range, Western Australia

R. R. Anand and C. R. M. Butt

Abstract

This paper summarizes the distribution and characteristics of the lateritic regolith of the Darling Range and presents models for its formation and evolution. Typical, complete, weathering profiles on granite average about 20 m in thickness and consist of gravelly soil, lateritic duricrust, saprolite and saprock. Lateritic duricrusts occupy gently sloping to horizontal upland areas and are either residual, or locally transported and recemented. In much of the Darling Range, the lower part of the duricrust, especially on hill slopes rather than crests or valley floors, is highly aluminous and forms an extensive resource of bauxite. Fragmental, fragmental-pisolitic, pisolitic and vesicular types can be identified on the basis of secondary structures. Fragmental duricrust largely consists of gibbsite, hematite, goethite and quartz and has resulted from direct gibbsitization of saprock or bedrock without forming the kaolinite-rich deep saprolite. Outcrops of duricrust with relict bedrock textures are common. In contrast, pisolitic duricrust with hematitemaghemite and χ-alumina rich mineralogy have a more complex history than fragmental duricrust with simple mineralogy. Vesicular duricrust is goethite-rich and is formed by the ferruginization of sandy detritus and quartz pebbles. The profiles show no condensed sequences, and individual rock types are traceable geochemically and mineralogically, but with increasing difficulty, from bedrock to the surface. The concentrations of Fe, Al, Si, Ti, V, Cr and residual quartz, particularly in fragmental duricrust, can be used to identify bedrock. Deep weathering profiles at Jarrahdale and Boddington yield late Tertiary palaeomagnetic ages and it appears that modification of these profiles to form bauxite is continuing today.