Physical properties of the regolith in the Lawlers area, Western Australia

Abstract

The regolith in the Lawlers district, Western Australia, can be divided into a number of geologically identifiable layers. Saprolite commonly forms one of the thickest and least dense of these layers, and appears to be by far the most porous material encountered in the section. Porosity derived from many samples averaged 37.8%. The solid constituents of saprolite are not solely clay, saprolite rather is an extremely weathered rock type with a retained fabric that supports its structure and which behaves as a clayey sand. While the constituent clays (predominantly smectite over mafics and ultramafics, and kaolinite over felsics) are inherently conductive, the very high porosity suggests that the measured conductivity of saprolite in-situ should be dominated by the salinity of the local groundwater, even if contained fluids are only slightly saturated. Previous work has suggested that the conductance and/or conductivity of the saprolite layer are different over different lithologies. However, physical property measurements indicate that the conductivity of saprolite cannot uniquely be used to determine parent lithology. These physical property studies indicated overlaps in electrical properties for metasedimentary, felsic, mafic and ultramafic saprolites and also overlying palaeochannel clays. However provided that groundwater conductivities are not too variable, diagnostic conductance differences may occur, particularly since the saprolite layer over felsic lithologies is usually reported to be thinner than that developed over mafics or ultramafics. Below the saprolite layer, a saprock layer is defined that contains the transition from virtually unweathered bedrock to heavily weathered saprolite. This transitional layer retains much of the structural strength and density of bedrock, and thus has very similar seismic velocity to bedrock. Its conductivity however, is far greater than that of the bedrock: possibly reflecting the influence of connected conductive paths that mark the passage of fluids during the weathering process. Above the saprolite, regolith materials have much lower porosities and higher densities. They also tend to be lower in conductivity than the saprolite. The geological processes of deep argillisation and surface and mid-level ferruginisation, accompanied by silicification are responsible for the physical property variations described above.