A comparison of the weathering of igneous rocks at Long Ridge and Black Hill, Western Murray Basin, South Australia

Abstract

In this study, microscopic, X-ray spectrometric and X-ray diffraction techniques have been used to investigate the weathering of a granodiorite and a norite exposed in the western Murray Basin, South Australia. Weathering along joints and near-surface weathering profiles from both sites were analysed. Clay formation around individual crystals adjacent to joints appears to have initiated the physical disintegration of the rock at both sites. Mineral stability and the mobility of elements follow the trends outlined by earlier workers, but different microenvironments produce on the one hand varied weathering products developed on the same parent rock, and, on the other, similar weathering products on both types of rock. Montmorillonite has formed in response to impeded drainage on both rocks, and non-tronite has formed in joints at Black Hill. Illite, kaolin and randomly interstratified material are formed from both rock types. Weathering products inherited from a previous environment can be distinguished at Black Hill.