A study of the post-Permian quartzites of south-eastern New South Wales

Silcretes are indurated, silicified quartz-rich rocks. In an attempt to understand the genesis of such rocks, twenty two deposits of post-Permiar. quartzites were chosen for investigation from the widespread occurrences of this rock type in the Tallong, Bungonia, Windellama, Nerriga, Lake Conjola and Cooma regions in south-eastern New South Wales.Pétrographie and field examination has shown that the quartzites are equivalent to the silcretes of South Africa and inland Australia and the name silcrete is used to avoid confusion with the metamorphic rock term.The silcrete deposits studied occur over a wide range of elevations and are associated with weathered rocks which overlie parent rocks of various types and ages. Pétrographie examination has shown that iron minerals have etched allogenic quartz and that chalcedony is not an essential mineral. Each region studied has a characteristic pattern of occurrence of the iron, titanium and aluminium oxides. It is considered that the silcretes formed under climatic conditions and favourable environments which existed at or near the time basalts were extruded in the region. Some poorly silicified silcretes represent siliceous laterites from which iron oxide or carbonate has been leached. Field and chemical evidence shows that silcretes have formed in the sub-surface as part of soil profiles and have passed through a viscous liquid phase during formation.