An overview of silicification in Cainozoic landscapes of arid central and southern Australia

Abstract

Silica diagenesis in the near-surface zones of subdued Tertiary drainage basins in the arid interior of central and southern Australia produced a mosaic of silcretes now exposed as relic duricrust on mesas that are significantly higher than the contemporary base-level of erosion. The silcretes comprise a variety of silica-cemented sediments and possibly soils that exhibit a range of textures and chemical compositions. Acid-weathering conditions in pedogenic environments near the margins of the Tertiary drainage basins are likely to have provided soluble silica for soil waters and groundwater moving slowly through the regolith into depositional zones in the lower reaches of the basins. The silica cement in silcretes ranges from ordered forms of opal to quartz overgrowths, and the existence of incomplete transformations between structural states and between textural forms suggests heterogeneous precipitation and recrystallization in response to local fluctuations in properties such as pH and moisture regime. High concentrations of titanium were precipitated from solution in bedrock fractures as well as in sediments in the piedmont of the ancient drainage basins, but as yet there is no unequivocal explanation for its occurrence. Apart from zircon, enrichment of zirconium and aluminium in areas of high titanium concentration is evident from electron probe microanalyses and X-ray scanning images, but the mineral phase or phases responsible for this distribution are unidentified.