Much of the Australian landscape is old. In places, such as the western shields, its origin can be traced to the Precambrian, but elsewhere it dates from the Palaeozoic to Recent and owes much of its character to the superimposition of multiple weathering events under different conditions. This has resulted in a complex regolith in which many of the features of the underlying bedrock have been greatly modified, thereby hindering geological mapping and concealing mineral deposits. Conversely, weathering processes have produced ore bodies, most importantly of aluminium, gold and nickel. The oldest landscapes are in the shields of the western plateau. Some are covered by thick and complex regolith as a result of long exposure and tectonic quiescence (e.g. the Yilgarn Craton), whereas others are almost free of regolith, owing to long-term physical erosion (e.g. Arnhem Plateau). In comparison, landscapes of the eastern third of the continent are younger, having been developed on tectonically active Palaeozoic rocks of the eastern highlands. The regolith is variable in thickness and complexity and dates mainly from the Mesozoic and Tertiary, although some Permian landscapes and regolith are preserved. Major depositional basins cover much of the western shield and Palaeozoic terrains. The landscape of these is comparatively flat and dates from the mid-Cretaceous to late Tertiary. It is typified by erosional remnants of deeply weathered rocks capped by duricrust (ferricrete, silcrete, bauxite), and in many areas gentle tectonic deformation. Deep weathering is a unifying feature of Australian landscape and regolith , and a deep weathering profile is present across the continent. It typically comprises, from the base, a zone of weathered rock in which the primary lithic fabrics are preserved (saprolite), commonly bleached towards the top, especially on felsic rocks, overlain by a quartz- and/or clay-rich zone with ferruginous mottling and an upper aluminous, ferruginous or siliceous horizon. This profile may be preserved or partly eroded, and buried beneath later sediments. Deep weathering mainly occurred during the wetter and, at times, warmer periods during the late Mesozoic and early Tertiary, although it is unclear whether it was cyclical or continuous through this period. Arid climatic phases during the Quaternary have left a marked imprint on the Australian regolith. Most notable are the widespread aeolian sediments which mantle much of the continent, and the addition of alkali and alkaline earth elements and associated anions (e.g. carbonate, sulphate and halide) as overprints to the leached regolith and the groundwater. Weathering can result in the complete alteration of mineralogical, chemical and petrographic characteristics of the rocks and any mineralisation that they host. However, the dispersion of the ore-related elements can crcate larger, though subtle, exploration targets.
