Australia: The Land Where Time Began
The processes that lead to the formation of the desert features that characterise so much of Australia's arid areas were started back in the days of the Early Tertiary when Australia, shortly after it took the final step in the breakup of Gondwana. The continent was wet and green, before the beginning of aridification that is still proceeding.
When the climate was warm and wet, deep weathering took place to depths of hundreds to thousands of metres, the chemical makeup of the rocks and soil being changed. The results of this weathering can be seen now in the form of the mesas of the arid zone, with a red, oxidised upper zone that grades via a mottled zone into the pallid zone (a bleached zone). It has now been found that deep chemical weathering can occur under a number of climatic regimes, from warm to cold. The only condition necessary is deep groundwater over long time periods.
Many parts of the world have undergone deep weathering during a warm, wet period that lasted from the Late Cretaceous to the Early Eocene. A suggested cause for the widespread occurrence of deep weathering at this time is the widespread volcanic activity that resulted from the breakup of continents. It is believed this high level of volcanic activity in many parts of the world would have produced acid rain on a global scale, greatly increasing the level and depth of chemical weathering above that occurring in less volcanically active times.
At this time a deep regolith (layer of weathered rock) was produced. Groundwater is held in weathered rock, the saturated zone being topped by a watertable that separates the anoxic saturated zone from the upper oxidized zone.
The Cretaceous and Early Tertiary weathered profiles were formed by water percolating down thorough the soil and rock to the watertable. On its way down to the watertable acid in the water, especially during times of acid rain, leaches iron and other minerals from the upper layers of the rock and carries them to the groundwater. Many Ore bodies originated in this manner, the minerals being concentrated by the process of leaching - copper, gold, uranium and opal.
Continental crust is composed mostly of silicate minerals, within which chemical reactions occur that break them down to clay minerals, which were then moved down with the percolating water. The result was the distribution of colour seen in the remaining rock profiles, iron-stained in the top zone, merging with a mottled zone then a pallid zone of white clay. This clay layer is were the silica that migrated from the upper levels of the rock was deposited to form opal as a result of deep weathering.
The appearance of the mottled zone results from the unequal distribution of iron. In this zone, as water percolated down through it, iron oxides were segregated, small crystals of haematite coalescing to for nodules of various sizes. This mottled zone would no doubt have been of the most interest to Australian Aborigines, as the nodules (ochre) are one of the pigments used by them extensively in ceremony and decoration in general.
All weathered material lies on a basement rock, the upper part of which is the saturated zone, that is the zone of groundwater discharge. It is at places the groundwater moves towards, springs or seepages, where it emerging at the surface. The emerging water carries the products of weathering in solution, such as dissolved silica (silicic acid). When it percolates through iron-rich regoliths it carries iron. A chemical gradient is maintained by the removal of weathering products as they precipitate out, the water being able to continue weathering the rocks.
In the lower part of the saturated zone, that is stagnant, as the products cannot be carried away by groundwater flow, they diffuse upwards by osmosis along a concentration gradient, and when they are precipitated out of solution the groundwater can continue with the process of weathering. When ferrous iron diffuses upwards it is converted to ferric hydroxide when it reaches the oxidising layer, the water discharge zone. It can be deposited in that zone as ferricrete, either in massive or nodular form.
Yowah Nuts are ironstone concretions, sometimes containing precious opal, that are found in the Yowah opal field in western Queensland. They result from the combination of a gel of concentrated silica solution and the precipitation of iron, the process taking place in the groundwater zone of a deeply weathered profile. Also in western Queensland, boulder opal is found in cracks and fissures in massively concretionary ironstone, that has formed by a similar process.
Dissolved iron in groundwater becomes concentrated in low-lying areas of restricted or slow flow, as when it flows down a shallow gradient. In these places it impregnates the soil, sediment or permeable layers of rock forming varying types of ferricrete. At these depositional sites the ferricrete is younger than the weathered profile it forms above, so is not part of the local deep weathering sequence. It is said that an unconformity separates it from the underlying sequence.
As the process of erosion abrades the surface around the ferricrete, lowering it below that of the ferricrete, that is more erosion-resistant than the surrounding surface, the result is a typical duricrust-capped profile. Erosion results in 'inversion of relief', where the duricrust, that formed in the lowest parts of a landscape, is now the highest part of the 'inverted landscape', as the softer surrounding surface is eroded away while the softer rock beneath the capping is relatively unaffected by erosion.
The same process occurs in the silcrete capped profiles of inverted landscapes. It is also said to rest unconformable on the profile beneath it.
Flat areas favour deep weathering, as it can only occur where the rate of weathering is greater than the rate of erosion. Australia was very flat at the time the duricrust was forming in the Late Cretaceous and Early Tertiary, and it had a much higher rainfall than now, ensuring the presence of deep groundwater, the sort of conditions ideal for the formation of silcrete and ferricrete. As Australia dried out and the rate of erosion became higher than the rate of deep weathering, stripping the regolith from around the silcrete and ferricrete that had already formed, the surface around the duricrust-protected areas was lowered, leaving the duricrust as the highest surface in the eroded area.
Old River Gravels
When some areas became elevated leading to more variation and drainages being rejuvenated, the progressive stripping of weathered material is seen in the old river gravels that indicate a steady change of composition. Quartz-dominant material is found in the early gravels, that would have been the only material available because of deep weathering of the entire landscape. Later gravels demonstrate a wider variety of rock types as new rock was exposed by erosion of the landscape. In the Miocene the gravel type changes from quartz to mixed.
Miners of old gravels call them deep leads. They often contain gold and other minerals that were concentrated in the watertable in the weathered profile before erosion removed it, only the more resistant and heavier minerals remaining in the river beds.
Deep weathering in arid areas can result in deposits such as gypsum and sometimes opal, as the water is more likely to remain in place. The Great Artesian Basin (GAB) is a good example of arid areas being underlain by a deep watertable, it can reach as much as 3000 m in depth, allowing deep weathering to occur down to this depth. The mineral content of the water can vary between different areas of the GAB, depending on the rock type passed through and the chemical processes taking place.
Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994
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