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Avon River CZO is located on Ridgefield, a 1588 ha property near Pingelly (S 32° 30’ 23” – E 116° 59’ 31”), in a major agricultural region (the ‘grain belt’) of Western Australia (Figure 1). The climate is Mediterranean and the average annual rainfall is about 445 mm, most of which falls in the cooler months (April to September).Geologically, the CZO is located on the southwestern terrane of the Yilgarn Craton, an extensive area of Achaean rock that covers 657,000 km2 of southwestern Australia (Anand & Paine, 2002; Anand & Butt, 2010). These ancient, billion years old rocks are the primary source of soil parent material for southwestern Australian soils (Moore, 2004).

Figure 1. A) General overview of the regions of Western Australia (WA) showing the location of the farm Ridgefield (white circle with a cross) in the Grain Belt of WA. B) Aerial photography of the Ridgefield with the eastern part of the adjacent Boyagin Nature Reserve; UTM coordinates zone 50H.

Regolith Formation

Much of Ridgefield is occupied by an old weathering profile (Gilkes et al., 2006) that can be traced throughout the Darling Plateau down to the south-west of Western Australia (Figure 1). The underlying bedrock of the Darling Plateau consists of the tectonically stable Yilgarn Craton, an Archaean igneous and metamorphic rock that has been exposed to aerial weathering since the mid-Proterozoic eon (Annand & Paine 2002). This long-term weathering resulted in a deeply weathered ancient lateritic profile similar to those currently developing in the tropics. A schematic overview of this weathering profile is given in Figure 2, and McCrea et al. (1990) and Gilkes et al. (2006) provide more detailed descriptions.

Figure 2. Generalised weathering profile in the Darling Range with site-specific images from Ridgefield. Weathering products have been eroded and re-deposited throughout the area as colluvium and alluvium and form the parent materials for most of the soils currently used for crops and pasture.

The craton has been exposed to the atmosphere since the Proteozoic and thus has an extensive history of weathering under a diversity of climates (Anand & Paine, 2002; Anand & Butt, 2010). Ultimately, weathering causes the least soluble minerals, such as kaolinite and hematite, and most resistant primary minerals, such as quartz, to remain in surface materials.

Landscape Evolution

The gently undulating landscape of the craton, a partial etchplain, was formed through aggradation and valley development on a variety of bedrock lithologies (Anand & Paine, 2002). Deep weathering has affected the majority of the craton, with the weathered mantle being as deep as 150m in some areas (Anand & Paine, 2002).

Figure 3. Geophysical lines (here ERT) characterize the general depth and shape of the weathering profile.

Characteristics of the South-Western Terrane

The southwestern terrane of the craton is composed of a granite-gneiss complex, with dolerite dykes frequently intruding the predominantly granitic rocks (Anand & Paine, 2002). The dykes average at 10m in thickness and extend subvertically through the profile (Anand & Paine, 2002). Sedimentary rocks, derived from felsic volcanics, and mafic and submafic volcanic rocks make up the granitic bedrock (Anand & Paine, 2002).

Young Soils on the Australian OCBIL?

Australia is considered an old, climatically buffered, infertile landscape (OCBIL), however, not all of the soils on this landscape are old. These older landscapes have been dissected by deep and intensive weathering, and soils have been continuously developing from these more recent weathered materials (Anand & Paine, 2002). Often forming new surfaces, such as ferricete and silcrete, which are then also eroded, with soils forming from these secondary weathered materials (Anand & Paine, 2002). Because of this, there is a complex mosaic of old and new soils across the Australian landscape (Anand & Paine, 2002) adding to the complexity of the critical zone architecture.

Current Land Use

Primary Land Use – Dryland Agriculture: Mixed Sheep and Wheat Farming

The area is predominantly used for rotational dryland wheat (Triticum aestivum) cropping and grazing pastures for sheep. Modified sheep grazing pastures include fodder plants such as subterranean clover (Trifolium subterranean) and annual medic (Medicago polymorpha) (Land Use Map; Field Notes; Latta et. al., 2001). Secondary Land Uses – Research and Infrastructure Smaller areas of land are used for research, with a 10ha area in the northern corner of Survey Area A being used as an experimental plot by the Ecosystems Restoration and Intervention Ecology Research Group (ERIE, 2014). Additionally, areas are used for farm infrastructure, such as the central ALVA farmhouse and various equipment storage sheds.