Landform Development and Soils

• Landform development and soils Introduction The soils of the Murchison have been mapped on a broad scale (1:2,000,000) by Bettenay et al. (1967). The soils of the north-east of the area were previously described for the 1:250,000 scale map sheet of Belele by Litchfield as part of a larger study of the lands of the Wiluna-Meekatharra area (Mabbutt et al. 1963). Teakle (1936), Stephens (1962) and Bettenay and Erosional landforms at the margins of 'Old Plateau' surfaces - Churchward (1974) described one of the unique the distinctive breakaway faces, residuals and footslope plains widespread features of this region, the red-brown of the upper units of the Sherwood land system. siliceous hardpan (locally known as MurchIson cement) underlying many soils, especially on broad wash plains. Other features such as the dominant red normally stable and well vegetated. They may occur soil colour, a paucity of organic matter, the highly regularly or intermittently and are often arranged in patterned distribution of nutrients within a small area water-modified patterns transverse to the direction of and shallow depth are typical of soils in this and other the sheet flow. parts of the arid zone of Australia. A siliceous red-brown hardpan occurs throughout much of the Murchison River catchment as an almost continuous cemented layer varying from a few Landform development centimetres to 30 m in depth. The pan usually lies The survey area exhibits a mainly Quaternary below the soil at a depth of 70 cm or less with landscape underlain by ancient igneous and occasional minor outcropping and is commonly metamorphic rocks first described in detail by Jutson exposed along drainage channels. (1934). Mabbutt (1963) hypothesised geological Sheet drainage is terminated partly by infiltration processes of uplift and dissection of an old plateau in and partly by discharge at or near major tributaries Tertiary times, which was characterised by lateritic and channels of the Murchison, Sanford and Roderick weathering and the subsequent formation of the Rivers (and more locally along the Wooramel and present lower plains that Mabbutt called the 'New Greenough Rivers). The channels of the major Plateau'. More recently, Ollier et al. (1988), suggest the tributaries may be up to 200 m wide but often are less landscape displays a recurring sequence of Old above than 100 m, while the associated flood plains are New above Old Plateau, showing a saw-tooth cross normally 3-10 km wide. These riverine areas are section of the land rather than one major undulating generally saline to some extent and often contain landscape feature. This suggests that weathered exposed calereted valley fill existing as restricted material was repeatedly deposited into valleys and raised plains dissected by sluggish drainage lines. Part the relief became inverted so that the alluvium of the survey area drains into salt lakes such as Lake remained as a duricrust capping. Both hypotheses Austin in the south-east, Lake Anneen in the north suggest weathering and erosion were the dominant east and the lakes on Muggon station in the west. forces leading to the formation of the current landscape. The local existence of tiered remnants of Old Plateau surfaces was confirmed during this survey, lending support to the hypothesis of more than a single Old Plateau surface having been formed. General stripping of the original landscape has intermittently exposed fresh country rocks of granite, gneiss and greenstone (metamorphosed voleanic rock) which form mainly low hills above colluvial slopes and peneplains. The heavily mantled colluvial pediments with more or less incised drainage lines drain onto broad flat 'wash' plains carrying sheet flow down extremely shallow gradients. The wash plains consist of alluvium partly derived from pallid zone materials of the older lateritic land surface and partly from weathered granite, gneiss and metamorphic rock. Shallow accumulations of surface sand and an extensive underlying hardpan are features of these One example of the most widespread geological formations in wash plains. The sandy banks (known locally as the region: exposed red-brown hardpan overlying granite, wanderrie banks), may be oblong or reticulate and are Woogalong Brook, Jingemarra station. 274 Deeply dissected Permian sediments of sandstone Soil profiles sharing similar characteristics have and siltstone occur on the western margin of the been combined to create soil groups. These soil groups survey area. This landscape is dominated by gently are used for soil descriptions throughout the text. undulating stony plains below breakaways with remnant low hills and plateaux with up to 30 m relief. Twelve major soil groups were identified in the The upland plateaux are often capped with a duricrust survey area. Sorted primarily on soil profile of silcrete. The northern portion of the Permian development, the soil groups are, in part, similar to landscape drains to the Wooramel River, while the those of Stace et al. (1968). Soil phases of Great Soil southern portion is associated with a lacustrine Groups have been introduced to differentiate the soils system. This palaeodrainage links the present according to their lithology or profile development. Wooramel River headwaters with the Murchison River The soils range from poorly sorted river sands to well (Denman et aZ. 1985). Drainage is very sluggish with structured deep cracking clays. many local foci which are often saline. The Permian and lacustrine systems are closely associated with extensive areas of aeolian sand sheet. The sand sheet Alluvial (juvenile) soils occurs as flat or gently undulating plains with occasional linear dunes and no surface drainage. This These are weakly or undifferentiated soils formed in is essentially an outlying section of the vast Victoria recent alluvium. They occur in or immediately Sand plain (Condon 1967) which dominates the adjacent to major and minor rivers or flow lines. These landscape further west. sediments may have many layers of coarse loose sand, clayey sand or silty sand. Layers containing river pebbles are visible on exposed creek banks. Depth is variable, but where it is less than 1 m, the soil is often underlain by rock or hardpan which, within drainage Soils lines on occasions has a thin calcrete veneer. Fluvial sediments are often found on levee banks associated The method of soil sampling used during the survey with the larger flow lines. They may be no more than and the data collected are described in the 'Survey poorly sorted coarse sands, depending on catchment methods' section of the report. The techniques and size and lithology. Catchments with metamorphic standards used were those given in the Australian Soil lithology tend to produce finer sediments than granitic and Land Survey Field Handbook (McDonald et al. based catchments where quartz sand predominates. 1984) and Northcote (1979). The data collected enabled Alluvial soils are often called Juvenile soils, they occur Northcote's key to rank soils into Principal Profile in lake beds, drainage foci or other low lying areas Forms (PPF's). receiving major run-on. Soil classification is a systematic arrangement of At drainage channels these soils are mostly weakly soils on the basis of similarities and differences in their acidic to neutral (pH 6.0-7.5) and are non-saline. Saline morphological features. Soils are generally composed varieties occur in areas of calcreted valleys, of the of three major layers designated A, Band C horizons. Mileura, Cunyu and Beringarra land systems. Soil The A and B horizons are layers that have been colour varies from dark red (2.5YR 3/6) to strong modified by weathering and soil development. The C brown (7.5YR4/6). horizon is weathering parent material. Occasionally D horizons occur. These are not related to the soil These soils generally support vegetation on the forming processes of the overlaying soil (e.g. red stable levee banks and floodplains but only occasionally brown hardpan). Northcote's key is based on in the river channels, indicating the observable soil profile features and uses alpha impact of intermittent river flows rather than a numeric coding system to identify classes. The first reflection on soil quality. These soils cannot be classified within the Northcote key. division is based on texture variation within the soil profile. Soils are mostly classified as one of three types - uniform (U), gradational (G) or duplex (D). Uniform soils show little, if any, change in soil Solonchaks texture throughout the profile. These soils are further Solonchaks are highly saline. They occur in lake classified as either coarse, medium or fine textures. A beds, on gypsiferous plains, in some drainage flats and uniform coarse profile is typically a sand; a uniform at terminated drainages. Solonchaks are stone-free and medium profile a loam and a uniform fine profile is very shallow (often less than 30 em). Textures range clay throughout. from clayey sands to clay loams often overlying Gradational soils show a gradual increase in clay gypsum or cemented carbonates. Soil colour ranges content throughout the depth of the soil profile. from strong brown (7.5YR 4/6) to dark red (2.5YR 3/ Further classification is based on the presence or 6). Some Solonchaks are characterised by a surface absence of calcium carbonate (lime). crusted with salt, showing polygonal cracking or exhibit a soft, loose, powdery surface horizon with Duplex soils show a sharp textural change between visible salt crystals. the A and B horizons such as a coarse texture (e.g. sand) overlying a fine texture (e.g. clay). Duplex soils With high salinity and susceptibility to prolonged are further classified on COIOUf, surface characteristics, waterlogging, these soils support little or no . soil reaction trends and structure. vegetation. Away from lake beds, the less salIne lake 275 + margins support salt tolerant vegetation such as Earthy sands samphire and saltbush.

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