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2.1.5 Plateaux and rises of residual Cainozoic landscapes (Dereel, Meredith, White Hills, Trentham)

2. Western Uplands (WU)
2.1 Dissected Uplands

The Palaeozoic rocks of the Western Uplands are capped in places by sands and gravels, which are remnants of Palaeogene and Neogene sediment deposition.

The Palaeogene White Hills Gravels and equivalents are the erosional remnants of an extensive sheet of fluvial sediments that once covered much of the Western Uplands. The coarse grained sediments form a quartz-pebble conglomerate, often ferruginised and sometimes silicified. The thickness varies from around 30 m (such as at Sago Hill quarry) to a few metres. Landforms include isolated flat-topped mesas overlying Palaeozoic rocks (Napoleons), break-of-slope deposits between the overlying basalt and the underlying Palaeozoic rock (Lal Lal Reservoir) and broad tablelands fringing the overlying basalt (Haddon-Nintingbool).

Neogene sands generally fringe the south of the Western Uplands as a dissected tableland. A remnant of the Late Palaeogene marine transgression, the sands are often ferruginised by groundwater processes. Erosion and salinisation are commonly associated with this landform.

Soil types include sands and texture contrast soils, often with ferruginised nodules (‘buckshot’).
Image: 2.1.5

Widespread deposition of sands and gravels are preserved as fringing sheets of capping material across Palaeozoic sediments and granitic terrains of the upper Wimmera River catchment. They are significant due to their preservation of underlying bedrock as resistant ferricrete and silcrete. Examples include the undulating rises near Great Western and gentle to undulating plains from Deep Lead to Glenorchy and Mokepilly.

Undulating rises and plains characterise the landform patterns of these Cainozoic landscapes. Generally the rises that are often capped with Cainozoic sands and gravels sit in a lower topographic position in comparison to neighbouring low hills and hills of granitic plutons and Palaeozoic bedrock. The extensive rises that fringe many of the higher landscapes as well as lining alluvial valleys and flats, often occur as discontinuous landforms with relief of 5–30 m and gently to very gently inclined slopes.

More subdued landscapes along Mount William Creek include level to undulating plains that have long gentle slopes (usually < 5%). Drainage is infrequent and is primarily confined to areas with slightly more relief than the sandy plains of Mokepilly between Lake Lonsdale and Lake Fyans. Little to no bedrock is exposed across these gentle slopes.

These Palaeogene and Neogene deposits (known as the Great Western Group, Denicull Formation and Parilla Sand) are generally preserved as hilltop capping (‘ironstone’), upper slopes of bedrock ridges and extensive planar landforms that mimic duricrust landscapes associated with the Dundas Tablelands and Brimpaen landsystem (Sibley 1967).

The Great Western Formation (or White Hill equivalents) has been found between 220 and 290 m AHD in the upper Wimmera River catchment (Hocking 1997). Lying above current alluvial deposits, the gravels are the erosional remnants of prominent fluvial systems that were active through much of the Western Uplands. Deposit thickness varies from less than 1 metre to in excess of 15 m on footslopes/terraces/rises of the Wimmera River valley and other terrane (e.g. Rocky Point undulating rises). Size and compositional sorting of the gravels is poor (Cayley & MacDonald 1995) with quartz reef derived clasts located within a matrix of fine to very coarse sand, minor silt and clay.

Deposition of these sediments is often followed by deep weathering including ferruginisation/silicification, and kaolinisation of underlying Palaeozoic bedrock (Taylor & Joyce 1996). This can be observed today as inverted landscapes through preferential denudation (e.g. deep lead development).

Later activity in the Neogene saw retreat of the sea from the Murray Darling Basin and deposition of fine to coarse-grained quartz sands as sand ridges and beach relicts (shoreline dunes). The Parilla Sand deposits form sub-parallel ridges that are often ferruginised and are more commonly expressed in the Millicent Coast Basin and Nhill-Kaniva areas of the Wimmera CMA region.

Developed on these sediments are a suite of texture contrast soils (yellow, brown, red; sodic and non-sodic) and red gradational or uniform soils.

The yellow and brown texture contrast soils (Sodosols and Chromosols) are often associated with Kandosols where soils are slightly acidic at the surface but become neutral or alkaline at depth. Brown sandy loams overlie conspicuously bleached massive loamy sands with variable amounts of coarse quartz fragments. Mottled yellowish brown medium clays provide a sharp texture change from above material. The brown to red texture contrast soils are similar with pronounced red subsoil mottling and ferruginous nodules (buckshot) common in the bleached subsurface horizon.

Red gradational or uniform soils are (Dermosols and Kandosols) acidic profiles with brown sandy loam surfaces containing variable amounts of coarse quartz fragments. This gradually changes to yellowish red sandy clay that also has quartz gravels. This soil has a greater capacity for water infiltration and drainage than the texture contrast variants.

Undulating landscapes dominated by sandy deposits have main vegetation communities including Heathy Woodland to Box Ironbark Forest and Plains Sedgy Woodland. Other vegetation communities present include Plains Grassy Woodland, Grassy Woodland, Grassy Woodland/Heathy Woodland Mosaic, Plains Woodland and Drainage-line Woodland. Landscapes further up Mount William Creek including plains at as Mokepilly and undulating rises at Bellellen have Heathy Woodland and Plains Grassy Woodland also with variants including Damp Sands Herb-rich Woodland, Red Gum Wetland, Heathy Dry Forest, Sedge-rich Woodland, Shallow Freshwater Marsh, Shallow Sands Woodland and Lateritic Woodland.

Landscapes dominated by gravels intermixed with sand, silt and clays include the vegetation communities Heathy Dry Forest, Grassy Dry Forest, Heathy Woodland, Plains Grassy Woodland, Alluvial Terraces Herb-rich Woodland, Grassy Woodland, Plains Woodland and Creekline Grassy Woodland.

The deeply weathered nature of these landscapes represents groundwater systems that are very slowly permeable generally owing to significant kaolin deposits with high salt stores. Saline discharge often occurs as localised events or sub artesian in lower landscape positions. Gully and tunnel erosion are prominent on moderate to gentle slopes. It is often a combatant or bi-product of saline discharge mechanisms that lead to exposure of vulnerable sodic subsoils and ultimately erosion.

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