5. North Western Dunefields and Plains (DP)

Victoria's Geomorphological Framework (VGF)

5.1 Calcareous dunefields
5.2 Siliceous dunefields (Sunset, Big and Little Desert)
5.3 Depressions
5.4 Clay plain with subdued ridges (Minyip)
5.5 Ridges with sand, and flats (Goroke, Nhill)
5.6 Hills and low hills (Mt Arapiles, Mitre rock, Mt Jeffcott)

The North Western Dunefields and Plains occur in the western part of the Murray Basin Plains submerged by Late Tertiary seas. Landscapes reflect diverse processes, including marine shoreline deposition, earth movements, lacustrine and alluvial deposition, accession of calcareous loess from coasts further to the west, groundwater discharge and finally dune development during Quaternary arid periods. Our understanding of the nature and distribution of the diverse landforms is being greatly enhanced by satellite images, particularly those showing elevation. The undulating landscapes are clearly defined from the Western Uplands to the south and from the Riverine Plains to the east. Riverine Plains also occur to the north along the River Murray, and in the south along several ephemeral stream courses which terminate in the central parts. Elevations decline from about 200 metres in the south to 40 metres in the north. The climate is mainly semi-arid, but it ranges from almost arid in the north to sub-humid in the south. Drainage systems have rarely been developed in the drier areas where rainfall is readily absorbed by porous soils. The centre and north are known as the Mallee after the growth form of the local multi-stemmed eucalypts. In the Wimmera the original vegetation was mostly grassy woodlands of eucalypts and Buloke (Allocasuarina leuhmannii).

The whole area was inundated when sea levels rose about 6mya, depositing Parilla Sand. As the sea retreated westwards over a period of some 4 my, it built up a regular series of curvilinear beach ridges which trend NNW-SSE, tending to NW-SE in the north and N-S in the south. The stranded beach ridges and intervening swales were then laterised, producing soils some metres deep, with deep sands of low fertility overlying somewhat clayey subsoils, mottled zones of iron oxide segregation and accumulations of rounded or angular ironstone.

This landscape was modified by movements in the Palaeozoic bedrock, producing a fall to the north west, several broad depressions, and also relatively large ridges which trend mainly NNW-SSE. These movements have greatly affected sedimentation in the north, particularly in the Tyrrell Depression and in the raak Depression to the east of the Danyo Fault which runs from near Murrayville north eastwards almost to Mildura. To the west of Murrayville, the Pinnaroo Block forms part of a large ridge which extends to the Lawloit-Diapur ridge in the Wimmera into South Australia.

Earth movement to the north west of the Pinnaroo Block dammed back the River Murray some 2.5 mya forming a huge lake system, Lake Bungunnia. Large lakes occupied the broad depressions, tending to erode and bury the stranded ridges. The lacustrine deposits, mainly grey clay (Blanchetown Clay) may be more than 30 metres thick in the major depressions, but are now mostly buried. Bores have revealed their presence in most of the Victorian Mallee except to the south –east of Lake Tyrrell, in the Noora Depression and beneath much of the Big, Little and Sunset Deserts, three east-west belts of land which continue on into South Australia. Blanchetown Clay has also been noted to occur discontinuously beneath about half of the Wimmera, mostly towards the west.

Alluvial and aeolian deposition also occurred during the Pleistocene. The alluvial Shepparton Formation accumulated between low lying stranded ridges. Very hard blocks and sheets of limestone a metre or more thick sometimes outcrop, particularly in the western parts of the Ouyen-Murrayville and Millewa aresa. Such deposits, known as “Bakara Calcrete” are widespread in South Australia. The material is thought to be derived from loess stripped from the coast. Leaching concentrated the carbonates in the subsoil which hardened when exposed by erosion. Surfaces now tend to be stable, being protected by a litter of limestone.

During the last 500 Ka, a series of arid phases during glacial periods led to the widespread development of dunefields. The lacustrine, aeolian and alluvial deposits gave rise to the Woorinen Formation, calcareous material of variable clay content on which linear dunefields are prominent. Where laterised Parilla Sand had remained at the surface, erosion produced Lowan Sand, the siliceous sand of the Big, Little and Sunset Deserts which contain both linear and parabolic dunes.

Blancehtown Clay was deposited in an ancient large freshawater lake called Lake Bungunnia during the late Pliocene and early Pleistocene (2.5-3.5 Ma - 0.7 Ma, An et al 1986). These deposits consist mainly of red-brown and green mottled clays (usually vertic and sodic).

References

Conn, B.J. 1993. Natural Regions and Vegetation of Victoria. In Flora of Victoria, Vol 1, Eds Foreman, D.B. and Walsh, N. G. Royal Botanical Gardens, Melbourne, Inkata Press.

Lawrence, C. R. 1976 Murray Basin. In Douglas, J. G. and Ferguson, J. A. eds. Geology of Victoria, pp 276-288. Geological Society of Australia Special Publication 5.

Wetherby, K. G. 1980. Shallow stratigraphy and dryland use of the Northern Murray Mallee, South Australia. In Aeolian Landscapes in the semi-Arid Zone. Australian Society of Soil Science Publication.

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