Geology and Geomorphology

The Birchip area lies within the Murray Basin - a large sedimentary basin that evolved as a result of subsidence at the beginning of the Tertiary period (approximately 65 million years ago). As global sea levels rose in the middle Tertiary period, this basin became flooded by the sea to form the ‘Murravian Gulf’. This gulf covered most of what is known today as the Wimmera and Mallee regions and extended into New South Wales as well.

The uppermost Tertiary sediments are referred to as Parilla Sandstone and this was deposited as the Murravian Gulf receded in the late Tertiary period (approximately 5 million years ago). The sea retreated in a number of stages, which has resulted in numerous coastal ridges forming. These are obvious today in many areas of the Wimmera and Mallee as NNW/SSE trending sandstone ridges and rises. As the sea retreated it left behind a sandy coastal plain over much of the area.

Much of the Tertiary sandstone was covered by aeolian (wind), alluvial (stream) and lacustrine (lake) deposits during the Quaternary period (from 1.6 million years ago). In the Quaternary period there were several major climatic oscillations with phases of global glaciation. In drier climatic periods, Parilla Sandstone was eroded and 'reworked' into quartz rich (siliceous) sand dunes. These white to yellow coloured sand dunes are common in the Big Desert, Little Desert and Sunset Country regions. In the Birchip region, the Parilla Sandstone material has largely been covered by Quaternary deposits of windblown sands, silts and calcareous clays. These deposits are referred to as the Woorinen Formation and occur as elongated east-west trending dunes (e.g. Site MP28) and as thin sheets of sediments. Sites MP22, MP26, ORZC11 and ORZC12 have alkaline soils derived from Woorinen Formation deposits that overlie part of an earlier acidic soil formed on Parilla Sandstone material.

In more recent geological times, sediments have been deposited by drainage systems related to present day streams. An example occurs around Birchip and has been termed the Tyrell Creek Land System by Rowan and Downes (1963). Site MP24 has developed on floodplain deposits from a former northerly extension of the Avoca River.

References

Land Conservation Council (1974). A Report on the Mallee Study Area.

Rowan, J.N and Downes, R.G (1963). A Study of the Land in North-Western Victoria. Soil Conservation Authority, Victoria. Tech. Comm. 2.