This section has been developed in association with Peter Dahlhaus, University of Ballarat.
Landscapes continually evolve and processes such as earthquakes, landslides, and even saline groundwater discharge are manifestations of this evolutionary process. The Corangamite CMA region has formed through landscape-building episodes over the past 500 million years (Ma) in a variety of environments from the deep sea to explosive volcanoes. The following brief history of landscape evolution has been summarised from a variety of sources (Webb, 1991; Tickell et al., 1992; Taylor et al., 1996; Edwards et al., 1997; VandenBerg et al., 2000).
Episode 1: Rock Formation in the Palaeozoic
The origins of the Palaeozoic sedimentary and igneous rocks of the Corangamite region, commenced about 500 Ma when it formed part of the deep sea floor. Fast-flowing rivers carried sediment from an ancient continent to the west (known as the Delamarian Highlands) to the sea, where it was deposited on a narrow continental shelf and ultimately swept to the foot of the continental slope by turbidity currents. The sediments built up in overlapping submarine fans thousands of metres thick. The sediment was compressed into rock – sandstone, siltstone, mudstone - during the collision of crustal plates, which shortened and thickened the extensive deposit of sediment. This lengthy tectonic event, known as the Benambran Orogeny, occurred around 455 Ma to 420 Ma and resulted in the rocks being folded into a series of steep anticlines and synclines along north-south trending axes. This mountain-building event was also the time when hot silica-rich groundwater was squeezed into the gaps in the rocks created by the folding and faulting. The fluid cooled to deposit quartz veins and in some cases, minerals such as gold. Around 350 Ma the sedimentary rocks of this mountain chain were injected with magma, which subsequently slowly cooled at depths of around 2 to 5 km to form granites.
Episode 2: Palaeozoic and Mesozoic Erosion
After the intrusion of the granites followed a long period of erosion, which stripped at least two kilometres and up to five kilometres thickness, from the surface. This erosive period of spanned about 200 million years and reduced the region to a planar landscape, known as the Mesozoic palaeosurface.
For part of this long erosive era, around the time of the Permian Period (~ 260Ma), this region of Victoria lay close to the South Pole and parts of the landscape were subjected to glaciation. Although there are very few known deposits of glacial rocks in the Corangamite region, it is assumed that ice sheets were present over the entire region and would have assisted in the erosion of the rocks. At the conclusion of this glaciation period, the major drainage direction in the region was to the north.
Episode 3: Gondwana Break-up and Rock Formation in the Cretaceous to Neogene
The next significant event was the break away of Australia from Antarctica. This started with the formation of a large rift valley in the south of the Corangamite region during the Jurassic Period (around 140Ma). Sediments washed into the rift valley by large braided river systems, now form the rocks of the Otway Ranges and Barrabool Hills. The Australian Antarctic break-up resulted in a down-warping to the north creating the Murray Basin, and an uplift that formed the drainage divide of the Great Dividing Range.
As the break-away proceeded, a sea-way opened between the two continents and a series of marine sediments began to accumulate. These now form the underlying rocks of the southern Corangamite region. As the sea-way developed into the Southern Ocean the depositional environments varied spatially and temporally, changing from terrestrial to littoral and marginally marine. This resulted in the deposition ot the Port Campbell Limestone in shallow warm seas during the Miocene.
In the northern parts of the Corangamite region, the uplift of the planar landscape started a new cycle of erosion, assisted by the associated increase in rainfall. The erosion deposited a vast sheet of coarse gravel during the Eocene epoch (~50 Ma). The gravel was deposited in outwash fans and along braided rivers in a high-rainfall environment, which was far greater rainfall than the current climate. The remnants of this gravel sheet now form sporadic deposits capping the Palaeozoic rocks. A period of intense weathering began in the early Palaeogene (formerly referred to as Tertiary), probably associated with the very wet climates. The result is that the Palaeozoic rocks in the northern Corangamite region are all deeply weathered.
By around 40 Ma the rainfall had lessened and dendritic drainage development formed deeper channels, which are preserved today as leads and deep leads. These channels filled with sediments derived from erosion of the highlands and cyclic reworking of the earlier deposits.
From the mid Miocene (~15 Ma) the direction of movement of the Australian Plate changed, resulting in a change in regional stress from extensional to compressional. This change resulted in renewed block faulting which has been responsible for the formation of the major physiographic features of the Corangamite region. The faulting formed the Central Highlands, Barrabool Hills, Bellarine Peninsula and Otway Ranges. Many of these faults remain active and seismicity (earthquake activity) continues to the present day.
Episode 4: Pliocene Marine Incursion and Volcanism
The events of the Pliocene (~ 4 Ma) – an invasion of the sea and the commencement of volcanism – have had a dramatic influence in shaping the detail in today’s landscapes. During a warmer global climate the sea rose and slowly invaded the land from the south east (around Geelong). The transgression reached as far inland as Scotsburn, and possibly extended up the valleys to Ballarat West. The subsequent sea retreat from the land, resulted in a thin but extensive deposit of sands. Post-Pliocene uplift of the northern highlands was at least 150 metres higher than the maximum sea level during the Pliocene.
The volcanism commenced around 4 Ma and continued to around 50 000 years before present. The courses of all the major rivers in the region were changed by volcanic activity during the past 2 Ma and the majority of the large lakes and wetlands were formed during this period. The volcanic eruptions were sporadic and mostly resulted in lobes of lava flowing from the eruption points, which overlap to form a variable thickness of basalt, interleaved with sporadic pyroclastic deposits of scoria and tuff. At times, lengthy breaks between eruptions allowed soils to form on the upper surface of the basalt flows which were subsequently covered by later eruptions, forming discontinuous buried palaeosols of variable thickness.
Episode 5: Today’s Processes
The climate changes during the past million years, especially the arid period of the mid Pleistocene (~500 ka) and the relatively dramatic changes in sea levels (up to 140 m) since the last Glacial (~18 ka) have left significant imprints in the regolith development and soil profiles.
The present-day geological formations are the deposits of stream alluvium, lake and swamp sediments, landslide debris, slope colluvium, beach dunes and anthropogenic fill.
The movement of water, the development of soil, and the establishment of ecosystems have all been influenced by the past landscape history. They continue to be influenced by the present-day processes of landscape evolution.