Regional Geology

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Old marine Palaeozoic rocks do not outcrop in the Glenelg region. They are found mainly 1 000 to 3 000 metres beneath the present day surface and are covered by Mesozoic and Cainozoic deposits as described below.

Mesozoic Sediments

In the early Jurassic period Australia and Antarctica were joined together. By the early Cretaceous period Australia and Antarctica began to separate. As they moved apart a large basin or 'rift valley' formed as a result of subsidence. Extensive deposits up to 4 500 metres thick, of felspathic mudstones and sandstones were deposited under freshwater or estuarine conditions. Sedimentation was mainly confined to the Port Campbell Embayment and occurred within the sea gulf between both continents. Deposition occurred during two major cycles of sea level change during the late Cretaceous. Some 3 000 metres of Cretaceous sediments were deposited. At the end of the Cretaceous period significant uplift and erosion occurred. Fossilised bones of dinosaurs have been found in the cliff faces of the Otway Ranges e.g. Dinosaur Cove, in Cretaceous period sediments.

Tertiary Period

During the Tertiary period from approximately 65 million years ago, Australia continued to drift away from Antarctica at rates of up to 6 cm per year. In the middle Eocene age of the Tertiary period approximately 45 million years ago, uplift occurred that formed the Otway Ranges. In this period the rate of sea-floor spreading between Australia and Antarctica increased. By the late Oligocene from 29 million years ago, the Southern Ocean had widened significantly. This created favourable conditions for the biological production of calcium carbonate e.g. shells, bryozoans and calcareous algae.

The sea level fluctuated significantly during the Tertiary period. When the sea advanced over former coastal plains, several hundred metres of limestone (Port Campbell Limestone) were laid down in the Port Campbell Embayment. An iron rich soil called laterite formed in areas above sea level. Port Campbell Limestone usually overlies Gellibrand Marl up to 400 m thick. Port Campbell Limestone is middle to late Miocene in age and was deposited in a continental shelf environment.

In the late Miocene approximately 5 million years ago, the sea retreated from the Port Campbell Embayment. Uplift in the Pliocene resulted in the formation of mountains to the north of the Embayment. An alluvial out-wash fan resulted in the Hanson Plain Sand formation, and covered some areas of the Port Campbell Limestone usually less than 10 metres thick. It formed a plain which was later lateritised and then uplifted and dissected. Laterite plateau remnants can be found today in some areas e.g. near Simpson. A deep lateritic soil developed on the Hanson Plain Sand under warm and wet climatic conditions which prevailed in Victoria for much of the Tertiary period. Temperate rainforest was widespread throughout the State at this time. In many areas these Tertiary deposits have been covered by subsequent volcanic material. At the end of the Tertiary period the Otway Ranges were uplifted to their present height.

Quaternary Period

The most recent geological period is the Quaternary that extended from 1.6 million years ago to recent times. During this period extensive volcanic deposits were laid down over much of western Victoria. A major period of volcanic activity began about 2 million years ago and lasted till about 20 000 years ago. Extensive olivine basalt flows referred to as 'newer volcanics' erupted from about 400 volcanoes, fissures and vents. For example, flows from Mt Rouse east of Penshurst, extended about 60 km. Eruptions mostly occurred from 2 - 4 million years ago. Some activity also occurred in the Pleistocene. It is estimated that the youngest volcanic activity occurred from Mt Napier south of Hamilton, about 7 000 years ago. During the Quaternary period, extensive lakes e.g. Lake Corangamite, developed when lava flows blocked the regional drainage.

The more recent of the volcanic deposits consist of scoria cones, maars (now crater lakes) and stony rise basalts. Scoria cones were active in the southern areas due to interactions between shallow groundwater and magma. Scoria deposits originated from eruption points such as Mt Warrnambool, Red Rock and Mount Leura. Lavas have weathered to a wide range of soils. The youngest soils have formed on lava flows that now form ‘stony rises’ country.

There were about 40 maar type volcanos between Colac and Port Fairy. These volcanoes have very large craters up to 2 km across, which are often filled with lakes e.g. Tower Hill. They were formed after some very explosive eruptions and resulted in deposition of volcanic ash, as well as some limestone from underlying deposits for several kilometres away.

During the Pleistocene period a number of ice ages occurred which resulted in fluctuating sea level changes. At the peak of an ice age, sea levels were significantly lower than present. Rivers flowed across what is now the continental shelf to a coastline which was located south-west of the present one. In interglacial periods the sea level rose to present day levels. During the most recent glacial period the icecap reached its maximum extent about 17 000 to 20 000 years ago. During this time the sea level along the Victorian coast was some 120 metres below present day levels. Consequently, carbonate rich sediments were exposed and provided the source material for calcareous coastal dunes to develop e.g. Bridgewater Formation consists of calcarenite dunes a mixture of shell fragments and quartz. During the glacial period, arid conditions prevailed and windblown silt and clay (loess) deposits mantled many areas.

References

Cochrane, G.W., Quick, G.W. and Spencer-Jones, D (editors) (1991). Introducing Victorian Geology. Geological Society of Australia (Victorian Division).

Land Conservation Council (1976). Report on the Corangamite Study Area.

Tickell, S.J., Edwards, J. and Abele, C (1992). Port Campbell Embayment. 1:100 000 Map Geological Report. Geological Survey Report 95. Department of Energy and Minerals.