Older Geology

The oldest sediments in this area are those deposited under deep marine conditions during the Ordovician [510 to 435 million years ago (mya)] when all except the far west of Victoria was under a deep sea. A major period of orogeny, which is the term given to periods of mountain building, faulting and faulting, occurred during the Late Ordovician – Early Silurian (440 – 420 mya). This orogeny is called the Benambran Orogeny and it resulted in most of Victoria becoming dry land.

Once 5 km north of Bairnsdale the parent rock mostly comprises Ordovician sediments, mostly fine-grained sandstone, siltstone and claystone. The Tambo and Nicholson Rivers and Clifton Creek derive most of their sediments from these rocks, so one would expect that the river sediment would be sand to clay with few cobbles, rather than gravelly and many cobbles like bed of the Mitchell and Avon Rivers. The sediments in the alluvial terraces would also reflect differences in parent material.

However, West Gippsland remained under a deep ocean throughout the Silurian (435 to 405 mya) into the Middle Devonian (about 380 mya), over which time a very thick sequence of sediments were deposited. To the east, a deep rift, called the Cowombat Rift, was also under sea. This extended from south of Bairnsdale north towards Corryong and during the Silurian it became filled with volcanic material and marine sediments.

The Late Silurian and Early Devonian (about 400 mya) was marked by another period of mountain building, the Bowning Orogeny. In Gippsland, this resulted in the sediments in the Cowombat Rift being uplifted. Many of the granitic intrusions into the Silurian and Ordovician sediments in eastern Victoria, for example at Granite Rock, north of Bairnsdale and around Dargo were also formed during this period. These mostly occur east of Dargo. The sediments around these granitic intrusions are often hardened due to contact metamorphism.

A dramatic major period of mountain building and deformation, the Tabberabberan Orogeny, affected most of Victoria during the Early to Mid Devonian (395 – 385 mya). This produced a major mountain range, called the Tabberabberan Highlands, in Eastern Victoria. Uplift associated with this event caused the sea to retreat to the east, so that all marine sedimentation ceased in Victoria.

A large sedimentary basin, 40 – 50 kilometres wide and extending from Valencia Creek to Mansfield, developed west of the Tabberabberan Highlands. This basin was first partly infilled by rhyolites and rhyodacites, which are volcanic rocks, during the Late Devonian Period (about 370 mya), and later, during the Early Carboniferous Period (about 350 mya), by a thick body of fresh water sediments. These sediments are mainly conglomerate, pebbly sandstone, sandstone and siltstone.

The steep hills that comprise Mt Taylor and Mt Lookout are predominantly Devonian rhyodacite with some Carboniferous sediments. More extensive Carboniferous sediments occur west of these hills.

Towards the end of the Early Carboniferous there was a widespread but not very intense deformation known as the Kanimblan Orogeny which uplifted Central and Eastern Victoria and split the southern part of the basin into three almost separate areas of outcrop. The Kanimblan Orogeny was the last orogeny to affect Victoria.

Much of the Macalister, Avon and the lower part of the Mitchell River catchments are based on the conglomerate, pebbly sandstone, sandstone and siltstone of the Early Carboniferous Period. The beds of these streams are characteristically very stony.

During the Late Carboniferous and Permian, about 290 mya, Victoria was positioned closer to the south pole and largely covered by glaciers. Erosion over this period as the glaciers moved northwards changed the Victorian landscape greatly. No glacial sediments occur in this area.

In the Late Jurassic and Early Cretaceous (160 – 96 mya) Australia and Antarctica began to split apart. As they moved apart, a long rift valley was formed in southern Victoria which was slowly filled up by up to 3000 metres of fresh water sediments, largely volcanic material, probably from volcanoes east of Australia. Included in these sediments are coal deposits (Wonthaggi) and plant and animal fossils, including dinosaurs. The Otway and Strzelecki Ranges are comprised of these sediments, which were uplifted during the Middle to Late Cretaceous when Australia and Antarctica split apart. Outcrops of these sediments occur between Yallourn and Tyers, just east of the area. The end of the Cretaceous (65 mya) was marked by a mass extinction of many animal groups, including the dinosaurs. There are none of these sediments in the area covered by this report.

South of the foothills of the Dividing Range the sedimentary basin that developed in the Early Cretaceous continued to widen and deepen, forming the Gippsland Basin in the Late Cretaceous to Early Paleocene (about 70 – 60 mya).

During the Tertiary (65 to about 1.8 mya) this basin was infilled, mostly by sediments eroded from the highlands. Some Paleocene basalts (57 –55 mya) may be found at depth from west of Lake Glenmaggie to south of Rosedale. Remnants of these basalts occur at Seaton. East of Kilmany, deposits of limestone, up to 300 m thick, were laid down during the Miocene (23 to 5 mya). These deposits have been covered by later sediments, but south and east of Longford they are exposed as a result of uplift associated with the Rosedale Monocline. The Pliocene (5 – 1.8 mya) was marked by a period of minor uplift of the eastern highlands, resulting in erosion and the formation of gravelly alluvial fans and extensive flood plains sloping towards the south-east of the area. On the Gippsland plains, these have been covered by later (Quaternary) sediments.

The foothills immediately north of Bairnsdale and extending to east of Orbost are based on sediments deposited during the Tertiary Period. Early deposits are marine, but the later deposits are alluvial. The end result was an extensive plain, which is now quite dissected. Some creeks, for example Prospect Creek has cut into marine deposits on which “terra rossa” (red friable soils with a lime rich subsoil) soils have developed.