Geology and Geomorphology

Diagrammatic representations of Terrace Development

Palaeozoic Era

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 occurred during the Late Ordovician – Early Silurian (440 – 420 mya). This was known as the Benambran Orogeny and it resulted in most of Victoria becoming dry land. However, West Gippsland remained under a deep ocean throughout the Silurian (435 to 405 mya) into the Middle Devonian (about 380 mya). Over this time a very thick sequence of sediments were deposited. To the east, a deep rift called the Cowombat Rift was also under sea. This rift extended from an area south of Bairnsdale, northward to Corryong. 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 Browning Orogeny. In Gippsland, this resulted in the sediments in the Cowombat Rift being uplifted. Many of the granitic intrusions in eastern Victoria were also formed during this period. Little evidence of these events can be found in the area covered by this survey.

The La Trobe mapping unit comprises soils that have developed on the sequence of marine sediments deposited during the Ordovician to Middle Devonian (510 to 380 mya). Most of these sediments are sandstones, siltstones and claystones.

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. The granites east of Orbost date from this period.

A large sedimentary basin, 40 – 50 kilometres wide and extending from Valencia Creek to Mansfield, developed west of the Tabberabberan Highlands. This basin was partly infilled by rhyolites and rhyodacites (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. Towards the end of the Early Carboniferous there was a widespread but not very intense deformation known as the Kanimblan Orogeny. This 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.

The Collins mapping unit comprises soils developed on volcanic rocks, mainly rhyolite and rhyodacite that were extruded during the Late Devonian. The Glenmaggie mapping unit comprises soils developed on freshwater sediments deposited during the Early Carboniferous, mainly conglomerate, pebbly sandstone, sandstone and siltstone.

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.

Mesozoic Era

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 eroded 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.

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

Cainozoic Era

During the Tertiary (65 to about 1.8 mya) this basin was in-filled, mostly by sediments eroded from the highlands. Some Palaeocene 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 Seaton mapping unit comprises soils developed on volcanic rocks (predominantly basalts) extruded during the Mid-Tertiary, probably during the Oligocene to Miocene Epochs (20 to 40 mya). The soils of Coongulla and Stockdale mapping units are developed on freshwater sediments, mainly alluvium and fan deposits, laid down during the Pliocene to Early Pleistocene (5 to 1.8 mya). The soils of the Stockdale mapping unit have a deeper and sandier topsoil.

The Quaternary extends from about 1.8 mya to the present and is subdivided into Pleistocene and Recent (10 000 years ago to the present). There were several ice ages during the Quaternary which caused considerable rises and falls in sea level. This was because as the ice caps grew in size, the water levels in the oceans dropped. The last ice age was about 17 000 to 20 000 years ago and resulted in a sea level fall of about 150 m and a land bridge between Australia and Tasmania. As well as being cold, the climate was dry and windy. The Latrobe, Macalister, Thompson and Avon Rivers cut deep valleys into their earlier flood plains, which then became partly in-filled as the sea level rose to its present level. This has resulted in a well-defined break between the old flood plain (upper terrace) and the present flood plain (lower terrace).

A number of sets of terraces were formed during the Pleistocene period in Gippsland but whether they were formed as a result of sea level change associated with ice ages or by uplift or a combination of both is uncertain. Geologists have identified five terraces with Qp1 being the oldest and Qp5 the youngest. An additional terrace Qp6, has also been recognised during this study.

Qp1
Minor occurrences of this terrace occur in the upper reaches of the Mitchell River and Perry River valleys. It does not occur in this study area.

Qp2
This terrace is the most extensive terrace stretching almost without interruption, from Stratford to Paynesville. It is very flat with an even SSE slope of about 1 in 200, with a maximum elevation of about 160 m near Briagolong and 125 m at The Fingerboards and a minimum elevation of 20–25 m at its southern margin. However, some geologists believe that more than one terrace may be present.

The sand hills and dunes were formed from sand blown from stream beds by the prevailing westerly winds, particularly during periods of aridity associated with past glacial periods.

Qp3
This is the least extensive terrace and is represented by three isolated terraces near Stratford, Wuk Wuk, and Lindenow South. Elevation is generally 20-30 m lower than Qp2.

Qp4
This occurs as wide, flat terraces in the Avon, Mitchell, Thomson and Macalister river valleys. This has a much gentler southward gradient of between 1 in 300 and 1 in 450. Thus, the difference between Qp2 and Qp4 is about 60 m at Briagolong but decreases steadily to 15 m near Stratford.

Qp5
This terrace has a similar distribution to Qp4 and is about 20 to 30 m lower. An extensive area between Toongabbie and Nambrok West is regarded as Qp5.

Qp6
This terrace is not delineated on the 1:250 000 geological sheets. Two phases are recognised: an earlier phase (Qp6a) that is slightly higher in the landscape and a later phase (Qp6b). Qp6b comprises former streams and levees, source-bordering dunes, back-plains and swamps. Qp6a may represent a flood plain that backed up behind Snake Ridge before a stream finally broke through, forming what is known as the Kilmany Gap.

Qra
This includes all modern flood plain deposits.

Subdivisions Of Gippsland Terraces