Geology of Victoria

From the birth of the state in the Pre-Cambrian through continental accretion and the folding of mountain chains in the Ordovician and Silurian, volcanoes and granite intrusions in the Devonian, glaciation in the Carboniferous and Permian, the age of dinosaurs and Australia’s separation from Antarctica in the Mesozoic and the dramatic sea level changes, volcanism and erosional processes which bring us through the Cenozoic to the present day.

The continent of Australia is a very familiar shape to most people, as are the tropical climates of Northern Australia, the arid grasslands of Central Australia and the icy Alps of South-Eastern Australia. But the rocks that make up Australia haven’t always existed harmoniously in their current positions. Thanks to Alfred Wegner and the Theory of Plate Tectonics, we now know that all continents have been moving around the Earth, building and destroying the land, for billions of years. Follow Australia in the interactive below to track its journey around the globe from the Cambrian to the Present.

Now, lets explore the rocks around Victoria! I’d recommend opening the geological map below in a new tab, so you can switch back between the text and the map easily to explore the different time periods.

550 – 495 MA

These rocks are the oldest in Victoria! The best way to see where they are located is to turn all of the other rocks off in the map above except Cambrian. You’ll see narrow north-south trending belts of Cambrian Rocks in Western, Central and Eastern Victoria.

In the Cambrian, Victoria was ocean-floor. It was a time of subduction and the growth and accretion of volcanic arcs. There are narrow windows of these volcanic rocks across the state, including: Mount Stavely Arc in Western VIC, the Heathcote Greenstones in Central VIC, the Jamieson-Licola Volcanics in Eastern VIC and the random outcrops of Cambrian Greenstone at Phillip Island and Waratah Bay. These rocks only outcrop at surface along major faults, but probably extend underneath all of Central Victoria.

The Mount Stavely Arc (Cambrian Greenstones) formed as a continental arc during subduction of an oceanic plate under Gondwana. It would have looked a lot like the Kermadec Arc (New Zealand Ring of Fire!), shown above as a bathymetric map.

The Heathcote Belt is thought to have formed in a back-arc basin, of volcanic rocks and pelagic sediments (deep sea; shale, chert).

Check out this section of the Marianna Arc, if you imagine the Mount Stavely Arc is the ‘Mariana Island Arc’ and the Heathcote Belt is the ‘Axis of Back-Arc Spreading’ (NOAA).

The Glenelg and Stavely Zone Volcanics experienced intense deformation (compression of rock which will then fold or thrust (fracture) to shrink in size), called the Delamerian Orogeny (~ 530–500 Ma). This created mountains. We can model deformation using sand boxes!

The Cambrian was also the time of the biggest evolutionary explosion in Earth’s History, with weird creatures called wiwaxia, trilobites and anomalocaris, a super predator of trilobites.

495 – 440 MA

During the Late Cambrian and Early Ordovician the Cambrian Volcanics were covered by an extensive sheet of turbidites, the St Arnaud and Castlemaine Groups in the West and Late Ordovician Adaminaby and Bendoc Groups in the East (you can select these layers which are lime green in the map!). These sands (like at the beach) and shales (muddy, like in ponds) were formed of sediments eroded from a mountain chain that formed during the Delamerian Orogeny, which you can watch in the videos above. Remember, at this point there were no land plants to slow erosion rates so soil washed right off of the land and into the sea!

The Digger Island Marlstone at Waratah Bay in south-eastern Victoria is a snapshot of a shallow-marine carbonate shelf (think Great Barrier Reef) during the Ordovician, with abundant trilobites, brachiopods, ostracods and algae. Elsewhere in Victoria, graptolites – extinct colonial and planktonic organisms – rule. Whilst limited to thin shales in the Castlemaine Group, they are prolific in the Mount Easton Shale of Eastern VIC. The end of the Ordovician was marked by a massive extinction event in which more than half of all the marine invertebrate species went extinct.

440 – 418 MA

Start video at 3:33 min.

The Silurian started with another big deformation event, this one called the Benambran Orogeny, which is the first in the long-lived Lachlan Orogeny (~450-340 Ma). This deformation event is very, very complex. And it gives a lot of Geologists a lot of headaches to the present day. In some places the crust is extending (imagine pulling a bug creamy cake apart), creating Metamorphic Core Complexes (Omeo, Kuark) and basins (Cowombat, Yalmy and Enano Groups) and in other places it is in compression, creating mountain ranges (think about what happens to dirt when you are digging a shovel in to scoop it up).

In fact, Geologists now think that during the Silurian all of the Delamerian mountain ranges and Ordovician sediments were wrapped clockwise around a rigid bit of Tasmania, called the Selwyn Block, shown below. If you look at the Geological Map you’ll see a repetition of the Ordovician (tan coloured) sediments either side of Melbourne… which is pretty cool!

(From Musgrave, R 2015).

The Grampians Group sediments were deposited in the Early Silurian and deformed in the Late Silurian, forming a fold and thrust belt which we enjoy hiking today!

A good way to understand this might be to imagine using clay to make a rainbow, each of the layers would need to be wrapped around your thumb (like the Selwyn block above! The inside layers will be compressed and the outside layers will be stretched.

The Murrindindi Supergroup sediments were deposited undisturbed in the Melbourne Zone (Selwyn Block, green in the image above) throughout the Silurian and Early Devonian. If you live in Melbourne you’ve probably seen these in a road cutting or two.

The Wagga-Omeo Zone was originally formed of the deposition of Ordovician Quartz-Turbidites, becoming metamorphosed (i.e. cooked) during the Silurian to become a metamorphic core complex (schists, gneisses and migmatites). These rocks are undeniably cool – check out the 3D rock collection below!

The Yalmy and Enano Group sediments were deposited in a series of extensional basins that formed during the Silurian, filled with shallow marine rhyolitic lava and high-level intrusions overlain by marine to subaerial volcaniclastic siltstone, sandstone and conglomerate below fine-grained turbidites. That is a lot of jargon! See the Buchan guide to learn more about these interesting basins.

Throughout the Silurian, the environment gradually transitioned from deep marine (graptolites) to shallow marine (brachiopods; McIvor Sandstone in Heathcote) to terrestrial (first land plants; near Yea). The Baragwanaithia fossils at the Yea Flora Fossil Site are amongst the earliest fossil land plants that occur anywhere in the world!

418 – 359 MA

The Devonian is known as ‘The Age of Fishes’. In Victoria, shallow seas, rivers and lakes teaming with fish, brachiopods, corals, gastropods, crinoids and trilobites, as seen in limestone at Buchan, predominate. The Devonian is an exciting time for the colonisation of land by the earliest Tetrapods (i.e. fish with fins-come-feet). Tetrapod trackways are found on the banks of the Genoa River in far western Victoria, which have been dated at about 380 million years old!

Volcanoes, some of the largest in Earth’s history, exploded across central and eastern Victoria late in the Devonian Period, covering several thousand cubic kilometers of Victoria. The volcanism resulted from crustal extension that affected much of the eastern Lachlan Fold Belt creating large rift-like basins where the felsic volcanics are overlain by limestone (i.e. Snowy River Volcanics, Buchan), and in much smaller cauldrons including Mount Burrowa, Dartella, Besfor, Mount Elizabeth, Carberean (Cathedral Ranges) and Mount Gelantipy. Mount Macedon and Mount Dandenong are also remnants of these eruptions. These volcanoes often create great hikes because the rocks are hard and have resisted erosion over millennia.

At Mount Ida near Heathcote you can climb the upturned beds of the Mt Ida Formation, part of the Murrindindi Supergroup sediments which continued to be deposited in the Melbourne Trough throughout the Devonian.

In western and central Victoria Granites intruded deep into the crust, becoming exposed at surface through weathering over millions of years, and the Rockland Volcanics erupted at the Cathedral Ranges. The Devonian was a busy time!

359 – 251 MA

The Carboniferous (359 – 299 Ma) was the time of giant insects and early reptiles, however the information we have about this time comes from Europe and America as there are no known Carboniferous units in Victoria.

Artists impression of the Carboniferous Period. Source: Walter Myers.

During the Late Carboniferous and Early Permian, Australia drifted South from the equator to the South Pole and joined with all of the other continents to form the supercontinent Pangea. This created an Ice Age that lasted for millions of years.

Throughout the Permian (299 – 251 Ma) Victoria was covered by a giant glacier which moved slowly North bringing rocks from Antarctica, with relatively few rocks from this time preserved at Bacchus Marsh and Werribee Gorge and, Lake Eppalock in Victoria. Rare Permian fossils are preserved on a little island off the coast of Tasmania called Maria Island.

As the ice retreated, forests developed with Glossopteris (seed ferns), tree-ferns, club-mosses and horsetails. These same species can still be seen in living ancient rainforests such as at the Tarra-Bulga National Park in the Strzelecki Ranges. Following the ice age there was a giant extinction event which killed 90% of the planet’s species. Thankfully animals like Sharks, Reptiles and… Mammals survived!

251 – 65 Ma

The Mesozoic is renowned for two things: Australia ripping away from Antarctica, creating giant basins (Otway, Bass and Gippsland) and… Dinosaurs!

TRIASSIC (251 – 200 MA)

Triassic rocks are rare in Victoria, and so our knowledge of the climate, animals and plants is limited. What we do know is that there would have been giant glacial valleys which covered the landscape, as is common all over the world including in New Zealand. There is an exposure of Triassic-aged rocks at Council Trench near Werribee Gorge in Victoria.

JURASSIC (200 – 145 MA)

In the late Jurassic and early Cretaceous periods (160-96 m.y.a), Australia and Antarctica began to split apart. This is evidenced by the extensive Dolerite (igneous rock) outcropping in Tasmania and abundant mafic dykes in Central Victoria which resulted from crustal stretching, and the formation of rift basins in southern Victoria.

The rift basins were filled with Early Cretaceous sediments (sandstones and mudstones) deposited by rivers and lakes, including swamp material which formed the Wonthaggi coal seams, plants and fish which have been preserved at Koonwarra and the remains of anything else living in the valleys at the time (dinosaurs and early mammals at The Otways and Inverloch!).

Volunteers from Museum Victoria dedicate a few weeks every summer to digging up the dinosaur fossils around The Otways and Inverloch/Wonthaggi areas. There is even a dinosaur trail being developed, called the Bass Coast Dinosaur Trail!

CRETACEOUS (145 – 65 MA)

It wasn’t until the Late Cretaceous that sea-floor spreading initiated between Australia and Antarctica, and Australia was able to move further North. A failed rift also developed between Australian and Tasmania, with the increased stresses resulting in multiple faults, volcanism and also the uplift of the Strzelecki and Otway Ranges. This is also when the Great Dividing Range developed from uplift in Victoria and NSW.

Cretaceous strata in the Otway, Bass and Gippsland basins are also home to important oil and gas reservoirs.

Dinosaurs lived in a polar world in Victoria with little sunlight, and adapted to their hostile environment with many being only the size of a chicken!

At the end of the Cretaceous there was yet again another mass extinction event, which resulted in the well known extinction of the Dinosaurs. There is strong evidence that the culprit was a huge asteroid that formed a large crater on what is now the Yucatan Peninsula of Mexico.

65 Ma – RECENT

The story of Victoria since the Mesozoic has been one of erosion, mostly. And relative climatic stability, resulting in the diversification of modern animal groups including birds, mammals and reptiles.

At the beginning of the Palaeogene Period, sedimentation in the Murray Basin was fluvial, much like the depiction below, producing the Murray Group.

The Eocene to Miocene (56 – 7.2 Ma) was an important time for the development of modern Australian plants, and it was also when the 700m thick Latrobe Valley Coal Measures developed in the Gippsland Basin between the Eastern Highlands to the North and the Gippsland Highlands to the South West. These coal seams are mined in the Yallorn, Morwell and Loy Yang Open Cut Mines for electricity generation.

In the late Miocene to Early Pliocene (7.2 – 5 Ma) there was a major sea level rise or transgression, where the sea transgressed from the present coast between the Fleurieu Peninsula and Mount Gambier in South Australia, forming a large gulf that extended into NSW (the Murray Basin). The sea level was approximately 65m above present levels. Lake Tyrrell is one of many inland lakes which formed as a result of this inland sea.

65 Ma – RECENT

Throughout the Pliocene (5.4 – 2.4 Ma) sea levels regressed and fluvial sediments were deposited in shallow waters. This gave rise to the beautiful red cliffs, formed of Sandringham Sandstone, we see at Half Moon Bay and Beaumaris today. The cliffs at Beaumaris are now renowned world-wide for the incredibly rich and diverse marine fossil assemblage. This includes Megascyliorhinus (shark), numerous shark teeth, seals, whales, penguins, albatross, turtles and even Diprotodonthids (the largest marsupials ever known!).

The Newer Volcanics Province (large grey layer on the map) is the result of hundreds of eruption centres which formed in the Cenozoic, and extensively covers much of Western and Central Victoria. Research has shown that in just a few million years there were over 704 eruption points from 416 volcanic centres, i.e. not exactly the sort of place you would want to live! In fact, the last eruption was ~5000 yeas ago at Mount Gambier in SA, so the province is still considered active.

Throughout the state we can see many different types of outcrops which formed as a result of this volcanism – from the large volcanic lakes (‘maar’), showing evidence of large phreatic eruptions (when magma comes into contact with shallow water) at Mount Eccles and Tower Hill, to the lava blister in Williamstown to the columnar jointing at the Organ Pipes and the sub-aerial eruptions seen at Eagle Rock, among many others.

In Daylesford, it is easy to distinguish between outcrops of the basalt and the Ordovician sediments by looking at the land use: nutrient rich farmland lies on top of the basalts which provide the overlying soils with essential nutrients such as magnesium and iron. The volcanics also create fantastic waterfalls.

You can download your own Geological Map of Victoria by accessing the dataset below. I recommend either dropping the KML files into Google Earth or downloading the free GIS program QGIS and dropping the shp files into the project.

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