Victoria is comprised of a relatively new (geologically speaking) land mass, with most rocks being younger than 500 million years (Ma). Read on below to see how the geological locations talked about on this site fit into the geological puzzle of Victoria.
Cenozoic (65 Million Years Ago – Present day)
NW-SE directed compression resulted in rifting between New Zealand and Australia, which continues today. The Otway ranges was the result of this. Some outcrop is visible in the north-east section of Werribee Gorge, which also provided the iron source for the orange-red sediments seen throughout the area.
The Newer Volcanics Province (large red layer on the map) is the result of hundreds of eruption centres which formed in the Cenozoic, and covers a large area in central- west Victoria. Throughout the state we can see many different types of outcrops from this event- from the large volcanic lake (‘maar’), showing evidence of large phreatic eruptions (when magma comes into contact with a large body of shallow water,) to the lava blister in Williamstown to the columnar basalts 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 Newer Volcanics 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 forest-land, lies on top of the nutrient poor Ordovician sediments.
Lastly, closer to the present-day (Pliocene), as the sea level was regressing, fluvial sediments were deposited in shallow waters. This gave rise to the sediments we see at Half Moon Bay and Beaumaris today. Towards the back of the beach you can climb the beautiful red cliffs, which are outcrops of the Red Bluff Sand. These were formed in the Pliocene (late Cenozoic) when sea levels were regressing and fluvial sediment were deposited in shallow waters.
Mesozoic (252-66 Million Years Ago)
The Mesozoic is defined by deposition in basins including Otway, Bass and Gippsland. This is because Australia and Antarctica began to split up (both were together as one big continent – Gondwana land – near the South Pole. This is the time of the dinosaurs! There are museum volunteers who are actively digging up the dinosaur fossils around The Otways and Inverloch/Wonthaggi areas, and you can visit these beaches yourself to see these areas.
Paleozoic (540 – 252 Million Years Ago)
During the Palaeozoic, a great deal of Victoria was formed, progressively, eastwards. A subduction system – where ocean floor slides beneath buoyant continental crust – developed outboard of the eastern margin (remember, near Ararat). This system is responsible for many of the large granitic plutons which litter the map of Victoria above, including the present day You Yangs, Wilson’s Prom and Labertouche. Deep-water sedimentation continued and as a result of the Delamerian Orogeny (a mountain building event), these sediments were uplifted in western Victoria. We can see deep-sea Silurian shales at the Cathedral Ranges, Studley Park and Buchan, and even older Ordovician sediments at Werribee Gorge and Daylesford Springs.
Early-on, the Earth was in the grips of a widespread ice age. Australia however being so close to the equator remained relatively warm. As glaciers and ice caps continued to form, the sea levels decreased worldwide, exposing more land on all shores (sea levels are rising now because ice-caps are melting).
Towards the end of the Paleozoic, Australia began drifting southwards, reaching the cold climates of the South Pole. Ice sheets formed over the entire state and then slowly began to retreat by the end of the Paleozoic.
Pre-Cambrian (541+ Million Years Ago)
Few rocks are preserved in Victoria from this time. This is because Australia as we know it did not yet exist. It was part of a giant continent, and the parts that would one day become Australia straddled the equator (between then and now it moved all the way to the pole and back up to its present position). The eastern margin of this continent ran through present day Ararat – or thereabouts – which meant most of eastern Australia was submerged in deep water. At Cape Liptrap the turbidite sequence shows underwater deposition, and at Kitty Miller we can see greenstone outcrops, which have been suggested to form at ancient oceanic spreading centers. They are composed of metamorphosed volcanic rocks, primarily basalts and sedimentary rocks.