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Victorian Geology

Introduction to Victorian Geology – Dr Monty Grover

The oldest rocks in Victoria are Cambrian in age (560 to 380 million years ago). During this time Victoria was covered by an ocean that extended far to the east and was deep in most places. The nearest coastline was to the north-west and the eroded material from this as well as material from large volcanoes scattered across the sea-floor contributed to the sedimentary material. Between the volcanoes there were also extensive eruptions of basalt lavas from long cracks in the bed of the sea. Limestone beds in north-east Gippsland indicate that the sea was shallow in at least one area. In the latest Cambrian and earliest Ordovician times a mountain building event caused the cessation of deposits of sediments and the folding and faulting of the Cambrian rocks. Rocks deep in these ranges were subjected to high temperatures and pressures and metamorphosed. Associated with these in the south-west are granitic intrusions, the oldest dating back to 500 million years ago.

Sedimentary rocks continued to be deposited during the Ordovician (500 to 420 million years ago) in a deep marine basin that extended northward into New South Wales and southward into Tasmania. To the east there was a line of volcanic islands that occur intermittently in a belt north of Victoria with the edge extending into north-eastern Victoria near Benambra. There, dacites and andesites occur within a narrow stretch of rocks. Nearly all the Ordovician rocks in Victoria are of deep water sedimentary origin. Most are interbedded sandstones, mudstones and minor shales of turbidite origin or thick sequences of deep water, black shales. The source material for these sediments is from the west as discerned from scour and ripple marks in the turbidite sequences.

During the Late Ordovician to Early Silurian (420 to 440 million years ago), Victoria was effected by large area deformation. Regions in both western and eastern Victoria were uplifted to form dry land. This orogeny caused the Ordovician sediments across Victoria to be folded and faulted. Another major period of deformation affected most of Victoria around 385 to 395 million years ago, probably due to a continent-continent collision. This was the final stage in the formation of what is now the south-eastern part of the Australian continent. It was probably the most dramatic geological event ever to occur to Victoria and caused major changes in the patterns of sedimentation. Uplift caused sedimentation to cease and a major mountain range was formed in eastern Australia. Associated with this orogeny was extensive granitic intrusion and intrusion of large numbers of parallel quartz to olivine rich dykes. Fractures within the dykes are filled with quartz which carries gold; historically Victoria's most economic mineral resource.

The late Carboniferous and Permian was a period of relative geological stability across Victoria with no major faulting or folding and very little rock formed. Glaciers covered most of the land, reaching their maximum depth and extent during the Late Permian. The glaciers changed the Victorian landscape eroding valleys and when they melted they left behind material carried by the ice.

Early Mesozoic outcrops are rare and occur, mainly, on the southern margins of the state. With the break up of the supercontinent Gondwana, major east-west fault lines caused rift valleys to form along the southern portion of Victoria causing extensive flood plains. Volcanic eruptions supplied most of the sediment.

During the Cenozoic, Australia drifted north away from Antarctica. Extensive river systems formed over the parts of Victoria not covered by shallow seas. These rivers were fast flowing, eroding deep channels and filling them with coarse gravels, including particles of gold derived from quartz veins intruding into the Palaeozoic bedrocks. There were swamps in all the Cenozoic basins, especially in the Gippsland Basin, where thick beds of brown coal accumulated. These basins contain most of the State's Brown Coal and oil and gas reserves. Shallow marine limestones are present along most of coastal Victoria, forming steep cliffs along the south-western Victorian coast. Thick Cenozoic deposits also occur in north-western Victoria, within the Murray Basin. These include heavy mineral sands within shallow Plieocene sediments containing world-scale heavy-mineral-sand deposits with good potential.

Volcanic activity occurred intermittently throughout the Cenozoic, almost to the present day. Basaltic eruptions began in the Late Cretaceous, about 90 million years ago. Outcrops of volcanics are scattered across southern Victoria, covering major portions of Ordovician basement sequences as well as more recent deposits.