VICTORIA'S GEOLOGY after the Carboniferous
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This section provides a brief overview of Victoria's geological history from the Carboniferous to the present. The significant earlier part of earth history in this region is described in a parallel page. Images of sites, rocks, and minerals can be found here, or through links in this page - underlining denotes links.
The subsequent history of Victoria is summarised in a companion page here. A splendid report of the entire history of Victorian rocks is found in The Geology of Victoria (3rd Edition), available from the Society.
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Late Carboniferous |
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After the Kanimblan Orogeny in the Early Carboniferous, all of Victoria went
through a tectonically stable phase lasting until the middle of the Mesozoic
era. No Late Carboniferous units are known in Victoria, and the State was probably
an area of non-deposition.
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Permian |
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Victoria lay at a high latitude in the Permian, and adjacent parts of Gondwana were covered by continental ice sheets during this period. Deposits of glacial and fluvioglacial origin are scattered around the state, with outcrops around Coleraine in the west and Bacchus Marsh and Derrinal in central Victoria. Permian sedimentary rocks also occur in several graben structures under the Murray Basin in northern Victoria (Netherby and Wentworth troughs in the northwest, Numurka Trough and Ovens Graben in the north).
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Outcrops in Victoria belong to the Bacchus Marsh Formation and include some classic glacial deposits of international significance. The most common deposit is diamictite consisting of boulders and cobbles of diverse rock types in a clay or sandy matrix. Rare varves also occur. Some deposits of conglomerate and sandstone are fluvial, and some are marine. Tillite deposited beneath, in front of and on top of glaciers is also present. Striated and faceted pebbles are common throughout the Bacchus Marsh Formation.
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A continental ice sheet probably covered most of Victoria at times during the Permian. Ice moved from south to north, and during retreat it left areas of fluvial outwash plains, while some areas were covered by shallow sea.
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Triassic |
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Triassic rocks are rare in Victoria, and it is assumed that little sedimentation occurred, with only mild denudation. One small outcrop of fluvial sandstone occurs at Bacchus Marsh, and the other occurrences are several small syenite and granite intrusions and trachyte lava flows in the Benambra area of northeast Victoria. These constitute the Mount Leinster Igneous Complex, with radiometric dates of around 210 and 230 Ma.
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Jurassic |
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By the Early Jurassic Australia showed early signs of rifting from Antarctica. Perhaps the first indication of this process is the Coleraine Volcanic Group in western Victoria, consisting of alkalic basalt and trachyte. These are exposed on the edge of the Otway Basin, which along with the Gippsland Basin of eastern Victoria, marks the rift valley that developed during the Jurassic and Cretaceous periods along the southern part of Victoria.
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Jurassic rocks in the Otway Basin are restricted to the subsurface Casterton Formation of fluviatile sedimentary rocks and some interbedded basalt flows, marking the onset of continental rifting. No rocks of this age are known in the Gippsland Basin. Some dolerite and lamprophyre dykes in central and eastern Victoria have been dated as Jurassic, and probably many of the thousands of similar dykes in Victoria are the same age. They may also reflect limited crustal stretching during the early stages of rifting.
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Cretaceous |
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Thick sedimentary sequences of Cretaceous age occur in the Otway and Gippsland Basins, representing the fill of a deepening rift valley separating Australia from Antarctica (Otway Basin) and Australia from the New Zealand continent (Gippsland Basin). These units continue offshore under the continental shelf. Tasmania was also partly rifted away to form the Bass Basin underneath Bass Strait.
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Early Cretaceous rocks are mainly fluvial sequences of the Otway
Group in the Otway Basin and the Strzelecki Group in the Gippsland Basin. These thick units are cut by a network of normal faults, forming graben and half graben structures, that developed as the crust was stretched and thinned.
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Important dinosaur fossils have been found in the Otway Group at Dinosaur Cove, and rocks of similar age in the Gippsland Basin at Koonwarra have yielded remarkable fossils of fish, birds, plants, insects and other invertebrates.
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A mid-Cretaceous phase of mild basin inversion was followed in the Late Cretaceous by alluvial, deltaic and shallow marine units in the Otway Basin (Sherbrook Group) and Gippsland Basin (La Trobe Group). This episode marks the incursion of the sea into the rift (the sag phase of basin development), and the initiation of the Southern Ocean and the Tasman Sea. By the end of the Cretaceous a classic passive margin had developed along the southern coast of Victoria.
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Important oil and gas reservoirs are located in Cretaceous strata in the Otway, Bass and Gippsland basins, mainly in fluvial, paralic and deltaic sediments.
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Palaeogene |
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The Palaeogene was dominated by deposition in three large basins, the Otway and Gippsland basins in the south, and the Murray Basin in the north of the state. Smaller basins in the central south of the state (Torquay, Port Phillip and Westernport basins) also received sediment in the Palaeogene. Basaltic lava flows of the Older Volcanics Group were erupted over much of the eastern half of the state at this time, covering fluvial gravel and sand deposits.
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The Otway and Gippsland Basins continued to receive sediments as they underwent thermal sag caused by Australia's drift northward away from the spreading ridge in the newly formed ocean to the south. Sea-floor spreading in the Tasman Sea had ceased by the Early Eocene.
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A general trend from non-marine clastic deposition early in the Palaeogene to marine conditions late in the Palaeogene is seen in these basins. In the Otway
Basin these units are dominantly littoral to shallow marine mudstone, sandstone and limestone of the Wangerrip, Nirranda and lower Heytesbury groups. In the Gippsland Basin the La Trobe, La Trobe Valley and Seaspray groups are the main Palaeogene units. The La Trobe Group (Palaeocene to Eocene) and La Trobe Valley Group (Oligocene to Miocene) are largely fluvial to deltaic siliciclastic sediments and coal measures, while the Seaspray Group (Oligocene to Pliocene) is composed of shallow marine carbonates. Within the La Trobe Valley Group are the giant brown coal deposits of the La Trobe Valley.
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The Torquay, Port Phillip and Westernport basins were predominantly fluvial and paralic in the Palaeogene, with the shallow marine Jan
Juc Marl deposited in the Late Oligocene in the first two basins as subsidence continued.
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Large reservoirs of oil and gas have been found in Palaeocene and Eocene strata in the Otway, Bass and Gippsland Basins, within fluvial, paralic and deltaic sediments.
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At the beginning of the Palaeogene the Murray Basin was initiated as a broad downwarp in the north and northwest of Victoria, extending into New South Wales and South Australia. The first sedimentation in the Palaeocene and Eocene was fluvial and paralic, producing the Renmark Group. A marine incursion during the Late Oligocene gave rise to the Murray Group consisting of shallow marine limestone and marl.
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In the highland areas of central Victoria fluvial gravels were deposited in high energy rivers, giving rise to the White Hills Gravel in the Palaeocene to Eocene. This unit has some placer gold. This was followed in the Oligocene by the Loddon River Group, also known as the deep leads, which is a series of fluvial gravels and sands that have been covered by valley basalt flows of the Newer Volcanics. These are the main source of palaeo-placer gold in central Victoria.
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Neogene |
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The southern basins continued their passive margin depositional styles into the Miocene, with a sea level rise peaking in the Early Miocene.
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Carbonates of the Heytesbury Group were deposited in the Otway Basin, including the well known Port Campbell Limestone, seen in the Twelve Apostles and other features along the southwest coast. Minor tectonic uplift in the Late Miocene produced a break in sedimentation, followed in the Pliocene by shallow marine to beach deposits (Whalers Bluff Formation and Hanson Plains Sand) resulting from a marine transgression in the Early Pliocene. Basalt flows of the Newer
Volcanics interfinger with Pliocene sediments at the northern edge of the Otway Basin.
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The Torquay, Port Phillip and Westernport basins had a similar history to the Otway Basin in the Neogene, with deeper marine carbonates in part, and paralic to fluvial deposition around the basin edges. The same Late Miocene uplift and Early Pliocene marine transgression recorded in the Otway Basin affected these basins.
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In the Gippsland Basin, clastic and coal deposition of the La Trobe Valley Group continued into the Miocene, and the more seaward parts of the basin accumulated the marine carbonates of the Seaspray Group into the Pliocene. Tectonic uplift affected the Gippsland Basin in the Late Pliocene, leading to an unconformity below the Late Pliocene Sale Group of marine to marginal marine clastics and carbonates. In the Late Pliocene to Pleistocene alluvial fans and floodplains covered much of the onshore basin, giving rise to the Haunted Hill Formation.
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The Murray Basin was largely covered by a shallow sea in the Miocene, with carbonates of the Murray Group continuing to be deposited until the Middle Miocene. Eastern parts of the basin received fluvial sediments of the Calivil Formation. After uplift in the Late Miocene, the sea began to retreat in the Pliocene, leaving behind the beach and nearshore sands of the Parilla Sand. A huge system of parallel strandlines remains today, hosting valuable heavy mineral sand deposits. As the sea retreated, a thin sheet of fluvial sand and silt (Shepparton Formation) covered much of the basin in the Late Pliocene.
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Quaternary
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During the Quaternary, deposition continued in the main basins in Victoria, with a declining marine influence. The Otway Basin was partly covered by shallow marine carbonates early in the Pleistocene, and as the sea retreated later in the Pleistocene the basin was covered by the calcareous beach dunes of the Bridgewater
Formation. Dunes are now forming along much of the southwest coast.
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The Gippsland Basin was mainly alluvial in the Quaternary, with barrier deposits forming near the present coast. This style of sedimentation has continued as the current barrier beach system of the southeast coast.
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After regression of the sea in the Pliocene, much of the Murray Basin was covered by Lake Bungunnia, a large saline lake that deposited the Pleistocene Blanchetown Clay. Fluvial conditions prevailed in the southern and eastern parts of the basin. As arid conditions developed the lake dried up and sand was blown in from the west to form the Lowan Sand and Woorinen Formation.
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The large Western District Volcanic
Province began in the Late Miocene and continued through to the present. Much of western Victoria was flooded by flows of basalt, forming the volcanic plains. Many valleys in the highlands were also flooded by basalt. As well as lava flows, numerous scoria cones, lava domes, maars and maar complexes (such as Tower Hill and the Red
Hill Complex) were formed. The youngest of these features, Pleistocene to Holocene in age, are exceptionally well preserved.
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