The geology of Tasmania is complex, with the world's biggest exposure of dolerite (diabase in North American English). The rock record contains representatives of each period of the Neoproterozoic, Paleozoic, Mesozoic and Cenozoic eras. It is one of the few southern hemisphere areas that were glaciated during the Pleistocene with glacial landforms in the higher parts. The west coast region hosts significant mineralisation and numerous active and historic mines.
Geological history
The earliest geological history is recorded in rocks from over . Older rocks from western Tasmania and King Island were strongly folded and metamorphosed into rocks such as quartzite. After this there are many signs of glaciation from the Cryogenian, as well as the global warming that occurred at the start of the Ediacaran period. An orogeny folded the older Precambrian rocks. In the Cambrian time the Tyennan block forming the south west and central Tasmania, was pushed up and slightly over the land of north west Tasmania, the Tyennan Orogeny. Then there were volcanic action and sediments from the Cambrian and Ordovician. The large ore deposits were formed on the West Coast. The north east of Tasmania began to form as part of the Lachlan Orogen with turbidity flows of mud and sand on to the ocean floor. In the Devonian the Tabberabberan Orogeny caused more folding, and intrusion of granite on the west and east coasts, and probably joined the east of Tasmania to the west.
In the Permian period, conditions were again glacial and the Tasmania basin formed, with low sea levels in the Triassic. A giant intrusion of magma happened in the Jurassic forming dolerite which gives many of the Tasmanian mountains their characteristic appearance. The continental breakup happened in the Cretaceous and Cenozoic Periods, splitting off undersea plateaus, forming Bass Strait and ultimately breaking Tasmania away from Antarctica. In the Cenozoic, a couple of basins extended inland from Macquarie Harbour and the northern Midlands. The higher mountains were glaciated during the Pleistocene.
Precambrian The oldest rocks in Tasmania from the Precambrian form several blocks. The blocks are King Island; Rocky Cape in the North West, Dundas Element in the mid west; Sheffield Element in the central north; Tyennan Element in the west central and south west; and the Adamsfield-Jubilee Element in the south central to south coast.
The island's oldest rocks seem to have originated when that part of the island was attached to western North America. Analysis of monazite and zircon in rocks of the ancient Rocky Cape Group in north-west Tasmania found that they are between 1.45 billion and 1.33 billion years old. These minerals, strongly resemble those found in Montana, Idaho and southern British Columbia. Fossils called Horodyskia have also been found in both sites. Importantly, at more than 1 billion years old, the fossils are some of the oldest visible to the naked eye. Previous theories had suggested that Tasmania emerged from central Australia as the supercontinents broke apart, but the recent spectroscopic and radioactive dating evidence contradicts this.
Tasmania's geographic location during the Precambrian is still unclear, but it is clear that some of it was linked to an area of ancient North America. The rocks of Tasmania are much older than those of the east coast of Australia indicating a different geologic history. Alternative ideas for the location are presented below.
On King Island now in Bass Strait, the oldest Tasmanian rocks are found. On the west side of King Island, there are basalts that have been metamorphosed by amphibolite grade metamorphism at . Sedimentary rocks such as feldspathic sandstone that have been altered to schist and quartzite. Detrital zircons have been dated to 1350, 1444, 1600, 1768 and 1900 Ma. A dolerite sill was intruded. Granite intruded in the Cryogenian. The granite contains inherited zircons from . The Wickham deformation affected the earlier rocks by heating to 470 to 480 °C at pressures below 300 MPa, and tight folding. This was followed later in the Neoproterozoic on the eastern side of the island with beds of diamictite, dolomite, mudstone, tholeiite, and picrite interleaved with conglomerate. Also dykes of augite syenite, picrite and 580 Ma tholeiite dolerite were intruded. An interpretation is that deposits occurred in a tidal area, with a continental rift allowing magma from the mantle to intrude. These newer Proterozoic sediments were then tilted and faulted.
The latter tholeiite igneous rocks have a strong magnetic signature, and this can be detected from the rock underneath Bass Strait. The band of rocks is 35 km wide. It extends north northeast to Phillip Island, Victoria, and also south 40 km to just off the west coast of Tasmania stopping at the Braddon River Fault
In the Rocky Cape Block west of Wynyard and north of Granville Harbour, the Precambrian rocks consist of the Rocky Cape Group from the Stenian period, with Cowrie Siltstone, Detention Subgroup, Irby Siltstone, and Jacob Quartzite. The sequence covers most of the block and is over 5700 metres thick. Currents travelled either northwesterly or southeasterly. The metamorphic belt titled the Arthur Lineament forms the limits of the Rocky Cape Group to the south east. The Burnie Formation followed in the Tonian period south east of the lineament with greywacke and slaty mudstone, and also some basic pillow lavas. The Oonah Formation has even more varieties of rock than the Burnie formation, also including conglomerate, quartz sandstone, dolomite and chert. The Bowry Formation in the Cryogenian was intruded by granite (Bowry granitoids) . These have been metamorphosed to the blueschist level. In the Smithton Synclinorium the Togari Group followed with conglomerate from the Sturtian and Marinoan glaciations and dolomite marking the end of Cryogenian and on into the Ediacaran and Cambrian. The Togari group contains greywacke, conglomerate, diamictite, mafic volcanic rocks, and quartz sandstone, and mudstone. The components of the Togari Group are called Forest Conglomerate and Quartzite, Black River Dolomite, Kanunnah subgroup (containing the lavas) and Smithton Dolomite. These rocks are important for determining the boundary between the Cryogenian and Ediacaran periods as they contain volcanics that can be dated and dolomites marking the end of glaciations and marking the period boundary.
Near Corinna the Ahrberg group is correlated with the Togari Group and the Success Creek Group. It contains the Donaldson Formation (a marine fan), Savage Dolomite which contains stromatolites; Bernafai Volcanics containing albite epidote actinolite chlorite; Corinna Dolomite, and Tunnelrace Volcanics. Where the dolomite has been dissolved away over millions of years it has left layers of very pure silica flour, an important mineral resource.
The Dundas Element lowest level starts with the Oonah Formation with greywacke, dolomite and basic volcanics. The Oonah Formation appeared between . It has three sections, Mount Bischoff Inlier, the Ramsay River Inlier and the Dundas Inlier. The Success Creek Group from the Cryogenian has diamictite, quartz sandstone (Dalcoath Formation), and mudstone. It includes the Renison Bell Formation named after the Renison Bell mine. The red rock member is hematite stained chert. The sediments slumped while soft forming folds and breccia and mélange. They were then capped with limestone. The group is up to 1000 metres thick. The Crimson Creek Formation consists of greywacke with tholeiitic basalt. It is from 4000 to 5000 metres thick. This formation could be as late as the early Cambrian. The basalt is probably the same as mafic lavas of the Kanunnah Subgroup.
The Sheffield Element extends from Wynyard past Devonport and the Asbestos Range on the north coast and as far south east as Golden Valley. It contains structural elements called Dial Range Trough, Forth Massif, Fossey Mountains Trough. The oldest Precambrian rocks are the Ulverstone Metamorphic Complex and Forth Metamorphic Complex. This is assumed to be the same age as metamorphic rocks from the Tyennan Block, at from the Stenian. This contains zircons predominantly dated , but also from 1710, 1851, the oldest being , and the youngest . The Burnie or Oonah Formation with Greywacke is possibly from the Tonian period, dated around . Slate from both the Burnie and Oonah formations is dated at . Both of these formations came from a shallow marine shelf. The Cooee Dolerite intruded the Burnie Formation at . Zircon grains in the Cooee Dolerite are from mostly .
The Barrington Chert is finely laminated and has flaggy bedding. It is found in the Dial Range and Fossey Mountain Troughs, up to 1 km thick. The Motton Spilite lies on top of the chert. It consists of pillow lava, massive lava flows, sediments made from volcanic fragments, and chert breccia. The basalt is an ocean floor type. The Badger Head Inlier consists of deformed Burnie Formation. The Andersons Creek Ultramafic Complex is west of Beaconsfield and east of the inlier with serpentinite, pyroxenite, gabbro and a sliver of oolitic chert introduced as a fault bounded block. To the west of the Badger Head Inlier is the Port Sorell Formation, a tectonic mélange of marine sediments and dolerite.
In the Tyennan block, the Precambrian basement that forms the central core of Tasmania there are two formations. First, the Oonah Formation contains turbidite with quartz sandstone interbedded with siltstone deposited by gravity flows. This has been deformed with tight folds that have been overturned, and exhibits crenulation cleavage and brittle faulting. Zircons in the quartzite have peak numbers aged and . Secondly, the Scotchfire Metamorphic Complex contains quartzite deposited in the sea from windblown desert sands, schist and phyllite possibly from a delta. Small quantities of dolomite and boulder conglomerate are also included. The complex includes boudinage structure and en echelon veins.Kathryn Harris: "The Geology of Central Western Tasmania: Context for a Major Mineralised Province", in Journal of Undergraduate Science Engineering and Technology Phyllite near Strathgordon has been dated at . Metamorphism to greenschist facies occurred at around 400 °C and 300 MPa. The Franklin Metamorphic Complex is near Mount Franklin. At Raglan Range the rocks are a mixture of quartzite and knotted schist. Metamorphism in this area was higher grade with almandine garnet forming. The Collingwood area experienced the highest grade of metamorphism with garnet-mica schist, mica schist and garnet-mica-kyanite gneiss present, and enough heat to form veins of migmatite. Eclogite and garnet amphibolite are believed to be the remains of basalt. The eclogite has been heated to 700° at 1520 MPa, a burial depth of perhaps 50 km. Metamorphism happened at the same time as the Cambrian ultramafic complexes were introduced.
In the Neoproterozoic in the Jane River basin, the very thick Jane River Dolomite appeared.