Geology of Tasmania
The geology of
Geological history
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An approximate time-scale of events in the Geological history of Tasmania. Axis scale is in millions of years ago. |
The earliest geological history is recorded in rocks from over 1,270 million years ago.
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 diabase, or 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
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.[2]
On King Island now in Bass Strait, the oldest Tasmanian rocks are found. On the west side of King Island, there are
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
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.[4] 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 780 million years ago was intruded by granite (Bowry granitoids) 777 million years ago. 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.[4] 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
The Dundas Element lowest level starts with the Oonah Formation with greywacke, dolomite and basic volcanics. The Oonah Formation appeared between 708 to 690 million years ago. 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.[4]
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 1,100 million years ago from the Stenian. This contains zircons predominantly dated 1,796 million years ago, but also from 1710, 1851, the oldest being 3,100 million years ago, and the youngest 1,400 million years ago. The Burnie or Oonah Formation with Greywacke is possibly from the Tonian period, dated around 735 ± 35 million years ago. Slate from both the Burnie and Oonah formations is dated at 690 million years ago. Both of these formations came from a shallow marine shelf. The Cooee Dolerite intruded the Burnie Formation at 725 ± 35 million years ago. Zircon grains in the Cooee Dolerite are from mostly 1,700 to 1,800 million years ago.
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.[5] Zircons in the quartzite have peak numbers aged 1,681 million years ago and 1,771 million years ago. 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.[5] Phyllite near
In the Neoproterozoic in the Jane River basin, the very thick Jane River Dolomite appeared.
The Adamsfield Jubilee element is east of the Tyennan Block. It has a strip exposed on the surface that includes the Florentine Synclinorium, The Adamsfield District, the Jubilee Region, and down to the South Coast at Precipitous Bluff and Surprise Bay. It also underlies the Tasmania Basin across southeastern Tasmania, but not including the east coast. The subsurface structure has been studied from a few outliers, boreholes, xenoliths, and gravity and magnetic surveys. The basement at 5 km deep is the same as the Tyennan metamorphic rocks (Scotchfire Metamorphic Complex). Its oldest exposed rocks are from the Clark Group, of pelitic rocks, some with stromatolites, and evaporites, and overlaid with orthoquartzite. The Weld River Group lies above, starting with 0.5 km thickness of conglomerate and sandstone, then up to 3 km of dolomite, interbedded with sandstone, mudstone and diamictite. Glacial dropstones are found in the interbedding, suggesting Cryogenian age, however δ13C profiles suggest Ediacaran age instead. Gravity and magnetic studies indicate that this sort of dolomite (dense and non-magnetic) underlies Hobart and Bruny Island in a north–south strip, and also in a region west of Hobart.
The Cape Sorell Block is a region of metamorphosed sediments from the Mesoproterozoic, to the south of the west end of Macquarie Harbour. It is separated from Neoproterozoic rocks by a low angle thrust fault. The Neoproterozoic rocks contain greywacke, mudstone and pillow lavas of the Lucas Creek Volcanics (matching the Crimson Creek Formation), mudstone, siltstone (matching the Success Creek Group) and dolomite (correlating with the Togari Group). South east of this is a metamorphosed belt of dolomite rich sediments correlated with the Oonah Formation. An ultramafic belt called Point Hibbs Mélange reaches the coast near Point Hibbs. This has been complexly faulted with Cambrian, Ordovician and Devonian sediments and limestone.
At the end of the Precambrian uplift there were several raised blocks forming land above the sea: the Tyennan Uplift in the central and south west Tasmania, the Rocky Cape uplift in the north west, and the Forth uplift, near Forth in the north. The far north west also had uplift as probably also did some region to the east. Basins formed were the Smithton Basin, Dial Range Basin, Fossey Mountain Basin and the Adamsfield Basin.
Early Cambrian
Next an
As part of this collision, three exotic suites of basalt were tectonically introduced into the Dundas Block. Near Waratah is a sub-alkaline basalt from an ocean floor, another is a high-magnesium andesite-basalt with chrome spinel and clinoenstatite named boninitic rock after the Bonin Islands. This magma produced the layered pyroxenite dunite in the ultramafic area. Thirdly there is a low titanium basalt-andesite with extreme light rare-earth element depletion that produced the layered pyroxenite-peridotite and associated gabbro cumulate.[4]
Two kinds of basalt from the Birchs Inlet–Mainwaring River Volcanics, occur in a belt north from Veridian Point and west of the south end of Birchs Inlet.
In the Adamsfield area The Ragged Basin Complex is a broken up formation of chert, sandstone, red mudstone and mafic magma derived rocks. The sandstone is derived from metamorphic and volcanic fragments. Ultramafic rocks are serpentinised. They are not ophiolites, but instead are cumulates of heavy minerals in a shallow magma chamber. The densest mineral, osmiridium has been concentrated and mined at Adamsfield. These rocks are allochthonous, meaning that they were inserted into position by tectonic processes.
Cambrian
Mount Read Volcanics
The Mount Read Volcanics are a 250 km long belt that is 10 to 20 km wide attached to the western edge of the Tyennan Block or eastern side of the Dundas Element. The volcanics consist of underwater eruptions interbedded with sediment. A range of lava from basic through intermediate to acid are present along with intrusions and volcanic clastics such as breccia and pumice. The breccia includes pieces of andesite, dacite and massive sulfide. The massive sulfides were formed by hot springs on the sea floor. These have become ore deposits for copper, lead, zinc and silver.[5] The volcanics extend south to Elliot Bay. The Noddy Creek Volcanics extend north of high Rocky Point to Macquarie Harbour with pyroxene and feldspar containing andesite as lava, breccia and intrusives.
The Sticht Range Beds form a sedimentary base sitting on the Tyennan Block metamorphic rocks. Parts of the volcanics were from 502.6 ± 3.5 million years ago, and the younger Tyndall Groups has a dating of 494 ± 3.8 million years ago. Fossils also indicate a late middle Cambrian age. Zircons in the volcanics have two age groups: 1,600 to 800 million years ago matching the metamorphic rock in the Tyennan block; and 600 to 530 million years ago without a satisfactory explanation.[4]
In the Dial Range Trough the middle Cambrian saw the deposition of the Cateena Group of conglomerate (of purple mudstone pebbles), sandstone with feldspar, mudstone and greywacke and some felsic volcanics. The age is
In the Adamsfield area the Trial Ridge Beds, Island Road Formation, and Boyd River Formation consists of conglomerate and greywacke. They contain fossils of agnostoids.
Cambrian granites
The Murchison Granite intruded east of the Mount Read Volcanics. It consists of dioritic granodiorite. Major mineral deposits were formed at Mount Lyell, Rosebery and Henty. Granite also intruded in the Cambrian at Low Rocky Point and Elliott Bay.
The north west element was altered by the Tyennan Orogeny around 500 million years ago. The Arthur Lineament was metamorphosed to phyllite, slate and
The Dove Granite intruded the Tyennan Block metamorphics with several small plugs in the north dated at 483 ± 35 million years ago.
Dundas Group
The Dundas group are Cambrian sedimentary beds that interfinger with the Mount Read Volcanics. They lie unconformably on the Precambrian basement. The kind of rock is sandstone, laminated mudstone and a pebble conglomerate in which the pebbles consist of quartzite, sandstone and green mudstone. The group was formed as a submarine fan. The conglomerate includes volcanic fragments where it borders the Mount Read Volcanics, indicating that it was deposited at the same time.[5] The Huskisson Group is from the same time period.
In the Smithton Synclinorium the Scopus Formation is from the same period between
The Fossey Mountains Trough contains Cambrian intermediate volcanics, and greywacke where trilobite fossils show the age as late Middle Cambrian. Boomerangian age fossils were found in Paradise.[4]
Ordovician
During the Ordovician Tasmania was near the equator and was joined to Gondwana. The Tyennan Block was uplifted with the Great Lyell Scarp as an active fault.
The Ordovician Wurawina supergroup comprises the Denison Group and the Gordon group. The Owen Conglomerate, part of the Denison group, lies conformably on the Dundas Group, but unconformably on the Mount Read Volcanics. The pebbles include quartz, quartzite, quartz sandstone, pale pink mudstone and chert, embedded in a matrix of sand. The Owen Group rocks are found on the West Coast Range. The conglomerate was derived from the highlands of the uplifted Tyennan Block and is up to 1500 meters thick. The lowest section is the Jukes Conglomerate, with Lower Owen Conglomerate and Middle Owen Conglomerate above. Upper Owen Sandstone is found in Queenstown, it formed while the Great Lyell Fault was active, resulting in folding of the lower parts. The Pioneer Beds are the top layer, containing chert and chromite. Correlated rocks also occur in a syncline south west of Birchs Inlet, around the upper part of the Wanderer River, and in the Dial Range Trough the equivalent unit is called the Duncan Conglomerate. The Duncan Conglomerate has pebbles mostly of chert, but also some of quartzite, hematite and lava. On the west side of the Dial Range trough at Penguin the Beecraft Megabreccia sits on top of the Burnie Formation. It consists of blocks of chert up to 120 metres long, embedded in conglomerate. The Teatree Point Megabreccia is similar and about 150 metres thick. The Lobster Creek Volcanics is actually an intrusion of plagioclase pyroxene hornblende porphyry dated at 480 ± 18 million years ago.
Conglomerate and sandstone in the Fossey Mountains Trough is exposed in a band on Black Bluff Range,
The Gordon limestone belongs to the Gordon group. It is formed over
In the central north of the Sheffield element is the Early Arenig age Caroline Creek Sandstone on a bed of chert conglomerate. The Cabbage Tree Formation is east of the Andersons Creek Ultramafic Complex, and is sandstone and conglomerate.
In North east Tasmania the Mathinna Group starts in the Ordovician with Stony Head Sandstone, a quartz sandstone formed in turbidity flows. Turquoise Bluff Slate formed from shale. Fossils are rare (mostly sparse graptolites), and ages hard to determine.[4]
The Wurawina Supergroup formed in the Duck Creek Syncline amongst other places. This syncline is oriented east–west, located on the west coast south of the mouth of the Pieman River. It consists of conglomerate equivalent to Mount Zeehan Conglomerate, sandstone, siltstone, shale and micrite equivalent to the Gordon Group, and finally equivalents to the Eldon Group (to Devonian). The Wurawina Supergroup also occurs in the Adamsfield Element with the Denison Group consisting of Singing Creek Formation (of quartzawacke), Great Dome Sandstone, Reeds Conglomerate, Squirrel Creek Formation. Then above this the Gordon Group consists of Karmberg Limestone, Cashions Creek Limestone, Benjamin Limestone, and Arndell Sandstone all from shallow marine conditions. Limestones are also found at Lune River, Precipitous Bluff and produced in deeper water at Surprise Bay on the south Coast.
Silurian
The Mathinna Group continued in the
In the Adamsfield element is the Tiger Range Group with Gell Quartzite, Richea Siltstone, Currawong Quartzite and possibly McLeod Creek Formation. Upper layers have been removed by erosion.
Devonian
In early to mid
In the north east of Tasmania the Mathinna Group received its last deposits in the form of turbidites in the Bellingham Formation and Sidling Sandstone containing more feldspar.
Granites were intruded in the east of Tasmania around 395 to 368 million years ago. The St Marys Porphyrite is an
Veins of gold were crystallised in the Mathinna-Alberton Gold Lineament, a line from Scottsdale to Blue Tier. The Scamander field originated from the edge of the Mount Pearson biotite adamellite-granite pluton, containing tungsten–molybdenum, tin–copper and silver–lead–zinc veins.
Folding and foliation occurred in northeast Tasmania during the Devonian after the granites appeared.
The Eldon Group finished forming in a shallow marine environment with quartz sandstone and mudstone lying conformably on the Gordon Group rocks. Fossils include
Granite
In the west of the state there were thirteen small granitic intrusions around 367 ± 10 million years ago. The western plutons were associated with mineralization at Zeehan.
The Heemskirk Granite is a D-shaped double intrusion of 120 km2. It has two parts, one part red, and another white granite that intrudes the red, it is high in tourmaline. Mineralisation from the Heemskirk Granite with cassiterite or tin and tungsten skarn, or silver lead and zinc veins occur in the Zeehan field. The Pieman Granite is a similar age at 347 ± 9 million years ago, but without useful mineral deposits. The Cox Bight granite is white aged at 376 ± 10 million years ago. The South West Cape Granite is dated at 319 ± 10 million years ago. It is foliated and white to cream with biotite and feldspar in large crystals.[7]
The Meredith Batholith contains biotite adamellite. It contains ten separate plutons. A
The Housetop Granite outcrops over 120 km2 at the western end of the Sheffield Element. It is a biotite granite solidified 380 to 343 million years ago. It produced some mineral veins with lead, silver, zinc, copper and tin-tungsten skarn at Kara. The Dolcoath Granite outcrops near Cethana but extends underground to the west. It has produced magnetite–fluorite–vesuvianite mineral deposits at Moina, and tin tungsten bismuth veins at the Shepherd and Murphy Mine. The Beulah Granite outcrops near Paradise and Beulah and it extends underground to the north and west.
Granite Tor Granite as it appears on the surface is just a small part of a large buried granite body that may connect with the Heemskirk Granite. Its age is 359 ± 15 million years ago.
Carboniferous
On the eastern side of King Island some small stocks of granite with dykes intruded. The granites are adamellite-granodiorite with large crystals of
Megakinking caused shortening in the NNW-SSE direction in north east Tasmania with blocks up to 9 km across rotated.
Permian
In the Permian, glacial conditions predominated with,
The Permian and Triassic deposits together are known as the Parmeener Super Group. The lowest levels are a discontinuous dark grey pebbly tillite up to several hundreds of meters thick. It has been found at Cygnet (
Siltstone with varves is found at Maydena, it is called laminite. Above the tillite is massive mudstone and siltstone with occasional dropstones, the Woody Island Siltstone in southern Tasmania and Quamby Mudstone in the northern half. The upper levels of the marine sequence are siltstone and sandstone with frequent dropstones and fossils. These are the Bundella Formation and Golden Valley Group. Oil shale forms a layer in the north and at Douglas River in the east. The shale is known as tasmanite. Above this are freshwater deposits of conglomerate, sandstone with pebbles, siltstone with quartz or mica. These freshwater beds can be up to 30m thick. They are called Faulkner Group, the Liffey Group and the Mersey coal Measures. Above this in south east Tasmania are more marine units that include Nassau Formation Berriedale Limestone, up to 60 m thick, siltstone and sandstone rich in fossils and dropstones (Malbina Formation, and Deep Bay Formation), and the upper part is dark grey siltstone rich in dropstones. The very top layers are coloured black, probably from an estuary (Risdon Sandstone and the Abels Bay Formation). Felsic volcanic ash is found near the top of the sequence of sediments.
Fresh water deposits form the Upper Parmeener Super Group. The layer commence with poorly cemented sandstone, mudstone, carbonaceous mudstone and coal (Cygnet Coal Measures). Glossopteris is frequently found as well as Dulhuntyspora. In the past coal was mined at Mount Cygnet and Adventure Bay[8] and at Mount Ossa. These sandstones were laid down by east flowing rivers.
Triassic
Continental conditions resulted in sandstone deposits, which represent the upper layers of the Parmeener Super Group. The lowest levels are a sparkling clean quartz sandstone free of coal. The uppermost parts have sandstone and beds of coal. Coal was mined at Newtown, Kaoota, Mount Lloyd, Strathblane, and on the Tasman Peninsula. The sandstone has also been heavily used as building stone.[8]
At
Jurassic
A major intrusion of dolerite occurred in the Jurassic. This was a widespread phenomena covering over one third of Tasmania, and possibly more in the past. This intrusion also affected
Tasmania has the largest exposure of dolerite in the world of 30000 km2 and a volume of 15000 km3.
Most of the intrusions are in the form of sills up to 500 m thick. Mostly the sills are in the Parmeener Super Group rocks. There are also stepped sills, inclined sheets, cones and some dykes. Closely adjacent country rocks were metamorphosed to hornfels. The upper parts of sills may be more coarsely grained. Dolerite is crushed to use as road metal, and aggregate.[8]
Mount Anne, Mount Mueller, and Mount Wedge in the south west are capped in dolerite, where it also makes contact with Precambrian rocks.
It appears that the magma came from the crust rather than the mantle. Solidification occurred at 174.5 ± 8 million years ago.
A Jurassic forest was buried in an andesitic volcanic eruption at Lune River. Here, beneath the lava flow, is mudstone with fossil wood and leaves.
On King Island, there was an intrusion of a biotite lamprophyre dyke at 143 million years ago. Xenoliths of granulite-facies metamorphic rock resemble those found in eastern Antarctica.
Cretaceous
In the Cretaceous continental breakup of Gondwana started near Tasmania. About 83 million years ago a rift entered the east coast of Tasmania from the south and split off the
This extension created a number of sedimentary basins: Bass, Durroon, Gippsland, Otway and Sorell Basins. They each contain several kilometres of sediment from the late Mesozoic to Cenozoic time periods. Bass Basin, between King Island, and north from the Tamar River, has up to 12 km of sediment, actually starting from the Jurassic. The lowest layer is the Otway Group of sandstone made from rock fragments. The Eastern View Coal Measures follow. The Latrobe Group found in the Otway Basin, closer to Victoria, is from the same time and produces the oil found in the area. From Late Paleocene to early Eocene there was an unconformity. A shale from Demons Bluff Formation follows in the Eocene, deposited in calm sea water. The Torquay Group reaches from Oligocene to the current day, with marl and limestone formed in open sea water.
The Durroon Basin is south east of the Bass Basin. Late Cretaceous rocks are conglomerate, with sandstones above. From 125 to 100 million years ago there was a high thermal gradient of 55° per km. Around 60 million years ago there was uplift and erosion of 900 m of sediment called Southern Ocean breakup unconformity. A layer of olivine basalt lies on this, followed by carbonaceous shale for 300 m called Durroon Mudstone 93 to 85 million years ago. This was deposited in a lake. Non marine sediment follow from Cretaceous, through Paleocene to Eocene 83 to 40 million years ago. From 40 to 33 million years ago the Demons Bluff Formation sandstone formed, and finally the Torquay Group with more sandstone and shale than in the Bass Basin.
Rocks from the Cretaceous include
Cape Portland is host to andesite, lamprophyre and porphyrite intrusions and eruptions from 102.3 to 101.3 million years ago. Musselroe Bay nearby has a lamprophyre and basalt from 98.7 ± 0.8 million years ago.
Cenozoic
Pre-Quaternary
Tectonics
Tasmania finally disconnected from Antarctica 45 million years ago. Several basins were formed by faulting. Faulting was connected with continental breakup. Most faulting was finished by the Eocene, but the Sorell Basin continued into the Oligocene. Cenozoic age deposits are found in the northern midlands (Tamar Graben), and south of Macquarie Harbour in the Macquarie Harbour Graben. In the south east are the Derwent Graben and the Coal River Graben. Thick layers of Cenozoic rocks are found in the estuary of the Derwent River, D'Entrecasteaux Channel, Sandy Bay, Taroona, Middleton, Craigow Hill, and Spring Bay. The rocks are mostly siltstone and clay. The deep estuary rocks are from the Paleocene. Travertine is found at Geilston Bay. Silcrete and laterite from this time is found too.[8]
The Macquarie Harbour graben deposits dating from Palaeocene and Eocene are poorly consolidated sand, and gravel, with some beds of lignite and clay.[7] Sediments are up to 500 meters thick, with the lowest layers consisting of dolerite boulders.
The Tamar Graben was an extension to the south of the Bass Basin onto the Tasmanian island. Sediments started in the graben at the very end of the Cretaceous, and into the Paleocene and Eocene with conglomerate, sandstone, mudstone and lignite. Basalt and conglomerate is buried south of White Hills. There is Eocene carbonaceous silt. The Longford Sub-basin extends inland south of the Tamar Graben, and is filled with 800 m of clay, sand and gravel, with some basalt towards the top layers, mostly from the Eocene.
The Devonport-Port Sorell Sub-Basin was formed in Paleocene with carbonaceous mudstone and sandstone. The Thirlstane Basalt is above at 38 million years ago, an alkali-olivine basalt. Then the Wesley Vale Sand follows, and the Moriarty Basalt is 50 meters thick at 29.5 million years ago.
The Sorell Basin forms the continental shelf off the west coast. It has sub-basins of King Island, Sandy Cape, Strahan, and Port Davey which were formed in the Early Cretaceous. The King Island Basin is terminated on the east by a normal fault. It is south of King Island and north west of Tasmania. It has a basement of the Rocky Cape Group from the Proterozoic. The first sediments are red conglomerate beds for 190 m. mid-Upper Cretaceous sandstone and mudstone follow, the same age as the Sherbrook Group. Then more conglomerate sandstone and mudstone matching the Wangerrip Group up to early Eocene. Quartz sandstone is above this, with marl, mudstone and limestone from Oligocene and Miocene age. There is an unconformity at the Pliocene base. Approximately 4 km of sediment is found in each subbasin.
Sea level
The sea level was high in the very early Miocene, and sandstone and calcarenite deposits are up to 30 meters above sea level in the north west and on King Island. In the late early Miocene sea level was up to 100 meters higher than now. There is Pliocene limestone on Flinders Island just above sea level. The Scottsdale sub-basin is up to 225 meters thick from the late Oligocene to early Miocene.
Basalt
Volcanic vents opened up 58 to 8 million years ago. Lava flows of basalt up to 20 meters thick were formed. Some volcanoes were explosive with bombs, and pyroclastic tuff. The eruptions are probably from the Oligocene and Miocene.[8] The earliest eruption was at Bream Creek on the east coast at 58.5 million years ago. From Weldborough it is 47 million years ago but mostly eroded.
In the south east, basalt from Sandy Bay dates from 26.5 ± 0.3 million years ago. Campania has an alkali basalt from 24.2 million years ago but it also has younger flows of olivine tholeiites. From near Hobart there is olivine basalt from 23 million years ago.
In north east Tasmania, there are many lava flows from middle Eocene to early Miocene. There are at least four types: alkali olivine basalt, quartz tholeiite 30.7 million years ago, alkaline basalt, and olivine nephelinite. Lava flows in the north east flowed down valleys to the sea.
In the north west, there was so much lava that valleys filled and overflowed. A plain resulted with up to 750 meters thickness, and maximum extent south of Wynyard and Burnie. In the late Eocene and early Oligocene lakes were formed near Waratah. Older alkaline basalt in the north west is from 26.3 million years ago, at Table Cape basanite from 13.3 million years ago and at Stanley basanite is dated to 12.5 million years ago and 8.5 million years ago. Mount Cameron West has olivine basalt from 15.5 and 14.4 Ma.
On the southern part of the Central Plateau, there are olivine melilite nephelinite, olivine nephelinite, quartz tholeiite lava flows. These ran south down tributaries of the Derwent River 24.3 to 22.4 million years ago. On the east side of the Central Plateau an olivine nephelinite is from 24.9 million years ago, and a flow of nepheline hawaiite is from 24.2 million years ago. In the western Midlands there is basalt from 36.3 million years ago, and hawaiites from 25 and 24.3 million years ago.
Around Launceston, igneous rocks were intruded into Cenozoic sediments forming dolerite and monzonite.
Quaternary
In the Ice age there were valley glaciers and a 1000 km2 ice cap. Glaciation on
Glaciers were in a number of locations on the west coast – at
Several caves have developed in dolomite and limestone. Well known are King Solomons Cave and Marakoopa Cave at Mole Creek, and the Newdgate Cave at Hastings.
Gravels are also left from rivers in Quaternary times. These include the
South Tasman Rise
A ridge of continental crust extends south of Tasmania below sea level. It contains quartz syenite from 1,119 ± 9 million years ago, and Cenozoic volcanics.[12]
Macquarie Island
The two different rock zones are separated by the Finch-Langdon fault zone. It consists of seven segments of faults, subsidiary faults and splays. The fault is a transform fault with a corner at the
Plate tectonics
Various theories describe the past history of Tasmania in relation to other continental masses. Most models have the south west Tasmania abutting East Antarctica. In the missing link model, Z. X. Li has south west China positioned off the east coast of Tasmania with rifting at 825 to 780 million years ago, with the Kamding dykes in China matching some granites from Tasmania.[1]
Tasmania can be subdivided into two terranes, separated by the Tamar Fracture System, on a line from the Tamar River to Sorell in the south east. The West Tasmania Terrane constitutes most of the state, including all the Precambrian and Cambrian rocks. The East Tasmania Terrane makes up the north east and east coasts dating from the Ordovician.[2]
In the model of Moore Betts and Hall, Precambrian Tasmania was part of a microcontinent called VanDieland. VanDieland was first proposed by Cayley in 2011. It consists of Western Tasmania, the
The Barrow River Fault is a major Paleozoic fault. In the section from the west of King Island it has a north-northwest orientation and had a sinistral displacement of 70 km. Just to the north of the western end of Macquarie Harbour the fault changes direction and heads south. But the southern section only has 5 km of displacement.[2]
The Rocky Cape Block which is mainly in the far northwest Tasmania also includes parts that have been shifted south by the Barrow River Fault. The Sorell Peninsula south of Macquarie Harbour, and the tip of Point Hibbs are also part of this element. The Eastern boundary of the Rocky Cape Block is marked by the Arthur Metamorphic complex in the north, and a fault in the south. These structures dip to the east 30°, and have had sinistral movement at 516 Ma. East of this block is the Burnie Zone. The Pedder Zone lies east, and makes up south and south west Tasmania. The Tyennan Zone is separated from the Pedder Zone by eastward dipping Mt Hobhouse Fault.[2]
Geohazards
The east side of the island of Tasmania is in a low earthquake area. The western highlands is part of a belt of seismic activity that includes highlands in eastern Victoria and New South Wales. Just off the north east coast is a point with a large number of tremors, believed to be an incipient volcano.
The last major earthquake in Tasmania was between magnitude 6.5 and 7 at the
On 4 June 1872, a large landslip collapsed part of the side of Mount Arthur. A huge debris flow descended Humphrys Rivulet, stripping the upstream parts of trees and regolith. Where Glenorchy is now, a flood 600 meters wide engulfed farms. Broken trees, boulders, and mud were deposited. Remarkably no one lost their life as all escaped to safety when hearing the rumbling in the distance.[20]
Minerals
Several unusual minerals are known from Tasmania:
Tasmanite the mineral named after Tasmania is in Dana's classification as an Oxygenated hydrocarbon. It consists of reddish brown scales about 1 mm across. It is insoluble in benzene, carbon disulfide, turpentine, ether or alcohol. It contains about 5% sulfur. It is found on the banks of the Mersey River. The shale it is present in, is a kind of oil shale.[22]
Pelionite is a name for cannel coal from Mount Pelion East and Barn Bluff. This term is no longer used.[23]
William Frederick Petterd was an amateur who studied minerals in Tasmania. He built up the Petterd collection which was donated to the Royal Society of Tasmania and stored at the Tasmanian Museum and Art Gallery. He discovered dundasite, named from the mine where it was found.[24] Dundaisite has formula PbAl2(CO3)2(OH)4.H2O. It is a silky milk white spherical aggregate.[25]
Philipsbornite, PbAl3(AsO4)2(OH)5.H2O was originally found in the Adelaide mine and identified as a new mineral by Professor Kurt Walenta. It was named after another German professor Helmuth Richard Hermann Adolf Friedrich von Philipsborn. It occurs as several other mines and appears as a greenish grey earth.[26]
Shandite, Ni3Pb2S2, was first discovered at Trial Harbour by P. Ramdohr in 1960.[27]
Geophysics
The geothermal gradient was measured at a gas seep in Smithton as 26.4 degrees/km. The gradient at Forest, Tasmania is 27.8 °C/km. The Otway Basin has a gradient of 36 °C/km.[28] Several companies are exploring for hot rocks for geothermal energy.[29][30] The granite areas have a gradient of 30°/km, whereas the Parmeener sedimentary areas have a gradient of 40°/km. Heat flow is between 85 and 159 mW/m2.
Active seismic exploration reveals the nature of the deep crust. It shows that the Tyennan block plumbs the depth to the moho which is about 33 km underneath. The Tyennan Block slopes below the Adamsfield-Jubilee Element. Under the Tasmania Basin the block is stretched, with faults in to several large blocks that have tilted down. Above these the Adamsfield-Jubilee Element sediments have filled in the topography. Below the north east element the moho is 36 km deep with alternating seismically fast and slow rocks in the mid crust.
The Tyennan Block and the Rocky Cape Element have a boundary that dips at 30° to the east to the base of the crust. The Dundas Element lies on top of this boundary.
Magnetic field measurements show that the different elements making up Tasmania have very different signatures. Wherever there is Jurassic Dolerite, the magnetic map shows fine ripples, so the Tasmania Basin stands out, as does the smaller intrusions in the other elements. The North east element is smooth, as is the Tyennan Block, and the Adamsfield-Jubilee Element. The Dundas Element has a smooth background with prominent north–south ridges. The Rocky Cape Element is densely packed with linear textures parallel to the Arthur Lineament, with the Smithton Syncline showing as a Y shape. King Island also shows north–south texture. Basalt south of Wynyard also shows a wrinkly magnetic signature.[31]
The stress field in the crust has not yet been measured.
Mining
World class mineral deposits of base and precious metals were found in western Tasmania. Major mines are at Mount Lyell, Rosebery, Zeehan, Que River, Henty and Savage River. Many are hosted in the Mount Read Volcanics. They are in the form of massive sulfides. The Mount Lyell mine extracts copper and gold. The Renison Bell mine was the largest primary tin producer in Australia. Mount Lyell gold and copper deposit was discovered in 1883, formerly the biggest copper mine, and operating till this day.[33]
The Savage River ore body is in the Bowry Formation in the Arthur Metamorphic Complex. It consists of magnetite, pyrite, chalcopyrite and tiny amounts of sphalerite, ilmenite and rutile. The ore was formed under the sea in association with volcanism. The Savage River area also contains deposits of magnesite in the form of marble.
At Beaconsfield, 1.95 million ounces of gold was mined from a quartz reef in a fault.[34] The largest Tasmanian gold nugget was found at Rocky River in 1883, weighing 243 ounces.[33]
An oil exploration boom happened in the 1920s with two companies making bold claims, but earning nothing from oil shale.[35]
Asbestos was mined from the Cape Sorell and Serpentine Hill ultramafic complexes.
History
A. W. Humphrey, a mineralogist, collected rocks and minerals from 1804.
Joseph Milligan sent specimens of a manganese mineral from
In 1882, Gustav Thureau was appointed Inspector of Mines, later called Inspector and Mining Geologist. In 1889, the position became Geological Surveyor.
Professor
The Tasmanian Seismic Net was established in 1957.
Professor Carey founded the Tasmanian Caverneering Club.
The film Walking with Dinosaurs was filmed in part in central Tasmania where forests of gymnosperms similar to vegetation in the Cretaceous still grow.[37]
References
- ^ .
- ^ S2CID 130356645.
- ^ R A Glen (28 July 2006). "The Tasmanides of eastern Australia: A Collage of Accretionary Orogens Documenting Neoproterozoic to Triassic Interaction with the Proto-Pacific Plate" (PDF). International Geoscience Program ICP-480. Ulaanbaatar, Mongolia. pp. 28–31. Archived from the original (PDF) on 17 December 2008. Retrieved 28 July 2008.
- ^ a b c d e f g h i j k l m D. B. Seymour and C. R. Calver Time – Space Diagram for Tasmania Mineral Resources Tasmania and AGSO 1998; and synthesis
- ^ a b c d e Kathryn Harris: "The Geology of Central Western Tasmania: Context for a Major Mineralised Province", in Journal of Undergraduate Science Engineering and Technology
- ^ Small Bass Strait Island Reserves Draft Management Plan Parks and Wildlife Service, Department of Primary Industries, Water and Environment 2000
- ^ ISBN 0-7246-0473-1
- ^ ISBN 0-7246-0486-3
- ^ Bacon, C.A.; Green, D.C. (1984). "A radiometric age for a Triassic tuff from eastern Tasmania" (PDF). Unpublished Report 1984/29. Department of Mines, Tasmania. Retrieved 28 June 2018.
- ^ After Ontong Java Plateau, Deccan Traps and the Siberian Traps.
- ^ Michael Reilly, "The Armageddon factor" in New Scientist 8 December 2007 page 42-45
- ^ ISBN 0-7246-4017-7
- ^ Rafael Osok and Richard Doyle: Soil development on dolerite and its implications for landscape history in southeastern Tasmania
- ^ C. Gaina, R. D. Müller, B. Brown and T. Ishihara: Microcontinent formation around Australia Archived 23 July 2008 at the Wayback Machine in Geological Society of Australia Special Publication 22, 2001 page 400-405
- ^ A. N. Mackintosh, T. T. Barrows E. A. Colhoun and L. K. Fifield: Exposure dating and glacial reconstruction at Mount Field, Tasmania, Australia, identifies MIS 3 and MIS 2 glacial advances and climatic variability Archived 17 December 2008 at the Wayback Machine in Journal of Quaternary Science Vol. 21(4) p363-376 2006.
- ^ a b K. D. Corbett, A. V. Brown: Geological Survey Explanatory Report Geological Atlas 1:250,000 series sheet SK55/5 Queenstown 1976
- ^ a b Karah L. Wertz et al.:Macquarie Island's Finch-Langdon fault: A ridge-transform inside corner structure in Frontier Research on Earth Evolution, volume 1
- ^ M Sandiford: Neotectonics of southeastern Australia: linking the Quaternary faulting record with seismicity and in situ stress Geological Society of Australia Special Publication 22, page 101–113
- ^ "Antarctic expeditioners unscathed by earthquake". News Online. Australian Broadcasting Corporation. 24 December 2004. Retrieved 5 April 2007.
- ^ C. Mazengarb, G. J. Dickens & C. R. Calver: A compilation of historical accounts of the 1872 Glenorchy landslide Tasmanian Geological Survey Record 2007/01
- ^ a b R. S. Bottrill: A mineralogical field guide for a Western Tasmania minerals and museums tour 2001
- ^ W. E. Ford: Dana's Textbook of Mineralogy John Wiley and sons 1932 page 776.
- ^ Letchworth George: Descriptive List of the New Minerals 1892–1938 p 176
- ^ "THE PETTERD COLLECTION". The Mercury. Vol. XCV, no. 12, 861. Tasmania. 14 June 1911. p. 4. Retrieved 24 May 2017 – via National Library of Australia.
- ^ "Dundasite: Dundasite mineral information and data". www.mindat.org. 5 May 2017. Retrieved 24 May 2017.
- ^ "Philipsbornite: Philipsbornite mineral information and data". www.mindat.org. 28 April 2017. Retrieved 24 May 2017.
- ^ Steve Sorrell: Turning Red Lead into Gold: The story of crocoite and other lead minerals of Tasmania 1999
- ^ P. W. Baillie: Investigations into gas of possible geothermal origin at Smithton 1999
- ^ Alison Ribbon (17 September 2007). "'Hot rock' miners win dig rights". Mercury. Retrieved 10 September 2008. [dead link]
- ^ "Hot rocks hot property in Tasmania". ABC News. Australian Broadcasting Corporation. 7 January 2008. Retrieved 10 September 2008.
- ^ .
- S2CID 35541910.
- ^ a b c Geoffrey Blainey: The Peaks of Lyell Melbourne University Press 1967
- ^ "Beaconsfield Gold Mine, Tasmania". Mining Technology. Retrieved 13 November 2019.
- ^ a b c C. A. Bacon: A Brief History of the Department of Mines – 1882 to 2000 2001
- ^ David Branagan: An early view of Permo-Carboniferous glaciation and its implications, from Australia (1906) in Episodes, Vol. 28, no. 3 2005
- ^ Haines, Tim. "The Making of... – Walking with Dinosaurs – The Arena Spectacular". www.dinosaurlive.com. Archived from the original on 15 January 2017. Retrieved 29 November 2016.
Maps
- Geological Map of Tasmania 1:506,880 Geological Survey of Tasmania-Department of Mines 1961
- Geological Atlas 1:250,000 series SK55-3 Burnie Geological Survey of Tasmania-Department of Mines 1973
- Geological Atlas 1:250,000 series SK55-4 Launceston Geological Survey of Tasmania-Department of Mines 1975
- Geological Atlas 1:250,000 series SK55-5 Queenstown Geological Survey of Tasmania-Department of Mines 1975
- Geological Atlas 1:250,000 series SK55-6 Oatlands Geological Survey of Tasmania-Department of Mines 1975
- Geological Atlas 1:250,000 series SK55-7 Port Davey Geological Survey of Tasmania-Department of Mines 1976
- Geological Atlas 1:250,000 series SK55-8 Hobart Geological Survey of Tasmania-Department of Mines 1975
- Mineral Resources Tasmania: Online maps