The geology of the Australian Capital Territory includes rocks dating from the Ordovician around 480 million years ago, whilst most rocks are from the Silurian. During the Ordovician period the region—along with most of eastern Australia—was part of the ocean floor. The area contains the Pittman Formation consisting largely of quartz-rich sandstone, siltstone and shale; the Adaminaby Beds and the Acton Shale.[1]
Most of the younger rocks are pyroclastic deposits from explosive volcanic eruptions, but the Yarralumla Formation is a sedimentary mudstone/siltstone formation that was formed around 425 million years ago.
In the 1840s fossils of brachiopods and trilobites from the Silurian period were discovered at Woolshed Creek near Duntroon by the Reverend William Branwhite Clarke.[2] At the time these were the oldest fossils discovered in Australia, though this record has since been far surpassed. Other specific geological places of interest include the State Circle cutting and the Deakin anticline.[3] [4]
The early European name for the district was "Limestone Plains". In 1820, following the discovery of Lake George and the Yass River, Governor Lachlan Macquarie decided to send a party, with provisions for one month, to discover the Murrumbidgee River. Joseph Wild was accompanied by James Vaughan, a constable, and Charles Throsby Smith, a nephew of the explorer Charles Throsby. Detailed instructions had been given to the explorers by Charles Throsby, who had accompanied the Lake George exploration party earlier in the year. They were provided with acid to test for limestone. On 7 December 1820, Smith recorded in his journal:
... Came on to one of the plains we saw at 11 o’clock. At half past 1, came to a very extensive plain, fine Rich Soil and plenty of grass. Came to a Beautiful River plains that was running thro’ the plains in a S.W. direction, by the side of which we slept that night. When we made the Hut this evening, we saw several pieces of stone that had been burnt by all appearances. I then examined some of it, which proved to be limestone. ...[5]
There is little limestone evident at the surface in the district, but an outcrop at Acton, near the Museum of Australia, by the shores of Lake Burley Griffin.These formations became exposed when the ocean floor was raised by a major volcanic activity in the Devonian forming much of the east coast of Australia.
Much of the western and southern parts of the Australian Capital Territory (ACT) are made from granite-like rocks. These are from the Murrumbidgee Batholith intruding during the late Silurian or early Devonian times.
Tectonics explains the large-scale structure of the Earth's crust and its constituent rocks in terms of blocks moving along faults, uplifted into horsts or downthrown into grabens.The ACT is positioned on the Australian continent, which was once a part of the supercontinent Gondwana. The ACT is in the Tasmanides, the deformed rocks of the orogen that make up the core of the old mountain range that makes up the Australian continent east of the Tasman Line. These rocks are an addition to the Proterozoic core of the continent. The Tasmanides are the result of compression, horizontal shortening, and vertical thickening of various "terranes" such as small continental fragments and volcanic island arcs that were plastered against the original continental margin as a result of plate tectonic movements.
The Tasmanides also extended into Antarctica in the south and northern China in the north, as these continental units were attached to Australia at the time, in Gondwana.[6]
The ACT is part of the Eastern Lachlan Fold Belt, which is located on a terrane that is called the Benambra Terrane in Victoria, but the Molong-Monaro Terrane in New South Wales.
also known as Canberra-Yass Synclinorial Zone East
also known as Canberra-Yass Synclinorial Zone west
A plate with a continent fragment on board bumped into the east coast of what is now Australia, forming the Delamerian Orogeny. The remains of the mountains of this orogeny can be found near Broken Hill and in eastern South Australia, western Victoria, and western Tasmania. This happened between 520 and 480 million years ago up to the Cambrian period. No associated ocean floor has been preserved by obduction in the ACT.
During the Palaeozoic era at least a few thousand kilometres of the ocean floor were subducted, taking of the order of a hundred million years. Sediments were deposited in the ocean floor in the form of fans formed by turbidity currents off the side of a continental slope. The flow was in the northerly direction, indicating that the continental slope was to the south. These deposits occurred during the Ordovician period. The ocean floor was distant from the continental source of the sediment. Towards the end of this period there were isolated parts where no turbidity currents reached and only fine clay and animal organic and silica debris were deposited into oxygen-depleted deep water. This ocean basin has been called the Monaro Basin. To the north west was a volcanic chain of islands, the Macquarie arc, with an associated submarine trench.
The subducting Pacific plate was old, cold and dense, easily sinking into the mantle at a steep angle. The hinge zone of the plate also migrated oceanwards over time. So the trench retreated oceanwards, and the old trench and ocean floor become part of the continental plate. Volcanoes formed inwards from the trench. The part of the oceanic plate attached to the continent was compressed, the suboceanic crust was severely shortened and thickened as well, giving rise to a duplex structure. This happened at the end of the Ordovician period and in the Early Silurian. The sediments were heavily folded and overthrusted resulting in severe crustal shortening. In the Canberra area the sediments were raised above sea level and eroded. The land to the west (around Wagga Wagga) was raised higher.An unconformity resulted between the Pittman Formation and the State Circle Shale and Black Mountain Sandstone deposited on top.
State Circle Shale and Black Mountain Sandstone deposited in a marine environment as turbidites. The source of the Black Mountain sand was near by from the west, from the Wagga Wagga area. The Canberra area was on the proto-Canberra-Yass Shelf. East of Canberra deep water of the Monaro Basin remained in the Captains Flat area.
A second unconformity occurred after the Black Mountain Sandstone was uplifted and eroded at the end of the early Silurian. This was called the Quidongan Deformation. The Canberra formation was deposited in shallow water with limestone, and shale forming. There were some small volcanic activities at this stage with dacite and ashstone layers included.
Several stages of volcanic activity followed. The first stage with Paddys River Volcanics, Walker Volcanics in west Canberra, Hawkins Volcanics in the north and Ainslie Volcanics in the north east had acidic lava volcanoes erupting.The next stage was Mount Painter Volcanics in middle Canberra, and Colinton Volcanics south of Queanbeyan and near Williamsdale. Then came a pause in volcanism at the start of the Upper Silurian with the Yarralumla Formation and Yass Formation sedimentary deposits. Volcanic activity resumed with Deakin Volcanics in the north west and south of Canberra. First rhyodacite was erupted followed by tuff, more rhyodacite, tuff with some underwater sediments, and finishing with rhyolite. At least four large eruptions made up this volcanic deposit. Over the top of this in the west near the Murrumbidgee River there was a further massive volcanic eruption, called Laidlaw Volcanics.
West of the Murrumbidgee is another different geological setting. The tectonic block is called the Cotter Horst. This was probably in a different position relative to Canberra, compared to the present. From here, ocean floor turbidite deposits occurred in the Ordovician period. The sediments were deeply buried by being compressed and faulted down. Melting occurred in deep sediments and in the basaltic oceanic crust beneath. The magmas mingled and intruded upwards. The Murrumbidgee Batholith was formed, with several intrusions. The faults were reversed and the granites from the batholith became elevated.
Small granite intrusions injected the rocks in the Canberra Graben around 408 million years ago. The Molong-Monaro Terrane was carried into position on the east coast of Australia.
The Bowning Deformation caused the north–south faulting and long folding in the area surrounding the ACT. This deformation was connected with the attachment of the terrane to the continent. During this stage metamorphism occurred. In the Canberra Graben and Cullarin Block, metamorphism mostly reached the upper greenschist stage, with shallow burial and temperature below 350 °C. This changed the volcanics and sediments with sericitisation, saussuritisation, conversion of plagioclase to albite, and conversion of biotite to chlorite, titanite, epidote and opaque minerals. On the western margin in southwest Belconnen, Duffy, and Kambah in the Laidlaw and Walker Volcanics the temperature was lower and prehnite-pumpellyite facies was achieved. This was not sufficient to convert plagioclase to albite.
More intense metamorphism occurred to the east of the Googong Dam, east of the ACT on the Molonglo Range and Yarrow Peak and Taliesin Hills. Psammitic schist and pellitic schist occur there. Within this region, there are two parallel belts of knotted schist even more strongly heated to over 525 °C. The temperature gradient in the area was high at 70 °C per kilometre.
In the east side of the Cullarin Block, in the east-pointing finger of the ACT, the Tabberabberan Orogeny also reached the upper greenschist facies again. East–west pressure caused ruptures forming the Winslade and Deakin Faults and other northwest- or northeast-trending faults.
A dyke of olivine teschenite intruded into the Red Rocks Gorge area of the Murrumbidgee River. The Kosiusko uplift elevated the land in the Snowy Mountains and Southern Highlands areas. This uplift reactivated the Murrumbidgee Fault, the Queanbeyan Fault and the Lake George Fault.
Minor mining operations have occurred historically throughout the region, both for precious metals including gold, silver, lead, and copper, and for construction materials. The only commercial operations now here are the Stockmans Quarry at Pialligo which excavates Camp Hill Sandstone, and a large quarry on Mount Mugga Mugga which excavates Mugga Mugga Porphyry for construction gravel and concrete mix.
A deposit of galena and copper carbonate was tunneled in the Balconnel Gold Mine around 1894. No significant gold was ever found there. The mine was 200 m downstream from the Molonglo River on the Murrumbidgee River with coordinates 35°S deg 14' 38.3", 148°E deg 58' 13.6".[7]
Paddys River Mine or Cowley copper mine is located 700 m upstream on Paddys River. The host rock is a skarn where Shannons Flat Adamellite baked limestone from the Paddy's River Volcanics. The bulk of the mineral here is magnetite, but other sulfide and minerals of copper, lead zinc and silver attracted miners. Thomas Coyle operated the mine in 1895, producing 2.6 tons of copper and 26.1 kg of silver. In 1907 the Cowley Copper Syndicate Ltd started more work cutting two more adits and winzes. It was abandoned by 1909 as the ore was too low grade. More than a million tons of magnetite remain. Many minerals such as aurichalcite, bronchanite, caledonite, cerussite, hemimorphite, hydrozincite, linarite, native silver, rozenite, and zincsilite have been found at the location.[8]
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Chemical analysis reveals low concentrations of sodium, calcium and strontium because of the low feldspar content.
The Ordovician turbidites, consisting of greywacke, feldspathic sandstone, micaceous siltstone, micaceous shale, chert, and phyllite, are very similar in all parts of the Tasman Orogen, including New Zealand and the Transantarctic Mountains. Detrital zircons from the turbidites have been isotopically dated with age ranges of 0.46 and 0.60 Gya, 1.0 and 1.2 Gya, and at ~1.8 Gya, ~2.2 Gya and ~2.7 Gya. These do not match the age of zircons from the interior of the Australian Shield, so the source of the sediments is from another continent. The other evidence from the Ordovician sediments are chemical composition indicates granitic source, with the absence of feldspar. Secondly the fine grained nature shows the sediments have been transported a long way from their ultimate, source, and could have been second cycle, derived from sedimentary rocks.[9] [10]
At the time these sediments were deposited the location was at least 3,000 km from the Australian continent. Underneath there is no Proterozoic continental basement, instead these sediments are lying on top of oceanic crust.
The sediments making the Acton Shale were deposited in the ocean in a reducing environment starved of oxygen, and lacking fresh sediment.
Canberra Formation can be found in the east part of South Canberra in Fyshwick, Kingston, Barton and Parkes. It is also found through North Canberra, excluding Campbell and Russell. It occurs through most of Gungahlin apart from Crace and Nicholls. The beds extend north in a wide band to 35 deg 03S near Bald Hill.[11]
Narrabundah Ashstone is a member of the Canberra formation[11] and is found is eastern Narrabundah and in a geological monument along Fairbairn avenue. Ashstone is a fine grained tuff.[12] In addition to the ashstone the Canberra Formation has layers of green-grey to reddish dacite, also tuff, quartz andesite, but mostly it is calcareous shale, limestone or sandstone. Much is deeply weathered, which has posed difficulties for building foundations.
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Named after Mount Ainslie, Australian Capital Territory where their type location occurs, the Ainslie Volcanics are composed of Dacitic ignimbrite and minor volcaniclastic and argillaceous sediments. The lithology is bluish grey dacitic tuff, which can be massive or foliated, also dacitic agglomerate and shale. These volcanics contain chloritised cordierite and some have red almandine garnet. Jasper is found on low hills on the north side of the Molonglo River. The thickness is at least 700 metres. The magma was formed by melting an aluminium-rich pelitic sediment. The eruption came from a volcano into shallow sea water. The deposits built above sea level as they progressed. The underlying sediments now make up the Canberra Formation. Between Hall and Nanima Hill on Spring Range, the Mount Painter Volcanics lie unconformably on top.These rocks date from the Late Wenlockian epoch of the Silurian period. They were formed about the same time as the Walker Volcanics and Paddys River Volcanics.
The Ainslie Volcanics occur on Mount Ainslie, Mount Majura, and in a band extending from Bonshaw and Harman north to the east of Woolshed Creek, through Majura and at least to Gooroo Hill and Old Joe on the NSW border. On Mount Ainslie the sequence starts with dacitic tuff, banded dacitic tuff, massive dacitic tuff, fifty metres of agglomeratic tuff, massive dacitic tuff, fifty metres of ashstone and topped with massive dacitic tuff. The rocks on top of the eastern ridge is altered.
Xenoliths include jasperised sediments and there are some beds of tuffaceous siltstone and sandstone. It was mostly deposited from the air without water.
Mount Painter Volcanics overlie the Walker Volcanics unconformably. The top is also an unconformity with Yarralumla Formation and Deakin Volcanics and Yass Formation. The age is of Late Wenlock epoch or early Late Silurian. These rocks form a belt from Coppins Crossing towards Narrabundah and Jerrabomberra Creek.[16]
Deposition occurred in a shallow marine environment with a delta. The sea level was relative higher during this depositional phase compared with the earlier and later subaerial volcanic deposit periods. Its age is early Ludlow. This has been determined by shelly marine fossils, and a tonalite intrusion southwest of Red Hill with age determined as 417±8 Mya. Outcrops occur on Red Hill (the hill) and throughout Deakin, Australian Capital Territory and in southern Yarralumla, and also Hughes, Australian Capital Territory. The formation extends from Red Hill and Woden in the South to Yarralumla and Lake Burley Griffin in the north. The formation is evidence of the last major period when eastern Australia was still covered by shallow seas. It shows fossil evidence of trilobites, coral and primitive crinoids.
Another band stretches from Lyons, Australian Capital Territory north north west towards the Molonglo River. There may also be a patch in Symonston, Australian Capital Territory. Exposures can be seen at the Deakin Anticline[18] —with pale brown siltstone; and also at the Yarralumla brickworks[19] with olive-green calcareous mudstone.
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The Mugga Mugga Porphyry is a lava flow. It is blue or mauve grey in a mass. The rock is veined with calcite, light green epidote, and deep red hematite. The phenocrysts are quartz, grey plagioclase, pink potash feldspar in lesser amounts and flakes of biotite. It is dated at 414±9 Mya. The Federal Golf Course Tonalite introduces some veins and saccharoidal galena.
Some shale and water deposited tuff occur to the west of Pine Island and Point Hut Crossing. North west of Mount Stromlo, on Uriarra Road, there is an occurrence of limestone over a few square kilometres.
Middle Silurian intrusions could be volcanic necks. A coarse green grey rhyodacitic intrusive outcrops over one square kilometre west of Holt in the Walker Volcanic sediments. It is likely to be the magma chamber for the uppermost eruption of Walker Volcanics.
A coarse green grey rhyodacitic intrusive with white prominent feldspar crystals appears between the arms of Lake Ginninderra, McKellar, Evatt, Nine Elms, east Spence, and Nichols. This covers a few square kilometres. A small outcrop of the same material occurs in Cook and Jamison Center. This rock matches the upper layer of the Hawkins Volcanics.
East and north east of Watson is an intrusion of grey and cream dacite. This may correspond to the lower layer of the Ainslie Volcanics.
Late Silurian to early Devonian intrusions could also be volcanic necks. If so they have been responsible for the Laidlaw Volcanics. They are all unnamed. The first three intrude Laidlaw Volcanics.
A coarse rhyodacitic intrusive outcrops to the north west of Charnwood intrudes Laidlaw Volcanics, with about half a square kilometre of surface area.
A line of outcrops of coarse pink-brown rhyodacitic porphyry outcrops on the north side of Canberra on Forster Hill, MCquoids Hill and Neighbour Hill, and on East side of Mount Taylor.
A distorted band of coarse grey rhyodacitic porphyry occurs along the Murrumbidgee River from 1 to 4 kilometres south of Point Hut Crossing.
Another pink and green rhyolite porphyry occurs in Holder, Weston and Lyons. It intrudes Deakin Volcanics.
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Shannons Flat Granodiorite is poor in xenoliths and is coarse grained. It was intruded after the clear Range Granodiorite.
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The Tharwa Adamellite resembles Shannons Flat Adamellite, but with more strongly zoned plagioclase (An50-20)contains microcline, it is poor in xenoliths and is coarse grained.
It is low in iron and calcium, and higher in potassium than other Murrumbidgee Batholith intrusions.
The rock consists of microphenocrysts of olivine altered to serpentine, pinkish brown titanaugite, brown kaersutite amphibole, and about 15% magnetite, embedded in a colourless analcime groundmass. Kaersutite contains titanium and has a formula near NaCa2(Mg4Ti)[Si<sub>6</sub>Al<sub>2</sub>O<sub>22</sub>]O(OH).
The dyke is believed to be Tertiary in age.
In the ACT the Moho depth is about 46–47 km below sea level. South south east of Canberra to the Tasman Sea is a region of lower Moho depth than the surrounding south-east Australia.[22] Seismic P-waves travel at 8.1 km/s in the mantle below this boundary. Above the Moho is an underplated layer with P-wave speeds of 6.7 to 7.0 km/s between 22 and 47 km. Above this layer is rock with speeds of 6.4 to 6.6 km/s from 22 to 15 km deep probably made from basic igneous rocks that were once oceanic crust. Above 15 km depth the P-wave speed is from 6 to 6.2 km/s and likely to be Ordovician turbidites as seen at the surface.[22]
Magnetic deviation through the centre of Canberra was measured at 12.00 degrees east in 1985. The contours run North east. Each year it shifts 2.2 minutes to the east. Magnetic field in all three components is measured continuously at a station in Canberra and made available online.
Magnetic Field Components at Civic at the[23] end of 2013:
Magnetic declination D = 12.273° field strength F = 58096 nT Magnetic inclination I = -65.856° dD = -0.008 deg/yr dF = -28 nT/yr dI = 0.014 deg/yr
The gravity anomaly over the ACT has been measured and published on the 1:100000 Canberra Geological Map.High level points in the gravity field occur at the head of Yass River just north of the east finger of the ACT of –320 μm·s−2. 1 μm·s−2 is 0.1 milligals, so this level is –32 milligals. Another high of –330 μm·s−2 is just north of the northernmost point of the ACT. The contours in the ACT run NW–SE. At the Canberra GPO is about −440, at Scriviner Dam −510, at Lake Tuggeranong Dam −600, and Banks −590. The western end of Kambah has the lowest level in Canberra at −610. Oaks Estate is −400, the highest levels in the metropolitan area are at Watson and Mitchell at −350, and at Mulligans Flat at −345. This means that a 50 kg person, if measured on a force scale rather than on a balance, would apparently weigh 1.3 grams more in Watson, than they would in western Kambah.[24]
A sensitive gravity measuring station is positioned on Mount Stromlo. This can measure changes in the gravity field over time.
The gravity lows are due to the Murrumbidgee Batholith which is compose of lighter rocks.[22]
Temperature gradients in the Canberra area vary from 20 to 30 °C/km. At Blundell's Cottage heat flow is 73 mWm−2 with a gradient of 27 °C/km.[22]
The ACT is moving 24 degrees east of north with a velocity of 54.5 mm/yr along with the rest of the Australian plate. It is sinking at a rate of 3.5 mm per year.[25]
Most soils in the ACT are Podzols. They have a duplex structure with red or brown clayey layer. Typical thicknesses are 2 metres.