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GEOLOGY The oldest rocks in the ACT date from the Ordovician around 480 Million Years Ago. During this period the region along with most of Eastern Australia was part of the ocean floor; the ''Pittman Formation'' consisting largely of Quartz-rich sandstone, siltstone and shale; the Adaminaby Beds and the Acton Shale. {Link without Title} Most rocks in the ACT are from the Silurian . These are mostly pyroclastic deposits from explosive volcanic eruptions. The ''Yarralumla Formation'' is a Mudstone / Siltstone formation that was formed around 425 Million Years Ago during the Silurian Period . 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 marine sedimentary period when eastern Australia was still covered by shallow seas. It shows fossil evidence of Trilobite s, Coral and primitive Crinoid s. The Yarralumla brickworks quarry and the Deakin anticline are places where the formation is exposed and easily studied. {Link without Title} In the 1840s Fossil s of Brachiopod s and Trilobite s from the Silurian period where discovered at Woolshed Creek near Duntroon . At the time these where the oldest fossils discovered in Australia, though this record has now been far surpassed. Other specific geological places of interest include the State Circle cutting and the Deakin anitcline.[http://www.nationalcapital.gov.au/planning/projects/state_circle_cutting.asp [http://www.act.nationaltrust.org.au/places/deak-anti.html . The early European name for the district was the "Limestone Plains". In 1820, following the discovery of Lake George and the Yass River , Governor 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, Charles Throsby 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. ... {Link without Title} There is, however, little limestone evident at the surface in the district. There is an outcrop at Acton , near the Museum of Australia by the shores of the Lake 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 ACT are made from granite like rocks. These are from the Murrumbidgee Batholith intruding during the late Silurian or early Devonian times. Tectonic context Tectonics explains the solid rock in terms of blocks moving along faults, uplifted into Horst s or downthrown into Graben s The ACT is positioned on the Australian continent, which was earlier part of Gondwana . The ACT is in Tasmanides, the part of the Australian continent east of the Tasman Line added onto the Proterozoic core of the continent. The Tasmanides also extended into Antarctica in the south and northern China on the north as they were attached at the time in Gondwana. The eastern part of the Tasmanides is the Tasman Orogen, on the Pacific margin of Gondwanaland. This was formed by accumulations of extended continental crust like Lord Howe Rise accreted and shortened. Plate tectonic processes in the southwest Pacific: a spatial and temporal context Barry Drummond, B.L.N. Kennett2, R.J. Korsch, B.R. Goleby & P.A. Symonds Penrose Conference March 1999 The ACT is part of the eastern Lachlin fold belt In Victoria the plate piece ACT is on is called Benambra Terrane. It moved 600 km south before attachment to Whitelaw Terrane in Devonian times. However in New South Wales this is called Molong-Monaro Terraine. Canberra Region Structure (Rocky Pic Horst) outside the east of ACT. :Narongo Fault Captains Flat Graben (easternmost 3 km of ACT) :Ballallaba Fault Cullarin Horst (north of Queanbeyan, Oaks Estate, most of Kowen) :Collingwood, Queanbeyan, Sullivans Faults Canberra Graben (north east ACT including Canberra, and Williamsdale also known as Canberra-Yass Synclinorial Zone East :Murrumbidgee, Winslade, Pig Hill Fault Cotter Horst :Tantangara Fault Goodradigbee Graben also known as Canberra-Yass Synclinorial Zone west :Long Plain Fault (Cowra Trough) outside the west of the ACT Geological History pre history 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, east South Australia, western Victoria, and Western Tasmania. This happened between 520 to 480 Million years ago up to the Cambrian Era. The oceanic floor was probably formed in the Cambrian Era or early Ordovician . This oceanic floor is deeply buried under the Ordovician sediments and is not exposed on the surface in the ACT. Ordovician History During the Palaeozoic Era at least a few thousand kilometres of ocean floor were Subduct ed taking of the order of a hundred million years. Sediment s were deposited in the ocean floor in the form of fans formed by Turbidity Current s 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 era. 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 Current s reached and only fine Clay and animal organic and Silica debris were deposited into oxygen depleted deep water. This ocean basin was called Monaro Basin. To the north west was a volcanic chain of islands, Macquarie arc, with an associated Submarine Trench . Benambran Orogeny The subducting Pacific plate is old, cold and dense, easily sinking into the mantle at a steep angle. The plate also snapped off and started sinking more 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 Age and in the early Silurian . The sediments were severely shortened, resulting in heavy folding and overthrusting. 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. Silurian deposition 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, Walker in west Canberra, Hawkins in the north and Ainslie Volcanics in the north east had acid volcanoes errupting. 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 there was a further massive volcanic eruption, called Laidlaw Volcanics. Cotter Horst History West of the Murrumbidgee is another different geological setting. This was probably in a different position compared to today's north east ACT. From here ocean floor turbidite deposits occurred in the Ordovician Era. 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 became elevated. Devonian 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. 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 degrees. This changed the volcanics and sediments with sericitisation, saussuritisation, conversion of Plagioclase to Albite , and conversion of Biotite to Chlorite , Sphene , Epidote and opaque minerals. On the western margin in southwest Belconnen, Duffy, 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. there is Psammitic Schist , and Pellitic Schist . Within this there are two parallel belts of knotted schist even more strongly heated to over 525 degrees. The temperature gradient in the area was high at 70 degrees per kilometre. The east side of the Cullarin Block - on 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 north west or north east trending faults. Tertiary 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. Mining The Stockmans Quarry at Pialligo excavates Camp Hill Sandstone. A major Quarry on Mt Mugga Mugga excavates Mugga Mugga Porphyry for use as construction gravel. So this stone is spread all over Canberra, on the roads, and in concrete. Lithology Ordovician Pittman Formation The Pittman Formation was described originally by Opic in 1958 who named it after the Pittman Valley, southeast of Aranda. The Pittman formation is about 800 meters thick near Canberra, but at Captains Flat it is over 1200 meters thick. The lower levels are greywacke, exposed east of Queanebeyan and north are very thick and heavily overturned and thrusted. At Etheridge Creek, the type locality is a repeating pattern of sandstone, micaceous sandy shale, mudstone, black argillaceous and radiolarian chert. In the sandstone beds there are occurrences of graded bedding, clay pellets, and current bedding. Fossils of graptolites, radiolarians, conodonts, and occasionally brachiopods and sponges are found. The geological formation east of Queanbeyan used to be known as the Muriarra Formation. This alternates between sandstone with a high quartz content and mica, and phyllite. Radiolarian chert is found in the central section. The railway forms the border between Queanbeyan in NSW and Oaks Estate in the ACT. West of the Queanbeyan railway station is a cutting where folding has overturned the beds, with axes dipping to the east at 50 degrees. 300 meters of thickness is exposed in this cutting. Llanvirnian age (Pygodus serrus conodont zone). Fossils found include Phyllograptus anna, Trigonograptus ensiformus, Pterograptus, Didymograptus, Isograptus, Hallograptus from the Darriwillian age. Near the top of the formation are fossils Dicellograptus sextans, D divaricatus, D salopiensis, which are Gisbornian. Greywacke, Feldspathic Sandstone, micaceous siltstone, micaceous shale, chert, phyllite. The Ordovician turbidites are very similar in all parts of the Tasman Orogen, including New Zealand and the Trans Antarctic Mountains. Derital Zircons from the turbidites have been isotopically dated with age ranges 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. Reference: CRUSTAL EVOLUTION IN SOUTHEASTERN AUSTRALIA: A ZIRCON VIEWPOINT Ian S. Williams and Bruce W. Chappell composition tables SiO2 71.23% TiO2 0.64% Al2O3 13.76% FeO 4.74% MnO 0.05% MgO 1.90% CaO 0.31% Na2O 1.02% K2O 3.54% P2O5 0.13% Rb 168 ppm Sr 64 ppm which is low in Na, Ca and Sr because they are low in feldspar. reference Basement of the Lachlan Fold Belt: the Evidence From S-Type Granites B.W. Chappell, GEMOC ANU At the time these sediments were deposited the location was at least 3000 km from the Australian continent. Underneath there is no proterzoic continental basement, instead these sediments are lying on top of oceanic crust. Acton Shale Acton Shale is a grey to black thinly laminated silceous Shale containing Graptolite s. It is generally leached and frequently silicified. Where weathered the colour changes to whitish Grey. The colour of the beds alternates between grey and black. The graptolytes appear on the bedding planes as black films. In Canberra the Acton Shale appers in several outcrops in Acton, in two bands through Aranda, through Bruce near the Calvary Hostpital, on the Bruce Ridge behind Lyneham. Also another band starts under the University of Canberra in Belconnen, and heads north east through Lawson and Giralang and folded and faulted into several bands in Crace on Gungahlin Hill. Another band is found on the west side of Queanbeyan, extending north to Dundee, and south around the east side of Jerrabomberra Hill. Acton shale is only preserved in the cores of synclines, being eroded from uplifed parts. Brachiopod s, Conodont s and Sponge s Fossil s are rarely found. The beds are up to 60 meters thick and appear high in the Pittman Formation. The age range is Gisbornian to Bolindian of the Ordovician Era. Lower beds contains fossils like Dicranograptus nicholsoni. Upper beds contain Climacograptus bicornbis, Chastatus, C tubuliferis, Dicellograptus elegans, and Dicranograptus hians which are late Eastonian in age. The sediments making the Acton Shale were deposited in the ocean in a reducing environment starved of oxygen, and lacking fresh sediment. Late Early Silurian State Circle Shale State Circle Shale was named by Opic in 1958. It is named after the street where it was described. Its age is Llandovery. Its constitution is shale, mudstone, siltstone and minor sandstone. In the type location there is about 60 m of non-calcareous sandy shale and dark grey shale with beds of fine-grained sandstone. Between Kings ave and Commonwealth Avenue, there is a good outcrop on State Circle consisting of buff-coloured laminated siltstone and shale with fine sandstone beds contorted by slumping. Its top is an unconformity with Camp Hill Sandstone on top. There is probably up to 200 meters thickness of this shale. The shale was deposited in the deep sea as turbidites. It can be found in Yarralumla, Parkes, Acton, north and south of Black Mountain, and from Lawson, to Crace and Ngunnawal. Black Mountain Sandstone Black Mountain Sandstone is deposited on top of State Circle Shale conformally. It is made up from thick beds of grey Quartz Sandstone mostly, but has some beds included of Siltstone and grey Shale . The grain size is fine to medium. It was originally named by Opic after the mountain - Black Mountain where it is found. Originally it was believed to be Ordovician, but is actually from the Silurian Era, late Llandovery . Some of the slopes of Black mountain are covered in Fanglomerate . The deposition was in a marine proximal turbidite fan, with the turbidity current flowing to the east. There are no fossils, but there is some sedimentary structure including plane, cross or convolute laminations, load casts, slump units and flute moulds. Tidbinbilla Quartzite This has been modified by granite intrusions close by. It consists of medium grained sandstone, partly silicified and changed to quartzite. Belts of silstone and sandstone are included becoming more more frequent at the top. The exposure is 300 meters thick. Low down there is a 2 meter thick bed of ashstone across a broad area that can be used as a marker bed. Late Middle Silurian Canberra Formation In the 1840s Fossil s of Brachiopod s and Trilobite s from the Silurian period where discovered at Woolshed Creek near Duntroon . At the time these where the oldest fossils discovered in Australia, though this record has now been far surpassed. {Link without Title} These fossils were from the Canberra Formation. 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. Narrabundah Ashstone is a member of the Canberra formation and is found is eastern Narrabundah and in a geological monument along Fairbairn avenue. Ashstone is a fine grained tuff. 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 and has posed difficulties for building foundations. Walker Volcanics Walker Volcanics appear as Purple, or greenish-grey dacitic ignimbrite. These volcanics contain chloritised cordierite and some have garnet. They are Wenlock age. They occur in southern Belconnen including Macquarie, Weetangera, Hawker, Page, Scullin, Higgins, Holt and the Pinnacle.
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