Geology of the Australian Capital Territory 3203656 221927324 2008-06-26T18:49:28Z Civil Engineer III 1001318 /* Bibliography */ fix [[Image:Australian rock ages-MJC.png|right|thumb|200px|Australia grew around three fused pieces of very ancient continental crust (cratons).]] 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&mdash;along with most of eastern [[Australia]]&mdash;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.<ref>{{Citation |url=http://www.ga.gov.au |title=Geoscience Australia |publisher=Australian Government |accessdate=2007-10-11}}</ref> Most of the younger rocks are pyroclastic deposits from explosive [[volcano|volcanic eruptions]], but the ''Yarralumla Formation'' is a [[sedimentary]] [[mudstone]]/[[siltstone]] formation that was formed around 425 [[Ma (unit)|million years ago]]. In the 1840s [[fossil]]s of [[brachiopod]]s and [[trilobite]]s from the Silurian period were discovered at Woolshed Creek near [[Duntroon, Australian Capital Territory|Duntroon]]. At the time these were the oldest fossils discovered in Australia, though this record has since been far surpassed. <!--<ref>http://www.environment.act.gov.au/__data/assets/pdf_file/13077/183.pdf {{dead link|date=October 2007|url=http://www.environment.act.gov.au/__data/assets/pdf_file/13077/183.pdf}}</ref>--> Other specific geological places of interest include the State Circle cutting and the Deakin anitcline.<ref>{{Citation |url=http://www.environment.gov.au/cgi-bin/ahdb/search.pl?mode=place_detail;search=place_name%3DState%2520Circle%2520cutting%3Bkeyword_PD%3Don%3Bkeyword_SS%3Don%3Bkeyword_PH%3Don%3Blatitude_1dir%3DS%3Blongitude_1dir%3DE%3Blongitude_2dir%3DE%3Blatitude_2dir%3DS%3Bin_region%3Dpart;place_id=13321 |title=State Circle Cutting, State Ccl, Parkes, ACT, Australia &mdash; Statement of Significance |publisher=Australian Heritage Database |accessdate=2007-10-11}}</ref><ref>{{Citation |url=http://www.environment.gov.au/cgi-bin/ahdb/search.pl?mode=place_detail;search=place_name%3DDeakin%2520anticline%3Bkeyword_PD%3Don%3Bkeyword_SS%3Don%3Bkeyword_PH%3Don%3Blatitude_1dir%3DS%3Blongitude_1dir%3DE%3Blongitude_2dir%3DE%3Blatitude_2dir%3DS%3Bin_region%3Dpart;place_id=13312 |title=Deakin Anticlines, Macgregor St, Deakin, ACT, Australia &mdash; Statement of Significance |publisher=Australian Heritage Database |accessdate=2007-10-11}}</ref> [[Image:LimestoneOutcropActonACT.JPG|thumb|Limestone outcrop at Acton]] The early European name for the district was "Limestone Plains". In 1820, following the discovery of [[Lake George, New South Wales|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 (constable)|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: <blockquote>... 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. ... <ref>{{Citation |url=http://www.abs.gov.au/Ausstats/abs@.nsf/0/4e782ee652d0db0aca2569de001fb2dd?OpenDocument |title=1301.0 - Year Book Australia, 1931 &mdash; Special Article - Canberra Past and Present |publisher=Australian Bureau of Statistics |accessdate=2007-10-11}}</ref> </blockquote> There is, however, little limestone evident at the surface in the district. There is an outcrop at [[Acton, Australian Capital Territory|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 [[orogeny|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. ==Tectonic context== [[Tectonics]] explains the large-scale structure of the Earth's crust and its constituent rocks in terms of blocks moving along [[fault]]s, uplifted into [[Horst (geology)|horst]]s or downthrown into [[graben]]s. The ACT is positioned on the [[Australia (continent)|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 onto 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 Tectonics |plate tectonic]] movements. The Tasmanides also extended into [[Antarctica]] in the south and northern [[China]] on the north, as these continental units were attached to Australia at the time, in Gondwana.<ref>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 </ref> 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. ===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== ===Ordovician=== 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 (Australia)|Victoria]], and western [[Tasmania]]. This happened between 520 to 480 million years ago up to the [[Cambrian]] Era. No associated ocean floor has been preserved by obduction in the ACT. During the [[Palaeozoic]] Era at least a few thousand kilometres of ocean floor were [[subduction|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 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 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 (Australian Capital Territory)|Black Mountain]] [[Sandstone]] deposited in a marine environment as [[turbidite]]s. 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, New South Wales|Williamsdale]]. Then came a pause in volcanism at the start of the Upper Silurian with the [[Yarralumla]] Formation and [[Yass, New South Wales|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 River|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 Era. The sediments were deeply buried by being compressed and faulted down. Melting occurred in deep sediments and in the [[basalt]]ic [[oceanic crust]] beneath. The [[magma]]s mingled and intruded upwards. The Murrumbidgee Batholith was formed, with several intrusions. The faults were reversed and the [[granite]]s from the batholith 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. 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 [[Sericite|sericitisation]], [[Saussurite|saussuritisatio]]n, conversion of [[plagioclase]] to [[albite]], and conversion of [[biotite]] to [[chlorite]], [[sphene]], [[epidote]] and opaque minerals. On the western margin in southwest [[Belconnen]], [[Duffy, Australian Capital Territory|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. [[Image:TescheniteDyke1.jpg|thumb|left|250px|Dyke of teschenite]] A [[Dike (geology)|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, New South Wales|Southern Highlands]] areas. This uplift reactivated the Murrumbidgee Fault, the Queanbeyan Fault and the [[Lake George, New South Wales|Lake George]] Fault. ==Mining== Minor [[mining]] operations have occurred historically throughout the region, both for precious metals including [[gold]], [[silver]], [[lead]], and [[copper]], and for construction materials. However the only commercial operations to continue at the present day are the Stockmans [[Quarry]] at [[Pialligo, Australian Capital Territory|Pialligo]] which excavates Camp Hill Sandstone, and a large Quarry on Mt Mugga Mugga which excavates Mugga Mugga Porphyry for use as construction [[gravel]] eg. for road surfaces, and in [[concrete]]. ==Lithology== ===Ordovician=== {| style="float:left" align="top" | {| class="wikitable" ! colspan = 2 | Pittman Formation Chemical Analysis |- | oxide |% or ppm |- | SiO<sub>2</sub> | bgcolor="FFFF99" | 71.23% |- | TiO<sub>2</sub> | bgcolor="FFFF99" | 0.64% |- | Al<sub>2</sub>O<sub>3</sub> | bgcolor="FFFF99" | 13.76% |- | FeO | bgcolor="FFFF99" | 4.74% |- | MnO | bgcolor="FFFF99" | 0.05% |- | MgO | bgcolor="FFFF99" | 1.90% |- | CaO | bgcolor="FFFF99" | 0.31% |- | Na<sub>2</sub>O | bgcolor="FFFF99" | 1.02% |- | K<sub>2</sub>O | bgcolor="FFFF99" | 3.54% |- | P<sub>2</sub>O<sub>5</sub> | bgcolor="FFFF99" | 0.13% |- | Rb | bgcolor="FFFF99" | 168ppm |- | Sr | bgcolor="FFFF99" | 64ppm |- |} |} ;Pittman Formation The Pittman Formation was described originally by Öpik in 1958 who named it after the Pittman Valley, southeast of [[Aranda, Australian Capital Territory]]. It is entirely of Ordovician age and about 800 metres thick near Canberra, but at [[Captains Flat]] it is over 1,200 metres thick. The lower levels are [[greywacke]], exposed east of [[Queanbeyan, New South Wales|Queanbeyan]] 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 minerals|argillaceous]] and radiolarian [[chert]]. In the sandstone beds there are occurrences of graded [[bedding]], clay pellets, and current bedding. Fossils of [[graptolite]]s, [[radiolarian]]s, [[conodonts]], and occasionally [[brachiopod]]s and [[sponge]]s 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, Australian Capital Territory|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 metres 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. Chemical analysis reveals low concentrations of [[sodium]], [[calcium]] and [[strontium]] because of the low [[feldspar]] content. The Ordovician [[turbidite]]s, 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]]. 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.<ref>''Crustal Evolution in Southeastern Australia: A Zircon Viewpoint'' Ian S. Williams and Bruce W. Chappell </ref><ref>Basement of the Lachlan Fold Belt: the Evidence From S-Type Granites B.W. Chappell, GEMOC ANU</ref> At the time these sediments were deposited the location was at least 3,000 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 appears 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 [[sea sponge|sponge]]s [[fossil]]s are rarely found. The beds are up to 60 metres 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 The State Circle Shale was named by Öpik in 1958. It is named after the street where it was described. Its age is [[Llandoverian]]. Its lithology is shale, mudstone, siltstone and minor sandstone. In the type location there is about 60&nbsp;metres 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 metres thickness of this shale. The shale was deposited in the deep sea as turbidites. It can be found in Yarralumla, [[Parkes, Australian Capital Territory|Parkes]], [[Acton, Australian Capital Territory|Acton]], north and south of [[Black Mountain (Australian Capital Territory)|Black Mountain]], and from [[Lawson, Australian Capital Territory|Lawson]], to [[Crace, Australian Capital Territory|Crace]] and [[Ngunnawal, Australian Capital Territory|Ngunnawal]]. <ref>[http://www.environment.gov.au/cgi-bin/ahdb/search.pl?mode=place_detail;search=place_id%3D105733%3Bkeyword_PD%3Don%3Bkeyword_SS%3Don%3Bkeyword_PH%3Don%3Blatitude_1dir%3DS%3Blongitude_1dir%3DE%3Blongitude_2dir%3DE%3Blatitude_2dir%3DS%3Bin_region%3Dpart;place_id=105733 Department of Environment and Heritage]</ref> ;Black Mountain Sandstone Black Mountain Sandstone is deposited on top of State Circle Shale conformably. 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 Öpik after the mountain - [[Black Mountain (Australian Capital Territory)|Black Mountain]] where it is found. Originally it was believed to be Ordovician, but is actually from the [[Silurian]] Era, late [[Llandoverian]]. 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 Tidbinbilla Quartzite 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 metres thick. Low down there is a 2 metre 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, Australian Capital Territory|Duntroon]]. At the time these were the oldest fossils discovered in Australia, though this record has now been far surpassed. [http://www.environment.act.gov.au/Files/heritageregister20010.pdf] These fossils were from the Canberra Formation. In the past these rocks were known as the Canberra Group with components of Turner Shale, Riverside Formation, and City Hill Shale. Canberra Formation can be found in the east part of South Canberra in [[Fyshwick, Australian Capital Territory|Fyshwick]], [[Kingston, Australian Capital Territory|Kingston]], [[Barton, Australian Capital Territory|Barton]] and [[Parkes, Australian Capital Territory|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, Australian Capital Territory|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 redish 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. ;Hawkins Volcanics Hawkins Volcanics is a green grey dacite or dacitic tuff. These volcanics contain chloritised cordierite and some have garnet. Age is Ludlow to Wenlock. The volcanics occur in northern Belconnen, in Dunlop Frase, Spence, Mt Rogers, Flynn, Melba and Hall. ;Ainslie Volcanics {| style="float:left" align="top" | {| class="wikitable" ! colspan = 3 | Ainslie Volcanics oxide & element chemistry |- | order | oxide |% or ppm |- | 1 | SiO2 | bgcolor="FFFF99" | 66.8% |- | 2 | Al2O3 | bgcolor="FFFF99" | 14.28% |- | 3 | FeO | bgcolor="FFFF99" | 3.42% |- | &nbsp; | Fe2O3 | bgcolor="FFFF99" | 0.79% |- | 4 | K2O | bgcolor="FFFF99" | 3.8% |- | 5 | CaO | bgcolor="FFFF99" | 2.61% |- | 6 | H2O | bgcolor="FFFF99" | 2.5% |- | 7 | MgO | bgcolor="FFFF99" | 2.15% |- | 8 | Na2O | bgcolor="FFFF99" | 2.05% |- | 9 | TiO2 | bgcolor="FFFF99" | 0.59% |} |} Named Named after [[Mount Ainslie, Australian Capital Territory]] where their type location occurs, the Ainslie Volcanics are composed of [[Dacite|Dacitic]] [[ignimbrite]] and minor volcaniclastic and argillaceous [[sediment]]s. The litholgy 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 [[pellitic]] 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 Namina Hill on Spring Range, the Mount Painter Volcanics lie disconformably on top. The age name for these rocks is Late [[Wenlock]]. 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, Australian Capital Territory|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. ;Mount Painter Volcanics This was named after Mount Painter in Canberra by Öpik, however he called it a porphyry<ref>A. A. Öpik, ''The Geology of the Canberra City District'', chapter in ''Canberra, a nation's Capital'', ed H. L. White 1954 [[Angus and Robertson]], p143</ref>. The description is a massive dark bluish-grey dacitic crystal tuff containing garnet and chloritised cordierite. There is a local appearance of agglomerate and pumice. There are prominent quartz and feldspar phenocrysts. 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]]. ;Yarralumla Formation In an intervening phase in volcanism predominantly sediments were deposited. Yarralumla Formation was named after the suburb of [[Yarralumla]] by Öpik in 1958. It consists mainly of mudstone which may be cemented by lime, or originally derived from tuff. There are some beds inserted of quartz sandstone or limestone. The bottom of the formation is on top of the Mount Painter Volcanics. The top of the formation grades up into pyroclastics from the Deakin Volcanics.<ref>[http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=20879 Geoscience Australia database record]</ref> 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 subarial 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, Australian Capital Territory|Red Hill]] and [[Woden Town Centre|Woden]] in the South to [[Yarralumla, Australian Capital Territory|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 [[trilobite]]s, [[coral]] and primitive [[crinoid]]s. The Yarralumla brickworks quarry and the [[Deakin, Australian Capital Territory|Deakin]] anticline are places where the formation is exposed and easily studied. 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<ref>http://www.tams.act.gov.au/__data/assets/pdf_file/0016/13363/78.pdf ACT Heritage Register</ref> - with pale brown siltstone; and also at the Yarralumla brickworks<ref>http://www.aussieheritage.com.au/listings/act/Yarralumla/YarralumlaBrickpits/2476 brickpits heritage</ref> with olive-green calcareous mudstone. ;Yass Subgroup Dated early Ludlow to Wenlock. This may outcrop in Belconnen and Florey. The composition is Calcareous and tuffaceous [[shale]], [[sandstone]], [[ashstone]] and [[limestone]]. These volcanics lack [[cordierite]] or [[garnet]].<ref>[http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=27031 Geoscience Autralia stratigraphic database record for Yass Subgroup]</ref> ;Deakin Volcanics {| style="float:left" align="top" | {| class="wikitable" ! colspan = 3 | Deakin Volcanics chemistry |- | order | oxide |% or ppm |- | 1 | SiO2 | bgcolor="FFFF99" | 72.99% |- | 2 | Al2O3 | bgcolor="FFFF99" | 13.33% |- | 3 | K2O | bgcolor="FFFF99" | 3.78% |- | 4 | H2O | bgcolor="FFFF99" | 2.98% |- | 5 | CaO | bgcolor="FFFF99" | 1.73% |- | 6 | FeO | bgcolor="FFFF99" | 0.9% |- | &nbsp; | Fe2O3 | bgcolor="FFFF99" | 0.53% |- | 7 | CO2 | bgcolor="FFFF99" | 1.6% |- | 8 | Na2O | bgcolor="FFFF99" | 1.12% |- | 9 | MgO | bgcolor="FFFF99" | 0.49% |- | 10 | TiO2 | bgcolor="FFFF99" | 0.22% |- | 11 | P2O5 | bgcolor="FFFF99" | 0.06% |- | 12 | Ba | bgcolor="FFFF99" | 540 ppm |- | 13 | MnO | bgcolor="FFFF99" | 0.03% |- | 14 | Rb | bgcolor="FFFF99" | 210 ppm |- | 15 | Zr | bgcolor="FFFF99" | 125 ppm |- | 16 | Sr | bgcolor="FFFF99" | 100 ppm |} |} [[Image:DVbandedTuff.jpg|thumb|right|Banded Tuff from the Deakin Volcanics]] [[Image:Deakin Volcanics Green rhyodacite.jpg|thumb|right|Deakin Volcanics rhyodacite modified to green colour]] [[Image:Deakin Volcanics red tuff.jpg|thumb|right|Deakin Volcanics red tuff]] The [[Deakin, Australian Capital Territory]] Volcanics can be seen in the road Cutting along the [[Tuggeranong Parkway]] between Hindmarsh Drive and Cotter Road. The visible base shows a weathered repeating sequence of [[interbedded]] [[Rhyodacite|rhyodacitic]] [[ignimbrite]], [[sandstone]], [[siltstone]] and red and yellow [[shale]]. Southwards the section passes up into a massive and partly banded rhyodacitic ignimbrite. The bottom of the Deakin Volcanics is exposed on nearby Heysen St. At least 400m thickness of beds are exposed in the Parkway cutting. The [[Tuggeranong, Australian Capital Territory|Tuggeranong]] area has the thickest deposits. Rock types in the Deakin Volcanics are Rhyodacitic ignimbrite, [[lava]] (Mugga Mugga Porphyry Member), [[tuff]], tuffaceous shale and minor quartz sandstones and volcanic [[breccia]] - units show reddish brown alteration. The unit is of Early Ludlovian age. The rock on Mount Rob Roy and Pemberton Hill used to be known as Tuggeranong Granite, but is actually ignimbrite. The ignimbrite forms escarpments, with the lower lying land being underlain by tuff. The Deakin Volcanics can be found south of the Deakin Fault between Belconnen and Charnwood and MacGregor. It also is found through Weston Creek north of Chapman and Fisher, through Woden apart from Curtin, and across to Hume, and south through Tuggeranong. The different colours found in the rocks is due to weathering, the red being from [[hematite]], and the green from clay minerals such as [[celadonite]]. The pink crystals are potash feldspar. 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 [[phenocryst]]s 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]] introduced some veins and [[saccharoidal]] [[galena]]. ;Laidlaw Volcanics [[Image:RedRockIgnimbrite.png|thumb|right|Ignimbrite from the Red Rocks Gorge has a strong red colour]] The Laidlaw volcanics are a [[Rhyodacite|rhydacitic]] [[ignimbrite]] in the form of pale to dark grey rhyodacitic to dacitic crystal [[tuff]]. They are the top layer of [[Volcanic]]s in the ACT. The Laidlaw Volcanics occur in a band along the [[Murrumbidgee River]] to the south west of Canberra, and also north west of [[Belconnen, Australian Capital Territory|Belconnen]]. They exist in the suburbs of Latham, Chapman, Kambah, and Greenway. The volcanics are of [[Ludlovian]] age 420.7±2.2 m.y. and have been used to set an absolute date for the [[Ludlow epoch]]. They are up to 850 metres thick.<ref>The Laidlaw Volcanics: a Late Silurian point on the geological time scale by D. Wyborn and M. Owen, W. Compston and I. McDougall in Earth and Planetary Science Letters Volume 59, Issue 1, June 1982, Pages 90-100</ref> 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. ;Porphyrys [[Porphyry]]tic [[intrusion]]s occur throughout the area. Middle [[Silurian]] intrusions could be [[volcanic neck]]s. A coarse green grey [[Rhyodacite|rhyodacitic]] intrusive outcrops over one square kilometre west of [[Holt, Australian Capital Territory|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 Mt 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. ;Glebe Farm Adamellite A coarse porphyritic micro-adamellite was intruded sometime from late Silurian to early Devonian. It intrudes the Hawkins Volcanics in Belconnen in Flynn, Melba, McKellar, the Town Center and Bruce. ;Sutton Granite Also known as the ''Greenwood Granite'', the Sutton Granite is a medium grained [[granite]] intruding the Pittman formation in the hills to the north east of [[Canberra Airport]]. The outcrop covers 4 square kilometres in the ACT. It was intruded during the late [[Silurian]] and is dated at 410+4Mya. The colour is pale grey with mineral white [[feldspar]], milky grey [[quartz]], black [[biotite]], and with [[hornblende]] rich [[xenolith]]s. Minor minerals are apatite and zircon. The surrounding Pittman formation rocks have been [[metamorphosed]] in a [[contact aureole]] to [[hornfels]] and spotted [[schist]]. A magnetic high matches the location indicating the presence of magnetite. ;Adaminaby Beds [[Image:Adaminaby Beds.jpg|thumb|right|Adaminaby Beds outcrop south of the ACT]] The Adaminaby Beds were formed from quartz [[turbidite]] capped with black shale. It consists mainly of fine to medium grained quartz rich sandstone and some mostly thin beds of siltstone, shale and slate. The sediments occur in a band on the west side of the ACT on Bulls Head, Mt Franklin, Mt Ginini and along the upper parts of the Cotter River. A separate outcrop extends into the ACT from the south in the area of [[Gudgenbury River]]. Yet another outcrop is on the Bullen Range. ;Paddys River Volcanics Paddys River Volcanics consist of dacite and tuff with some shale phyllite and limestone. They occur to the west of the Bullen range along the lower parts of the Paddys River. They were deposited on top of the Ordovician Adaminaby Beds, and intruded by Shannons Flat Adamellite. ;Uriarra Volcanics Uriarra Volcanics consists of [[dacite]] [[lava]] flows and [[pyroclastic]] deposits of [[tuff]]. A fine [[ashstone]] bed called Tarpaulin Creek Ashstone Member outcrops in a distorted north south line acts as a marker. Tuff and flows above and below the ashstone member contain obvious pink [[feldspar]] crystals. The tuff shows [[bedding]], and the flows have banded flow structure. The Cotter [[Porphyry (geology)|Porphyry]] to the north of the Cotter Dam is actually a dacite flow. There is a [[limestone]] lens north of Uriarra Crossing. The outcrop goes from Mountain Creek Road in the west to the Murrumbidgee river in the east. It extends a few kilometres to the north of the ACT border and south to the Winslade Fault near the Cotter River. A wedge extends to the south south west including Pierces Creek. ===Late Silurian to Middle Devonian=== ;Murrumbidgee Batholith Formed by melting different sediments to the Ordovician Pittman Formations, as the granites contain more feldspar elements Ca Na K. At Cooma there is migmatite where the sediments have been partially melted in order to produce the plutonic rocks in the batholith. Variations in their composition are explained by partial mixing with melted oceanic crust. Largely crystallised before emplacement, made space by displacement rather than dissolution. ;Clear Range Granodiorite {| style="float:left" align="top" | {| class="wikitable" ! colspan = 2 | Clear Range Granodiorite Mineral Composition |- | Quartz | bgcolor="FFFF99" | 33.9% |- | Microcline K-feldspar | bgcolor="FFFF99" | 8.9% |- | Plagioclase | bgcolor="FFFF99" | 34.2% |- | Biotite | bgcolor="FFFF99" | 16.1% |- | Muscovite | bgcolor="FFFF99" | 1.8% |- |} |} Clear Range Granodiorite covers 475 square kilometres from [[Tharwa, Australian Capital Territory|Tharwa]] to [[Thredbo, New South Wales|Thredbo]] on the west side of the Murrumbidgee batholith on [[Clear Range, Australian Capital Territory|Clear Range]]. Clear Range ridge forms the ACT border in the south east. Described as [[Foliation (geology)|foliated]] with numerous [[inclusion (mineral)|inclusions]]. The inclusions are metamorphosed sediments and are common in biotite. It contains quartz and microcline Feldspar and brown biotite and also muscovite. It has more biotite and plagioclase than Shannons Flat Granodiorite. The muscovite is distinctly foliated and there is also blue quartz. Texture is fine to medium grained. Close to the Murrumbidgee Fault the texture is [[mylonitic]] and the rock is easily weathered. ;Shannons Flat Granodiorite Shannons Flat Granodiorite is poor in xenoliths and is coarse grained. It was intruded after the clear Range Granodiorite. [[Image:ShannonsFlatAdamellite.png|thumb|left|Shannons Flat Granodiorite from the [[Canberra Deep Space Communications Complex|Tidbinbilla]] Tracking Station]] {| style="float:right" align="top" | {| class="wikitable" ! colspan = 2 | Shannons Flat Granodiorite Mineral Composition |- | Quartz | bgcolor="FFFF99" | 36.0% |- | Microcline K-feldspar | bgcolor="FFFF99" | 21.0% |- | Plagioclase | bgcolor="FFFF99" | 34.7% |- | Biotite | bgcolor="FFFF99" | 8.3% |- |} |} {| style="float:right" align="top" | {| class="wikitable" ! colspan = 2 | Tharwa Adamellite Mineral Composition |- | Quartz | bgcolor="FFFF99" | 34.1% |- | Microcline K-feldspar | bgcolor="FFFF99" | 29.7% |- | Plagioclase | bgcolor="FFFF99" | 29.1% |- | Biotite | bgcolor="FFFF99" | 7.1% |- |} |} <br> ;Tharwa Adamellite 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. ;Booroomba Leucogranite Booroomba [[Leucogranite]] is found at Mt Tennant and [[Booroomba Rocks]]. The total outcrop is 52 square kilometres. It is coarsely [[crystalline]] and pale in comparison to other [[granites]]{{Fact|date=June 2007}}. [[Muscovite]] occurs in unweathered rock. This leucogranite intrudes [[Clear Range, New South Wales|Clear Range]] [[Granodiorite]] and [[Shannons Flat, New South Wales|Shannons Flat]] Granodiorite. It is low in [[iron]] and [[calcium]], and higher in [[potassium]] than other Murrumbidgee Batholith intrusions. ;Olivine Teschenite from Red Rocks Gorge [[Image:Teschenite3.png|thumb|left|Black teschenite from the Red Rocks Gorge]] A [[Dyke (geology)|dyke]] of [[teschenite]] occurs in the Red Rocks Gorge near Allens Creek. It is oriented north west and heads towards [[Kambah]] Pool Area. A similar dyke also outcrops on the west side of the Murrumbidgee River near Pine Island. The Dyke is less than 0.5 metres thick. The rock is black showing up against the reds and browns of the Laidlaw Volcanics. The rock consists of microphenocrysts of [[olivine]] altered to [[serpentine]], [[titanaugite]] coloured pinkish brown, [[kaersutite]] [[amphibole]] coloured brown, and [[magnetite]] about 15%, embedded in a colourless [[analcime]] groundmass. Kaersutite contains [[titanium]] and has formula NaCa2(Mg4Ti)Si6Al2O23(OH)2. The dyke is believed to be Tertiary in age. ==Geophysics== In the ACT the [[Moho]] depth is about 46km below sea level. ===Magnetic Field=== 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. At Weston creek [[Magnetic declination]] is 11.757 deg, Field strength is 43045 nT, Inclination = -65.882 deg, rate of change of declination is 0.004 deg/yr, rate of change of field strength is -24 nT/yr, rate of change of inclination is 0.016 deg/yr. ===Gravity Field=== 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&nbsp;[[metre per second squared|μm·s<sup>-2</sup>]]. 1&nbsp;μm·s<sup>-2</sup> is 0.1&nbsp;[[milligal]]s, so this level is -32&nbsp;milligals. Another high of -330&nbsp;μm·s<sup>-2</sup> 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 Mitchel at -350, and at Mulligans Flat at -345. This means that a 50&nbsp;kg person, if measured on a force scale rather than on a balance, would apparently [[human weight|weigh]] 1.3 grams more in Watson, than they would in western Kambah. A sensitive gravity measuring station is positioned on [[Mount Stromlo]]. This can measure changes in the gravity field over time. ===Movement=== 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<ref>[http://rses.anu.edu.au/geodynamics/tregoning/17.pdf P. Tregoning:''Is the Australian Plate deforming? A space geodetic perspective''] Geological Society of Australia Special Bulletin 22, 2003, p 46</ref>. ==Soil== Most soils in the ACT are [[Podzol]]s. They have a duplex structure with red or brown clayey layer. Typical thicknesses are 2 metres. ==References== <references/> ==Bibliography== *Richardson, S.J. and Barron L., 1977 Michelago 1:100000 Geological, Sheet 8726, Geol Surv Sydney *Richardson, S.J., 1976, Geology of the Michelago 1:100000, Sheet 8726 *Canberra 1:250000 Geological Sheet, SI 55-16, 1964 *Strusz, D.L., 1:250000 Geological Series-Explanatory Notes Canberra *Packham, G.H., 1969, The Geology of New South Wales *Henderson, G.A.M. and Matveev, G., Geology of Canberra, Queanbeyan and Environs 1:50000 1980. This was used to determine the rock types in each suburb. *Henderson, G.A.M., Geology of Canberra, Queanbeyan and Environs Notes to Accompany the 1980 1:50000 Geological map. [[Category:Australian Capital Territory]] [[Category:Geology of Australia]]