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.
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.ACT Heritage Council: Woolshed Creek Fossil Site 2004 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.
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. ...
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.
Tectonic context 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.Plate tectonic processes in the southwest Pacific: a spatial and temporal context Barry Drummond, B. L. N. Kennett, R. J. Korsch, B. R. Goleby & P. A. Symonds Penrose Conference March 1999
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 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.
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 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.
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 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.
Mining
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".