Limestone outcrop at Acton
Is a small town in the north of the Australian Capital Territory (ACT), Australia. It was founded in 1882, named after the first landholder in the area, Henry Hall. In 1911 Hall was included within the boundary of the land allocated for the Australian Capital Territory, close to the north western corner, near the New South Wales border. Although considered a village, similar to Tharwa, in practical terms Hall might now be considered an outer suburb of Australia’s capital Canberra.
In 1954 the main street of Hall (which had been part of the state highway system since 1935) was named the Barton Highway. In 1980 the alignment of the highway was shifted to the south as part of a dual carriage upgrade, bypassing Hall completely.
Some features of note within Hall village are the equestrian park, playing fields, showground, and former Hall Primary School, along with a small shopping centre and three historic churches. The showground area plays host to the Hall Markets, a popular monthly charity market showcasing hand-produced goods.
Geology
In the Hall area the rocks are all Silurian age volcanic rocks. Green grey and purple quartz andesite and dacite from the Hawkins Volcanics are under the village and extending up the Halls Creek valley. Green-grey dacite and quartz andesite from the Hawkins Volcanics cover the surrounding areas including up to the New South Wales border, north to One Tree Hill, south east to the Harcourt Hill and south to the north edge of Belconnen. Dark grey to green grey dacitic tuff is found over the other side of the New South Wales border, and also within the Australian Capital Territory near Gooromon Ponds Creek.
Geological history
Ordovician
Dyke of teschenite
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. 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 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 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 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 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
Deakin Volcanics Green Rhyodacite
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 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 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, sphene, 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.
DV banded Tuff
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.
Hall Village, Australian Capital Territory, Australia
Hall is situated on the Northern border of the Australian Capital Territory and named after Henry Hall who was the first resident landholder of the Ginninderra district. Originally from Loughborough in England, he arrived in New South Wales in 1823 and immediately got a job working for the Australian Agricultural Company. Ten years later, he obtained a grant of 3,492 acres in the Ginninderra district. He married Mary Fisher, had four daughters, six sons and named his property ‘Charnwood’ for reasons unknown.
By 1861, settlement in the Ginninderra district had reached an advanced stage. The largest property was owned by William Davis and his wife Susan Adriana. It included homesteads, workers’ cottages, a store and a post office. However, it wasn’t until 1881 that it was unofficially recognised as a village, and surveyor Charles Potter was instructed to survey the area for an official village site. He chose a location two miles away from the established village.
Working with uncharacteristic speed, the new village was proclaimed official in 1882 and named Ginninderra, which was also the name of the unofficial village, suggesting the government of the time wanted to replace the original with their new improved version. Those in the unofficial village complained, and the new village was named Hall, which was far easier to spell. The new village layout was a simple grid, a common layout at the time. Despite the speed of proclamation the first actual sale of allotments didn’t occur until 1886, which was followed by three more sales in 1892, 1895 and 1902. During this time houses and shops were established, saving the residents the two mile walk to the unofficial village.
Hall continued on its merry way until 1911, when New South Wales gave some land to the Commonwealth for a new Capital they had in mind and Hall was included within the boundaries. Just to make it interesting the Barton highway was eventually run through the centre of the village. After World War II, the village went into decline until 1967 when Hall finally obtained a water supply and pre-Yuppies ‘discovered’ it. Then in 1980 they built a bypass around Hall which had much the same effect as sticking a small cute creature in formaldehyde. Periodically, attempts are made to make the village more ‘rural’ and ‘rustic’ by upgrading and modernising the facilities.
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