The geology of New South Wales has shaped the settlement as well as the economic and social development of the state and provided the basis for our agricultural industries.

The history of rocks that form the backbone of New South Wales go back almost 2000 million years. We know less about rocks formed in the first 1000 million years, but rocks deposited during the last 1000 million years reflect four large plate tectonic movements, the break-up of the Rodinian supercontinent 800 to 600 million years ago, complex collision history between the developing Australian plate and the old Pacific ocean plate from 530 to 230 million years, the break-up of the Pangaean and Gondwanan supercontinents (223-65 million years ago) and opening of the Tasman Sea, beginning about 90 million years ago.

We can be more certain about the recent plate movements since the effects on our landscape have not been completely worn away. The main topographic elements that resulted from these movements, i.e. the Great Dividing Range, the low Murray Basin and the western agricultural 'plateau' (average height 300 m), are still present. Drowning of some river valleys by sea-level rises produced estuaries that became modern population centres (e.g. around Sydney Harbour).

However, it is the older rocks that have had the most impact on the economic and social development of NSW, and these rocks even today still contain important deposits of metallic minerals, coal and hydrocarbons that create jobs, especially in regional New South Wales, bring in export dollars and generate wealth for the whole community.

Gold from older lower Palaeozoic rocks (470 to 400 million years old) sustained the large population increases in both Sydney and inland centres from the middle 1800s, and set NSW on its present economic course. Other fabulously rich areas such as Broken Hill with rich deposits of silver lead zinc, in rocks around 1700 million years old, put Australia's manufacturing on a firm base. The mining of coal from the Permian (280-260 million years old) from parts of the Sydney Basin triggered the development of the Lithgow, Hunter and Illawarra regions, led to the production of cheap electricity and added substantially to export wealth. Country towns such as Parkes and Cobar survive periodic downturns in agriculture because they are also thriving centres of mining.

The best way to describe the geology of New South Wales is in terms of plate tectonics, a revolutionary concept from the late 1960s in the USA (and advocated even earlier by Professor Sam Carey at the University of Tasmania) that affected the way geologists thought about the geology of the world.

Using the ages of rocks as starting points (either from fossils or from the decay of radioactive elements) we assign rocks to certain geological ages. The biggest unit is eons (Proterozoic or Phanerozoic), then eras (Palaeozoic or Cainozoic), periods (e.g. Jurassic) and then smaller subdivisions where appropriate. At the same time we try to assign the rocks to a setting according to their place either within tectonic plates or at the boundaries between tectonic plates.

Of course this plate tectonic approach can be speculative and subjective but it is also more fun since it helps to bring old rocks alive and makes geological history more exciting and dynamic. And it might help to find new metal deposits that keep our economy healthy.

How NSW was assembled

The first thing to realise is that NSW is a part of the developing Australian continental plate and that this plate moved around the surface of the earth in response to plate motions which themselves reflect convection systems deep in the earth's mantle. Continental crust is largely a thin (30-50 km thick) rigid skin on top of mantle rocks that are able to flow at rates of 5-10 cm/year (the rate of fingernail growth). Where plates are moving apart, continental crust is stretched. Stretched crust subsides below sea level, and the deposition and accumulation of sedimentary rocks in generally submarine conditions leads to the formation of sedimentary basins. If the crust is stretched even more, it breaks apart. Mantle rocks flow into the cracks to form oceanic crust and as the ocean basin widens, sea floor spreading starts with new igneous crust forming at an oceanic ridge, called a spreading centre.

As oceanic crust spreads away from the ridge it gets older, colder and starts to sink, forming a subduction zone. As plates converge across this zone, one plate sinks (is subducted) beneath the other. Subduction zones are generally marked by lines of volcanoes that form from melting at depth of around 100 km. As tectonic plates come together and eventually collide, oceanic crust is subducted. Continental crust reacts in a different manner, its light density gives it buoyancy, so any piece of continent caught between the colliding plates is scraped off, crumpled up and pushed up into the air to form mountain ranges. This bending, breaking and doubling up of crust produces folds and faults in rocks. Intrusions of granitic rocks form by heat 'cells' deep in the crust that cause rocks to melt; they also start chemical reactions in surface rocks pushed down in the crust where they are changed to metamorphic rocks. Rocks with all these characteristics occur in orogenic belts. Most of the mountain building in New South Wales was over before dinosaurs appeared (about 200 million years ago) so that in some areas orogenic belts have been covered by younger Mesozoic and Cainozoic sedimentary basins.

NSW geology from oldest to youngest

Precambrian supercontinent

The oldest rocks in NSW occur in the NW corner of the state around Broken Hill, and are placed into the Willyama Supergroup. These rocks represent sediments and volcanic rocks deposited in one or more rift basins about 1700-1600 million years ago. The rocks were strongly deformed, bent, broken and piled up on top of themselves around 1600 million years ago, but we don't know why. We do know that the rocks now at the surface at Broken Hill were originally buried to 12-20 km (forming rocks like those we believe occur in the middle to lower crust).

These rocks host the fabulously wealthy Broken Hill silver-lead-zinc deposit, and mining profits since 1883 have underpinned much of the mineral industry and other economic development in Australia.

These Proterozoic rocks formed part of the Rodinian supercontinent that was a forerunner to the well-known supercontinent called Gondwanaland that existed for much of the Palaeozoic (540-250 million years ago) and which fleetingly became part of Pangaea around 320 million years ago. In the Rodinian supercontinent North America lay east of Australia and Antarctica. It was assembled around 1100 million years ago but started breaking up about 800 million years ago. It lasted a mere 300 million years!

Breaking up supercontinents - the Delamerian Orogen

The break-up of Rodinia is recorded in the rocks that now form the Late Proterozoic to Cambrian (1000 to 490 million years old) Delamerian Orogen. This orogenic belt mainly occupies the eastern half of South Australia where the Adelaide Rift Complex contains mixtures of sandstones, conglomerate, shales, limestone glacial deposits and local volcanic rocks that record rift and sag phases of crustal extension.

The eastern part of the Delamerian Orogen extends into far western New South Wales where rocks north of Broken Hill form part of the Adelaide Rift Complex. These rocks were deposited in shallow water while to the east there was deeper water in which felsic and mafic volcanic lavas accumulated at the edge of the Australian plate. Similar rocks occur to the south in western Victoria and are also found in parts of Antarctica, which at that time lay south of Kangaroo Island.

The break-up of Rodinia took around 200 million years (from 800 to 600 million years ago). Stretched crust thinned to form rift basins, some with volcanics and glacial deposits in them, and thinned even more to give way to oceanic crust, resulting in sea-floor spreading.

Sea floor spreading changed to subduction about 530 million years ago and a convergent plate boundary formed with development of an east dipping subduction zone and formation of Cambrian island arc volcanics and associated rocks. Although these are best represented in western Victoria, they are also inferred from aeromagnetic data to underlie the Murray Basin south-west of Broken Hill.

Rocks of the Delamerian Orogen were deformed (the mountain-building phase of the Orogen took place) near the end of Cambrian around 490 million years ago when the island arc collided with the older Broken Hill forming the eastern margin of the Australian continent. This deformation caused the shovelling together of rocks so they were folded, faulted, became metamorphosed and intruded by granites.

Palaeozoic plate interactions with the Proto-Pacific Ocean - the Lachlan Orogen

Mountains formed by the Delamerian deformation event extended from western New South Wales westwards into South Australia and then southwards into Antarctica. Their erosion shed vast amounts of mud and quartz-rich sand into ocean basins to the east where they covered Cambrian basalts and other rocks that formed the oceanic igneous crust. These sediments are now preserved as the widespread Early to Middle Ordovician (490-460 million year old) turbidites that occupy much of the Lachlan Orogen.

Andesites and basalts (volcanics) are part of the Ordovician story too. Destruction of the Cambrian subduction zone caused a new subduction zone to form hundreds of kilometres to the east and a new island arc system (the Macquarie arc), best seen in central and southern New South Wales, developed above a west-dipping subduction zone. Several phases of volcanism are present in the arc from earliest to latest Ordovician (490-440 million years ago). Breaks or quiet periods in volcanism are marked by the formation of tropical limestone reefs. Arc volcanism died out at the end of the Ordovician with the intrusion of monzonites ('granodiorites') before the plate tectonic movements in the Early Silurian (around 435 million years ago) caused the arc to collide with the back arc basin turbidites, resulting in the major Benambran deformation that caused the folding and faulting of older rocks and generation of new granite magmas.

The Macquarie arc is a world-class porphyry copper-gold province. Monzonite in the arc host gold-copper deposits at Cadia, Northparkes, Lake Cowal, Browns Creek and major mineral accumulations at Cargo and Copper Hill. There is considerable potential for more discoveries.

This deformation ended the first stage in the development of the Lachlan Orogen.

Because the Australian and proto-Pacific plates were still converging, a new subduction zone was formed several hundred kilometres to the east (off the present coastline) after the Benambran deformation event. For the remainder of its history, the Lachlan Orogen was in a back arc position. (Note that a different story took place in the New England area.)

Back arc areas commonly undergo extension, and from the Middle Silurian (425 million years ago) to middle Devonian (470 million years ago), the Lachlan Orogen was largely in extension except for small periods of local convergence and deformation.

Rifling of the old crust led to formation of sedimentary basins and mixed sedimentary plus volcanic filled basins, as well as the emplacement of large amounts of granites, some hosting tin and gold deposits. The Macquarie arc was rifled into several belts, separated by rift-sag basins. These rifts were closed by the mid-Devonian deformation (the Tabberabberan) which marks the end of the second stage in development of the Lachlan Orogen.

Mid Silurian to Mid Devonian extension in the Lachlan Orogen also coincides with two major metallogenic episodes. Mixed sedimentary-volcanic filled basins formed a major volcanic province with numerous volcanic hosted metal sulphide deposits. Deposits with strong structural controls occur at Captains Flat, Woodlawn, and nearby Currawang, Mt Bulga and Lewis Ponds. Other deposits occur in the Hill End, Mt Hope and Rast troughs. Elsewhere, sediment-rich basins, such as the Cobar Basin in the central part of the state, also contain several large, structurally controlled deposits.

The Tabberabberan deformation was followed by the localised development of rift basins with volcanic fill. However, the remainder of the Devonian period was mainly typified by the deposition of fluviatile (river and floodplain) sedimentation. The enigmatic but widespread early Carboniferous Kanimblan deformation event, possibly related to events in the New England Orogen to the north, followed, and was itself followed by emplacement of the post-tectonic granites of the Bathurst Batholith.

Thomson Orogen

The Thomson Orogen lies north of the Lachlan Orogen and extends north into central Queensland. In NSW the orogen has an east-west orientation and although most of it is covered by younger sedimentary sequences approximately 30% of units making up the orogen are less than 300 metres from the surface. The Thomson Orogen is currently considered a NSW new frontier.

Recent work on the Thomson Orogen suggests that it may have had a similar tectonic history to that of the Lachlan Orogen to its south, as it also contains Ordovician oceanic island basalts and turbidites, Mid Silurian to Mid Devonian rock packages and possible Late Devonian basins. Recent drilling has found calc-alkaline andesitic rocks in the Bourke region with a subduction-related geochemical signature. Airborne magnetic data and ground acquired gravity data are suggestive of an arcuate amalgamation of these rocks divided into smaller geophysically distinct domains interpreted as deformed volcanic belts which are variably crosscut by elliptical domains interpreted as younger intrusive complexes. Seismic reflection data suggests that the faulted contact of the Lachlan and Thomson orogens extends to, and significantly offsets the Mohorovicic discontinuity. The seismic data shows a north-dipping fault with the Thomson Orogen to the north overiding the Lachlan Orogen to the south. The fault also truncates a Late Devonian sequence hence it can be concluded that the fault was active since Late Devonian times.

New England Orogen

The multi-phase history of the New England Orogen began with the fragmentary Cambrian to Ordovician convergent margin volcanic and volcaniclastic rocks as well as disrupted Cambrian ophiolites and Ordovician blueschists. The second, ?Silurian to Early Devonian phase was marked by plate convergence between the Australian plate with the proto-Pacific plate, causing the mix of intra-oceanic arc and accretionary prism rocks (a block of rocks trapped between the plates, in the subduction zone).

More is known about the third phase, when the New England Orogen was the site of a Late Devonian (370-355 million year old) continental margin arc of mafic character, above a west dipping subduction zone, passing eastwards into a forearc basin and accretionary prism. The arc continued into the Carboniferous (to about 325 million years old), but changed in composition to produce more felsic lavas and volcaniclastic rocks.

Multiple deformation, metamorphism, and emplacement of granites (some antimony-bearing) occurred in the accretionary complex rocks in the Late Carboniferous to Early Permian, and the forearc belt started to be deformed into a fold and thrust belt. The Peel Fault System, between the subduction complex rocks and the forearc basin, became a major fault containing Cambrian ultramafics, with scattered occurrences of asbestos, chromite and gold.

In the Early Permian (around 300 million years ago), convergence along this plate margin changed into extension, coupled with strike-slip faulting. This led to the formation of small rift basins (some with felsic volcanic-hosted mineralisation, such as at Halls Peak), and formation of a major back arc rift basin that formed the early stage of the Sydney and Gunnedah basins. Renewed plate convergence in the Late Permian to Triassic led to volcanism, formation of epithermal gold and base metal deposits and emplacement of granites.

Sydney and Gunnedah basins

The Sydney and Gunnedah basins lie between the Lachlan and New England Orogens. Starting as back-arc rifts in the earliest Permian, they developed into foreland basins. Most of their fill was generated by uplift in the New England Orogen and this alternated with lesser fill coming from the Lachlan Orogen to the (south)west. Westward migrating deformation persisting until the mid-Triassic gradually converted these basins into west-verging foreland fold and thrust belts. Coal deposition was associated with fluviatile events and led to the formation of thick Permian seams of bituminous black coal. Natural gas has been found from basin sediments and there is considerable potential for extraction of coal seam methane.

Permian coal deposits also occur in the southern part of the state, in successor basins developed above the Delamerian and Lachlan Orogens, and preserved as infra basins below the younger Cainozoic Murray Basin.

Mesozoic basin formation

The Mesozoic Era lasted from 250 to 65 million years and was a period of worldwide rifting and basin formation as the supercontinent Pangaea broke up.

In the north-eastern corner of New South Wales, the Clarence-Moreton Basin developed in several stages - from Triassic oblique extension - to a Jurassic sag basin - into an Early Cretaceous half rift. The movements resulting from extension associated with early stages of opening of the Tasman Sea. Basin inversion (uplift and closure) occurred in the mid-Cretaceous. Triassic and Jurassic coal and oil and gas shows are present, and there is considerable potential for the extraction of methane from coal seams.

Further west, Jurassic and Cretaceous rocks were deposited in the Great Australian Basin which covers older rocks in the northern inland of New South Wales, and adjoining areas of Queensland and South Australia. Two subdivisions of the basin, the Eromanga and the Surat basins, lie partly in New South Wales. Both range in age from Early Jurassic to (Late) Cretaceous and contain low grade coal deposits and rich oil and gas fields in the adjacent states. These basins are under-explored in NSW but contain promising gas discoveries. Mid-Tertiary weathering led to formation of valuable deposits of opal.

Cainozoic basin formation and tectonics

About 90 million years ago, the Tasman Sea between Australia and New Zealand began to open by seafloor spreading. The western edge of this rift basin was tectonically and thermally uplifted to form the Great Dividing Range and there were outpourings of large volumes of intraplate basalts and other volcanic and volcaniclastic rocks which contain economic diamonds and sapphires. Rivers draining eastwards to the north coast of New South Wales led to formation of valuable beach sands sourced from granites of the New England Orogen.

Formation of the broad shallow Cainozoic Murray Basin is also linked to this uplift. The New South Wales part of the Murray Basin developed on a basement of Palaeozoic rocks and above narrow elongate infra basins that contain rocks of Devonian to Cretaceous age. Basin sediments are generally undeformed, and contain bituminous coal deposits and concentrations of heavy minerals, and oil and gas.

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