Geology of the Curnamona Craton Geology of the Delamerian Orogen Geology of the Lachlan Orogen Geology of the New England Orogen Geology of the Thomson Orogen

Regional Geology of NSW - Craton and Orogens

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Curnamona Craton

The Proterozoic Curnamona Craton, in western New South Wales, is represented by the Broken Hill and Euriowie Blocks, and is bounded along its eastern margin by the Delamerian Orogen. The cratonic units consist of strongly deformed and metamorphosed sedimentary and igneous rocks termed the Willyama Supergroup. These rocks represent sediments and volcanic rocks deposited in one or more rift basins about 1700–1600 million years ago and subsequently strongly deformed. The rocks now at the surface at Broken Hill were originally buried to 12–20 km depth (forming rocks like those we believe occur in the middle to lower crust) and host the extremely rich Broken Hill silver–lead–zinc deposit. Mining profits from Broken Hill have underpinned much of the mineral industry and other economic development in Australia since 1883.

Delamerian Orogen

The Delamerian Orogen is a major orogenic belt that occupies the eastern half of South Australia and the western parts of Victoria and New South Wales. In New South Wales the Delamerian Orogen is poorly exposed, outcropping in the Koonenberry Belt and as part of the Adelaidean rift sequence flanking the Broken Hill and Eurowie Block. The Stawell Zone, well defined in Victoria, can be traced through into south-western New South Wales on the high resolution magnetic data where it comprises the easternmost structural zone of the Delamerian Orogen. The Stawell Zone of the Delamerian Orogen is entirely under variable thicknesses of cover. On the western side of the Stawell Zone, north of Mildura, the zone has potential for Stawell or Magdala-type orogenic gold deposits under approximately 400 m of cover.

The Adelaidean sequences to the north of Broken Hill record the progressive rifting of continental crust and formation of the proto-Pacific Ocean as the Precambrian supercontinent Rodinia broke up. Rocks of shallow-water origin in the west become deeper water in the east, where they include deformed narrow linear belts of Precambrian and Cambrian sea-floor volcanic rocks. Plate convergence began about 520 million years ago, with formation of a subduction zone and development of island arc volcanism. This tectonic regime was destroyed late in the Cambrian around 500 Ma, when the arcs collided with thinned passive margin of the Australian continent. The resultant Delamerian Orogeny consisted of several phases of deformation from the late Middle Cambrian to Early Ordovician.

Lachlan Orogen

The Lachlan Orogen, east of the Delamerian Orogen, is a broad orogenic belt that records Cambrian to the Carboniferous convergence between the Australian craton and the proto Pacific Ocean. It is not a collisional orogenic belt.

The Lachlan Orogen has a northwest to meridional structural grain. The structural style is one of thick- and thin-skinned thrusting, coupled with strike-slip faulting on major faults. Deformation was episodic, with major events in the early Silurian, mid-Devonian and in the early Carboniferous. Main features of the Lachlan Orogen are as follows.

  • The Ordovician period was largely represented by the deposition of Early to Middle Ordovician quartz-rich deepwater turbidites (and chert) that pass up into Late Ordovician black shales. The turbidites are quartz-rich and are craton-derived, sourced from the uplifted Delamerian Orogen to the west and its continuation in Antarctica, the Ross Orogen. In central and southern New South Wales, Ordovician turbidites and shales are structurally faulted against Early to Late Ordovician intermediate to mafic volcanics that represent the core and fringe of a 1000 km-long intraoceanic island arc that was subsequently split into four structural belts by later extension. This arc hosts major Au–Cu deposits. This stage of convergence between the Australian part of Gondwana and the proto-Pacific plate ended when the arc collided with the back-arc sedimentary rocks in the latest Ordovician to early Silurian. Chert-rich rocks around Narooma and Batemans Bay on the south coast of New South Wales were formed on the floor of the proto-Pacific ocean and represent an exotic terrane that drifted towards, and accreted with, the developing Lachlan Orogen.
  • The Silurian to Middle Devonian stage of development of the Lachlan Orogen was characterised by extension behind a new arc that developed in the northeast area of New South Wales. The orogen underwent rifting, leading to formation of mineral-rich sedimentary basins and mixed sedimentary and volcanic basins, as well as the emplacement of large amount of S- and I-type granites. These rifts were closed by a mid-Devonian deformation.
  • The final stage of development of the Lachlan Orogen was characterised by widespread mid- to Late Devonian fluviatile sedimentation that was preceded by local rifting and localised emplacement of A-type igneous rocks. The rocks were deformed in the early Carboniferous and then intruded by I-type granites in the Mudgee–Oberon region of the state.

Thomson Orogen

The Thomson Orogen lies north of the Lachlan Orogen and extends north into central Queensland. In New South Wales 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 m from the surface. The Thomson Orogen is currently considered a New South Wales 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. These are interpreted as deformed volcanic belts which are variably cross-cut by elliptical domains of younger intrusive complexes. Seismic reflection data suggest that the sutured contact of the Lachlan and Thomson orogens extends to, and significantly offsets, the Mohorovičić discontinuity and also truncates a Late Devonian basin. Hence, it can be surmised that the suture was active since Late Devonian times.

New England Orogen

The New England Orogen is developed east of the Lachlan Orogen, with main growth from the mid-Devonian until the mid-Triassic. From the Devonian to Carboniferous, the orogen developed as a classical orogenic belt, with subduction complex rocks in the east and a forearc basin and Andean arc (now missing) in the west. The Peel Fault, hosting Cambrian ultramafics with asbestos, chromite and gold, separates the subduction complex rocks from the forearc basin. Multiple deformation, metamorphism, and emplacement of granites occurred east of the Peel Fault system in the late Carboniferous to early Permian. In the early Permian, convergence along this plate margin changed into extension coupled with strike-slip faulting. This led to formation of small rift basins and a major back-arc rift basin (that became the basal parts of the Sydney and Gunnedah basins) containing basal mafic and felsic volcanics. Renewed plate convergence in the Late Permian to Triassic led to volcanism, formation of epithermal gold and base metal deposits and emplacement of I- and A-type granites that represent the roots of a new continental margin arc which intruded the former accretionary complex rocks.

Regional Geology of NSW - Sedimentary Basins

Over 60% of New South Wales is covered by sedimentary basins. In the east, the coal and coal seam methane rich Permian-Triassic Sydney-Gunnedah-Bowen Basin system is overlain by the gas-prospective Jurassic-Cretaceous Great Australian Basin, comprising the Clarence-Moreton, Surat and Eromanga Basins. To the west, the largely Devonian gas-prospective Darling Basin is also overlain by the Great Australian Basin and the Cainozoic Murray Basin. A number of older infra-basins, such as the Oaklands Basin, Gilgandra Sub-Basin and Paka Tank Trough are prospective for coal and gas.

Links to the detailed geology of major sedimentary basins in NSW are listed below.

Map showing major NSW sedimentary basins, click to enlarge