Australia: The Land Where Time Began

A biography of the Australian continent 

Westward Subduction Beneath Eastern Australia

490-458 Ma.

Early Ordovician. The Delamerian-Ross-Beardmore Orogen was in low latitudes during the Ordovician. In this tropical situation, the orogen was rapidly denuded, labile minerals and rock fragments forming large volumes of sediment downslope to New Zealand-Australia. Westward subduction beneath eastern Australia generated a 30,000 km long volcanic arc stretching to the north in front of a marginal basin. The Delamerian Highlands shed quartzose turbidite to the east (transversely) and the volcanic arc shed volcanogenic sediment. Vast abyssal fans were formed, comparable to the modern Bengal fan, mostly from turbidite shed northward (longitudinally) from Antarctica along the fore-arc basin and abyssal plain. The Larapinta Sea crossed  the interior, behind the shelf, to the western (Tethyan) shelf margin. Along the Pacific margin of Antarctica and Africa, shelf quartzose sand was deposited.

Marginal basins close

The Benambran Highland and related highlands formed during the Early Silurian as a result of the closure of the marginal basins of eastern Australia. At about 460 Ma, the  cooling of granulite in the Harts Range and Strangways Range and the coarse sediment of the Carmichael Sandstone and the Mereenie Sandstone indicate uplift in central Australia associated with the Rodingan Movement. The Sahara Formation was deposited in a restricted sea in the Canning Basin, and the Ajana Formation, of shallow-marine origin, in the Carnarvon Basin, that was the first marine deposit in this basin. In the Bonaparte Basin, salt was deposited some time in the Late Ordovician-Middle Devonian lacuna, 460-382 Ma, the salt structures piercing the sediments from the Late Devonian and later sediments. Shelf deposition is indicated by reworking of conodonts and shelly fossils in New Guinea.

Parts of Tasmania were covered by a shallow sea to the south of the intra plate tear. Metamorphism terminated turbidite deposition in New Zealand. This metamorphism also affected Marie Byrd Land, where calc-alkaline granite was emplaced, and the Crashsite Sandstone was deposited in the Ellsworth Mountains in the Antarctic Peninsula.

418-394 Ma

Early Devonian. Marine fossils in South Africa, Ellsworth Mountains, Ohio Range, and non-marine fossils in the Pensacola Mountains, Beardmore area and South Victoria Land, indicate a shelf along the Pacific margin. Marine fossils of this age were deposited on the Buller Terrane and the Takaka Terrane in New Zealand that amalgamated during the Tuhua Orogeny of the earliest Devonian, about 415 Ma. At about 400 Ma, northeast and west Tasmania amalgamated. In between, a west-dipping subduction slab underlay North Victoria Land. Magmas rising into the suture zone of the Bowers Terrane between the Wilson Terrane and the Bay Terrane were generated from this slab. Re-location of the trench in southeast Australia was accompanied by granite emplacement and wide back arc extension. The Adavale Basin was initiated with the Gumbardo Andesite. To the north, Plutons flanked marine deposits, inner shallow and outer deep deposits. The Tandalgoo aeolian deposits and playa deposits in the Canning Basin and the redbeds of the Kopke Sandstone, lay to the west, past a group of non-marine deposits in the centre.

394-362 Ma

Middle and Late Devonian. Apart from the eruption of the older Admiralty intrusives in North Victoria Land during the continued westward subduction, there is no change to the South African and Antarctic margin. In Tasmania, similar rocks were intruded, as well as in Victoria and New South Wales, CVMP (Central Victorian Magnetic Province) including, after the Tabberabberan Orogeny, volcanic rifts. Behind the Baldwin volcanic arc, in the Barka Basin, Lambie Basin and Hervey Basin, fluvial sediment accumulated, and in the fore-arc basin and the accretionary prism, that extended north as the Connors-Aubern arc and associated complex,

Following uplift of the Arunta Block in Central Australia, alluvial fans of the Pertnjara Group were shed. Local folding and erosion along the northern margin of the Amadeus Basin climaxed with 5 km of uplift. The overthrusting Musgrave Block shed fans in the Munyari Trough. Flows from central Australia through the Adavale Basin, that had been to the east, changed to the north, parallel with the Connors-Auburn arc in the Drummond Basin. In the latest Devonian, sediments that had accumulated in the Broken River Embayment were deformed.

The Goulburn Graben was initiated in the north. At the foot of the Cockatoo Fault, in the Bonaparte Basin, alluvial fans were deposited in the Frasnian. These were followed by the Ningbing reef complex of the Famennian. Southward from the Halls Creek Fault, sand was deposited in the Ord Basin. Gravel fans from the Kimberley Block mixed with a reef complex on a platform in front of a deep axis on the north side of the Fitzroy Trough, in the Canning Basin. To the south, the paralic Tandalgoo Formation was deposited in an arm of the sea. In the Carnarvon Basin, shelf limestone and inshore quartz sand give way to reef-like facies in Pandock-1.

Sources & Further reading

Veevers, J.J., 2001, Atlas of Billion-year earth history of Australia and neighbours in Gondwanaland, GEMOC Press Sydney.

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Last Updated 27/08/2010 

 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading