Australia Geology

Australia Geology

In Australia the main geological regions correspond roughly to the topographical divisions; four are the largest on the continent: a) the peneplane of paleozoic and archaic rocks which occupies almost all of Western Australia, Northern and Central Australia and the north-western part of South Australia; b) the mountainous reliefs of the eastern part, consisting mainly of paleozoic soils with small synclines of Triassic rocks, resulting from an uplift with fractures and weak folds; c) the great Artesian Basin of western Queensland, of the Mesozoic age; d) the Murray Basin, which occupies New South Wales and the state of Victoria, consisting of deposits from the Upper Tertiary and more recent.

Paleogeography. – Australia did not undergo noticeable changes during the Mesozoic and Tertiary, contrary to what happened for the adjacent regions of New Guinea and New Zealand. Sir Edgeworth David thus sums up the history of the continent.

The last retreat of Australia dates back to the Carbonic. The permic rocks of Gympie in southeastern Queensland are steeply tilted but never have tight folds. Large peneplanes formed later, then the sea covered a large area to the east, from Sydney to Townsville, and series of fresh water including substantial amounts of coal were deposited in the synclines. In the Triassic and Jurassic periods, a series of great lakes stretched from Tasmania to central Queensland, and possibly as far as Lake Eyre. During the Upper Jurassic there were huge intrusions of dolerite all over the earth, especially in Australia and Tasmania, and it is possible that during this same period the submersion of the land of Gondwana, which already linked Australia to India, took place. Further major transgressions produced an epicontinental ocean from the Bass Strait to the Gulf of Carpentaria. The fossils of this Cretaceous sea differ greatly from those of the contemporary deposits of the extreme west coast, similar to those of India; it would appear, therefore, that a land barrier divided the eastern Australian waters and those of the western ocean. During the Upper Cretaceous, about half of Australia was covered with lakes, but the region was no longer submerged, except for some southern strip, invaded several times by the Tertiary sea, as evidenced by the marine limestones of the Great Bay and the Lower Murray Valley. The deformation of the great penepian formed in the early Epochs of the Tertiary seems to have occurred mainly during the Pliocene and the Pleistocene: thus the Miocene lands of Adelaide were brought to the height of Mount Lofty (711 m.); large lava effusions occurred at the same time. The last interesting phenomenon was the Pleistocene glaciation, when the area of ​​Mount Kosciusko, above 1800 m., And even lower levels in Tasmania, were invaded by glaciers and suffered their erosive action.

Stratigraphy. – Precambric system. – This formation covers nearly a third of Australia, mainly in the west and center. The shales described by Gibb-Maitland occupy an area of ​​almost 800,000 sq km in Western Australia. In the Kimberley, similar rocks underlie lower Cambrian fossil beds, but the exact age of these shales is not well determined; the Gibb-Maitland divides them into two groups: gneissic and granite rocks, and crystalline schists. All these rocks were more or less irregularly compressed and bent by a general NO.-SE push. Basic rocks are included and, associated with these, gold deposits (Kalgoorlie) meet; amphibolitic and Andalusitic rocks are also common, as well as jaspers, perhaps due to the alteration of silica and limestone. Above the crystalline rocks there is a large series of sedimentary layers which do not present any fossils, but seem to also belong to the Cambrian and which are particularly extensive in the Pilbarra and Nullagine regions in the extreme western portion: they are mostly quartzites, shale and conglomerate clays, crossed by important inclusions of gold-bearing quartz. In Nullagine dolomitic limestones abound with dolerite thresholds. In the rest of the continent, precambricum rocks are not so widespread. It seems that the Petermann Range and the M. Musgrave, in Central Australia, are of this age; in South Australia precambricum rocks are represented mostly by slates and shales, which are found near Port Lincoln, in the Gawler Mountains and also in the Flinders Mountains.

Cambric. – It is very well developed in South Australia. According to Howchin, the Cambrian deposits were deposited in a large syncline and measure from 5 to 6000 m. thick. The layers are very inclined and the lower ones consist mainly of tillites and limestones with a notable series of Lower Cambrian tillites (glacial beds) which appear in a typical way in the Sturt River, near Adelaide: in some places they form a layer of 450 m. thick, full of erratic blocks. Tillite is found over a distance of 700 km. from Cape Jervis almost to Lake Eyre. In the upper Cambric strata of the Flinders Mountains, limestones at Archeocyathinae are noteworthy, formerly coral reefs in the Cambrian sea; they extend throughout the range, but the best preserved fossils are found near Beltana in the north. Precambric strata with Olenellus are found in the Kimberley region and also in Barkley Tableland in Northern Australia. The other main deposits are given by the Heathcotian rocks of Victoria (a series of shale and siliceous clays with Protospongia) and by a series emerging from Broken Hill in New South Wales.

Ordovician. – These lands are mainly found in Central and Southeast Australia. In the Mac Donnell Mountains they occur in limestone schists with Asaphus. In Victoria, the most productive gold fields, at Bendigo, Ballarat and elsewhere, are found in the Ordovician rocks, consisting mainly of slates and sandstones. They also extend into the southern part of New South Wales, where they are accompanied by slate lavas and contain good iron ores in Cadia. In Iron Knob, South Australia, rocks of a similar type are the main sources of iron ore on the continent. Typical fossils are Graptolites, such as Climacograptus and Diplograptus.

Siluric. – It is widespread only in the eastern states of Australia, especially in the southern part of New South Wales, where, in Yass, it is the best known locality for fossils; HalysitesFavositesHeliolites are the most common corals. Limestones are rich in brachiopods (Pentamerus) and trilobites (Phacops). In the same territory there are many other similar limestone soils which present, in various places, beautiful caves (Jenolan, Wombeyan, Wellington); the copper deposits of Cobar and the adjacent districts in the center of the state are associated with rocks of this age. Rocks and similar fossils are found in western Tasmania. especially towards the Gordon River. The lands surrounding Melbourne are also of the same age, and numerous fossils are found in Lilydale. In Queensland, the best known torpedo rocks are found in the NE., At Chillagoe, locations whose fossils closely resemble those found at Yass.

Devonico. – These lands are also found mostly in Eastern Australia, but there is also a good number of them in the Kimberley, especially in the Napier Range. Various species of CyathophyllumPachiporaAtrypaLoxonema and Proetus determine the age of the limestones. In the eastern part of New South Wales, and in particular along the upper Murrumbidgee (near Burrinjuck) and even further north, near Tamworth, there are large areas of lower Devonico rocks, including StromatoporaSyringopora, and Receptaculites are the characteristic fossils. At Mount Lambie (112 km. West of Sydney) there are shale clays containing brachiopods of the upper Devonico, such as Spirifer and Lingula ; Lepidodendron trunks are also not rare, constituting the most ancient flora found in Australia. In the state of Victoria, Buchan Caves are carved out of devonic limestone. In Queensland, compact limestones of the Burdekin series, especially rich in corals such as Pachypora and Stromatoporella, appear to measure 2000 m. thick. For Australia 2011, please check

Carbonic. – In Australia there is no hard coal at all in the soils of this age, which extend in the three eastern states, and especially in eastern Queensland near Townsville (the layers of Star with Lepidodendron), Rockhampton, Gympie and in the Belyando valley; and in New South Wales, especially north of the Hunter River. Sussmilch says they are nearly 6000m thick. and formed of sands, clays and tuffs. Numerous lava flows occurred during this period; the flora is much better preserved than that of the devonic rocks. The most abundant plant is Rhacopteris, but many species of Lepidodendron are also known. The sea contained an abundant fauna of invertebrates, especially Brachiopods such as SpiriferOrthisProductusChonetes. There is also a few trilobites such as Phillipsia and only one Blastoid found by G. Taylor. The Mansfield strata in Victoria and the sands of the Grampians are uncertainly placed among the soils of the Carbonic: there are many fossil weights, such as GyracanthidesAcanthodesCtenodus.

Permo – Carbon. – This system, as David writes, is perhaps the most interesting in Australia, first of all for the admirable evidence of a remote glacial action, secondly for the remarkable development of the flora in Glossopteris and Gangamopteriswhich replaced the flora in Lepidodendron ; thirdly, for its marine fauna belonging to two distinctly different types, the western similar to the permo-carbonic of India, and the eastern with peculiar characters, in many respects dissimilar from any other contemporary fauna of the earth.

Unlike Carbon, whose layers are folded, this formation is horizontal or almost everywhere, except at Gympie. The soils of the system are widespread throughout Australia and half of Tasmania is covered with them. At Wynyard in the NO. there are several tillite horizons, which seem to demonstrate the existence of glacial and interglacial eras at the base of the period. In Victoria there is the famous town of Werribee (west of Melbourne) where you can see a beautiful series of tilliti filling an ancient U-shaped valley; the local granites are also heavily streaked with ice, indicating its origin from the south. Similar striated surfaces are seen at Hallett Bay, near Adelaide, where the glacial layers measure 300 m. thick. At the River Inman large tracts of the ancient glacial landscape have been uncovered by recent denudation. Further west, the glacial layers reappear in the Collie carbon fields (southern corner of Western Australia), and can be followed northwards almost to the Tropic. Along the entire north-western coast, from the Irwin River to the Gulf of Carpentaria, there are permo-carbonic rocks with Indian-type fossils. The coal deposits are located in the eastern basins, from Cape York (Little River) to Tasmania: in Comet there is the thickest layer that has been encountered in Australia (24 m. Thick). Good Anthracite is found along the Dawson River near Baralaba. In New South Wales there are the main soils of the period: in the north, in Ashford and Undercliffe, there are small deposits of coal and graphite; but from Gunnedah the most important deposit in Australia extends southwards for 480 km, without interruption, to the River Clyde (near Ulladulla). The best district is in the vicinity of Maitland and Newcastle.

Australia Geology