Australia is one of the most uniform regions in the world, considering its size: and this is all the more surprising, as it is flanked to the north and east by two of the most disturbed regions of the earth’s crust. In New Guinea there are in fact two zones of active volcanoes, numerous traces of a strong uplift, high mountain ranges and some of the largest recent dislocations in the world; in New Zealand almost the same topography is encountered. The highest peak in Australia, on the other hand, reaches only 2230 m., And only a very small part, about 5%, of the territory is located above 600 m. This is because the great “mobility zone” of the western Pacific passes around what is, in fact, a very resistant portion of the earth’s crust, a “shield” against which the movements of the Pacific bed have come to a halt, exerting a very weak action there, except along the present eastern coasts. The most characteristic feature of the Australian topography is therefore constituted by this ancient shield, now represented by the Great Penepian. Probably during the Middle Tertiary the whole of Australia was barely elevated above sea level and the coasts extended further north, east and south-east than today. Since the Miocene epoch, the western half of the continent has been raised in mass by about 350 m., While a band 300 km wide. along the east coast it has been brought to various heights with a series of large plates surrounded by fractures. Between these and the penepian the ancient surface subsided perhaps slightly, but the floods carried by the rivers have filled the depression and most of it is now at about 150 msm. In Australia there are thus three major topographical divisions, corresponding quite exactly to the geological divisions dealt with elsewhere. They can in turn be subdivided into a number of smaller areas in relation to the structure. The classification of the topographical regions is summarized in the following framework:
- A) Great Penepian:
In Western Australia: 1. Kimberley Plateau; 2. internal desert; 3. north-western coastal region; 4. Land of the Swans; 5. Salt Lakes region; 6. Nullarbor syncline.
In Northern, Central and Southern Australia: 7. coastal area; 8. internal area; 9. Barkley plank; 10. Mac Donnell and Musgrave Mountains.
- B) Lowlands area:
- large artesian basin; 12. Murray-Darling basin.
- C) Eastern highlands:
- Queensland Highlands; 14. New England Mountains; 15. Monti Azzurri; 16. Monaro plateau; 17. Flinders Mountains; 18. Tasmania.
If we consider the relationship between the topographical surface and the geological structure in a section crossing Australia from Perth to Kermadec Island (north of New Zealand), we can see why the relief becomes less and less pronounced as it proceeds from east to west. Just to the east of the Kermadec there is one of the deepest areas of the ocean, in the form of a long meridian ditch. Here the collapse of the crust seems to have been localized, which must have given rise to the folds and curls existing between the Kermadec pit and the stable and resistant Australian shield: the highest is the curl closest to the pit itself which rises for 6000 m. from the ocean floor to form the Kermadec Islands. Two other folds further west emerge from the ocean in the Norfolk and Lord Howe Islands. A fourth, of 5140 m., constitutes the margin of the continental base and rises above the sea level up to a height of about 1200 m. in the Blue Mountains. Further on, the movement of the crust is gradually lost in light waves, three of which are visible: the small fold of Cobar, that of Broken Hill and the somewhat larger Flinders range, which constitutes a 600m ripple. high. From here, for more than 1600 km. towards the Indian Ocean, up to Perth, the continent maintains a height of about 300 m: there is in fact no change in the topography until the Darling fracture plate near Perth is reached. For Australia 1998, please check constructmaterials.com.
Triassic. – These soils are found mainly in Tasmania, where they also include the important carboniferous deposits of Fingal, and in New South Wales, where they fill a large syncline with brown shale clays (the Narrabeen series) and tuffs with metallic copper. Estheria is a very common fossil crustacean in them. A series of 300 m high follows the clays. of sandstone with fossil ferns, such as Thinnfeldia and Phyllotheca. An upper clay series (Wianamatta series) occurs in the center of the basin immediately west of Sydney; it has provided remains of Labyrinthodon and many fish including Cleithrolepis. The Sydney Triassic deposits probably formed in a large shallow lake communicating with the sea.
Jurassic. – During this period the lake sediments were deposited which constitute the seat of the artesian aquifers in most of Australia. They extend from Brisbane to Lake Phillipson and contain major coal deposits in Queensland (Ipswich), South Australia (Leighs Creek) and Victoria (Wonthaggi). In New South Wales, Talbragar, there are beautiful Leptolepis beds and a few coal-poor beds in the Clarence Basin. Among the plants are characteristic Taeniopteris, Podozamites, among the animals Unio and Ceratodus.
Cretaceous. – It is widely represented, especially in the artesian basin of Eastern Australia. The lower series is marine and is covered by freshwater deposits that form the so-called desert sandstone, once widespread over a third of the continent; contains opalised opals and sandstones. Maccoyella is a common shell, while large reptiles (Ichthyosaurus, Plesiosaurus) are found in the lower layers.
Tertiary. – It is not very common except in the Murray basin. Miocene coals of considerable thickness are found in Morwell in Victoria. In the Great Southern Bay, and along the north coast of Tasmania, limestones from the Upper Miocene occur. The aforementioned Pliocene limestones follow (Adelaide, Launceston). The clayey deposits of many salt lakes containing the remains of extinct marsupials such as Diprotodon and Nototherium date back to the Pleistocene age.