Australia - Geological Framework

Australia: The Land Where Time Began

Australia - Geological Framework

Ancient massifs, or shields, composed of crystalline rocks that have been dated to billions of years old, as is the case on the other continents. Sedimentary rock zones are included in many of these shields, some of which have been found to be relatively undisturbed, though most of them have been contorted by ancient orogens (fold belts). Underlying the Great Australian Basin are several sedimentary basins that are of various ages, some of the deeper ones have been seriously deformed. The Eromanga Basin, the Surat Basin and the Carpentaria Basin, which together with the Murray Basin form a series of structural platforms or regions where the strata is relatively undisturbed extending from the north coast to the south coast. Within the shield areas remnants of old orogens and platforms are present as outliers. They represent the various cratons when taken together, the terms 'craton' and 'block' being virtually synonymous, though following Trendall (1990, pp. 3-7), a 'block' represents an ancient relatively stable crustal unit that has evidently always been a tectonic high, in contrast with the consistently depressed basins that are adjacent to it.

Geologically, Australia is an ancient continent, physical dating showing the the Earth is immensely old, being formed 4.5-4.6 Ga (e.g., Dalrymple, 1991), and many meteorites have been found to be of that order of antiquity, though the Moon rocks have been dated to about 3.5 Ga. Though not yet demonstrably the oldest known, according to the author¹, (see Jack Hills), many of the rocks exposed in the shield lands of Australia have dates of more than 4 Ga, (Wilde et al., 2001; Laeter & Trendall, 2002), are among the oldest analysed from any place in the world. The dates were obtained from zircon crystals in quartzite exposures near Mt Narryer, Murchison River catchment, northern part of the Yilgarn Craton. Weathering and erosion of an older rock from an unknown location led to the quartzite rocks containing the detrital zircons. The rocks of which the zircons were originally a part may have been weathered and eroded down completely, and it is possible there may still be rocks that are even older than those so far found.

There are also suggested signs of former life - in the form of fossil bacteria or bacteria-like organisms, prokaryotes, from about 3.4 Ga that have been found in the area of Mt Narryer (Schopf & Packer, 1987; Rasmussen, 2000; but also see Brasier et al., 2002). Eukaryotes, complex single-celled organisms have been found in rocks from the same area that date from about 3.1 Ga. Australia is therefore the location of both rocks and life that is as old as, and possibly older than, any in the world that have been discovered so far.

Plates and lineaments¹

The Australian continent was part of Rodinia, described by the Author¹ as a super-supercontinent, for about a billion years, following which Rodinia broke up and the constituent continents drifted apart and the crustal blocks formed the supercontinents Gondwana and Laurasia, both of which formed another super-supercontinent, Pangaea, which existed from 300-200 Ma. According to plate tectonic theory (Homes, 1931; Hess, 1962; Vine & Mathews, 1963; Morley & LaRochelle, 1964; but also Meyerhoff & Meyerhoff, 1974, and other chapters of Kahle, 1974; also Larin, 1993) Pangaea and Gondwana began to breakup about 200 Ma. The component land masses of Gondwana began drifting apart as eastern South America, Africa, apart from the Atlas Mountains that are younger, Antarctica and Peninsula India, and these movements continue at the present. When Australia broke from Antarctica it moved towards the Equator, and at the present it is continuing to move at a rate of 65-70 mm/year in a direction of slightly east of north (e.g., Ludbrook, 1980, p. 91; Parker, 1993) to collide with the Indonesian Archipelago. These lateral movements are the most recent of a series of crustal movements that have profoundly affected all the continents, including Australia, as they have introduced crustal stresses and strains into the rocks and blocks of the crust that are most obvious in fracture patterns.

In all places structure is an important part of landscape analysis, as becomes apparent in discussions of regional patterns. Many strait or linear sectors are seen along the coast of Australia and rivers, as well as other large-scale features of the topography. They are coincident with lineaments, long strait, or gently arcuate, structures that are deep-seated, of which they are an expression (Hobbs, 1904, 1911; see also Hills, 1946, 1956; O'Driscoll, 1986, 1989; O'Driscoll & Campbell, 1997). Many are fault zones (O'Leary et al., 1976), while others are linear zones of strain, that are deep-seated, that are susceptible to weathering (e.g., Russell, 1935; Turner & Verhoogen, 1960, p. 476; Nabarro, 1967, p. 4). Some of them are very old, forming as soon as the crust of the Earth had solidified and was cool and brittle (see Skobelin, 1992), though they vary in age. They result from torsion or shearing of the crust (e.g., Kalb, 1990) that is related to migrations of the crust, and they have been active and remain recurrently active. They are part of a global network (e.g., Vening Meinesz, 1947), and in Australia (Hills, 1946, 1956), as occurs elsewhere, they are aligned roughly NE-SW and NW-SE. Latitudinal trends are also associated with conjugate, or genetically related, shear zones in the basement rocks that crisscross the land masses.

The Australian continent has been moving roughly in a northerly direction for at least 90 million years, and as it is composed of blocks with different compositions and strengths joggling induced by lateral movement, caused and has continued to produce shearing or deformation that result from torsion, or excess transverse stress. Therefore the many fracture patterns, that are essentially orthogonal, that are seen in outcrops of brittle rocks (Vening Meinesz, 1947; Kalb, 1990; O'Driscoll, 1986). The continued and continuing differential migration of blocks, as well as the distortion within them is attested to by the large number of earthquakes and tremors that have been recorded, and in some cases felt (e.g., Gordon & Lewis, 1980; McCue, 1990; Bowman, 1992; Greenhalgh et al., 1994; Twidale & Bourne, 2000a; Bourne & Twidale, 2005). The scarps and zones of weakness that resulted are believed to be of tectonic origin, resulting from deep-seated crustal movements. The neotectonic features are those that post-date the Miocene.

Morphotectonics¹

Lineaments are still expressed in the landscape, in spite of their ancient origins. Morphotectonics is the study of the relationship between structure and surface at the regional or 'mega' scale (Hills, 1961). Therefore the alignment of several major coastal sectors, such as the west coast of Eyre Peninsula and the Kimberley Block, the Bonaparte Gulf (Elliott, 1994;) and the Gulf of Carpentaria, are coincident with lineaments or major fracture crustal fractures, also the the western margins of 'Lake' Torrens and 'Lake' Eyre (actually salinas) and the outlines of the Murray and many other structural or framed basins.

There are also other features that are remarkably strait, such as the courses of several major rivers, of which the Darling River is a prime example. but sectors of other rivers, such as the Georgina, Diamantina, Lachlan, Thompson and Cooper, are also remarkably linear, yet these rivers flow through young alluvial sediments that are not brittle and lack regular systems and sets of fractures. Underprinting, or the imposition from below of fractures in the underlying basement on to the overlying sediments (Hills, 1961; Firman, 1974; Twidale, 2006), and the author¹ suggests this may be accounted for by slight joggling of deep fractures. An alternative suggestion is that groundwater flow might be attracted by the underlying fracture zones, which leads to weathering and subsidence (Twidale & Bourne, 2000b). According to the author¹ it is clear that ancient structures still find morphological expression, even in geologically young terrains, at local and site scales, as well as regionally, (see Kalb, 1990), whatever the mechanism. With certain arcuate stream patterns may also be accounted for by underprinting (Woodall, 1994; O'Driscoll & Campbell, 1997). A link between them and meteorite craters in the primordial crust, which has been suggested by some to be termed the Mohorovicic discontinuity (Skobelin, 1992; Twidale, 2006).

Many salient features of the Australian landscape are explained by lineaments, though they also pose a problem, as they formed at least 1 Ga, at a time when Australia was incorporated into Rodinia. The difficulty referred to is that the lineaments form part of a global network in every contemporary continent. The implication of this appears to be that they have retained their original orientations, or rotated through 90^o, or multiples thereof, as they underwent lateral migration, which the author¹ says is unlikely, suggesting that the pattern could possibly be more apparent than real, random trends being subconsciously fitted into a pattern that is preconceived, though he believes this is also unlikely, as the pattern has been reported as statistically real.

Exhumed surfaces and forms

Erosional and destructional surfaces

Sources & Further reading