Australia: The Land Where Time Began
The MacDonnells are close to the exact centre of Australia, and extend for 500 km in an east-west direction. They were formed by folding at least 1000 million years ago, but the sediments that comprise them are 1400-2400 million years old. They were originally about 4500 m high but after millions of years of weathering and erosion they are now only 400-500 m high.
The first phase of the rise of the MacDonnell Ranges, including the Chewings Range and Harts Range, took place at least 1.0 billion years ago. These original ranges reached a height of about 4500 m. Erosion debris from the ancestral ranges was deposited in the nearby seas. About 500 Ma the next phase of folding raised the south-running ranges, parallel to the ancestral ranges. These include the George Gill Range, Krichauff Range and the Waterhouse Range.
The final phase of folding occurred about 300 Ma. By this time the older ranges had been greatly reduced. For the last 300 million years there has been only minor uplift of the MacDonnells, but erosion has continued, removing the softer rock, the remaining exposed rock being in the form of folded ridges, separated by long, flat alluvial valleys. The, but erosion has continued, removing the softer rock, the remaining exposed rock being in the form of folded ridges, separated by long, flat alluvial valleys. The Finke River and its tributaries cut gorges through the ridges during periods that were wetter than the present.
The surrounding plains are now about 600 m above sea level, the MacDonnells being about 400-500 m above that. The predominant rock of the ranges is red iron oxide-stained quartz. There are also strata composed of dolomite, limestone, sandstone, conglomerate, siltstone and shale. Because of the complex nature of their construction, the rocks have weathered into a variety of patterns. Along the walls of the valleys, the rock is in the form of a number of patterns, in places being scalloped layers of different colours, from reds to creams, and in other places the rocks are sharply fractured and of a rusty red colour.
In the Chewings Ranges are a pair of peaks of more resistant rock, Mt Liebig and Mt Zeil, both above 1,500 m, and some others are a bit lower, Mt Heughlin, 1450 m, Mt Edward, 1,480 m, Mt Sonder, 1334 m, and Mt Hay, 1,349 m.
For at least the last 20,000 years, the only surface water in the region is found in the gorges of this and other ranges, in the form of springs, rockholes and, in some places, larger pools. On the rare occasions when heavy rain in the headwaters puts enough water into the streams to reach as far as the Macdonnells the pools and rockholes are joined as part of a flowing stream, usually for a short time. The white sandy watercourses are lined with river redgums (Eucalyptus camaldulensis). In some of the shaded gorges there are also rare cycads (Macrozamia macdonnellii) that are believed to have grown in the area for about 200 million years.
There are a number of other trees found throughout the Macdonnells, including bloodwood (Eucalyptus terminalis), corkbark (Hakea suberea), ironwood (Acacia estrophiolata) and ghost gums (Eucalyptus papiana). The ghost gums grow on the flat, sandy and grassy areas, as well as on high craggy ledges.
The mulgas, and other acacias, are the predominant low trees in the area. The most popular with the local Aborigines was the witchetty bush (Acacia kempeana), as the witchetty grubs are found in its roots. For most of the year the groundcover is comprised of spinifex (Triodia species), T. clelandii and T. longiceps. There is a transient flourish of wildflowers after rain and in spring.
The scenery is most widely known through the paintings of the local Aboriginal artist Albert Namatjira.There are some relict 100 million-year-old Cretaceous surface rocks on the tops of some parts of the Macdonnell Ranges, but most of the crests are Miocene duricrust.
Macdonnell Ranges and associated uplands5
A batholith of Proterozoic age is exposed in the Arunta Block. Here there are jagged peaks and the prevailing topography is of slopes. To the south folded and faulted strata of Middle Palaeozoic age underlie the Macdonnell and associated ranges, James Range, Krichauff Range and Waterhouse Range. Ridge and valley topography is well developed. The Macdonnell Ranges have been described as 'the only known contemporary land surface that has persisted unburied since pre Tertiary times' (J.A. Mabbutt, in Brown et al., 1968, p.304). This is a claim that the author5 says was reasonable at the time but cannot now be maintained as it has been demonstrated, or at least suggested strongly, that surfaces from the Mesozoic have been found to be quite widely preserved. The ranges have been uplands for many millions of years, and the author5 says the bevelled crests may be of Cretaceous age.
Prominent bevels have been preserved at 2 levels above the present floors of the valleys. On some quartzite ridges the lower summit bevels have been preserved that may be part of a landscape that is represented in the intervening valleys by plateau remnants that have been duricrusted. In the north, near Alcoota (Woodburne, 1967), they are lateritic and predate the Late Miocene, but near Glen Helen mesas that are silcrete-capped are preserved (Mabbutt, 1966) and in the southern Piedmont of the Ranges. Silcrete cappings with associated kaolinitic weathering profiles extend far south and to the Ooodnadatta area across the South Australian border on the western side of the Simpson Desert dunefield, and the Birdsville-Innamincka area (Sturts Stony Desert) on the east (Mabbutt, 1965; Wopfner, 1978). It is difficult to establish the age of the silicification, but a date of the Late Eocene in the Lake Eyre region is the most consistent with the stratigraphy of the deposits in the basin (Wopfner et al., 1974; Wopfner, 1978; Benbow et al., 1995b). It is implied by these valley-floor remnants that in the Macdonnell Ranges the ridge and valley topography was established by the Early Tertiary; by the Eocene if the silcrete is of that age. Local stream rejuvenation may be indicated by the Alcoota laterised surface, though it may also be older than the Miocene, an Early Miocene or pre-Miocene age being indicated by field evidence. Since that time there has been only a few metres of erosion from the valley floor. There are also remnants that are silcrete-capped in the piedmont of the upland, as well as on the plains to the south. South of Alice Springs an etch plain, that is related to a silcreted regolith that is widespread, has been identified on the plains (Mabbutt, 1965). The author5 suggests the summit of Chambers Pillar may be part of this weathered land surface.
According to the author5 as the summit bevel of the Macdonnell Ranges is substantially higher than the duricrusted remnants in the valley and bevelled lower ridges, so is therefore older than them. He5 also suggests it may be part of the land surface from the Cretaceous that was drained by rivers running into the Cretaceous seas (Twidale, 1994). The Gosses Bluff bolide or meteorite that impacted about 142 Ma forming a crater and amphitheatre (Crook & Cook, 1966; Milton & Sutter, 1987) demonstrates that previously an older land surface of Cretaceous age existed in this region.
There are many examples of transverse drainage, including breached snouts, in this region, though the courses of many streams are determined by structure. There have been a variety of explanations suggested, such as one that implied that the rivers are superimposed from a Cretaceous cover and it was also stated that the incised meanders of Finke River, where it flows through the Krichauff Ranges, are inherited from a previous low relief surface (Ward, 1925). This explanation persisted for several years (e.g. Campana, 1958b), though it had been pointed out (Mahard, 1942) that such features are authigenic, developing during incision, also (Twidale, 1955).
They have been attributed to antecedence, that is, they developed then survived subsequent movements of the Earth by cutting through any blocks that were uplifted. There is no known evidence of localised differential uplift, such as block faulting, the author5 suggesting Hills (1961, p.82) may have been correct to label some elements of drainage that are inherited, deriving their pattern from an old weathered landscape such as the silcreted surface which is still evident in this part of central Australia (Mabbutt, 1965). Some are understandable in terms of the deep erosion, as has been described in the Flinders Ranges, and drainage patterns that are imposed from higher levels. Unusual 'in-and-out' rivers and valleys, rivers that have worn meander loops in steep hill slopes, that alternate with valley floor segments. Some patterns such as those that have been investigated on the Finke River (Pickup et al., 1988) are still to be understood, in spite of these various explanations.
Prominent beehive forms (cf Bungle Bungle Range) developed in sandstone of Palaeozoic age are notable structures in the Gill Range, with Kings Canyon at its western end (Bagas, 1988), The author5 suggests the summit bevel may be a remnant of the same high planation surface that is preserved in parts of the Macdonnell Ranges, standing higher in the landscape than the remnants of the silcreted surface. The summit bevel of Uluru, in the Amadeus Basin appears to be an etch form from the Maastrichtian (latest Cretaceous), being more than 70 My old (Twidale, 1978a; Twidale & Bourne, 1978; Harris & Twidale, 1991). At Kata Tjuta, the crests of some of the lower domes and the bevelled summit of Mt Connor can be correlated with the surface that has been preserved on Uluru. On Uluru the summit bevel has been postulated to be of an age that is regarded as speculative as it is based on the assumption that the unconformity, a former land surface that is situated between Cambrian strata below and strata of latest Cretaceous above, that has maintained a similar inclination throughout, and according to the author5 it may not have. To the west of Uluru strata from the Eocene are present beneath the plains, and boreholes situated just south of the southern face of the residual encountered rocks of Tertiary age (Twidale, 1978a). Rocks of Precambrian, Palaeozoic and Mesozoic age have been found to lie unconformably beneath various Tertiary rocks that include duricrusted deposits and accumulations on the desert plains (Wells et al., 1970). Silcreted surface remnants from the Eocene that were preserved have been found to have a widespread distribution on the plains to the south of the Macdonnell Ranges.
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