Old, Flat and Red—Australia's Distinctive Landscape

226 SHAPING A NATION | A Geology of Australia Old, flat and red —Australia’s distinctive landscape Most of Australia’s landscape is stark, and the climate is harsh. The landscape is also remarkably flat, with an average elevation of only about 325 m and local relief never more than 1500 m. As a result, many of Australia’s major rivers are slow flowing, commonly into salt lakes in the arid interior. A thick regolith also blankets much of the continent. Sustained weathering has resulted in the formation of abundant iron oxides in the regolith, giving the Australian landscape its distinctive red colour. The regolith holds great wealth for Australia, but can also present challenges for mineral exploration. Human occupation over the past 50 000 years, and especially the last 200 years, has left a significant mark on the landscape. Colin F Pain,1,3 Brad J Pillans,2 Ian C Roach,3 Lisa Worrall 4,3 and John R Wilford 3 1University of Seville, 2Australian National University, 3Geoscience Australia, 4Zeus Uranium Ltd Old, flat and red—Australia’s distinctive landscape 227 Image by Mark Gray Figure 5.1: Locality and features map, also showing Australia’s deserts, rivers and lakes. 228 SHAPING A NATION | A Geology of Australia Regolith The aridity, combined with the flatness of the landscape, has created river systems that are slow The Australian landscape is remarkably flat, flowing. The slow-moving rivers, many draining and around 80% is covered by a thick, and in into the interior, have struggled unsuccessfully to places ancient, blanket of red-coloured regolith remove the wind-blown salt from the continent. (Figure 5.1, Box 5.1). The regolith forms as a result As a result, the nutritionally poor soils are also of the interaction between rocks, water, air and prone to salinisation. life (Figure 5.2a), which drive processes operating at or near the surface, such as weathering, erosion, The soils and landscape differ markedly between transportation, sedimentation and cementation Australia and much of the Northern Hemisphere. (Figure 5.2b). Regolith can be thought of as A key determinant of these differences was the role everything between fresh rock and fresh air; it is (or lack) of glaciers. The glaciers of the last ice ages somewhat synonymous with what has been called transformed the Northern Hemisphere’s landscape the ‘critical zone’. Regolith dictates the nature of the and soils, but the ice was restricted to small upland Australian landscape, and has had a profound effect areas in southeastern Australia. This lack of glacial on the development of Australia and the way people reworking and regeneration of soil and landscape have lived, and continue to live, on the continent. in Australia was due to the low overall elevation Regolith may vary in thickness from a few and the latitudinal position of the continent at the centimetres to hundreds of metres and can time (Chapter 2). have a complex architecture. Components of Australia’s landscape and regolith have been key the regolith range in age from very young—for determining factors in shaping where and how example, sediments deposited by a recent flood Australian people live. The regolith underpins our (Figure 5.3a)—to very old—for example, Permo– economic, social and infrastructure systems; we live Carboniferous oxidation of iron in weathering on it, we grow our food in it, it is the foundation of profiles exposed by mining in the Tanami region of many major engineering works, and much of our the Northern Territory (Figure 5.3b). The fact that water supply is stored in it (Chapter 7). Regolith ancient landscapes have been preserved is linked to also hosts or hides valuable mineral deposits, the overall tectonic stability of the continent. This and mineral exploration through and within the stability and landscape longevity have resulted in regolith is one of the greatest challenges facing a continent that is deeply weathered and soils that mineral explorers in Australia in the 21st century are nutritionally poor and fragile. (Chapter 11). Figure 5.2: (a) The regolith is the critical interface between four of Earth’s spheres: the lithosphere, hydrosphere, Geography and plate position are driven by The richness and diversity of Australia’s regolith biosphere and atmosphere. (Source: Taylor & Eggleton, 2001). tectonics. After the breakup of Pangaea– makes a fascinating story, so let us first look back (b) Role of regolith processes in the geological rock cycle. (Source: Wilson, 2004) Gondwana, Australia drifted northwards through to how the continent’s landscape and climate were the Cenozoic into a geographical position that transformed after the continent was finally released has made it the most arid inhabited continent. ‘out of Gondwana’ (Chapter 4). Old, flat and red—Australia’s distinctive landscape 229 dolphins and giant flamingos. Indeed, fully desert a. Cenozoic climate changes of the Australian landscape conditions were probably not established until around 3 Ma (Late Pliocene). For example, from Australia is the second driest continent in the cosmogenic 21Ne and 10Be dating, it is clear that world, Antarctica being the driest. There is ample stony deserts in northern South Australia formed evidence of dramatically different climates in the at 2–4 Ma, while major dunes formed somewhat past compared with the present (Chapter 3), with later, from around 1 Ma (Figure 5.4). the strongest imprint on the landscape occurring during the Cenozoic. The period from 65 Ma to With the increase in Antarctic ice, subtropical about 35 Ma (broadly equivalent to the Paleocene high-pressure systems moved from the south to and Eocene epochs) saw a significantly warmer dominate the continental interior (Figure 5.5). One outcome of this increased aridity was that b. world, with no Antarctic ice sheet, sea-levels perhaps some 60 m higher than present and some rivers draining to the south were unable to adjust their gradients to the post-Miocene atmospheric CO2 concentrations more than three times that of modern levels. During that time, the tectonics, especially on a continental scale to the Australian continent lay well to the south of its northeast, and became disorganised or moribund, current position, and the dominant vegetation was with a number of swampy lakes adjacent to the warm to cool temperate rainforest, even in central tilt axis. These lakes contracted and disappeared, Australia (Chapter 2). with marked faunal changes as the land dried up (Chapter 3). From about 35 Ma (latest Eocene), the Antarctic ice sheet progressively increased in size, global The cooling of Earth’s climate during the temperatures decreased, sea-levels fell, and central Quaternary glaciations, starting about 2.6 Ma, led to wetter and drier cycles related to glacial Figure 5.3: (a) Sediments deposited by recent floods on Australia became progressively more arid as the Darling River Floodplain near Menindee, western New the Australian continent moved northwards to and interglacial periods, and concomitant South Wales. (b) Weathering profile (more than 30 m deep) fluctuation in the amount of water flowing into exposed in the wall of an openpit gold mine, Tanami region, straddle the zone of subtropical high pressure that Northern Territory. is characterised by mid-latitude deserts on both the interior. During the cold, dry glacial periods, sides of the equator. Pollen evidence suggests that aeolian landforms (dunes) spread out from central seasonal aridity may have been the norm in parts of Australia and moved into surrounding areas, where central Australia as early as 40 Ma (middle Eocene). they can still be seen today as more or less stable. Nevertheless, around 30 Ma the moisture-loving Australia does not have as wide a variety of dune ancestors of Wollemi pines and southern beech forms as the Sahara Desert, but the dunes can trees were growing in the now semi-arid Pilbara be dramatic; most are longitudinal, some tens of in Western Australia, and around 25 Ma large kilometres in length. The pattern of dune fields central Australian freshwater lakes, forerunners to mirrors the anticlockwise pattern of the dominant Lake Eyre, had formed, inhabited by crocodiles, winds in Australia (Figure 5.5). 230 SHAPING A NATION | A Geology of Australia WHY IS THE AUSTRALIAN OUTBACK RED? (BOX 5.1) In a word, the answer is hematite (from the Greek haimatites, meaning bloodlike). This iron oxide (Fe2O3) is formed by weathering and is largely responsible for the pervasive red colour of rocks, soils, dust and sand that characterise central Australian landscapes. Every visitor knows how a combination of red dust and sweat produces stains on clothes that are almost impossible to remove. Hematite is also the predominant pigment in red ochre, one of the clays favoured by Aboriginal artists for their paintings. The iconic Red Centre of Australia is broadly encompassed by the 500 mm mean annual rainfall isohyet, suggesting that redness is related to aridity—which is certainly true. Aridity came rather late to central Australia, geologically speaking, probably beginning in mid-Cenozoic times as the Australian Plate moved northwards to straddle the dry mid-latitudes, and progressively intensified during the last 2–3 Myr. However, much of the red colour in the rocks originated prior to the last 3 Myr, at times when the climate was significantly wetter than the present— for example, between 80 Ma and 60 Ma. Furthermore, the red colour is not just a surface feature; it commonly extends to depths of more than 50 m, as seen in openpit mine exposures throughout Australia (Figure 5.3b). On the other hand, the red sand dunes are much younger, mostly less than 1 Ma according to luminescence and cosmogenic nuclide dating. The sand dunes are composed mostly of white quartz grains, but a thin coating of hematitic clay gives the reddest dunes their distinctive colour.

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