Australia: The Land Where Time Began

A biography of the Australian continent 

Australia’s Deserts – Ages and Origins

The deserts of Australia date to the Early Pleistocene (2.0-0.9 Ma), the time when the arid landforms, such as dune fields, salt lakes and stony deserts, are first seen in the Cainozoic record of Australia. Many elements of dryland biota are present much earlier, in the Pliocene (5.0-2.6 Ma) (Martin, 2006), though in Australia the development of aridity appears to have to have progressed in a stepwise fashion (McLaren & Wallace, 2010).

Smith suggests it is necessary to look to the Late Tertiary to understand the arid landscapes of Australia. Prior to 2.4 Ma Australia was a humid, forested land, but since then it has been transformed gradually into an arid continent. Beginning about 15 Ma in the Mid-Miocene, the transition to drier conditions occurred as the continental plate on which the Australian landmass is situated had carried it north into the mid-latitudes. At the same time allowing the Antarctic Circumpolar Current to strengthen as the gap between Australia and Antarctica widened, so provided less impediment to its movement around the globe, and this current drove changes in the global circulation (Bowler, 1982). Large perennial lakes dominated the central Australian landscape during this period. Distinctive sedimentary units of lacustrine clays and dolomites remain across much of this region that were deposited in these lakes. The Etadunna Formation in the Lake Eyre Basin is the best known of these, that has been found by palaeomagnetic dating to have formed in the Late Oligocene, 26-24 Ma (Woodburne et al., 1993), though the upper parts of the formation are suggested by biostratigraphy to have been deposited in the Late Miocene. Clays and dolomites of the Etadunna Formation underlie most Quaternary sequences in the Lake Eyre region, whatever their age is. An extensive shallow alkaline lake, or a series of lakes, is recorded in the Etadunna Formation, which fluctuated in depth and extent, and was surrounded by alluvial plain and fan sediments (Wasson, 1982; Alley, 1998). An abundant aquatic fauna was supported in these lakes, that included catfish, lungfish, crocodiles, chelid turtles, and river dolphins, and there was also a diverse assemblage of water birds, that included waders and flamingos (Miller, 1963). In Central Australia there were equivalent units from the Late Miocene, which shows that large freshwater lakes were also present at Lake Lewis (Anmatyrerre Clay, English et al., 2001; and Lake Amadeus (Uluru Clay, Chen & Barton, 1991; Chen, Bowler & Magee, 1993). Lakes Lewis and Amadeus, as with many saltlakes, are part of chains of playas that follow ancient palaeodrainage networks that had significant flow earlier than 15 Ma (Van de Graaff et al., 1977; Magee, 2009). Laterites forming extensive hardpans across the central and western parts Australia have been shown by palaeomagnetic dating to have undergone the main period of laterite formation in the Late Oligocene-Early Miocene (Alley, 1998), which is consistent with climatic conditions being warm and wet throughout the Miocene. 

It can be seen that the lake Eyre Basin was covered by forest until about 24 Ma, based on the plant fossils that have been found, with rainforest and swamp in the bottoms of valleys, and slopes and ridges were vegetated by sclerophyll forests that were seasonally dry (Truswell & Harris, 1982; Hill, 1994; Martin, 2006). At this time there is not much evidence of grasslands (Martin, 1990). It had been believed that the Livistona palms that are present in the Finke Gorge in Central Australia at the present were relict of the climate of the Tertiary (Latz, 1975; Wischusen, Fifield & Cresswell, 2004; Crisp et al., 2010), though they have now been shown by recent genetic work to have been introduced during the Late Pleistocene (Kondo et al., 2010. Evidence has been found in the Pilbara that until the Middle Miocene dry forests, mainly composed of eucalypts and casuarina, with rainforest being found in small patches, extended across the interior (MacPhail et al., 1994: 229).

After 15-12 Ma the continued desiccation of the continent is reflected in the lakes at the end of the Miocene, though these basins continued to structure, though less extensive, lacustrine phases. Lake Amadeus, in Central Australia, changed at 1.6-0.91 Ma from shallow freshwater conditions to a playa that was controlled by groundwater, with the formation of shoreline dunes of gypsum by 0.98-0.73 Ma, which marked the onset of pronounced seasonal aridity in the heart of the continent (Chen et al., 1993). At about 330,000 years ago Lake Lewis made the same transition to saline conditions (English et al., 2001).

Formation of widespread silcrete duricrusts that cap the Etadunna Formation and comprise the distinctive landscape of tabletop hills and breakaways in Central Australia reflect the drier conditions that prevailed in the Pliocene (Mabbutt, 1971b; Alley, 1998). The stripping of mantles of soil from these silcrete tablelands may have begun by 4 Ma, based on cosmogenic dating, which led to the formation of the stony deserts of Australia between 4 and 2 Ma (Fujioka et al., 2005). In the dunes of the western Simpson Desert it is also shown by exposure dating that by 1 Ma this very large dunefield had begun to form (Fujioka et al., 2009).

Corresponding changes are also shown by the palaeovegetation, as indicated by plant fossils from the area. There was a shift to dry eucalypt and casuarina woodlands, chenopod shrublands, and grasslands underwent a major expansion. It has been pointed out (Martin, 2006) that the vegetation of Central Australia included many typically arid-zone taxa, such as eucalypts, casuarina and chenopod, long before the region became arid, where these arid-zone taxa occupied other niches such as sandy or saline lake shores. The effect of desiccation was to remove taxa that could not thrive in the new climatic conditions. According to Smith the stage was set by these environmental changes for a rapid evolutionary diversification of animal taxa with the required adaptations to cope with the new conditions. After 15 Ma forests and rainforests in Australia were lost as aridity of the continent increased and this resulted in the marsupial ‘radiation’, a proliferation of dryland-adapted species that eventually led to the ancestors of most of the living marsupials of Australia (Long et al., 2002; Dawson & Dawson, 2006; Dickman & Ganf, 2007; Byrne et al., 2008). The Macropodines, the earliest known grazing kangaroos, and wombats made their first appearance in the fossil record in the Early Pliocene, and their lineages diversified rapidly to exploit the expanding grasslands. In the fauna of the Late Tertiary there were also a family of large terrestrial birds, the dromornithids, which included Genyornis and Dromornis. One of the few fossil assemblages that date from the Miocene in Central Australia about 8 Ma, is the Alcoota site, to the north-east of Alice Springs (Megirian, Murray & Wells, 1996; Murray & Vickers-Rich, 2004). This site is believed to represent a high mortality of animals that were tethered to a waterhole at a time of drought, the taxa of the site being dominated by dromornithids and diprotodontoid herbivores, though there are also thylacines, crocodiles, large varanid lizards (Megalania), ducks and flamingos.

Accompanying this diversification was a trend towards larger body mass which reached its most extreme expression in the Diprotodon optatum, which weighed an estimated 2.8 tonnes. The Australian marsupial megafauna were, in this sense, a product of the development of drylands in the Pliocene and Quaternary. According to Smith larger herbivores were better able to cope with a diet that was high in fibre and low in nutrients, though the advantage of large size was off to some extent by longer generation times, smaller populations and a greater reliance of surface water (Archer & Hand, 2006). The corollary of this is that while these animals were adapted to drylands, they were not able to cope with open desert or high environmental stochasticity.

Much of the desert landscape of the present had taken shape by the Early Pleistocene, as suggested by the collective data, at a time when the ice ages of the Quaternary were initiated by global cooling. Grasslands and stony desert, habitats of open dryland, had developed in the Pliocene, accompanied by a dryland biota expansion. It was not until the Early Pleistocene that these diverse components of modern desert biome were assembled.

Sources & Further reading

Smith, Mike, 2013, The Archaeology of Australia’s Deserts, Cambridge University Press