Australia: The Land Where Time Began
In central and western parts of Australia there are extensive plains of dunefields, basically all being similar in consisting to a large extent of fields of linear, longitudinal or seif dunes (sword-shaped isolated longitudinal dune). Close to the sources of abundant sand, such as river channels, there are areas of reticulate or chain dunes (Twidale & Campbell, 2005).
Linear dune formation
Water is important in the formation of dunes, such as the rivers in the catchment of Lake Eyre, the lowest point on the Australian continent. The rivers of this catchment, some rising in the monsoonal areas of the north, bring large volumes and sand, as well as salt, to the lake, though at present these rivers flow infrequently, as in the unusually wet summer of 2010/2011, though at earlier, wetter, times it occurred more often. The large dunefields are found downwind of main deposition areas of sand in these river systems, in depressions, such as salinas, claypans, river channels and valleys, much of the sand accumulated in dunefields having originated in river systems. The sand is picked up by the wind when the deposition sites dry after a flood event, the sand being deposited on the lee side of the depression, which is the northern side in central Australia, as the prevailing winds are mostly from southerly directions. These deposition sites are called source-bordering dunes of lunettes.
Orientation of Dunefields
Large dunefields advanced, expanding to the east across the Murray Basin during the glacial maximum, the height of the dune-building activity, forming the Big Desert and the Little Desert. The dunefields of the Strzelecki and Simpson Deserts were extended well beyond their previous limit. Desert dunes encroached into western New South Wales and northern Victoria.
The orientation of the dunes on the continent shows an anticyclonic wind direction pattern. The most characteristic form of dune is longitudinal, extending parallel to the direction of the wind. The trend of dunes in any given part of the desert area is constant. The southern Strzelecki dunes trend northeast, those of the Simpson Desert, Sturt's Stony Desert and Tirari Deserts back to 10o West. The trend of the dunes in all areas is consistent with the direction of the anticyclonic wind in their particular area. Strong prevailing winds produce some linear dunes up to 300 km long. Strong prevailing winds determine the length of the dunes, crosswinds determine the width and height of the dunes. The distance between dunes varies widely, in some areas there may be up to 15 per km, while in other areas there may be as few as 3 per km.
On the down-wind margin of lakes lunette dunes form that are transverse instead of longitudinal. They lie at right angles to the wind, as with barchan dunes, their ends curving down-wind forming horn-shaped structures, as the wind blows past them as well as over them. The lunette dunes of the Willandra Lakes, including Lake Mungo, have been found to contain evidence of aboriginal occupation. They also provide evidence of changes in lake water levels, from lake-full to the present condition.
The dunefields of the Simpson Desert and Strzelecki Desert are longitudinal dunes. These deserts are in the Lake Eyre sub-basin and the Callabonna sub-basin, dated to the Cainozoic, by the Kopperamanna Gap through the Gason Range, between the Corryanna Dome and the Gason Dome. In both basins the Eocene Eyre Formation is overlain by Miocene deposits of limestone, dolomite and clays, the Ettadunna Formation (Lake Eyre sub-basin) and Namba Formation (Callabonna sub-basin). During the Tertiary, the edges of the basins were uplifted by tectonic processes, raising the Gason Range between them, and downwarping the Kallakoopa Pans in the Eyre sub-basin and around Moomba in the Callabonna sub-basin. River and wind activity spread sediments over the Namba Formation and Ettadunna Formations. The dunefields of today overly these sediments.
Colour of Dunes
The colour of the dunes are not all the same, some are red-brown, others pale orange or cream, yet others are a glaring white. The red grains of the longitudinal or serif-type of dunes get their colour from the oxidised iron on the grain surface. This coating was acquired as they were moved from their original source. The crests of the dunes are sinuous, and the distance between them, and in any given area they are of equal distance apart. Occasionally the parallel dunes converge to form a Y-junction. It is not certain why this occurs. One suggestion is that the wind blowing between the dunes sets up what is called the venturi effect. The basis of this effect is that when the air stream is narrowed, the air moves faster through the narrowed section as the air entering it is forced through the reduce space. Based on research on the dunes of the Strzelecki Desert and Simpson Deserts, it has been found that the colour increases with distance from the source, and that there are many sources of the sediment that is piled up in dunes. Dunes with the highest percentage of quartz, 98 %, and therefore the lowest percentage of clay, are the reddest of the dunes.
With accumulated sand on the lee side of playa lakes, salt pans or flood plains, the linear dunes merge upwind. These dunefields bordering their source may be up to 50 m high. To the east of Strzelecki Creek, in the Strzelecki Desert, the dunes are formed from several sources, redistributed Miocene lake sediments, beach deposits and lakeshore dunes. To the west of the creek, clay-rich muddy alluvium, deposited at the same time the dunes were forming, was the source for the dunes.
In the southern part of the Simpson Desert, the source of sediment was deflation of pans that are controlled by groundwater levels and salinity. Deflation of the saline lakebed sediments result in clay pellets. The dune sand, mostly quartz with very low clay content, in the northern Simpson Desert are of alluvial origin. Near modern river channels fine, paler sands tend to occur. Further from the source, the sand is coarser and redder. Dunes tend to be formed from the sand of the region they are in, not transported from other regions.
As the colour of dunes depends on their composition, the form and nature of the dunes can be determined by their colour. Nearly always, the red-brown are the simple longitudinal type, lying on top of alluvium or bedrock and gibbers. In these dunefields, the swales (interdune corridors) are all claypans or hardpans, at about the same elevation as each other.
Pale-brown longitudinal dunes usually occur in groups, each longitudinal feature on a swell of sand. The basis of the swales is aeolian sand, dune and swale both contain clay pellets and support calcareous soils. In the Strzelecki dunefields, near Cooper Creek, the swales with their longitudinal dunes extend downwind from transverse dunes of the river floodout. In the Simpson dunefields, the pale dunes are downwind of pans, salt lakes and some river channels. Near modern streams in both dunefields, the topographically lowest areas are occupied by pale-brown compound dunes.
Dune Colour and Geomorphology
The geomorphology of a region can also be indicated by dune colour. Sediments from large rivers, such as the Diamantina River and Cooper Creek, draining areas of rocks from the Jurassic and Cretaceous in southwestern Queensland, often fine-grained sandstones and mudstones, provides sediments for pale dunes. River sediments from the Cretaceous rocks of the Grey Range in the east, in the south, channels to the east of Lake Frome, that used to enter the area, draining crystalline rocks of the southern part of the Barrier Range and Olary Arch, and from the northern parts of the Barrier Ranges, that are part of the ancient Gawler craton,
The Hale palaeoriver, Hay palaeoriver and Plenty palaeoriver in the Simpson Desert carried sediments from the Arunta Block, as well as well as sediments from the sandstones and mudstones of the Eromanga Basin. The dune colour often relates to the sources of the sediments composing them. Dune colour indicates both distance from the sediment source and the rock types from which those sediments derived.
White sand, reworked from the pale brown dunes that underlie it, caps many areas of the Strzelecki dunefields.
The dunefields of the Simpson Desert are still advancing, its brilliant red dunes are moving onto the Strzelecki floodplain, smothering entire eucalypt forests in the process. In places, the only indication that a forest was there are the occasional eucalypt crown protruding from the sand. The 130,000 km2 of the Simpson Desert comprise half the dune-desert area of Australia. At present, 40 % of the continent is covered by dunes.
The parallel central desert dunes date from about 300,000 years ago, which was a time of intense aridity, Ever since then mobile dunes have characterised arid areas. In the Canning Basin and Lake Eyre Basin, the sand-ridge deserts are places of spectacular scenery. The ridges of the narrow, reticular, and tens of km long dunes are between 12 and 30 m high, and the corridors separating them range from 250 - 450 m wide. The longest dunes are more than 300 km long, sometimes converging to asymmetrical Y-shaped junctions. These connections usually point in a similar direction in any particular area. The Y-shaped connections point in the downwind direction, the regional down-wind patterns of the dunes shows that the prevailing winds haven't changed much through time. Another sort of dune occurs in parts of Queensland and the Northern Territory. These dunes are of reticular plan, thought to be the result of the presence of 2 wind systems that are of about equal strength but blowing from different directions.
Another cause of collapse can be the drying of dune sand in the sun. Observers of such dune collapses report a bang like guns being fired. There were reports by the explorers Mitchell and Sturt of 'sounds like distant artillery'. Wills and King, of the Burke & Wills expedition of 1861 heard the 'noise of an explosion, as of a gun', that they attributed to Burke, but found when they returned to camp that he was not responsible. The authors suggest it was most likely to have been the result of the sun drying sand exposed on the slope of a dune causing it to collapse, the sound heard being the thud of the collapse.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|