Australia: The Land Where Time Began
Simpson Desert see
The area of the present Simpson Desert was covered by glaciers about 270 Ma. For some time after the glaciers melted the area was covered by an epicontinental sea, and eventually by a number of shallow freshwater lakes. It was during this time when the lakes were in the area that the Pedirka Basin formed, containing large amounts of mud and sand, as well as organic matter. By about 225 Ma the area was relatively dry, then downwarping of the crust resulted in the depression of the Eromanga Basin. Freshwater sediments from the lakes and rivers were deposited in the basin. The rocks formed by the compression of these sediments placed a seal above the hydrocarbons that had formed from the organic matter. During the Jurassic large amounts of sand were then deposited, to become the aquifers of the Great Artesian Basin (GAB).
Subsidence of the Eromanga Basin during the Cretaceous was followed by another marine transgression. The relatively impervious cap of the aquifers of the GAB resulted from the compaction of the marine deposits in this sea. The formation of the GAB took a total of about 150 million years. Once the sea had gone more freshwater sediments were deposited, and eventually the area was dry land again.
In the Early Tertiary the Lake Eyre Basin began to form about 60 or 70 Ma, the crust of the area subsiding to a lesser extent than in previous episodes of downwarping. The result was a broad, shallow depression, much of which was covered by shallow lagoons and slow flowing streams that meandered across the wide floodplains. Much of the present Simpson Desert overlies up to 200 m of sediments deposited at this time.
The area of the Simpson Desert was affected by the climatic swings during the Pleistocene, the lake and streams drying up during the last glacial maximum about 18,000-16,000. Dating of the dunes in the southern part of the desert and fossils in the sediments at the base of the dunes have indicated the dunes were actually formed in the Holocene 10,000-8,000 BP, making them relatively young.
The Simpson Desert, one of the driest areas of Australia, with rainfall less than 125 mm/year, is characterised by its dunefields, making up 73 % of the desert. The long straight dunes are oriented in a north-northwest/south-southeast direction. Some dunes converge to form Y-junctions with neighbouring dunes, but some remain straight and unbroken for 200 km. The largest dunes in Australia occur in the Simpson Desert.
As well as the dunes, the dunefields are comprised of the swales, also called the interdune corridors, between them.
The dunes are composed of siliceous, wind-driven sand. It is often mobile on the crests. The eastern slopes are usually steeper than the western slopes. The swales, the interdune space, some of which are of sand and others are of clayey sand, gibber surfaces or even alluvial deposits from old flood plains. The swales sometimes contain large claypans or saltpans (salinas). The height of the dunes varies between 10 and 40 m. The average space between the dunes is about 500 m. The greater the height the lesser the frequency of the dunes. The height and frequency of the dunes is influenced by the substrate of the swales. 15 m high dunes are about 200 m apart where the swale between them is sand, 30 m dunes with gibber swales can have a separation of up to 600 m.
Overall the Simpson Desert dunes appear to be uniform, but closer examination reveals a large amount of variation between the different regions. Based on a number of dune characteristics, the dunes of the Simpson have been categorised into 11 different systems of dunes. These characteristics include height, length, spacing, crest shape, convergence, swale nature, as well as characteristics of soil and vegetation. Fringing dunes, on the edges of the dunefield, have been separated into 9 types of minor dune systems - watercourses, salt lakes, freshwater lakes and floodplains.
A number of characteristics of the dunefields of the southern part of the Simpson, such as spatial organisation pattern, whether or not there are playa lakes, soil nature and the type of vegetation, have led to the development of a number of ecological associations.
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 (size estimates vary) of the Simpson Desert comprise half the dune-desert area of Australia. At present, 40 % of the continent is covered by dunes. Because of the fine-grained sediment, that prevents the organic matter being completely decomposed, the area is being explored for gas and oil.
Rivers and streams flowing into the Lake Eyre depression brought sand as alluvial sediment to the area of the dunefields, the material being collected from the vast area of the inward-draining catchment. Lake Eyre, the associated playas, and the wide floodplain of the Diamantina River to the southeast of the Simpson are the depocentres for this sediment (Twidale & Wopfner, 1990).
Dune formation begins with the surface sediment being picked up by wind and deposited in mounds on the leeside, on the northern margins of playas and floodplains, to form lunettes. Wind passing over the lunettes is deflected leading to the increase of turbulence resulting in the development of spirals and vortices. Sand is then moved by the wind to the 'dead' areas between these spirals, where it forms small sand ribbons or tongues, the origin of the dune that extends downwind as sand is gradually added by the wind (as identified by Grartz et al.).
Two prevailing wind systems of the Simpson produce and maintain the unique alignment of the sandridges. The prevailing winds, mostly from the southeast, but also from the southwest, produce dunes aligned along the axis of the combined windflow that results, generally NNW/SSE, as well as gradually extending the dune northwards along its axis. As long as a source of sand is available the dune growth is relatively self-sustaining.
As a result of the deposition of sediment that occurs whenever the rivers of the Channel Country in southwestern Queensland, Cooper Creek, Eyre Creek and the Diamantina River, flood sufficiently to continue all the way to the Lake Eyre region, dune formation can occur. The growth of dunes in the Simpson, mostly to the north, occurs episodically, advancing several metres in some years, remaining static in others.
The sand of the dunefields of the Simpson Desert is of a range of colours, from white to dark red, the lightest coloured sand being nearest to the source sediments in the south of the desert, becoming progressively darker to the north. The basic material of the dunes is white quartz sand, the particles of which are of a diameter of about 0.3-0.4 mm. The dunes comprised of this sand are white, but there is often a small amount of clay mixed in with the sand. As the dunes age any iron in the clay oxidises, the sand grains becoming coated with this iron oxide, the longer the weathering they have undergone the darker the red coating on the grains. At the downwind end of the dunefields, in the north, the darkest dunes are found, those farthest from the source sediments. The dark colour in these northern dunes is contributed to by sediment carried to the local area by a number of rivers, such as Hale River, Plenty River and Hay River that are sourced in the Central Australian Ranges such as the MacDonnell Ranges.
Ecological associations of the dunefields of the southern Simpson Desert
Wangkangurru Ecological Association. Dunes of red siliceous sand have a gentle western slope and steeper eastern slope, with sandhill canegrass on the dune crest. Lobed spinifex vegetates on the dune flanks and the swale. Acacia woodland in the swale.
Karanguru Ecological Association. Dunes of white or yellow siliceous sand are more symmetrical than those of the Wangkangurru Association. There is sandhill canegrass on the dune crests, but no lobed spinifex on the flanks or in the swale. A playa lake in the centre of the swale, the swale is vegetated with mixed Acacia and Hakea woodland and low chenopod shrubland.
Jeljendi Ecological Association. This association has the highest dunes in the Simpson Desert, and in the eastern desert the swales are wider. The vegetation of the swales, that are composed of clayey alluvium, is Acacia georginae woodland.
Land systems of the Simpson Desert
Surrounding the dunefields, the best known feature of the Simpson Desert, are a number of other landforms. Among these are major watercourses with associated floodplains, the Finke River and the Macumber River, on the western side, Warburton River and Kallakoopah Creek, on the southern side. On the eastern side are the Diamantina River, the Mulligan River and Eyre Creek. This drainage system was originally part of a much enlarged Lake Eyre, known as Lake Dieri, though some doubt this lake actually existed as a single, very large water body.
There are also a number of rivers flowing into the desert from the northwest. These are the Hale River, Plenty River, Hay River and Field River and Illogwa Creek. The catchments of these streams are in a number of ranges, the MocDonnell Ranges, Jervpois Range, Tarlton Range and Adam Range.
As a result of the inflow from these watercourses a flora and fauna have developed that is richer and more diverse than those in the dunefields of the central parts of the desert. As with the other parts of the arid zone, the rivers and creeks only flow after the very occasional heavy rainfall in their catchments. At these time the desert blooms, with a profusion of plants, many with brightly coloured flowers, that soon attract swarms of insects which in turn attract large numbers of birds and animals that eat insects. Ephemeral saline wetlands, that become progressively more saline as they approach Lake Eyre, occur on the floodplains of Kallakoopah Creek and the floodplains of the Warburton and the lower Macumba River in the far southernmost section the desert. These wetlands are unique to the Simpson, being actively fed by the rivers and creek.
There are also sandplains, gibber plains, breakaway country, or dissected residuals, low hills and tablelands.
A vast river system has been discovered flowing 35 m below the dunes of the Simpson Desert. The river system, including large rivers and smaller streams, is believed to have flowed on the surface about 50 million years ago in the Late Cretaceous, when the climate was much wetter than the present, eventually being buried by blowing sand that formed the dunes, at times when the climate was at its driest. The dunes covering the river system are believed to be about 1 million years old.
Fish - fossils
Metaceratodus wollastoni, Upper Cretaceous or Tertiary
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|