Australia: The Land Where Time Began

A biography of the Australian continent 

Lunettes or source-bordering dunes

Lunettes are crescent-shaped, fixed dunes along the edges of playas and river valleys in arid and semi-arid lands. Lunettes are well developed in semi-arid parts of southern Australia (including Tasmania), on the eastern side of lake basins.  They have now been found in the monsoonal north of  Australia. In north Queensland along the borders of lakes in the Great Divide. In the Northern Territory on the western edge of Lake Woods near Newcastle Waters. In the arid interior, where some claim they play a crucial role in the initiation of linear or longitudinal sand dunes.

Lunettes are crescent-shaped in plan, looking similar to barchans, but whereas barchans are mobile, lunettes are fixed. Another difference between these 2 sand structures is that the horns of a barchan point downwind, those of a lunette point upwind. And the horns of a lunette are close to the margin of the lake basin. Barchans are steeper on the downwind slope, lunettes are steeper on the upwind slope, with the crest close to the lake edge.

They vary in size, Australian lunettes are up to 6.5 km long, 1 km wide and 50 m deep. They can be composed of silt, gypsum, but mostly of sand. Some have been found to have cross-bedding and gravel has been found near the bottom of some. Some lake basins have only 1, but there can be more. They can be of very complex structure.

Lunettes in desert regions, such as those on the northern margin of Lake Eyre, deflect the southerly wind, increasing turbulence in the air flow. This leads to initiation of the fields of linear or longitudinal dunefields, in other deserts as well as Australia.

A number of mechanisms have been proposed for the formation of lunettes. They were reported by the explorer Thomas Mitchell in western Victoria. Lunettes near Echuca in Victoria were suggested to be the result of deposition by the wind. E.S.Hill was the first to study lunettes in Australia scientifically, his study including many lunettes from northwestern Victoria. In that region lunettes are mostly formed from fine-grained silt, with the result that Hill's interpretation was based on that material. According to his conclusion the lunettes formed by the trapping of atmospheric dust by the moist air being blown from the lake basin at times when water was present, the trapped dust being deposited on the eastern margin of the lake, being carried by the westerly winds.

This mechanism cannot explain the formation of the many sand lunettes that are present elsewhere in southern Australia. According to the 'wet hypothesis', lakes being involved in the formation, the lakes being required for the formation of lunettes to occur, but have an origin that is independent. It was later suggested that lunettes formed when the lake basins were dry, the 'dry hypothesis', the prevailing westerly winds carrying the sand particles to the basin edge by saltation, the building into mounds or lunettes resulting from the trapping of the sand by vegetation. Both wind erosion of the lake basin and particle transportation by wind are involved in this mechanism. This would account for the cross bedding that is occasionally found in these structures. The packing of sediment, especially clays or silts, is disrupted by the crystallising out of salt in the lake basin. This makes the surface susceptible to wind erosion as a result of roughening (Stevens & Crocker, 1946). According to this proposal, the lake basin forms where the drainage is interior, with areas of fresh alluvial deposits on which no vegetation has not established. In this case the lake and lunette develop together, the lunette being formed of the sediment of the lake basin (Twidale & Campbell, 2005).

Twidale & Campbell suggest that this proposal implies that the mineralogy and grain size should be similar in the lunette and the lake basin sediments. This is usually the case, but some of the very fine components of the lake sediment is not present in the lunette. This is probably explained by this very fine-grained component being carried further away in suspension than the lunette. Salinas, usually with a salt crust, mostly of halite or gypsum,  comprise many of the lakes that are bordered by lunettes. Gypsum is the material forming some lunettes, but there are also sandy lunettes that are found bordering salinas. The problem with this suggestion is that it must explain how sand can be blown to form lunettes when it is covered by a crust of salt. Another problem is that for a lunette to be formed by winds during the dry season they need to be deposited on the shore opposite to that from which the wind is blowing.

In southern Australia summer is the dry season. If the prevailing winds at this time of year are from the northwest, for example, the lunettes resulting should be on the southern margin of the lake. What little wind data is available suggests that the lunettes result from wet season, winter, wind patterns, suggesting it is not consistent with the 'dry hypothesis'. A confusing factor is that some lunettes are relic features, so modern wind patterns may not apply to their formation. Fresh sand is still being deposited on lunettes in southern Australia at the present, meaning these lunettes are still actively developing (Twidale & Campbell, 2005).

The authors suggest the problems associated with the proposed mechanisms could be overcome by another proposed mechanisms that involves water in the lake basins, debris being carried to the shore by wind-driven waves. At this stage deposition of detritus on the beaches occurs. In the later winter or summer the detritus is blown into the vegetation zone where it is trapped to form the lunette. This could also explain the finding by J.M.Bowler of gravelly material in some lunettes. Even in the shallow water of lakes, gravel can be carried by waves. It also overcomes the problem of getting sand from beneath a crust of salt, the salt going into solution in the water, allowing the sand to be moved by the waves to the shore.

In desert regions, lunettes such as those on the northern margin of Lake Eyre, deflect the southerly wind resulting in increased turbulence in the airflow. This mechanism has an important role in the initiation of linear or longitudinal dunes in dunefileds in the deserts of Australia and elsewhere.

Sources & Further reading

  1. Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000
  2. Twidale, C.R. & Campbell, E.M., 2005, Australian Landforms: Understanding a Low, Flat, Arid, and Old Landscape, Rosenberg Publishing Pty Ltd

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 05/04/2011 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading