Australia: The Land Where Time Began

A biography of the Australian continent 

Linear Dune Formation: Local Wind Rifting or Longitudinal Elongation - Strzelecki Desert, Central Australia

Linear dunes dominate the desert dunefields that form a large whorl, in a clockwise direction, across more than 1/3 of the surface area of the continent of Australia (Wasson et al., 1988). Mechanisms that are responsible for the formation on a regional scale of linear dunes in time and space are not well known, in spite of the extensiveness of the dunefields. According to the author1 linear dunes are formed broadly parallel to the resultant vector of the winds that shift the sand, their length varying from several hundred metres to several hundred kilometres. Several models have been proposed for the mechanism of formation of linear dunes on large scales. It is proposed by the downwind extension model that linear dunes extend longitudinally along their length by progradation (Mabbutt & Sullivan, 1968; Wopfner & Twidale, 2001). According to the author1 in such cases the upwind derived sediment is dominant over the sediment that is locally derived, the result being that dune ages are increasingly younger downwind, and evolution of sediment. Aeolian scouring (wind rifting) of sediment from local interdune swales which is added to dunes results in upward sediment accretion that is limited by the supply of sediment (Hollands et al., 2006; King, 1960). Deflection of sand grains for short distances along dune axes maintains the longitudinal morphology of linear dunes (Tsoar, 1978) which results in downwind propagation of dune morphology. According to the author1  this model would display no age decline downwind and not rely on sources of sediment from upwind.

 The author1 suggests a better understanding of the processes involved in the formation of linear dunes as a function of both space and time may be gained by knowledge of linear dune activity along a longitudinal transect. On the southwestern margin of the Strzelecki Desert in central Australia  the region of linear dunes to the east of the large playa Lake Frome is suggested as a possible site for testing the models of dune formation that have been proposed, as multiple stratigraphic horizons, separated by palaeosols, have been preserved in the dunes of that region, and overlie fossil source-bordering dunes that are transverse. The occurrence of multiple dune-building events, at different points along longitudinal transects, is indicated by the preservation of dune stratigraphy, and the relative availability locally of sediment is indicated by the proximity or otherwise of fossil transverse dunes. Lake Frome originated to the east of its present position location, having migrated westward to its present location over time, hence the fossil transverse dunes are designated as such, in spite of not being present adjacent to the Lake Frome of the present, as they formed as shorelines adjacent to the ancestral Lake Frome to the east of the present-day lake. Episodes of migration of lake depocentre or regression was followed by preservation of a fossil transverse dune, all of which were degraded over time until they are now degraded features of the landscape. Dunefields are largely relict features that formed under wind conditions that differed from those of the present, as indicated by their orientation not aligning exactly with the prevailing wind regime of the present.

The author1 took samples from 3 sites along about 20 km of a longitudinal transect of an individual dune to test the role in dune formation of downwind extension verses local aeolian wind rifting. Site I represents the point of genesis, site II represents the site approximately halfway along the length of the dune, and III represents the terminus of the dune. 2 fossil transverse dunes, that are topographically subdued to the extent that in the field they are only rarely discernable, are traversed by the longitudinal profile, and these features have been covered by longitudinal dunes that have migrated longitudinally over them. The author1 suggests the transverse dunes were probably the source of the sediment over time, and possibly having been reactivated at the same time as the linear dunes were forming (e.g. Fitzsimmons et al., 2007a).

All 3 sites along the linear dune display sedimentology that is very similar, with substantial proportions of feldspar and rock fragments, and it is suggested by zircon provencing studies that the derivation of sediments across this region were from basement rock of the Flinders Ranges to the west (Fitzsimmons et al., 2009; Pell et al., 2000). It was found that at sites overlying transverse dunes, as well as within the transverse dunes, the feldspar content was especially high, though there is no systematic variation of feldspar content with the distance from the Lake Frome playa or Flinders Ranges at this scale (Fitzsimmons et al., 2009). According to the author1 it is suggested by this that the formation of dunes may rely on local windrifting. At site I there are clay pellets that were deflated from adjacent swales that were inundated periodically. The local windrifting hypothesis as the dominant mechanism involved in the formation of linear dunes is further supported by the instability of clay pellets under long distance transport.

The samples from each of the stratigraphic units identified at sites II and III were dated by OSL and at regular intervals of depth at site I (Fitzsimmons et al., 2007b). Activity at site I is indicated between 34 ka and 28 ka by the oldest reliable ages for the longitudinal transect. The 3 ages are within 2σ of each other which indicates they effectively represent at least 3.2 m of sediment deposited in a single event. Aeolian activity is implied to be effectively at all 3 sites between approximately 16.5-13.6 ka by the uppermost age at site I being within 2σ of the ages of all ages that are reliable at the other sites. The ages at site II are predated by the lowermost ages at site I, suggesting to the author1 that it possibly indicates transport downwind over time, though it is unlikely given the age of about 52 ka obtained at the lowermost linear dune horizon at site III which is further downwind. 

There is no evidence that is unequivocal for a downwind age decrease, as would be expected if dune development is by downwind extension, in the case of this transect in the southwest Strzelecki Desert. No decrease of feldspar content downwind, which might also occur with downwind extension, is suggested by the sedimentological evidence. The author1 suggests luminescence dating is useful for assisting the testing of geomorphic processes that have been previously unassessed, as the chronological evidence is consistent with the evidence from sedimentology of material that has been derived locally. According to the author1 the windrift model of reworking and accrtion locally is favoured, based on the combined evidence from geochronological, sedimentological and stratigraphic methods.

Sources & Further reading

  1. Fitzsimmons, K.E., 2010, Linear Dune Formation: Longitudinal elongation or local wind rifting? An example from the Strzelecki Desert, Central Australia, Key Concepts in Geomorphology, Vignettes, UVM
Author: M. H. Monroe
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Last updated 13/03/2013

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