DO LINEAR DUNES FORM BY LINEAR EXTENSION?

Broad-crested linear dunes cover about 40% of Australia and constitute about 40% of the world's dune area. Given this extensive distribution, it is surprising that so little is known as to how they actually form. In central Australia, the easterly extent of the MacDonnell Ranges borders the Simpson Desert. In Camel Flat basin, widely spaced strike ridges intercept and isolate pockets of broad-crested linear dunes. Between about 75 ka and now, regional changes in Late Quaternary climate and flow regime associated with periodic avulsion of the Todd River have destroyed portions of the older dunefield, producing desert surfaces and associated dunes of younger age. This chronological diversity has enabled a study of the evolution of linear dunes. Large, widely-spaced fine-grained red dunes, 75-65 ka in basal age, occur on the western floor of the basin that has been isolated from fluvial activity. They are ramped against the foot-slopes of the Train Hills that block their migration northwards. The paucity of aeolian sand accumulated against this range, and the detailed luminescence chronology of the dunes in the basin, indicate negligible downwind sand and dune migration over this considerable period. We propose three models of possible linear dune formation: 1) Long distance sand transport with linear extension; 2) Windrift linear-extension; 3) Windrift vertical-accretion. Linear dunes in the northwestern Simpson Desert are formed mostly by wind rifting involving the vertical accumulation of sand swept from adjacent swales and dunes (Model 3) and some linear extension as sand becomes trapped in the lee of the nose of the existing dune (Model 2), but not as has been previously proposed by long-distance sand transport with linear extension (Model 1). Linear dunes evolve by becoming larger and more widely spaced, growing by vertical accretion (Model 3), as they cannibalise adjacent, smaller, more closely-spaced dunes.