The provenance of the Australian continental dunefields

Abstract

This study of the Australian continental dunefields focuses on the origin of the sand-sized material presently in the dunes and on the distances of its transport within and between dunefields. The sand dunes which currently make up the Australian continental dunefields are part of a sedimentary system which has been developed over millions of years. This system has received sediment from large numbers of protosource areas both within and outside the present margins of Australia. Some of these protosource areas underlie the current dunefields while others are hundreds of kilometres away, indicating significant distances of sand transport. Each individual dunefield comprises material from several different protosource areas. Material from certain protosources is widespread over the entire desert, while that from others is found concentrated only in particular regions. It is suggested that the mechanisms for the transport of sand from these protosource areas into the sedimentary basins currently underlying many of the dunefields involved tluvial activity and marine transgressions and regressions. Most of this sedimentary transport is believed to have occurred prior to the change to a more arid climatic regime in Australia during the Late Tertiary. Following this climatic change and commonly coincident with periods of glaciation, the Australian continental dunefields have been formed through the vertical . corrasion of local/underlying sedimentary material, accompanied by lateral .displacement of sand and upward growth of nearby ridges. Very little evidence has been found in this study to suggest significant distances of aeolian transport of sand. The main function of wind in the Australian dunefields is thought to be in the shaping of dunes. At the present time, the Australian continental dunefield is largely stabilised, and all but the crests of dunes are covered by vegetation, thereby reducing the possibility of aeolian transport. The determination of protosource areas for the desert sands has been based predominantly on U-Pb SHRIMP studies of zircon grains separated from the sand samples. This method provides direct evidence on provenance, albeit the provenance of zircon grains and not bulk sand samples. Studies of the oxygen-isotope values of quartz have also provided useful information on sand protosource areas, although this technique was hampered by the large numbers of protosource areas involved and by the presence of small amounts of low-temperature quartz in many samples. Characterisation of sands from the major dunefield areas in Australia in terms of their physical and mineralogical properties shows that there are significant differences in these parameters across individual dunefields. These differences are commonly found in sands from dunes overlying the boundaries between different underlying rock types. The apparent lack of mixing between sands derived from different underlying rock types indicates that transport within dunefields has not been substantial. On a wider scale, despite the possibility of the long-distance aeolian transport of sand around the continental "whorl" of the Australian dunefields, there appears to have been only a limited amount of such transport between dunefields. Similarities between the characteristics of sands from dunes directly overlying basement areas and the properties of the basement areas themselves, suggest that dune sands were derived from the weathering of the underlying bedrock, and could therefore be considered as remnant lag deposits. Dunes in these areas tend to be smaller, more scattered, and in many cases, merge with sand plains. In contrast to this, the major dunefield areas in Australia overlie sedimentary basins where sand supply is plentiful. In general, there has been negligible recent (post Late Tertiary) sedimentary input to the Australian continental dunefields. The exception to this is in the Simpson, Strzelecki and Tirari Deserts, where rivers and creeks flowing into the Lake Eyre Drainage Basin have deposited small amounts of sediment. After its fluvial deposition, this material has not been transported significant distances within the dunefields and is concentrated in areas near the rivers and creeks. In some areas of the southeastern Simpson and Strzelecki Deserts however, it is thought that limited amounts of aeolian transport of sand occur, although this is considered to be an exception to the general stability of the dunefields and is only very localised. Show more