Formation and age of desert dunes in the Lake Eyre depocentres in central Australia

Abstract

The sand dunes of the deserts surrounding Lake Eyre record the latest episode of climatic development from the warm, humid conditions of the Early Tertiary to the hot and arid climate prevailing today. The dunes are typical seif dunes which rest on a substrate of Late Pleistocene fluviatile and lacustrine deposits. The sand ridges are thus essentially of Holocene age, although earlier cycles of aridity are evidenced by remnants of older dune fields. The dunes are still advancing across the desert plains, blocking river channels and causing tracks to be diverted. The dune sands are white in the source areas but they become increasingly red downwind, due to the development of a hematite coating on the sand grains. The dunes derive from sand accumulations bordering the lee side of sand sources such as playas and alluvial plains. Large numbers of short sand ridges develop from transverse, barchan-like forms. The incipient dunes coalesce downwind from the generating area, and progressively fewer, but larger dunes evolve from the many small ones. Variations in dune spacing, both along and normal to the dune trend are related to sand supply and reinforcement mechanisms. Dune height is directly related to the transverse spacing. Internal structure and observed short term changes in dune asymmetry caused by different wind directions suggest that the dunes are depositional and that, once beyond the generating zone, the dunes are maintained by a bidirectional wind regime. The resultant of the dominant wind directions determines the long axis of the sand ridges and the trend of migration direction. Several man-years of field observations let us reject helicoidal flow regimes as a significant dune-forming mechanism.