SURFACE-FEATURES OF SAND GRAINS FROM THE AUSTRALIAN CONTINENTAL DUNEFIELD

Post-depositional features dominate the surfaces of sand grains from the Australian Continental Dunefield. Variations in the type and form of these features are evident among dunefields and also within individual desert regions. These variations are used to differentiate between sediment sources, and to provide evidence for the means of sediment transport into the deserts. Chemically precipitated features observed include previously unreported forms and are interpreted in terms of the environmental and climatic characteristics of the sand source regions. These forms here termed Type I sheet precipitation and Type II plate precipitation are believed to be intimately associated with the absence or presence respectively, of significant quantities of clay or clay pellets within the dune system. In the absence of clay the re-precipitation of silica from pore waters leads to the formation of Type I precipitated sheets. Type II plate precipitation results when grain surfaces covered by clay coatings are overlain by a secondary cover of Type I precipitated sheets. Such a cover would act as a resistant barrier to chemical attack and therefore protect the clay coatings from removal. Polygonal cracking found associated with Type II precipitation results from the dehydration of these underlying clay layers causing the disintegration of their silicate lattice structure. The predominance of chemically produced surface features and the poorly rounded and irregular nature of most grains indicate that the majority of sand in the Australian Continental Dunefield is currently stabilised and has not been transported long distances during its sedimentary history.