Identification of aeolian dust mantles in the semi-arid Flinders Ranges, South Australia

Abstract

Sediment samples have been analyzed from ridge-top and valley-fill profiles and Lake Torrens in the central Flinders Ranges, South Australia. Ridge-top samples show evidence of addition of silty clay rich aeolian dust. The aeolian dust is characterized by its uniform texture (silty clay to fine sandy clay) and colour (dark red brown) over a range of topography and lithology. On the ridge-tops this material ranges in depth from 5 - 30 cm and is distinguished by a well-sorted particle size distribution and significant particle population in the 2 – 63μm size fraction. Kaolinite and illite dominate the clay mineral suite in the ridge-top samples. Trace elements Zr, Ti, Th, La, Ce, Y, Cr and Nd are enriched in the ridge-top profiles compared to the underlying bedrock. 143Nd/144Nd ratios of the ridge-top, valley-fill and Lake Torrens samples show clear correlation with the Bunyeroo and Brachina Formations. 87Sr/86Sr residue ratios show a similar pattern with ridge-top, valley-fill and Lake Torrens samples similar to the Bunyeroo and Brachina Formations. 87Sr/86Sr leachate ratios are notably lower, ranging from (0.710- 0.715), showing the influence of the carbonate rock units of the Flinders Ranges and marine derived windblown carbonate. Excluding sample LT-6, the Lake Torrens samples have the same clay mineral suite, similar major and trace element geochemistry patterns and similar 87Sr/86Sr and 143Nd/144Nd ratios as the ridge-top and valley-fill samples along with the Bunyeroo and Brachina Formations. The particle size, geochemical and isotopic data all imply a significant recycling of sediment between the Flinders Ranges and Lake Torrens. During drier intervals fine sediment is blown eastwards from the Lake Torrens playa to form the ridge-top and hillslope aeolian dust mantles, later reworked by slopewash and incorporated into the fine-grained valley-fill deposits. During the more humid phases fine sediment is transported westwards by streams from the Flinders Ranges out to the Lake Torrens playa.