Age and origin of Terra Rossa soils in the Coonawarra area of South Australia

Crown copyright © 2004 Published by Elsevier Science B.V.The famous Terra Rossa soil in the Coonawarra area, South Australia, is dominated by locally derived aeolian detritus, which probably accumulated over the last 120–130 ka. Four soil profiles and associated limestone and lunette deposits were investigated using the following methods: mass balance geochemistry of bulk soil samples (major and trace elements), quantitative X-ray diffraction (XRD) mineralogy, strontium isotopes (87/86), as well as grain-size analysis and cation exchange capacity. These data show that the Terra Rossa soil from the Coonawarra has a thick, clayey B-horizon which is geochemically homogeneous and dominated by smectite and kaolinite. Mass-balance calculations show unrealistic weathering scenarios when plotted using silicate residuum from the underlying limestone as parent. Realistic weathering scenarios are produced with fine-grained silicate material from local lunette deposits as parent. Strontium isotopes of silicate residuum from Gambier Limestone (0.78) contrast strongly with the clayey B-horizon (0.726). Strontium isotope ratios of silicate material from a local lunette (0.725) are similar to the B-horizon soil values. Strontium isotope ratios from regional geological units indicate that the strontium signature in the lunette and soil B-horizon is dominated by weathering products from the Palaeozoic Kanmantoo shales, extensively exposed upwind to the west on Kangaroo Island and the Fleurieu Peninsula. Optical (optically stimulated luminescence, OSL) dating of 61 individual quartz grains (single aliquot) from three samples in the Coonawarra soil profile (one from the A-horizon and two from the B-horizon) shows that most of the quartz sand grains have been buried for only a few thousand years. Many of the grains, however, have been buried for tens of thousands of years with three grains having exposure ages of between 105 and 109 ka. The large population of young exposure dates represents quartz sands recently exposed in the A-horizon and which have been translocated down to the B-horizon. The older exposure dates are interpreted as representing grains that were buried during or soon after the accumulation of wind-blown silt and clay. Our current model concerning the timing and conditions of aeolian deposition of the Coonawarra soil is that much of it accumulated during the relatively wet, last interglacial period around 120–130 ka. During that time span, it is thought that the playa–lunette systems in the low-lying areas to the west were particularly active and generated a significant local dust fluxAija C. Mee, Erick A. Bestland and Nigel A. Spoone