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1a Age and origin of Terra Rossa soils in the Coonawarra area of South Australia

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

Aija C. Meea, Erick A. Bestlanda,*, Nigel A. Spooner

Earth Sciences (SoCPES), Flinders University, GPO Box 2100, Adelaide 5001, South Australia, Australia Research School of Earth Science, Australian National University, Canberra 0200, ACT, Australia
Received 6 September 2002; received in revised form 9 April 2003; accepted 10 April 2003

Geomorphology 58 (2004) 1 –25

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 finegrained
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 flux.
Crown Copyright D 2003 Published by Elsevier B.V. All rights reserved.
Keywords: Terra Rossa; Aeolian accessions; Mass balance geochemistry; Strontium isotopes; Optically stimulated luminescence dating

* Corresponding author. School of Chemistry, Physics and Earth Sciences, Flinders University, GPO Box 2100, Adelaide 5001, Australia.
Tel.: +61-8-8201-2306; fax: +61-8-8201-2676.
E-mail address: Erick.Bestland@flinders.edu.au (E.A. Bestland).

0169-555X/$ – see front matter. Crown Copyright D 2003 Published by Elsevier B.V. All rights reserved.