GEOLOGY AND SOILS OF THE COONAWARRA WINE REGION
In 1998, the Australian Wine and Brandy Corporation approached Dr Doug Mackenzie and requested his opinion on the geology and soils of the Coonawarra wine-producing district, as interpreted from airborne geophysical (radiometric) data. This information was to form part of the criteria for defining the "Geographic Indications" (GI) boundary of Coonawarra. The central part of the task was to identify the radiometric "signature", and map out the full extent, of the famed "Terra Rossa " soil.
| COONAWARRA RADIOMETRICS |
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This illustration is an RGB (red-green-blue) "false-colour" image derived from multi-channel gamma-spectrometric data obtained by aircraft flying at 80 metres (northern third of image) and 160 metres (southern two thirds) above ground level. The main sources (channels) of this low-intensity gamma radiation are potassium, represented by red tones, thorium (green)and uranium (blue); the intensity of each colour is proportional to the abundance of the corresponding element in the top ~45 centimetres of soil.
Dark tones represent quartz sand, very sandy soil, or very wet soil (water absorbs gamma radiation). Pink, scarlet, purplish and orange tones represent dark clay soils, with a calcrete layer, developed on calcareous sand and silt (Padthaway Formation); subdued green tones represent clay soils developed on calcarenite (Gambier Limestone); and brighter green tones represent relatively well-drained, variably sandy clay soils, also with a calcareous layer, developed on a particularly sandy part (facies) of the Padthaway Formation. This sandy area may at its time of deposition, about 500,000 years ago, been an off-shore sand bar or a beach.
Blue areas are ponds and swamps, blue lines creeks and water courses; the green line is the proposed ("interim") Coonawarra GI boundary as at the end of 1998.
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The radiometric image clearly shows that the "cigar" of Coonawarra's "Terra Rossa" soil (the bright green tones) lies along the eastern edge of an elongate area of soils formed on sediments of the Padthaway Formation, which are characterised by relatively high potassium contents (generally between 0.6 and 1.2 weight percent K). This area is flanked by soils (and sandy deposits) with very low-intensity radiometric signatures which indicate that the soils are quartz-rich, very wet, or both.
| CLASSIFICATION OF COONAWARRA RADIOMETRICS |
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This map shows the result of a supervised classification of the radiometric data. The supervision used measurements of the radiometric "signature" of areas of known soil type in a formula that assigned a different colour to each class.
The extent of the "Terra Rossa" according to this classification is shown in bright red. Note that the area of bright red corresponds well with the Penola Land System (approximately concident with the "Terra Rossa") as mapped by the South Australian Department of Environment, Heritage and Natural Resources, outlined in white.
NOTE: This image extends about 4 km farther to the north and to the east than does the image above.
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Copyright © Terroir Australia Pty Ltd, 2002
