Geological maps available at the time, published by the Geological Survey of South Australia, were inadequate in both detail and accuracy to be useful at the scale of the area of interest. It was necessary, then, to make use of some of the "black box" data that geologists use today to unravel the geology of areas where little rock is seen at the surface: gamma-ray spectrometric data.

Gamma-ray spectrometric (or 'radiometric') data are obtained by low-flying aircraft fitted with a very sensitive multi-channel spectrometer which measures the very low levels of gamma radiation being emitted from the uppermost ~35-45 centimetres of the land surface. Most of this radiation comes from radio-isotopes of potassium, thorium and uranium. It is unaffected by vegetation cover, buildings, roads, etc., and is therefore a direct measure of key chemical properties of the land surface.

The three main components of the radiometric data - potassium, thorium and uranium - as three primary colours: red, green and blue, respectively. The combination of these colours produces what is termed a false-colour image which, in turn, is used to produce a map showing the pattern of variation in the composition of the land surface.

	GAMMA-RAY SPECTROMETRIC IMAGE OF THE SOUTHERN ADELAIDE HILLS, SOUTH AUSTRALIA
			This illustration shows the geological rock-type boundaries, as known when this project began, superimposed on a radiometric false-colour image of the southern Adelaide Hills region. The "wedge" of relatively bright, reddish tones on the left represents ancient (Palaeoproterozoic; about 1,600 million years old) metamorphic rocks of the Barossa Complex. The areas of darker, greenish tones represent mainly dark-grey shales of the Burra Group (about 700-800 million years old), and the very dark areas represent mainly sandstones within the Burra Group, along with the Stonyfell Quartzite (far left, centre) and 'damp' alluvium. It is clear from this image that there are many inaccuracies and omissions in the published geological map; the following image and map show the results of the Terroir Australia revision of the geological mapping.

In the case of the Piccadilly Valley, additional information obtained from airborne magnetic data (collected simultaneously with the radiometric data) was used to interpret deep structural features such as faults. The interpretation of the radiometric and magnetic data was then used, along with existing geological information, visual interpretation of 1:20,000-scale colour airphotographs, and extensive field investigation to revise the geological map in the Piccadilly Valley area.

	RADIOMETRIC IMAGE OF THE PICCADILLY VALLEY			GEOLOGY AND VINEYARDS OF THE PICCADILLY VALLEY, SOUTH AUSTRALIA
				Deep Brown = Barossa Complex gneisses   Olive = calc-silicate rocks;   Violet = possible calc-silicate rocks   Dark Green = Barossa Complex schists   Light Brown = Castambul Formation black shales   Grey = Woolshed Flat Shale grey shale; siltstone; fine sandstone   Light Green = Basket Range Sandstone   Light Blue = Stonyfell Quartzite;   Pale Yellow = alluvium.   Note: arcuate light brown area extending through Group 2 should be grey.
	This image shows detail of the area outlined in the illus- tration above. The dark area that extends from the major geological boundary (crossing the image from upper right to lower left) to the small cross represents 'calc-silicate' rocks with special chemical properties, explained below.			This map shows the various rock formations of the Piccadilly Valley, in the southern Adelaide Hills of South Australia, with the Petaluma chardonnay vineyards (in red) superimposed. The main groups of vineyards are numbered: 1 - Tiers; 2 - Eric's; 3A - Summit; 3B - Mt Bonython; 4 - St Margaret's

Group 1 vineyards.

The Tiers group of vineyards is underlain by Palaeoproterozoic calc-silicate rocks* that have chemical compositions not unlike the 'average crustal' composition - i.e., without any of the major elemental deficiencies that are caused by the weathering cycle. Most importantly, these rocks are rich in calcium relative to sodium have weathered to form thick (>1 metre) soils that, because of the mineralogical and chemical composition inherited from their 'parent' rocks, are clay-rich but quite free-draining. Their chemical composition is favourable for plant growth, and their deep red-brown colour is favourable for fruit ripening in the cool climate of the upper Piccadilly Valley.

* Also known as 'granofels': very 'dirty' limestone, or limey sandstone, metamorphosed to form minerals similar to those found in some 'granitic' rocks - similar rocks elsewhere in the Adelaide Hills have been called "Houghton Diorite" (diorite is a type of granitic rock, with abundant dark, Fe-Mg-Ca-rich minerals).

Chardonnay made from Tiers vineyard fruit is typically ripe, 'white peachy', quite full-flavoured, and very long and even-textured ('seamless') on the palate. It seems to mirror the deficiency-free - 'complete' - chemical composition of the soils and the rocks beneath.

Vineyards in Group 2 ("Eric's", or "Pfitzner's") vineyards are underlain by dark grey Woolshed Flat Shale, part of the Burra Group. The viticulture in these vineyards was not fully under the control of Petaluma at the time of the investigation, so they were not studied in detail.

The Summit vineyards (Group 3A) are underlain by grey shale, sandy shale, and quartz-rich siltstone/fine sandstone of the Woolshed Flat Shale, which is particularly pale and quartz rich in this area. These rocks are deficient in elements such as calcium, magnesium, manganese, sodium, phosphorus, cobalt and, commonly, iron when compared with the calc-silicate rocks beneath the Tiers vineyard. Soils are shallower and stonier, their clays 'heavier', and their drainage a little poorer than soils of the Tiers vineyard.

Chardonnay made from Summit vineyards fruit typically has a more delicate aroma, is lighter-bodied, slightly less ripe ('nectarine', 'melon rind' and 'grapefruit' rather than 'white peach'), has more prominent acidity, and is less 'seamless' than the typical Tiers vineyard chardonnay.

The Mount Bonython vineyards (Group 3B) are also underlain by Woolshed Flat Shale, but there are several differences with the Group 3A area.

• The rocks are less quartz-rich and finer-grained overall, but pyrite (scattered cubic crystals in some black shales) is more common.
• Soils are thicker, more clay rich and better watered, partly because of a slightly lower position in the landscape.
• Aspect is less favourable, with more easterly and southeasterly facing slopes and more afternoon shadow (from Mount Bonython).

Chardonnay made from Mount Bonython fruit is typically a little less intense and 'ripe' (closer to 'grapefruit' and 'melon rind') in aroma and flavour, and has more 'dips and hollows' in the palate, than chardonnay from the Summit vineyards. The contrast is even stronger with Tiers vineyard chardonnay.

Vineyards of Group 4, the St Margaret's vineyards, are underlain by Basket Range Sandstone, which consists of little else other than quartz sand (with a little feldspar that has weathered to kaolinite, and traces of white mica [muscovite]). The weathered sandstone is highly depleted in all plant-nutrient elements, and the soil is intrinsically quite acidic. However, decades of vegetable growing, using copious amounts of fertilisers, have rendered the soil productive, but made its composition unbalanced, with near-toxic levels of some additives.

The St Margaret's vineyards produce chardonnay that contrasts dramatically with that from Tiers: it is much lighter in body, 'citrussy' in aroma and flavour, much more noticeably acidic, and has distinct 'dips and hollows' in the palate. The very fresh acidity is useful in blending, but not appealing in an unblended chardonnay.

3. THE "BOTTOM LINE"

As a result of the findings of this study, Brian Croser and Petaluma were satisfied that there is indeed a rational terroir-related (in this case basically geological) reason for the outstanding qualities of Tiers Vineyard chardonnay. The vineyard is underlain by very old, very unusual rocks with properties that are particularly favourable for full-flavoured fruit. Petaluma was able to use this information as the essence of its marketing strategy for the Petaluma Tiers Chardonnay, a very rare and sought-after wine that retails at a premium of approximately $90 per bottle over its blended stablemate, the Petaluma Piccadilly Valley Chardonnay. That represents a net return on the investment in Dr Mackenzie's input of, conservatively, $200,000 (before tax) in the first year.

[ Back ]   [ Home ]

Copyright © Terroir Australia Pty Ltd, 2002