Detailed geological analyses of plots belonging to the «AOC Gaillac» area have been carried out in order to adress one of the main natural component ruling the terroir effect process. These plots belong to terraces of the left bank of the Tarn river which coincides with one of the three main terroirs of the AOC area. Precisely, the analysed plots are localised on the rissian-aged (≈ 200 000 yrs B.P.) terrace composed of alluvial shelves crosscut by small valleys where the Oligocen (ca. 28 My) marly molassic basement outcrops. Three different Basic Terroir Units (BTU) have been identified : terrace shelf, terrace slope and comb. Each of them has specific viticultural potentialities related to its topographical, geological and pedological characteristics. Representative profiles have been analysed in each BTU. Field analysis has evidenced that all rocks material have derived from Rissian alluvial deposits due to solifluxion processes when part of the alluvial material deposited on the terrace shelf has slept onto the slope overlying the marly Oligocen molassic basement. This solifluxion phase has taken place during the late-glacial Würmian climatic oscillations interval (Böllering-Alerod episode ca. 12,000 years BP). Afterwards, during the Holocene period (i.e. the last 10,000 years) the alluvial-derived material has suffered pedogenetic alteration. The nature of the resulting alterites depends on the initial topographic situation inherited from the late-Würmian solifluxion phase. On to the terrace shelf the soil sequence begins by a reddish clayey horizon (up to 0, 6m) because of the erosion of the eluvial horizon during the last 10,000 years. It is followed by a thick (≈ 1m average) reddish coarse-pebble horizon rich in clays and iron oxydes. On the terrace slope, characteristics luvisols have developped composed by an eluvial silty-sandy horizon (up to 0.60 m) overlying an illuvial pebble-sand level (up to 3m) where clays and ferrous oxydes are moderatly accumulated. Finaly, the thick (> 2m) dark silts and clays sequence (with scattered gravels and small pebbles) of the comb derive from the deposition of eroded soil material of the above terrace shelf and slope units (colluvium).
On the basis of the role of high qualitative limiting factor played by the water stress parameter such as quality of drainage, permeability of soils, the down-side slope terrace unit appears as the most appropriate unit because of its slope gradient combined with the occurrence of a thick permeable pebble-sand sequence. Finally, combination of physical and chemical results - acidic pH and very low CEC - permits to recommand the Gravesac rootstock adapted to well-drained acidic soils and Syrah/Fer Servadou climatic-adapted grapevine varieties as the most suitable vegetative material. In addition, the knowledge of the geological component at the scale ot the basic units allows for the adaptation of some cultural practices in order to enhance the viticultural potentialities of the plots. In order to encourage the vine's roots to dig deep and reach the sandy-pebbles horizon, two cultural possibilities are proposed: inter-row grassing associated with the « inter-plant » method or earthing down under the row associated with inter-row ploughing. The choice will depends on the soil erosion amplitude if the inter-row ploughing method is used.
The analysis demonstrate the efficiency of detailed geological survey using BTU concept as an operational tool. Further, it enhances that the geological component can be regarded as an amplification point of the terroir system as any alteration - even of minor importance - of the geological parameters, may have noticeable consequences on the resulting terroir effect.
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