Geological characterization of plot belonging to the left bank terraces terroir of the Gaillac vineyard (Tarn, Midi-Pyrénées). Consequences on determination of choice of vegetative material

Detailed geological analyses of a plot belonging to the « AOC Gaillac » area have been carried out. This plot belongs to the left bank terraces of the Tarn River which coinciding with one of the three main terroirs of the AOC area. It is localised on the rissian-aged (≈ 200 000 yrs B.P.) terrace composed of alluvial shelves crosscut by small valleys where the Oligocene (ca. 28 My) marly molassic basement outcrops. It spatially coincides with the terrace slope on which typical luvisols have developed composed by an eluvial silty-sandy horizon (up to 60 cm) overlying an illuvial pebble-sand level (up to 3 m) where clays and ferrous oxides are moderately accumulated. The slope terrace appears to be a unit with great potential for production of high quality wine because of its high topographic gradient combined with the thick permeable pebble-sand sequence, both triggering a high drainage coefficient. Further, combination of physical and chemical results – acidic pH and very low CEC – permits to select Gravesac rootstock adapted to well-drained acidic soils and Syrah/Fer Servadou climatic-adapted grapevine varieties as the most suitable vegetative material.