Les calcrètes palustres (tuïres) du Pliocène supérieur de la plaine du Roussillon. Pierres monumentales d’usage historique ancien

Abstract : During the late Pliocene, the Roussillon plain was the site of repeated accumulations of fine-grained silty deposits, locally intercalated with sandy-gravelly flood deposits. Such sediment-accumulation processes implied that, around 4 Ma ago, the manifestations of structural collapse of the plain had virtually ceased, and that subsidence had become much attenuated. Our understanding of these alluvial deposits has long remained fragmentary, as the stream-beds generally were fairly small and the deposits within them mostly devoid of plant- and animal remains. Some beds that are strongly cemented by calcite and locally called “tuïres”, generally correspond to fine-grained sediment deposited at some distance from the flood channels, but locally also to more coarse-grained sand. Their CaCO3 content can be as high as 50 to 60%. The Terrats cliff, dominating the Canterrane River, has allowed a detailed analysis of this late Pliocene alluvial succession. None of the beds exceeds 1-m thickness, but their lateral extension can be several hundred metres. The alluvial sand is commonly poorly sorted, with multi-modal distribution curves. The alluvial system was controlled by a semi-arid climate with contrasting seasons. The first stages of soil formation developed under hydromorphic conditions, leading to more-or-less bluish gley soils. Precipitation of the carbonate cement took place over several stages, starting with the hydromorphic pedogenesis and ending under evaporitic conditions, i.e. close to the surface. The beds thus formed, here qualified as calcretes, can correspond to sandy limestone of calcareous sandstone. The degree of initial pigmentation generally defined the final colour of the ‘tuïre’, which can vary from near-white to dark-brown. In parallel with the geological work, we studied the use of the ‘tuïre’ stone in the architecture of several Romanesque and earlier churches of the Roussillon and even in some buildings dating from the late stages of Roman occupation. These observations have led us to envisage that such monumental stones were re-used throughout the ages. The properties and aspect of these stones are very similar to those of the calcretes studied in outcrops. In the absence of clear vestiges of true quarries, the places where they were extracted are unknown, but several indications point at the fact that huge blocks, fallen into the Canterrane river bed from eroded cliffs, may have provided opportunistic stone-mason sites. Here, the blocks were intermittently shaped into the stones used in the medieval and earlier constructions of the median part of the Roussillon plain between Terrats and Villeneuve-de-la-Raho, in particular including the villages of Terrats, Nyls and Trouillas with its Templar fortress of the Mas Déu.
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Submitted on : Thursday, December 1, 2016 - 12:07:32 PM
Last modification on : Thursday, January 11, 2018 - 6:25:51 AM

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Pierre Giresse, Michel Martzluff. Les calcrètes palustres (tuïres) du Pliocène supérieur de la plaine du Roussillon. Pierres monumentales d’usage historique ancien. Géologie de la France, 2016, 1, pp.7-25. ⟨hal-01406546⟩

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