Résumé
Après avoir rappelé les diverses interprétations concernant les formations détritiques situées entre le Massif central et la Sologne, les auteurs font une analyse géomorphologique et sédimentologique des dépôts en profitant des nouvelles coupes levées lors de la construction de l’autoroute A71. La détermination des minéraux lourds et l’altération du matériel mesurée par la teneur en hydroxydes de fer et en argiles permettent de distinguer deux ensembles d’âge différent. Les premiers épandages, mis en place au Miocène lors du remblaiement de la cuvette de Sologne, ne sont conservés qu’exceptionnellement, à la faveur de subsidences locales, comme au nord de Culan. Par contre, la masse principale des dépôts est contemporaine des éruptions rhyolitiques (3,1 Ma) du Mont-Dore et de l’accumulation des Sables du Bourbonnais. La tectonique joue un grand rôle dans la localisation des dépôts et dans l’évolution du réseau hydrographique. Les profils longitudinaux des différentes nappes tertiaires et quaternaires mettent bien en évidence les déformations postérieures à la mise en place des épandages pliocènes.
Mots-clés : Sédimentation détritique, Miocène, Pliocène, Minéral lourd, Contrôle tectonique, Quaternaire, Cher, Bassin du Cher
Abstract
The sedimentary plateaux of Boischant and Berry, lying between the basement rocks of the Massif Central and the surficial deposits of the Sologne basin, contain detrital azoic formations (Fig. 1). After reviewing the various interpretations concerning the age and deposition of these azoic formations, the authors provide a geomorphological and sedimentological analysis of the deposits (Table 1) using the latest sections (Figs. 2 and 3) uncovered during the construction of the A 71 highway. The use of several methods, such as grain-size analysis, chemical analysis, percentage of iron hydroxides (Table 2 and Fig. 4), and determination of clay minerals (Fig. 5) and heavy minerals (Table 3 and Fig. 6), make it possible to identify several detrital beds with different ages: - The Sologne sands (D: 7, 44, 46, 51, 52, 53) contain kaolinite and/or montmor-illonite, but hardly any iron hydroxides; their specific feature is the association of staurolite-zircon and tourmaline. - The Pliocene deposits, with high grades of iron hydroxides and containing some wind-borne volcanic deposits (pumice quartz and volcanic zircons), form two layers. The upper gravel layer (B2) contains more tourmaline, staurolite and andalusite than the underlying sands (C) in which zircon predominates. Kaolinite is predominant over illite in the sands (C), whereas the upper level (B2) shows the reverse. - The less weathered Quaternary alluvium (A) incorporates a far greater mineralogical variety. The first deposits, dating from the Miocene when the Sologne basin was filled, are preserved only exceptionally as a result of local subsidence, as at Culan. The main mass of the deposits is contemporary with the 3.1 Ma eruptions of Mont Dore and the accumulation of Bourbonnais Sands. As with the Pliocene deposits in the west of France, two superimposed levels are found: the upper part of the deposits, coarser grained and less weathered than the underlying clay sands, reflects an erosion accelerated more by the tectonism than by climatic changes. The topographic steps cut into the Pliocene deposits result from successive stages during the incision of the Cher river, caused by uplift of the northern border of the Massif Central, whereas the Quaternary terraces, composed of less weathered and generally better graded materials, have a climatic origin (periglacial phases). The reconstruction of longitudinal profiles through the Tertiary formations and terraces shows the evolution of tectonic deformation since the deposition of the Miocene deposits (Figs. 7, 8 and 9). This deformation is not a local phenomenon since the Allier basin is also affected.
Key words: Detrital sedimentation, Miocene, Pliocene, Heavy minerals, Tectonic controls, Neotectonics, Quaternary, Cher (France), Cher basin
Dernière mise à jour le 02.07.2015