La thèse de Paul DEMANGEON un demi-siècle après : une relecture actualisée dans le bassin de l'Orb (Languedoc, France)

• Main Authors: Robert Étienne, Jean-Pierre Larue
• Format: Article
• Language: English
• Published: Physio-Géo 2008-01-01
• Series: Physio-Géo
• Subjects: erosion surfaces; heavy minerals; Languedocian piedmont; sedimentology; Aerology; Climatology
• Online Access: http://journals.openedition.org/physio-geo/766

A half-century after the P. DEMANGEON thesis (1959) and despite numerous researches concerning the southern Central Massif, there are no certitudes about the chronology of the erosion surfaces and the uplift stages of the Massif Central. The use of P. DEMANGEON data, advances in the Montagne Noire metamorphism (M. DEMANGE, 1998) and our own results allows us to record mineralogical variations in the Neogene Languedocian piedmont (Tab. I, Fig. 3), then to explain them by erosion and tectonics in the Orb basin. A north-south cross-section in the Saint-Chinianais is used to analyse relationships between erosion surfaces and piedmont deposits (Fig. 1, 4, 5). A sharp change in the heavy minerals composition occurs between Miocene and Pliocene: tourmaline which dominated during the Miocene is replaced by staurolite during the Pliocene (Fig. 3). The figure 2 shows that this massive arrival of staurolite takes place when erosion reaches the X 14 and X 15 micaschists which are thrusted by the Cambro-Ordovician Avant-Monts. The fine sedimentation in the Cruzy-Montouliers basin shows that a low morphogenesis occurred from the Burdigalian to the Tortonian and was favourable to the elaboration of the Villespassans erosion surface. The Messinian regression of the Mediterranean Sea caused a deep valley incision up to Cessenon for the Orb River and to Montouliers for the Cesse tributary. However, an uplift of the basement with gentle distortions in the Languedocian piedmont seems necessary to explain the upstream erosion in the X micaschists. The Villespassans surface was distorted while the lower Fontcaude surface was not affected. These Mio-Pliocene tectonic movements are responsible of the size increase and of the mineralogical change in the Pliocene deposits.