Variety and relationships of weathering features along the early Tertiary palaeosurface in the southwestern French Massif Central and the nearby Aquitaine Basin

Palaeoweathering features and palaeosurfaces in the southwestern French Massif Central and its margins provide a basis for understanding the evolution of Tertiary continental areas. Recurrent sequences of weathering periods, erosion phases and sedimentary deposits have been identified and related to major sea-level changes, tectonic movements and palaeoclimates. The palaeoweathering features are different according to their palaeogeographic position, from basin margins to uplands, and in limestone platforms or crystalline areas. On the basin margins palaeokarst features prevail. Claystones together with lignite accumulated in these sinkholes. Later, weathering and oxidation of these organic-rich deposits led to highly acidic environments, resulting in dissolution of the primary clay minerals and formation of secondary clay deposits with gibbsite, halloysite, montmorillonite, nontronite, but also of opal-CT silcretes and ferricretes. Palaeoweathering profiles, fossilised at the edge of the subsiding basin, show red and ochre sands on which red-brown silicified duricrust developed, later invaded by palygorskite-rich calcrete horizons. On the limestone borderland, behind the subsiding area, various palaeokarsts developed extensively. Surficial palaeokarsts, developed after withdrawal of the Jurassic sea and buried by Cretaceous sandy deposits, have been reactivated during the early Tertiary. Later, the deep subsurface palaeokarsts from the early Tertiary were sealed and then reactivated as they were brought back to the surface by erosion.