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

Abstract

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. The hinge zone between the basin and the uplands appears relatively stable despite a long history. Palaeolandscapes here are mostly polygenic, with exhumed surfaces related to early Mesozoic continental evolution which formed the basis for the development of younger morphologies. The Tertiary landscapes are characterised by inselbergs of unweathered crystalline rocks rising abruptly above radiating low-angle glacis. The wide crystalline uplands were early exhumed from their Mesozoic cover and underwent deep erosion. Only scarce marks of a long continental history remain on these upland areas, and palaeoweathering features are mainly restricted to grabens. The graben deposits show red kaolinitic palaeosols at their base which were later impregnated by silica deposition. Green smectitic claystones, interbedded with calcrete horizons overlay the kaolinitic palaeosols. Extensive silcretes developed at the top of these deposits. Despite the wide variety of the palaeosurface compounds, there are many similarities. The climate change, from warm and very wet during the Late Cretaceous and early Tertiary toward warm and dry during the Late Eocene and Early Oligocene, is the main factor that led to these similarities. The oldest testimonies indicate kaolinite-rich soils developed on smooth undulated hills. The onset of drier climates led to the development of a contrasted mineralogical sequence, with inheritance of the kaolinitic materials from the palaeosols, development of silicified duricrusts on the piedmonts and glacis, and calcretes with Mg-rich clays in the flood plains and lacustrine environments. Tectonics also played a major role. The early Tertiary alpine movements led to the erosion of the old weathering mantles, and to reshaping of the landscapes by several erosion cycles. Drop of the base level led to incision of large valleys on the uplands and development of wide glacis and deep karst network on the margins of the basin. On the other hand, subsidence or general rise of the base level protected some parts of the landscape from further weathering and erosion. The inheritance of pre-existing landscapes also is of noteworthy importance. The thick kaolinitic mantle inherited from the humid climate has strongly prepared the shaping of the subsequent landscapes. The clearing of soft weathering cover exhumed ancient surfaces in the hinge zone between the basin and the uplands, whereas in the crystalline uplands it exposed a knobbly etch surface reproducing the ancient topographies. Present landscapes of Aquitaine and southwestern Massif Central still exhibit many remnants of a long and complex continental history that track back to the early Tertiary and beyond.