Les grands épisodes d’altération continentale en France en relation avec les déformations lithosphériques

Abstract
 

This article presents the preliminary results of a study concerning the relationships between weathering and instances of vertical deformation of the west European lithosphere in France during the post-Paleozoic period. The continental alterites and associated continental sediments, which developed horizontally along planation surfaces, are considered here to constitute a continuous series, ranging from a subtractive “negative” pole (laterites in the broad sense) to an additive “positive” pole (siliceous crusts, and calcretes, along with lacustrine limestone and evaporites). During the weathering period, the alterite facies often shift from one pole to the other, enabling weathering sequences to be defined. Comparison between the weathering sequences and the timing of vertical deformation of the lithosphere in the Alpine foreland shows that, in spatiotemporal terms, negative weathering coincides with upwarping of the lithosphere (domes), while positive weathering coincides with downwarping of the lithosphere (depressions). Two major periods clearly demonstrate these relationships: (i) the Early Cretaceous, when lateritic profiles developed on the shoulder of the North Biscay rift on the one hand, and on a dome forming part of the southern edge of the North Sea on the other hand; (ii) the Tertiary, when lateritic weathering occurred along the “peri-Alpine ridge” resulting from lithospheric buckling during the compressional phases experienced by the Pyrenees (Palaeocene to Middle Eocene) and the Alps (Late Miocene to present day), while continental carbonates developed from the Late Eocene until the Early Miocene over a depressed surface area near sea level.

Key words: Weathering, Paleosurfaces, Deformation, Lithosphere, Silcrete, Early Cretaceous, Tertiary, France