The Late Paleocene - Early Eocene interval as a potential period for weathering in Western Europe: the case of the Morialmé section (Belgium)
Résumé
Dating of weathered rocks based on radiometric K/Ar and Ar/Ar methods on potassic manganese oxides (cryptomelane of hollandite group) is considered as suitable by the scientific community (Vasconcelos, 1999). These methods have been widely used to highlight many weathering phases around the world (e.g. Thiry et al., 1999; Beauvais et al., 2008). Refined dating of weathering related minerals allow to place the weathering phases in their paleo -geological and -geomorphological contexts and also to better decipher the different weathering processes leading to the formation of these rocks. In Wallonia, weathering took place in several relatively short phases: Late Permian - Early Triassic, Early Cretaceous, "middle" Cretaceous, Late Paleocene - Early Eocene and Early Miocene (Dupuis et al., 1992 ; Yans, 2003 ; Barbier, 2012). Datings on saprolites of the Haute-Lesse (Transinne and Villance) and the Entre-Sambre-Et-Meuse (Morialmé) areas illustrate these different weathering periods. In the Entre Sambre-Et-Meuse region, a single weathering phase, Late Paleocene - Early Eocene in age, 55.75 +/- 2.35 Ma, has been identified based on radiometric da ng (K/Ar) on birnessite (hydrated form of hollandite) forming concretions coating cracks crossing the upper and lower part of the weathering profile (Barbier, 2012). In this area, the weathered profile is overlain by a fluvial channel not weathered and dated as earliest Eocene. Given the geological and geometric relationships of the dated Mn oxides with the whole weathering profile developed and the terrestrial sediments deposited at Morialmé, this age is supposed to be the last weathering phase affecting the area. This does not preclude that any older phases could have contributed to the weathering profile, and as shown by Yans (2003) in the Haute-Lesse area at Transinne. The mineralogy and geochemistry related to the weathering have also been studied in details at Morialmé. The result of the XRD analysis of the clay frac on and the Weathering Index confirm the presence of different weathering degrees along the profile, defined on macroscopic criteria. The main characteristic of Morialmé's quarry is the presence of an alloterite (sensu Wyns et al., 2004) at the top of the profile. Such a degree of weathering (i.e. very strongly weathered rocks) is most probably quite rare in Wallonia.