Etude de paléoaltération météorique (Crétacé Inférieur à Néogène) sur socle silicoclastique hétérogène : caractérisation et essai de cartographie d’altérites en Région Wallonne (Belgique)

Abstract

Weathered heterogeneous siliciclastic rocks are ubiquitous in the Walloon Region. This study first aims to characterize and secondly to map the weathered rocks, the characterization remaining an essential step of the mapping work. Firstly the characterization was performed on the Ardennian Allochthonous (Ardenne Anticlinorium and Dinant Synclinorium) and the Brabant Parautochthonous (Brabant Massif). Based on macroscopic observations, classification of weathering degrees of alterites is proposed. The mineralogy (XRD, thin section) and the geochemistry (major elements, trace elements, REE) of the studied sites are quite consistent with these categories. The evolutions of clay parageneses and Vogt’s weathering index (1927) provide representative results of the weathering degree at the regional scale. The dated alterites do confirm the presence of five weathering periods in the Walloon Region: Permian-Triassic, Early Cretaceous, “middle” Cretaceous, Paleocene-Eocene and Miocene. Within these periods, the weathering is induced during favorable phases. A chronological model of saprolitisation is proposed for the Walloon Region: a relationship between weathering phases, climate and relative sea-level changes is suggested. Secondly the knowledge acquired by the characterization of the weathering profiles in the Walloon Region facilitates the mapping of the weathered rocks in the Haute-Lesse area. The tools and methods tested in this work show several advantages and disadvantages for the mapping of weathered siliciclastic rocks. Those selected allow a distinction between the different types of alterites. A methodology based on the airborne radiometry to map the weathered heterogeneous siliciclastic rocks at the regional scale is developed. Confirming the uplift of the Ardennes, the airborne radiometry is a suitable tool for geomorphology. The mapping of weathered rocks in the Walloon Region would be useful through a field survey on topographic maps at the 1/10.000 scale. The explanatory notes would supply the link between the weathering profiles characterization and their mapping. The weathering processes in the Walloon Region are fairly uniform from one site to another. On the other hand, the results of weathering processes on different lithologies are different in intensity and thickness. A preliminary characterization study of the weathered rocks in the Walloon Region is required for mapping, regardless of the scale and accuracy.

Date of Award	13 Feb 2012
Original language	French
Awarding Institution	University of Namur
Supervisor	Johan Yans (Supervisor), Florence Quesnel (Jury), Christian Dupuis (Jury), Frédéric Boulvain (Jury), Michel Everaerts (Jury) & Vincent Hallet (President)