The Mousty Formation (Brabant Massif, Belgium): state of the art

Abstract

The Mousty Formation has unique and particularly interesting lithological, mineralogical, sedimentological, metamorphic and tectonic characteristics which have been the subject of much research in connection with the new geological map of Wallonia, but also within the framework of a large amount of academic research, sometimes unpublished. An effort to synthesise and upgrade it in an updated framework seemed necessary. After a history of its definition, mapping and outcrop areas, the following topics are successively addressed in detail and illustrated with diagrams and photos: its lithostratigraphy with the probable discovery of its base near the Court-St-Etienne anticline, its well-constrained biostratigraphy and chronostratigraphy, a new estimate of its minimum thickness, its sedimentology—a subject little addressed until now—, and the resulting depositional sedimentary environment. Its geochemistry, never addressed, is comparable to black shale international standards, except for abnormally high manganese and very low calcium contents; high radon levels in buildings are related to the black slate presence in outcrops. Its mineralogy is rich in manganese-bearing metamorphic minerals, its metamorphism in which we go beyond the strict framework of the formation to deal with the entire southern outcropping rim of the Brabant Massif. Finally, the relationship between the Mousty Formation and the tectonic is discussed, with additional field data, in the context of the innovative and unifying concept of a low-angle extensional detachment called the Asquempont Detachment System. All these observations, some old and some very recent, discussed in the updated framework of the geology of the Caledonian basement of Brabant and in the global stratigraphy and palaeogeography of the lower Palaeozoic, allow us to renew in depth the vision we had of the Mousty Formation and of its place in the Brabant Massif.