Frasnian reef mounds in the Durbuy–Bomal area (eastern border of the Dinant Synclinorium, Belgium)

This paper focuses on the Frasnian reefal development in the eastern border of the Dinant Synclinorium. Classical sections from the Durbuy–Bomal area were reevaluated for lithostratigraphy, microfacies, magnetic susceptibility (MS) and diagenesis. As regards the middle Frasnian succession, the studied area is located in a transitional zone between the Pont de la Folle/Philippeville Formations and the Moulin Liénaux/Grand Breux Formations. This succession is topped by Petit-Mont Member reef mounds. Massive mound microfacies are characteristic of the Petit-Mont and Lion Members, with fossil associations respectively dominated (from deepest to shallowest) by sponges; sponges, crinoids and corals; corals, crinoids, stromatoporoids and cyanobacteria; and microbes. Flank and off-mound microfacies consist of microbioclastic, bioclastic, crinoidal or lithoclastic bedded limestones. MS values are related to the depositional environment and regularly decrease from the off-mound to the reef mound microfacies. The reef mound diagenetic sequence is similar to that identified in other Petit-Mont buildups: cementation in the marine phreatic zone preliminary to drowning, then the development of a meteoric lens at the time of a marine regression, with dysoxic facies in the distal zones of the aquifer and, finally burial cementation and dolomitization during the Variscan tectonism.