Sedimentology of the Bieumont Member : influence of the Lion Member Carbonate mounds (Frasnian, Belgium) on their sedimentary environment

This is a sedimentological study of Middle Frasnian Bieumont Member fore-reef and off-reef carbonate sediments exposed in the Chimay-Couvin region (Belgium). It is based on four stratigraphic sections: the Lompret quarry, the Frasnes railway section, the Leus quarry and southern parts of the Lion quarry. The Lompret section is here described for the first time and represents the main focus of this work. The Bieumont Member consists of a bedded sequence of argillaceous and bioclastic limestone. At Lompret, the Member is exposed with a thickness of 48 m (the basal contact is not exposed). The Member consists of 6 lithological units (in stratigraphic order) : 1. argillaceous limestone with episodic intercalations of reef debris (distal reef talus), 2. an alternation of marl and argillaceous limestone (basinal background sediment), 3. thickly-bedded bioclastic limestone (channel deposits), 4. a sequence of argillaceous limestone (inter-channel deposits), 5. bioclastic limestone with sporadic occurrence of framestone (proximal fore-reef deposits) and 6. fine-grained, bioclastic limestone interbedded with prominent layers of reef debris (distal fore-reef deposits). Within unit 5, an isolated reef block transported down the paleoslope is present. Microfacies analysis revealed 16 microfacies units which together with the lithological subdivisions were used to reconstruct the dynamic sedimentary history of the Bieumont Member at Lompret. There are two orders of depositional rhythmicity. The reef growth cycle starts with the first significant influx of reef-related facies at the beginning of unit 3 and from thereon is well differentiated from the Bieumont Member stratotype which largely consists of lithologies equivalent to units 1 and 2. Progradation was directed to the South and related shallowing-upward conditions culminated during the deposition of unit 5. On a lower scale, rhythmic bedding within units 2 and 4 could be related to orbitally forced cycling. The vertical and lateral variations of facies within the Bieumont Member were used to reconstruct a depositional model for the Lompret area. In addition, the documentation of facies variation on a regional scale was enhanced by using data from the Focant borehole and geological surveys of Han-sur-Lesse and Barvaux regions.