Sea-level changes, Kellwasser horizons and coral-brachiopod crises during the Late Frasnian in the Namur-Dinant Basin (Belgium)

In the Namur-Dinant Basin, the Lower (LKW) and the Upper Kellwasser Events (UKW) are marked by the development of argillaceous deposits with dysaerobic-anaerobic facies and correspond respectively to the maximum of relative sea level (maximum flooding surface) of the two late Frasnian -earliest Famennian third-order sequences.The initial decline of the rugose corals within the Namur-Dinant Basin is recognized in the Lower rhenana Zone, and is correlated with the beginning of the rise in sea level triggering the transgressive system tract of the first late Frasnian sequence and following the fall in sea level which marks the top of the middle Frasnian limestone formations and bioherms.This decline was not due to the LKW sensu stricto, which happened later and induced strictly no extinction in corals and brachiopods as those occurring below the LKW were still present above.Rugose corals disappeared progressively, along with the tabulates, in the Upper rhenana Zone, before the UKW.The brachiopod decline occurred in three steps within the interval spanning the Lower rhenana Zone to the linguiformis Zone.Most brachiopod orders suffered severely and the major losses occurred at the top of the Upper rhenana Zone.These extinction episodes were linked principally to diachronous regional facies changes related to transgressions.For example, atrypids disappeared at the top of the Lower rhenana Zone in the deeper part of the basin, just before the deposition of the dark shales of the Matagne Formation, but persisted within the Upper rhenana Zone in its shallow parts.