The Upper Frasnian sequence of the Namur–Dinant Basin in southern Belgium consists of mixed siliclastic–carbonate succession of a ramp setting, where the sequence spans the rhenana–linguiformis conodont zones. Earlier studies investigated the chemostratigraphic variations during the Frasnian–Famennian event, but little has been yet known about the nature of the counterpart variations that immediately preceded that time interval. Despite the scarcity of well-preserved brachiopods, sixty-one calcitic shells were collected mainly from beds of the Neuville and Les Valisettes formations (Lower and Upper rhenana zones), to investigate biochemostratigraphic profiles of oxygen-, carbon-isotope and rare earth element (REE) variations of the time interval immediately before the Frasnian–Famennian boundary. The δ18O and δ13C values of the well-preserved shells range from − 9.5 to − 5.6‰ VPDB (− 7.7 ± 1.1, n = 33) and from − 1.8 to 3.8‰ VPDB (1.1 ± 1.7, n = 33), respectively, which are within the documented global values. The C- and O-isotope profiles exhibit parallel shifts, particularly at the top of the Neuville Formation (top of the Lower rhenana Zone), which are associated with a sea-level rise and shrinkage in the brachiopod community. Also, the Th/U (0.9 ± 0.6, n = 16) and Ce/Ce* (2.2 ± 0.5, n = 16) ratios suggest deposition under reducing conditions consistent with sea transgression.