The Miocene carbonate–siliciclastic deposits of the Pirabas Formation (Brazilian equatorial margin) are affected by significant diagenetic processes and limited biostratigraphic resolution, resulting in persistent chronostratigraphic uncertainties. This study integrates U–Pb carbonate geochronology (LA–ICP–MS), calcareous nannofossil and palynological data, petrography and bulk geochemistry–stable isotopes from the Primavera core (FPR-483) to constrain the timing and diagenetic evolution of this system. U–Pb analyses performed on micritic matrix and microcrystalline carbonate cement yield ages between 25 ± 0.7 Ma and 16 ± 0.7 Ma. Petrographic observations and cathodoluminescence imaging indicate that these carbonate phases include recrystallized and diagenetic components, suggesting that the obtained ages likely record episodes of carbonate stabilization and diagenetic modification rather than strictly primary depositional timing. Calcareous nannofossil assemblages (NN2(?)–NN3) provide broad biostratigraphic constraints within the Burdigalian, while dinoflagellate cysts suggest Chattian in deeper interval, although preservation is generally poor. Geochemical trends (CaO–SiO2–MgO) indicate increasing siliciclastic input and progressive dolomitization with depth. Covariant δ13C–δ18O values and pronounced δ18O depletion reflect strong diagenetic processes, including meteoric and burial processes. These results demonstrate that U–Pb carbonate ages provide useful chronological constraints on diagenetic evolution within mixed carbonate–siliciclastic systems, but must be interpreted cautiously in conjunction with petrographic and geochemical evidence. The study refines the chronostratigraphic framework of the Pirabas Formation while emphasizing the limitations of applying carbonate U–Pb geochronology in diagenetically altered successions.