ABSTRACT: This study investigates the calcareous nannofossil assemblage and composition of 62 field samples collected from a calcareous sedimentary sequence exposed in Mamburao, Occidental Mindoro, Philippines. The sequence consisting of foraminifera-rich, alternating beds of mudstones and fine- to medium-grained sandstones was subjected to stratigraphic logging and detailed calcareous nannofossil analysis. The nannofossils were used to interpret the biostratigraphic assignment of the sequence relative to the identified formations in the study area. Index calcareous nannofossil species reveal a Late Pliocene to Early Pleistocene age (~1.67 to 4.13 Ma) of the sequence, which suggests it was formed from continuous sedimentation after the Miocene collision of the Palawan-Mindoro Block with the Philippine Mobile Belt. Based on its lithologic description and age, the sequence represents the northwest extension of the Balanga Formation, a sedimentary unit reported to be extensively distributed in southeast Mindoro.