The age range of three major Tertiary index taxa of larger foraminfera have been calibrated to the geological timescale using the results of Sr isotope stratigraphic studies of the Darai Limestone directed at hydrocarbon exploration throughout the Papuan Basin. These data indicate that the disappearance of both Austrotrillina and Miogypsina are approximately coincident with the middle/late Miocene boundary (11.0 to 11.5 Ma) and the large eustatic 3rd order sea level fall of that age. The disappearance of Miogypsina at approximately the same age on the Northern Marion Plateau as indicated by Sr dating of samples from ODP 194 suggests it is widespread and linked to global sea level fall. The disappearance of Lepidocyclina at approximately 7.5 Ma in PNG and the Southern Marion Plateau at approximately 7.5 Ma may also reflect a eustatic event. This newly resolved late middle Miocene larger foraminferal faunal event (here designated the top of the Lower Tf or Tf (sub 1-2) T-Letter stage) indicates the potential importance of this and other Oligo-Miocene faunal events to the sequence stratigraphic interpretation of Cenozoic carbonate sequences in southeast Asia.