Strontium isotope stratigraphy of the Oligocene-Miocene Otekaike Limestone(Trig Z section) in southern New Zealand: age of the Duntroonian/WaitakianStage boundary

The Otekaike Limestone Trig Z sequence in southeastern South Island, New Ze aland, is the type section for the local Waitakian Stage, and contains the Duntroonian/ Waitakian Stage boundary, previously thought equivalent to the Oligocene/Miocene Epoch boundary. Strontium isotope analysis of molluscan (pectinid) macrofossil fragments and mainly benthic foraminifera from the O tekaike Limestone has, together with foraminiferal and nannofossil biostrat igraphy, provided a chronology for the section and established a correlatio n with the global time-scale. Strontium isotope ratios of macrofossil and f oraminiferal extracts are identical and indicate an age for the section ran ging from c. 25.5 Ma at the base to c. 23.7 Ma at the top, placing most of the section within the late Oligocene (Chattian). The Duntroonian/Waitakian boundary is set at c. 25.2 Ma (intra-Chattian) and is not coeval with the Oligocene/Miocene boundary(23.8 Ma), which occurs near the top of the secti on.