Strontium isotope stratigraphy of the Oligocene‐Miocene Otekaike Limestone (Trig Z section) in southern New Zealand: Age of the Duntroonian/Waitakian Stage boundary

Abstract

The Otekaike Limestone Trig Z sequence in southeastern South Island, New Zealand, 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 Otekaike Limestone has, together with foraminiferal and nannofossil bio‐stratigraphy, provided a chronology for the section and established a correlation with the global time‐scale. Strontium isotope ratios of macrofossil and foraminiferal extracts are identical and indicate an age for the section ranging 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 section.

Keywords:

• Trig Z
• New Zealand
• Oligocene
• Miocene
• foraminifera
• calcareous nannofossils
• strontium isotopes
• sea‐water curve
• absolute age