A radiometric age constraint from a tephra bed at the Miocene‐Pliocene boundary in New Zealand

Abstract

A new isothermal plateau fission‐track age (5.74 ± 0.34 Ma) for a tephra bed c. 6 m below the Miocene‐Pliocene unconformity at Mangapoike River, Gisborne region, along with microfauna above the unconformity, constrains the enclosing reversed magnetic polarity interval to Chron 3r (c. 5.2–5.9 Ma). This is equivalent to the Messinian‐Zanclean Stage boundary position in the Mediterranean region. The new magnetic polarity interpretation indicates that the unconformity at Mangapoike River is of shorter duration than previously stated (c. 0.9 Ma) and could represent the younger of the two terminal Miocene global glacial events. There is a need to utilise tephra beds in New Zealand biostratigraphic sequences to obtain radiometric ages for global correlation.

Keywords:

• Miocene
• Pliocene
• tephra
• fission‐track
• geochronology
• Mangapoike River