²³⁰Th-²³⁸U age of Rotoehu Ash and its implications for marine terrace chronology of eastern Bay of Plenty, New Zealand

Abstract

Rotoehu Ash, from the Wairoa River mouth, is dated at 71 000 ± 6000 years by the ²³⁰Th-²³⁸U radioactive disequilibrium system. The age is significantly older than the commonly quoted ¹⁴C age of 41700 ± 3500 years B.P. Marine deposits of Te Papa Terrace in the Bay of Plenty area, younger than Otamaroa Terrace of the last interglacial maximum (stage 5e), are usually conformably overlain by Rotoehu Ash. Thus, Te Papa Terrace, which emerged prior to the deposition of Rotoehu Ash, was probably formed during a global high sea-level stand at c. 85 ka. Some locations of Te Papa Terrace, however, may have been cut during an older high sea-level stand at c. 107 ka, because a peat layer occurs between marine deposits and Rotoehu Ash. Te Papa Terrace should be subdivided into two terraces which represent two successive relative high sea levels following the major high sea level of the last interglacial.

Keywords:

• Rotoehu Ash
• ²³⁰Th-²³⁸U age
• Te Papa Terrace
• late Quaternary
• sea level
• Bay of Plenty
• Wairoa River mouth