Fission-track ages of late Pleistocene tephra on the Bay of Plenty coast, North Island, New Zealand

Abstract

Abstract In the area surrounding Opotiki on the Bay of Plenty coast, tephra deposits are classified into 3 units: post-Hamilton tephras, Hamilton Ash, and Tablelands Tephra Formation. Each is separated by well-developed paleosols or unconformities. The Rotoehu Ash and the Ohinewai Tephra are located at the bases of the post-Hamilton tephras and the Hamilton Ash, respectively. Tablelands Tephra Formation (new) is defined and tentatively subdivided into upper and lower members by paleosols or unconformities that commonly occur at different exposures. This formation contains several characteristic pumice layers which are very useful for tephrochronological correlation. Obsidian fragments from 3 of the pumice layers in the lower member were collected for fission-track dating, which yielded ages between 0.21 and 0.18 m.y. The fission-track ages of these tephras are consistent with their stratigraphy. Among the marine terraces along the Bay of Plenty coast, the Otamaroa and Matakaoa Terraces are inferred to have been formed in the last and the penultimate interglacial stages, respectively, judging from their geomorphological positions, tephro-chronological horizons, and fission-track ages of tephras overlying the terrace deposits. The late Quaternary uplift rate in the eastern Bay of Plenty, North Island, New Zealand, is much higher than that estimated by Chappell in 1975. The lower member of the Tablelands Tephra Formation resulted from violent volcanic activity which distributed a large amount of rhyolitic tephra over an extensive area in a short time. The pumice layer Ta-3 of the lower member is probably correlated to the Mount Curl Tephra and the Ohariu tephra, based on their similar ages and mineral compositions.