Torao Yoshikawa

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

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 hav...