Geologic age of the Miocene Zaimokugawa Member of the Ikokuma Formation in the western part of Shimokita Peninsula, Northeast Japan

Abstract

Diatom fossils were detected from the Miocene Zaimokugawa Member of the Ikokuma Formation in the western part of Shimokita Peninsula, Northeast Japan. In this report, this unit is proposed as member for the Zaimokugawa Formation in previous work. The flora from this member contains Denticulopsis praedimorpha v. praedimorpha suggesting that this formation is assigned to D. praedimorpha Zone (NPD5B) (around the boundary between middle and upper Miocene). This is the first report on the age of this member. Furthermore, this member intercalated in the lower part of Ikokuma Formation consisting of andesitic pyroclastic rocks. The age of the Zaimokugawa Member constrains the age when the eruption of that pyroclastics started.