Volcanic Stratigraphy of the Transition Period (1.9-0.3 Ma) from the Pre-Unzen to Unzen Volcanoes in the Southern Part of Shimabara Peninsula, SW Japan

Abstract

This paper presents a revised stratigraphy of the volcanic rocks, pyroclastic materials, and marine deposits transitional from the Pre-Unzen to Unzen volcanoes during the period of 1.9-0.3 Ma in the southern part of Shimabara Peninsula, Kyushu. The geological units, in ascending order, include the Kazusa Formation, Mejima Formation, Minami-Kushiyama Formation, Hojodake basalt, Saishoji Formation, Otani Formation, Kita-Arima Formation, Suwanoike basalt, Ideguchi Formation, Tonosaka andesite, Takaiwasan andesite, and Older Unzen volcanic fan deposits. Among these units, Saishoji and Kita-Arima Formations are shallow marine sediments deposited during a quiet period of volcanic activity, and the Otani Formation, an exotic marker tephra, is intercalated between them. In this study, these formations are newly defined as the Uppermost Kuchinotsu Group (1.0-0.6 Ma), and the Upper Kuchinotsu Group (1.9-1.0 Ma) and Older Unzen volcano (0.6-0.3 Ma), were defined as two active volcanic periods separated by the quiet period (Uppermost Kuchinotsu Group). The continuity of activity ages and similarities in rock chemistry imply that the Suwanoike basalt, Tonosaka andesite, Takaiwasan andesite, and Older Unzen volcanic fan deposits were associated with the Older Unzen volcano. This means that Older Unzen volcano become active after the quiet period of 1.0-0.6 Ma. The Ideguchi Formation, also an exotic marker tephra, and the Otani Formation were excluded from the volcanic activity in this area. The eruption sources of these exotic tephras could have been derived from other regions in Kyushu, but the source was not identified in this study.