Takashi Kudo and Makoto Kobayashi (2013) Radiometric Dating of tephras from Pre-caldera and Caldera-forming stages, Towada volcano, Northeast Japan. Bull. Geol. Surv. Japan, vol. 64,, 2 figs, 2 tables.Abstract: Towada volcano is an active volcano located in the northern part of the Northeast Japan arc. We carried out fission-track (FT) and AMS radiocarbon ( 14 C) dating of tephras from Towada and Hakkoda volcanoes to establish a detailed eruptive history during the Pre-caldera and Caldera-forming stages of Towada volcano. The obtained FT ages were 0.23 ± 0.05 Ma for Shirobeta tephra (WP), 0.19 ± 0.05 Ma for T-6 tephra, 0.08 ± 0.03 Ma for Zarame 1 tephra (ZP1), and 0.09 ± 0.03 Ma for Okuse Pyroclastic Flow Deposits. Although these FT ages are consistent with stratigraphy, they have large error ranges of several tens of thousands of years. Therefore, further examinations including application of other chronological dating methods are necessary to obtain more precise estimates of the eruptive age of these tephras. The obtained 14 C age was 17,730 ± 70 BP for Biscuit 2 tephra (BP2). Based on stratigraphical consistency, this 14 C age is appropriate for the eruptive age of BP2.

The Kamo area is situated in the eastern margin of the Niigata sedimentary basin, Northeast Japan Arc. The Lower to Middle Miocene successions in the Kamo area are divided into the Tonoiri, Otani, and Nanatani Formations in ascending order. In this study, we show newly determined fission-track ages, chronostratigraphy, and regional stratigraphic correlations of the Lower to Middle Miocene successions, and discuss basin development and tectonics in the Kamo area. The age of each formation is as follows -approximately 18-17 Ma, 17-15.2 Ma and 15.2-12.3 Ma for the Tonoiri, Otani, and Nanatani Formations, respectively. On the basis of the age framework of the successions studied, we can correlate the successions with stratigraphic successions in other areas of the northern Niigata sedimentary basin. In particular, a glauconite sandstone bed in the uppermost part of the Nanatani Formation can be correlated to that intercalated in the boundary horizon between the Nanatani and Teradomari Stages in the Niigata sedimentary basin and that in the boundary horizon between the Nishikurosawa and Onnagawa Stages in the Akita region. Lithofacies and paleobathymetrical data show rapid subsidence and transgressions during the deposition of the Tonoiri and Otani Formations. Furthermore, the Tonoiri and Otani Formations show abrupt lateral changes in lithologies and thickness, which suggests that differential subsidence occurred in the basin, probably in response to rifting of the Eastern Japan Sea Rift System during the Early Miocene between 18 and 15.2 Ma. In contrast, the overlying Nanatani Formation has almost a uniform lithology with minor lateral changes in thickness. This suggests that the rifting may have ceased at around 15.2 Ma and subsequent uniform subsidence seems to have occurred between 15.2 and 12.3 Ma. Consequently, the syn-rift stage sediments can be assigned to the Tonoiri and Otani Formations and the post-rift stage sediments can be assigned to the Nanatani Formation.

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