Abstract
The Middle Pleistocene Atsumi Group, distributed in Atsumi Peninsula, central Japan is composed of the Futagawa, Tahara and Toyohashi Formations, in ascending order. Each formation shows a depositional sequence intensely affected by glacio-eustatic sea-level change. We investigated the Atsumi Group exposed around Ikobe Town, Toyohashi City, and revealed the depositional age of the Tahara and Toyohashi Formations based on the tephrostratigraphy.
We analyzed two tephras, At-3up in the Akasawa Mud Member of the Tahara Formation and Ikb-1 in the Terasawa Mud Member of the Toyohashi Formation. At-3up contains volcanic glass shards (T type (pumice shape) and O type (no bubbles): refractive index (n) =1.505−1.508) and green hornblende (refractive index (n2)=1.674−1.683). Based on these refractive indices and the chemical composition of the glass shards by the EDX analysis, At-3up is considered to be correlative with the Takatsukayama tephra distributed around Rokko Mountains, Kinki District. Ikb-1 is composed exclusively of volcanic glass shards (H type (bubble-wall shape): refractive index (n) =1.501−1.503). The refractive index and chemical compostion by the EDX analysis of the glass shards of Ikb-1 are comparable with those of the Kakuto tephra (Kkt) which is a well-known widespread tephra in Japan. The Takatsukayama and Kakuto tephras are believed to have fallen at transgressive stages of MIS 12-11 and 10-9, respectively. Therefore, the depositional sequences of the Tahara and Toyohashi Formations are considered to have formed at MISs 11 and 9, respectively.
