Estimation of volcanic activity deduced from early Pleistocene tephra in the Obama Formation, Inubo Group, central Japan

The purpose of this study is to reconstruct the early Pleistocene volcanic activity based on the frequency of tephra emplacements and chemical composition of tephras in the Obama Formation, the Inubo Group in the Choshi area, central Japan. As Choshi area is located east of the Kanto and Chubu regions, many tephras erupted from the Quaternary volcanoes in the Kanto Mountains and the most northern part of the Izu-Bonin Arc are preserved. Because the Obama Formation has been deposited continuously showing no gap in sedimentation without unconformity, the frequency of tephra emplacements represents the volcanic activity around the Choshi area. The depositional age of the tephras can be estimated using calcareous nannofossil datum and magnetostratigraphy. In addition, source volcanoes can be estimated from the chemical composition of volcanic glass in the tephras. The occurrence of FGGT in 1.8-1.4 Ma is more frequent than in later periods. In that period, many tephras had derived from Hida Mountains, Chubu region continuously. The occurrence of CGT is frequent in 1.8-1.5 Ma, especially type A (K_2O: 0.5-1.0 wt.%, CaO: 3.6-5.0 wt.%). In that period, many plinian tephras had derived from volcanoes located in the vicinity of Kanto region continuously. In future, establishing a detailed tephra stratigraphy in the entire southern Kanto region and research on the source volcanoes will lead to a detailed history of the volcanic activity in this period