Abstract
The Ohta Tephra Bed in the Tokai Group (Chita Peninsula, Japan) is of different purported age from correlated tephra beds in the Kobiwako Group (Mie Prefecture), Himi Group (Toyama Prefecture), and Hamatsuda Formation (Niigata Prefecture). We attempted to resolve the age of the Ohta Tephra Bed by establishing its correlation with a tephra bed in the Miura Group (Boso Peninsula). The detailed petrography and chemical composition of the tephras show that the Ohta Tephra Bed is correlated with the An85 Tephra Bed in the Miura Group; further more, the An77 and An53 Tephra Beds of the Miura Group are correlated with the Kosugaya and Sakai Tephra Beds of the Tokai Group, respectively. Based on average sedimentation rate of the Miura Group as determind from magnetostratigraphy and biostratigraphy, the age of the Ohta Tephra Bed is estimated to be about 3.9 Ma. The Kosugaya and Sakai Tephra Beds are 3.9 Ma and 4.1 Ma in age by the same estimation. This study is an important step in establishing a comprehensive stratigraphy of the Lower Pliocene formations in central Japan.
