Abstract
Previously reported planktonic microfossils and radiometric ages of volcaniclastic layers of the Miocene marine sequence in the Tomioka area are reviewed. Based on the biostratigraphic and radiometric ages, sediment accumulation history and geologic structure, more than 3, 600m-thick marine sequence of the Tomioka area is divided into lower Tomioka Group (partly modified) and upper Annaka Group (newly difined). The Annaka Group covers the Tomioka Group, unconformably (Niwaya Unconformity). High sedimentation rate of 200 cm/1, 000yr. characterizes the Tomioka Group. In contrast, sandstone to sandy siltstone succession of the upper half of the sequence (Annaka Group) shows relatively slow sedimentation rate of less than 40 cm/1, 000yr. High sedimentation rate of the Itahana Formation is due to the development of submarine delta related to regression.
