2019 Volume 70 Issue 1-2 Pages 17-41
The middle to upper Miocene Shida Group is widely distributed in the northeastern Sendai area, Miyagi Prefecture, Northeast Japan. This group is divided into the Irisugaya, Bangamoriyama, Aoso and Nanakita Formations in ascending order. Among them, the Aoso Formation is mainly composed of ocean current-dominated sedimentary facies and yields abundant molluscan and marine vertebrate fossils. Therefore, this formation has been focused for paleoenvironmental, paleogeographic and paleontological studies of Northeast Japan. In this study, we established integrated biostratigraphy using planktonic foraminifera and diatom for the Shida Group in the northeastern Sendai Plain, which includes the type locality of the Aoso Formation. We also report fission track (FT) ages of two pyroclastic layers of Aoso and Nanakita Formations. Consequently, the Irisugaya Formation is correlated with the diatom zone NPD5B. The Aoso Formation is assigned to the diatom zone NPD5D and planktonic foraminiferal zone N.16–N.17A. The Nanakita Formation is correlative with zones NPD6A and NPD6B. FT ages of the uppermost part of the Aoso Formation and the lower part of the Nanakita Formation were determined at 9.3 ± 0.4 Ma and 7.6 ± 0.7 Ma, respectively. These results are concordant with each other and support previous chronostratigraphic data of adjacent areas.
