江津層群・島の星層のFT(フィッション・トラック)年代

The Plio-Pleistocene Gotsu Group is widely distributed in the Masuda to Oda districts of the coastal area of Shimane Prefecture．The Gotsu Group in the Hamada area is composed of clay，gravel and interbedded tuffs，and is divided into three formations，the Tsunozu，Shimanohoshi and Murogamiyama Formations，in ascending stratigraphic order．However，stratigraphic correlation between districts is difficult，because the tuffs in each formation are strongly weathered，vary markedly in thickness，and thus are not useful marker beds．
For this study，we carried out fission track dating of several tuffs and pyroclastic samples in the Shimanohoshi and Tsunozu Formations around the Gotsu and Ida areas，to examine the stratigraphy from a geochronological viewpoint．FT ages of zircons from the lower Shimanohoshi Formation were 1.05 to 1.24 Ma，respectively．The FT ages of the Shimanohoshi Formation（about 1.1 to 1.2 Ma）are comparable with previously reported pale magnetic data，which indicate the Jaramillo subchron（0.99～1.07 Ma）．FT ages of 1 Ma from four localities in the south previously mapped as Minakami（equivalent to Tsunozu ＞ 1.8 Ma）suggest these are actually Shimanohoshi Formation．Consequently，the distribution of this Formation may extend further south than mapped at present．
In future work，we will measure FT age from a wide area，spanning Masuda to Oda，to establish the Pliocene to Pleistocene stratigraphy of the San-in disthct．