Fission track ages of tuffaceous sandstone from the Toki lignite-bearing formation of the Mizunami group in the Tono district, Gifu prefecture, central Japan

Sasao, Eiji   ; Iwano, Hideki*; Danhara, Toru*

Depositional age of the Toki Lignite-bearing Formation of the lowermost part of the Miocene Mizunami Group, which is distributed in the southeastern part of the Gifu Prefecture, central Japan, has been investigated by fission track (FT) dating and petrographic description. The samples used here are four tuffaceous sandstones collected from drilling cores and two granites of the Toki Granite, which is possible source of detritus of the Toki Lignite-bearing Formation. Tuffaceous sandstones are estimated to be mixture of andesitic tuff and granitic detritus. Two FT age clusters, 17-21 Ma (early Miocene) and 56-67 Ma (late Cretaceous to early Tertiary) are obtained by a statistical treatment of the distribution of zircon grain age. The younger age cluster is regarded as the age of andesitic tuff, indicating the depositional age of the Toki Lignite-bearing Formation. The older age cluster is nearly same as the zircon FT age of the Toki Granite (59-61 Ma). The depositional age of the Toki Lignite-bearing Formation is inferred to range from 18.5 to 21 Ma, taking the previous study on the depositional age of the overlying formation into consideration.