Abstract
The 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 Gifu Prefecture, central Japan, has been investigated by fission track (FT) dating and petrographic description of four tuffaceous sandstones.
An investigation of the refractive indices of plagioclase suggests that the tuffaceous sandstones are a mixture of andesitic tuff and granitic detritus. Two age populations, 17-21 Ma (early Miocene) and 56-67 Ma (late Cretaceous to early Tertiary) were obtained by a statistical treatment of the distribution of zircon grain ages. The younger age cluster is regarded as the age of andesitic tuff and the older one is probably the age of detrital zircon derived from the Toki Granite (59-61 Ma). The younger age implies the depositional age of the middle part of Toki Lignite-bearing Formation. Based on the obtained and previously reported FT ages, the depositional age of the Toki Lignite-bearing Formation except for the basal conglomerate is inferred to range from 18 to 20 Ma.
