Abstract
The well documented Hitoyoshi Formation, consisting mainly of lacustrine clastic sediments distributed in the Hitoyoshi Basin in southern Kyushu, has well preserved tuff beds. The Funato Tuff Member intercalated in the basal part of the formation, and the Yamada-1 and -2 Tuffs in the upper part were examined to estimate timings of the sedimentation. The former consists of the lower pumice layer and the upper pyroclastic flow deposit, and plagioclase separated from the middle part of the pyroclastic flow deposit gives 2.58±0.08 Ma in K-Ar age. The later consists of chiefly volcanic ash and pumice, and hornblende separated from the lower part of the Yamada-2 Tuff is dated at 2.72±0.25 Ma in K-Ar age. These results clearly indicate that the Hitoyoshi Formation is Late Pliocene in age.
