14C age of the talus deposits distributed in the middle course of the river Obirashibe, northern Hokkaido, Japan

The samples for the determination of the 14C age of the talus deposit were collected from Takinoshita, Obira town, Rumoe district, Hokkaido. The stratigraphy and the sequence of deposits were as follows: 1) T1 terrace deposit, 2) mud flow deposit, 3) T2 terrace deposit (lake deposit), 4) talus deposit, 5) large rock slide. The obtained 14C age of 23750+-620 Y.B.P. showed the lower limit of talus deposit and the upper limit of lake deposit. The 14C age 29060+-2180 Y.B.P. of the mud flow deposit was regarded as the lower limit of the lake deposit. The formation of lake deposit was estimated to be in the period of 29000-24000 years ago. The obtained age showed good correspondence with the results of pollen analysis which indicated the climate-mitigating period in last glacial epoch. Consequently, the lake deposit of this area was regarded as to be formed in the similar weather condition mentioned above. Talus deposit contained wood pieces in its basement, but fossil pollens and the relics of lage plants were not found. This means that the talus deposit was formed continuously without vegetation. The weather condition in which the deposits were formed are as follows. 1) Mud flow deposit was formed by the large collapse of mountain side slope in the last stage of [Biraotori moor period]. 2) Lake deposit was formed in the period of weather mitigation of 29000-26000 years ago. 3) Talus deposit was formed under the cold and dry periglacial condition 24000 years ago and on. (Ishimitsu, A.)