14C-age of volcanic ash soils in the environs of Jumonji-machi, Hiraka-gun, Akita Prefecture

This is a brief note on the age and origin of volcanic ash soils in the environs of Jumonji-machi. These volcanic ash soils, called kuroboku, cover river terraces developed in the south-eastern part of Yokote Basin. Of these terraces, the upper and middle groups are topped with kuroboku, but in the lower terrace area, kuroboku is buried by alluvial soils. 14C-age of kuroboku was determined by Prof. Kigoshi, and that in the middle and upper terraces was 6,390+-140 years B.P. and 5,650+-140 years B.P., respectively, while that in the lower terrace was proved to be some 1,000 to 2,000 years B.P. From these facts, the authors suppose that kuroboku in the lower terrace area is of redepositional origin. (Shibata, I.)