14C age of terraces developed in the upper basin of Kitakami River

Riverside terraces have well developed in the upper basin of Kitakami River between Morioka and Koma. The cause of formation of the terraces was presumed as follows. The Takizawa mud flow from Iwate Volcano checked Kitakami River between Shibutani and Kuriyagawa, and the lakes in Koma field and Hiradate basin were formed upstream. The depositional surfaces on the lake bottoms at the time are the Koma terraces, and the erosion terraces formed by dissection thereafter are the Makibori terraces. Kitakami River cut these terraces, and formed Ueda terraces and Kawamata terraces. It was presumed that the Koma and Ueda terraces had been formed before the fall of volcanic ashes from Iwate Volcano because they are covered with volcanic ashes, while the Makibori and Kawamata terraces had been formed after that. This time, the 14C ages of the wood pieces in terrace deposits and the humic acid extracted and refined from peat layer in Koma and Ueda were measured, and the new findings obtained are reported here. The measured age of the wood pieces was 21,120 +- 780 Y.B.P., and that of the humic acid was 6,050 +- 150 Y.B.P. It is problematic to assume that the formation of the Koma terraces was caused by checking Kitakami River. The Koma terraces are the fluviatile terraces formed in the Wuerm maximum period, and the Ueda terraces are the accumulation terraces formed in the most active period of Jomon transgression. (Kako, I.)