Detection of Accumulation-erosional Terrace Association formed in the Last 20, 000 Years

Abstract

A radiocarbon dating analysis clarified that the latest accumulation terraces, which were found at eight river-basins in the Ou and the Dewa mountains (fault and fold mountains in the northern Japan), were simultaneously completed around 20, 000yr B. P. At the age of the terrace completion, the polar frontal zone began to recede to the present position, and subsequently caused a substantial increase in heavy-rainfall frequency due to a typhoon attack even in the northern Japan.Detailed field survey and tephrochronological analysis for the Kariya river-basin in the upwarped Kitakami mountains, located in almost the same latitude as the Ou and the Dewa mountains, revealed that the latest accumulation terrace was also completed around 20, 000yr B. P. Simultaneity of the terrace formation regardless of their great geomorphological difference among those mountains described above, proved that a great increase in the heavy-rainfall frequency was primarily responsible for the initiater of the accumulation terrace formation. Accordingly, it was safely concluded that the formation of the latest accumulation terraces occurred around 20, 000yr B. P. by the climatic change, not crustal movements in the northern Japan.A sizable volume of previous papers concerning the accumulation and the erosional terraces formed in the last 20, 000 years throughout Japan, was intensively reviewed, and the terrace distribution was mapped out. Consequently, it was confirmed that the formation of the accumulation terraces surely extended to the northern to central Japan, and that it commonly accompanied the erosional terrace formed at latest by 10, 000yr B. P. This accumulation-erosional terrace association characterized the river-landform development during the last 20, 000 years.In the southern Japan, only the erosional terrace without the accumulation terrace was identified at the Miya, the Ishite and the Mimi river-basins. Since fossil periglacial landforms in the last Glacial time, widely distributed in Japan, are known to be closely associated with the accumulation terrace, the accumulation-erosional terrace association was plausible to be formed in the southern Japan, and the erosional terrace, therefore, was considered to be the counterpart of the terrace association.In addition to the terrace association in the last 20, 000 years, if a similar key landform is detected in the landforms before 20, 000yr B. P., there is a great possibility to develop a new topographical analysis method.