Subsurface Structure along the Middle and Lower Reaches of the Kitakami Valley

Abstract

1. The Kitakami river cuts through gorges in its lower reaches along the boundary between the areas of the Paleozoic rocks of the Kitakami mountains and the Miocene rocks of the back-borne range, and flows into the Sanuma lowland enclosed with terraces and hills consisting of Pliocene and Pleistocene sediments.The depth of basement of alluvium (Figs. 1 and 3) is more than 95 meters below the sea level at maximum in the lower gorges, and more than 25 meters b.s.l. at Kozenji-Oizumi dividing the middle reaches from the lower. (Fig. 4-F)During the last maximum regression the gorges were cut down rapidly and were filled up during the next transgression. The intermediate marine clay bed with mollsuca proves that the transgression reached to the Sanuma lowland through the lower gorges.The Masaka surface, the upper part of alluvial plain of the Sanuma lowland, may have been formed in accordance with the highest level of transgression at the early Jomon age (Second Phase) and terraced by the following minor regression in the last Jomon age (Fifth Phase).2. In the middle reaches, the alluvial plain is lower than 20 m.a.s.l., and the altitude of the basement of alluvium equals to sea level at Ichinoseki.The Mizusawa Terrase surface has a scarp 10 meters in height above the alluvial plain, but it desends downstream converging into the alluvial plain, but the thickness of the deposits is distinguishable. (Fig. 4-B, C, D)3. The Hanaizumi Bed, deposits of the Hanaizumi Terrace (Fig. 4-G) along the Kinryu river, a tributary of the Kitakami, was dated between 35, 000 and 16, 050 years B.P. by C-14 method (Kanto Loam Research Group 1961 and others). The Hanaizumi Terrace were formed perhaps by the effect of base level dropping along the main valley through the gorges.4. Differences in the relative height of the Lower Terrce surface from the alluvial plain and thickness of deposits along the Iwai, Koromogawa, Isawa and other tributaries suggest the tectonic movement on the Isawa-Yushima warping line.But the wide development of the terraces along the middle reaches, correlated to the Hanazimui Terrace may have been brought by the dropping of the base level along the main river.5. Hanaizumi, Mizusawa and other terraces are correlated each other by a lack of pumise fall bed. But all these terraces were not formed at a same time. The delay of the upstream effect of base level fluctuation must be considered (Fig. 5).6. As mentioned above the terraces can be correlated chronologically if separated far by the gorges, based not only on tephrochronological method also on the upstream effect of base level change.