Earth Science (Chikyu Kagaku)

From the results so far arrived at with regard to mechanism of crustal movement as reveald in the tectonic pattern in East Asia, or the Northwest Pacific Tectonic Zone of the present writer (TATEIWA, 1976, 1979), he has come to understand in recent years that the tectonic pattern in question has originated in oroganic forces which have lifted into fold and pressed up the insular arcs, including the central arcs of Japanese Islands, toward the Pacific Ocean. In this paper, the fundamental conception of the writer, cited above, is emphasized by introdusing results of the well known paper experiments of Tokuda (TOKUDA, 1926-27, 1931, a) together with the detailed field observations by KRAUSKOPF and others on the Girloy slide, California, U. S. A. (KRAUSKOPF, et al. 1939).

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The northern part of the Uonuma hills is one of the most suitable area for the study of the development of the Uonuma sedimentary basin, which is one of the biggest scale sedimentary basin in Japanese islands at the Pleistocene in age. Because, we can observe there the changes of the facies, thickness and geological structure from the central part to the marginal of the Uonuma sedimentary basin. In this paper, the writers intend to describe the stratigraphy of the Quaternary strata. The geological structure and the geologic development of the Quaternary in this area will be stated in the next report. Results of the research are summarized as follows; 1. A remarkable contrast is indicated between the southern central part and the northern marginal or the overlapping part in the Uonuma sedimentary basin. The Uonuma formation, in the central part of this basin, is about 2000 m in total thickness, and conformably overlies the Wanazu formation. We can observe there a nearly complete succession ranging from the basal part to the upper member of the Uonuma formation. On the contary, in the marginal or overlapping part of this basin, the Uonuma formation is 150 m in total thickness, and unconformarbly covers the Tertiary system. Furthermore, the unconformity between the upper member and the lower member of the Uonuma formation is developed there, and the middle member of the Uonuma formation is thinning out to the marginal part. The gravel beds becomes more conspicuously abundant in the marginal part of the basin than in the central part. 2. The Sumon volanic products, distributed in the northern part of this area, unconformably overlies the Uonuma formation, and is covered with the terrace gravel beds. Therefore, activity of the Sumon volcano is estimated to be post deposition of the Uonuma formation and pre formation of terrace. 3. Terrace deposits of the middle to late Pleistocene in age are divided into four stratigraphic group, namely, Terrace I, II, III, IV group in ascending order. Massmovement deposits are observed to be correlative with each terrace groups.

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