GEOMORPHIC DEVELOPMENT OF THE TAKANOSU BASIN, AKITA PREF., NORTHEAST JAPAN

Abstract

In the Japanese Islands, geotectonic movements have been, generally speaking, very active since later Tertiary and the crustal movements which have immediate influences on present landforms are also supposed to have no small significance in geotectonic development. Accordingly, it is an extremely important problem in geomorphology to clarify the relation between geomorphic and geotectonic developments. From this point of view, the writer investigated geomorphic development of the Takanosu Basin, Akita Pref., northeast Japan, which was located in so-called Green Tuff Region, one of the Neogene orogenic zone in the Japanese Islands. The results are summarized as follows: 1) In the first half of Pliocene, Japan Sea spread over the Takanosu destrict. 2) In the latter half of Pliocene, Nanakura mountains began to upheave, defining the western boundary of the Takanosu Basin, and Mitsukita formation, composed of lacustrine and coastal sediments, was deposited on its eastern foot. 3) At about the end of Pliocene, the crustal movement became more active. Nanakura mountains were elevated and the eastern boundary of the Takanosu Basin was defined by step faults. Thus the Takanosu Basin was formed and in accordance with the subsidence of the basin lacustrine Yuguruma formation was deposited in it. 4) With the retardation of the crustal movement, the basin was filled up with Yuguruma formation and an erosion surface was formed in the marginal zone of the basin by lateral erosion of rivers flowing into the basin. 5) After the formation of the erosion surface, the crustal movement became active again. The whole region was upheaved against the base level, being relatively warped down in the central part of the basin. The bottom of the basin was trenched by the Yoneshiro River and its tributaries and several levels of fluviatile terraces were made. It is inferred from the character of the terrace deposits and the distribution of the terrace surfaces that the crustal movement of the same tendency as what formed the Takanosu Basin was in action during and after the formation of the terrace surfaces.