GEOMORPHIC DEVELOPMENT OF THE HANAWA BASIN AND THE ÔDATE BASIN, AKITA PREFECTURE, NORTHEAST JAPAN

Abstract

As one of the case studies of geomorphic development of the young mobile regions, the author investigated two adjacent basins in the northern part of Akita Prefecture, Northeast Japan. They are the Hanawa Basin on the east and the Ôdate Basin on the west, and the Yoneshiro River which flows westward through the Takamori Mountains connects them with each other. These two basins are located in the so-called Green-tuff Region, a young mobile region which has developed since the biginning of Miocene. In Akita Prefecture the Green-tuff Region can be divided into two parts, one of which occupies the west and coastal side and is characterized by the upper Neogene Tertiary showing a folded structure with axis of N-S trend. The other part is characterized by lower Neogene Tertiary and Pre-Tertiary divided into many blocks by faults of various trends. The two basins are both located in the latter part and the shapes of the basins and the crustal movements concerned to the geomorphic development of the basins reflect the geologic structure above mentioned. The geomorphic development of each basin is summarized as follows. The Hanawa Basin-In a certain age of Pliocene the basin area, where mountainland of not low relief had existed, began to subside to become a depositional basin. Subsidence was caused by a southward tilting of a wedge-shaped block separated from the surrounding mountainl and by a NNE-SSW trend fault on the east edge and a NNW-SSE trend fault on the west edge. Thickness of the basin deposits, though varied reflecting the relief of the basal surface, becomes larger from north to south and reaches above 500m at the southern margin of the central part of the basin (few data are available in the southern part). In middle or late Pleistocene the basin area changed its movement to uplift and fill-top surface of the basin deposits was dissected to become separated terrace surfaces. At present these fill-top terrace surfaces are, though small in extent, left at 90m (in the northern _??_central part) _??_160m (in the southern part) higher than riverbeds. The southern part of the basin is separated from the central_??_northern part by a fault of E-W trend and has been uplifted faster so that the fill-top surface is higher and more levels of younger fluvial terraces are found. The Ôdate Basin Before the beginning of deposition the area now occupied by the Ôdate Basin was already of far lower relief than the surrounding mountainland, the reason of which was supposed that this area had been uplifted more slowly than the surroundings so that denudation by running water had proceeded more effectively to develop low relief topo-graphy. Then in a certain age of Pleistocene this area began to subside to become a deposi-tional basin. The crustal movements which caused the subsidence of the Ôdate Basin were similar to those of the Hanawa Basin, that is, southeastward tilting of a wedge-shaped block separated from the surroundings by a N-S trend fault on the east edge and by a NW-SF trend fault on the southwest edge. The basin is still subsiding except marginal parts and the thickness of the basin deposits is less than 100m, which is far thinner than those of the Ha-nawa Basin. From the latest Peistocene downward, Towada Volcano, which is situated to the northeast of the Hanawa Basin, threw out pyroclastics for several times, which deposited also in the Hanawa Basin and the Ôdate Basin. Soon after the deposition the Yoneshiro River and its tributaries dissected them to take back the former profiles. Terraces composed of these pyro-clastics, therefore, do not indicate crustal movements.