RELATIONS BETWEEN PYROCLASTICS OF QUATERNARY PERIOD AND TERRACES IN THE DRAINAGE BASIN OF THE YONESHIRO RIVER, AKITA PREFECTURE, NORTHEAST JAPAN

Abstract

Northern part of Akita Prefecture, Northeast Japan, is the drainage basin of the Yoneshiro River, which rises from the Ou Mountain Range and flows to the west to pour into Japan Sea, passing through Hanawa, Odate and Takanosu Basins and Noshiro Plain. At the northeast corner of its drainage basin, there is Towada Volcano whose double caldera is filled with water to form the Towada Lake. Towada Volcano threw out pyroclastics several times at the last stage of the Quaternary period, and some of them went down as far as Japan Sea along the Yoneshiro River. The author investigated the pyroclastic deposits distributed in the range from Hanawa Basin to Noshiro Plain and devided them into 8 members. They are (1)Nagadoro tuff, (2)Kosaka pumiceous ash, (3) Ogata lithic ash, (4)Takaichi pumiceous ash, (5)Torigoe pumiceous ash, (6)Sarugano pumiceous ash, (7)Oyu A pumiceous lapilli, (8)Kemanai pumiceous ash from the older to the younger. Oyu pumiceous lapilli is a pyroclastic fall deposit and others are pyroclastic low deposits. The last two (7, 8) are the deposits of the 2nd stage of Towada Volcano (the stage of a central cone in the older Towada Caldera) and others are ones of the 1st stage (the stage of a large strato volcano before the formation of Towada Caldera) or other volcano older than Towada Volcano. Some terraces developing along the Yoneshiro River have intimate relations with the pyrcolastic deposits. For instance the depositional surfaces of Takaichi, Torigoe and Kemanai pumiceous ashes which are distributed more extensively than others can be recognized as terrace surfaces. Among them that of Torigoe pumiceous ash develops most extensively in Hanawa and Odate Basins. The depositional surface of Takaichi pumiceous ash is found only at the western part of Takanosu Basin and that of Kemanai pumiceous ash is distributed in the narrow belt along the Yoneshiro River in Odate and Takanosu Basins, Some interesting phenomena are recognized in the states of distributions of these pyroclastic flow deposits. The height of the depositional surface of Torigoe pumiceous ash is lower at the central part than at the marginal part of the basins, and relative height from alluvial plain is almost constant. Accordingly it is said that the height of the depositional surface does not always become lower with the distance from the crater, but is influenced strongly by the existing land forms. In Takanosu Basin, Takaichi and Torigoe pumiceous ashes cover the existing terraces of fluvial origin with almost equal thickness (less than 1.5 m), as if they were pyroclastic fall deposits. This phenomenon leads to the idea that the flowing cloud of pyroclastics covered the basin as far as a certain height and left thin deposits upon the terraces. The relative height of the depositional surface of Kemanai pumiceous ash and the floor of the Yoneshiro River is 15 m at the gorge between Odate and Takanosu Basins, and about 8 m in both basins. It is presumed, therefore, that at this gorge Kemanai pyroclastic flow was obstructed to flow smoothly and left thicker deposits than in both basins.