GEOMORPHOLOGICAL NOTES OF THE MIDDLE DRAINAGE BASIN OF THE ABUKUMA RIVER

Abstract

Rising from the eastern foot of Volcano Nasu, the Abukuma River flows through the western margins of the Abukuma Mountains, and pour into the Pacific Ocean near Iwanuma, Miyagi Prefecture. Koriyama Basin in the middle course of the river is a waste-filled basin where two levels of land surfaces composed of lacustrine or marshy deposits can be found. They are called Yabuki Hills (Fig. 1-4) and Koriyama Uplands (Fig. 1-8) in descending order. After Shirakawa Dacitic Welded-tuffs dammed up the river, Yabuki Gravels and Tuffaceous Sands composing Yabuki Hills were deposited in the upper drainage basin up to Sukagawa. On the other hand, Koriyama Sands and Gravels unconformably overlapping Yabuki Gravels and Tuffaceous Sands compose Koriyama Uplands, which are distributed over a wide area in the basin. Koriyama Sands and Gravels were deposited in the basin due to the subsidence of the basin, the centre of which was the region around Koriyama City. In this basin, the older the land surface, the upper the drainage basin of the river it develops: And both Yabuki Hills and Koriyama Uplands have been warped towards north (Fig. 2).