仙台付近の第四系および地形(3)

宮城県北部の第四系と地形

抄録

This paper is mainly concerned with the river terraces and their deposits developed in Osaki Plain and along the Nanakita River, north of Sendai, Miyagi Prefecture. The basement rocks of this area consist mainly of Pliocene sediments and partly of Pleistocene pyroclastic flow deposits of Kitagawa dacite. The Pliocene sediments are a part of those, which are widely distributed in the northern part of Miyagi Prefecture.
The river terraces of the area are grouped into four horizons, that is, from the older to the younger, IV, III, II and I. The IV, III and II terrace surfaces in Osaki Plain are covered with volcanic ash layers. The writer accomplished a tephrochronological study using these layers and made clear the development of the terraces.
On the basis of the geomorphological features and the sequence of the terraces, the IV, III, II and I can be correlated with the Aobayama, Dainohara, Upper and Middle Sendai, and Lower Sendai (or Miyagino Plain) terraces in Sendai respectively. The successive developement of the IV, III, II and I terraces in this area is considered to be conformable with the opinion of Nakagawa (1961) and the succession of the IV, III, II and I Plains of him.
Along the northeastern and southwestern margins of Osaki Plain, we can observe vertical dislocations of the IV and III terrace surfaces. The writer regards them as due to northwest-southeast trending faults and flexures. The Honjo-Sendai tectonic line detected in the Tertiary System of the area (Kitamura, 1963, and others) may not affect the terrace surfaces.