抄録
In this report the author intends to present one of the case studies on the geomorphic development of the Uetsu folded zone—the western half of Northeast Japan, where crustal movements have been active since the early Miocene and the folded Neogene strata are widely distributed. The area surveyed is located at a marginal part of the depositional basin of the Uonuma group upper Pliocene_??_lower Pleistocene which is composed of littoral and fluvial deposits. Landforms of this area are therefore supposed to have been developed throughout the Quaternary period under the distinctive influences of crustal movements.
After the deposition of the Uonuma group the folded structures were developed in some extent. Then a rather large fan was formed over the area covering two drainage basins of the Igarashi River and the Kariyada River. This is concluded from the distribution of the fanglomerate (Odaira formation) and the fact that the gravels transported by the Kariyada River are contained among the fanglomerate now located in the drainage basin of the Igarashi River. The Odaira formation is for the most part 20_??_30m in thickness and is underlain by the folded Uonuma group or the older strata. The summit level of the hills where the Odaira formation is distributed indicates the tectonically deformed fan surface (Fig. 3).
After the deposition of the Odaira formation the area concerned has kept upheaving and the main rivers have eroded by 50_??_200m downward. Many levels of terraces were formed during the dissection. They are classified into 6 levels (Ig-I_??_Ig-IV) as shown in Fig. 2. Most of these terraces are strath terraces, but some exceptions can be found. Fig. 4 shows longitudinal profiles of the terrace surfaces along the Igarashi River. It is clearly shown that the terrace surfaces converge toward a syncline between Ogibori and Sasaoka (A). The terrace deposits are thicker near the syncline than at the other localities. This indicates that the crustal movement which is recognized from the structure of strata older than the Odaira formation is of the same tendency as what is recognized from the deformation of terrace surfaces. It is also shown in Fig. 4 that terrace surfaces have been deformed by faults or flexures near Shimoppara (B, C), but this is not geologically confirmed.
