Abstract
Situated within the so-called Green Tuff Region, or an orogenic zone since Miocene, Uonuma District has experienced very active crustal movements even in the Quaternary Era. It is the object of this article to investigate the relation between the geomorphic development and the crustal movement of this district.
Geomorphologically, Uonuma District is divided into four parts running in the direction of NNE_??_SSW, that is, Echigo Mountain Range, Muikamachi Basin, Uonuma Hills and Tôkamachi Basin from east to west. Uonuma Hills and Tôkamachi Basin are an anticlinal height and a synclinal basin respectively, in a folded region of the Niigata Oil Field where sedimentary rocks have been deposited since upper Miocene. Echigo Mountain Range is, however, composed of granite and other plutonic rocks, and of sedimentry rocks of Palaeozoic and Lower Miocene. Muikamachi Basin lies between the two geomoephologically and geologically different regions the older Echigo Mountain Range to the east and the younger folded region to the west.
The geomorphic development of the area studied has been interpreted by the auther as follows:
Uonuma Hills and Tôkamachi Basin were not yet differentiated in the first half of the Uonuma Stage, or Plio-Pleietocene_??_Pleistocene, when Echigo Mountain Range supplied deposits to form an extensive depositional plain where Uonuma Hills and Tôkamachi Basin now exist. Then followed elevation of Uonuma Hills, resulting in the formation of Muikamachi and Tôkamachi Basins. This is indicated by the deposits of the latter half of the Uonuma Stage.
After the Uonuma Stage, Uonuma Hills and Tôkamachi Basin kept upheaving from the base level. During the upheaval, there wes a rather inactive stage of crustal movement, when two levels of terraces which were more extensive than others wens made successively. With no such stages of inactive crustal movements since then, the River Shinano and its tributaries have trenched the basin making several terraces of lesser extent. The folding which formed the synclinal Tôkamachi Basin and the anticlinal Uonuma Hills continued to take place during the upheaval and it deformed some terrace surfaces. Muikamachi Basin has remained as a depositional basin after the Uonuma Stage and there few terraces are found.
