<国立科博専報>"世田米褶曲" : 南部北上山地における石炭紀末の変動 "Setamai Folding"
The Permian and Carboniferous systems distributed in the southern part of the Kitakami Mountainland, Northeast Japan, are summarized on the basis of rock facies as follows : The most striking feature of the geology of the area is the structural difference between the Carboniferous and Permian systems. The Permian System lies unconformably on the inclined older formations separated by an erosion surface. This unconformity, so-called "Pre-Sakamotozawa unconformity", represents the intense disturbance which is known as the "Setamai folding". The erosional surface can be traced over the southern part of the Kitakami Mountainland, and also recognized in the Abukuma Mountainland and the inner zone of Southwest Japan. However, there are many areas where no evidence of the disturbance can be recognized, and if any, the resultant features vary from area to area. Fig.2 shows the distribution of the Carboniferous formations before the deposition of the Permian beds. Distribution of the Hikoroichi, Arisu, and Odaira formations is limited in a N-S extending area of Setamai. The Onimaru Formation is developed on the outerside from the developmental area of these older formations, and the Nagaiwa Formation is widely distributed on the farther [table] outerside. The above-mentioned distributions seem to suggest that before the deposition of the Permian beds the Carboniferous System formed a large anticline or dome with a N-S axis, which extends over the southern part of the Kitakami Mountainland. As a whole, the structure is considered to be the result of a gentle upwarping, although it may have been destroyed and deformed by the older disturbances. The Carboniferous System, accompained by the "Setamai folding", may have been separated into several blocks, and subsidence or uplift of the blocks must have been reflected in the sedimentation of the Permian System. During the "Setamai folding" there must have been a tendency to form the faults of a N-S trend, which is parallel to the long axis of the upwarping structure.