Geologic structures of the Pombetsu fault district, central part of the Ishikari Coal Field

Abstract

There are remarkable differences of stratigraphic sequence, geologic structure and lithofacies between northern and southern parts of the Pombetsu thrust fault. They maybe considered to have resulted from the crustal movement which brought about the geologic structure of the Sorachi Coal Field and the existence of the Minenobu barrier. The structure which was caused by a compressive force from the NOTE-SSW direction took the form of NE-SW thrusts accompanied simultaneously by horizontal displacement to the SWW direction in the southern part of the Sorachi Coal Field, i. e., Minenobu barrier.
The primary structure of the Sorachi Coal Field, which was folded by a compressive force from NNE, rotated clock-wise. (latter force.) The western part of the Sorachi Coal Field (the plain side) slided to NE, and simultaneously the: eastern part (the mountain side) moved to SW. And then, the southern part moved to SW as it was. thrust upon the Minenobu barrier. Furthermore, the fact that the Yayoi fault was thrust up due to a compressive force from SE has. been verified by the geological data obtained through the surface and underground survey of the Pombetsu. Coal Mine. Consequently, the age of the crustal movement which formed the geologic structures is clearly different between the Sorachi Coal Eield and the Yubari Coal Field. The author emphasizes that the structure of the Yubari Coal Field was formed after the completion of the structure of the Sorachi Coal Field.