Geological Studies on the Central Part of the Ishikari Coalfield

Abstract

Existence of the "Minenobu barrier" which separated the Sorachi basin from the Yubari basin during the deposition of the lower half of the Ishikari group, has been pointed out by many geologists.
Another fact that the formation of the embryonic structure of the Sorachi anticline had been going on during the period when the Minenobu barrier existed has also been confirmed. The author assumed that the Minenobu barrier was connected with the embryonic structure of the Sorachi anticline; in othe words, the southern part of the embryonic structure had emerged above the water. The combination of the Minenobu barrier and the Sorachi anticline is named by the writer "Akabira-Ikushumbetsu upwarping zone."
During the Neogene Tertiary, the Sorachi anticline formed due to compression from the east. At this stage, an extremely strong force was exerted on the southern end of the anticline. Therefore, the southern end was bent toward the west, and folds and faults took place on the concave side, i.e., on the west side of the anticlindl axis. Based on the above-mentioned assumption, some tectonic features of this area may be explained, especially the formation and structure of the Pombetsu-fault and the fact that the general trend of the strata in this area does not agree with the general N-S direction of this coalfield.
There is a remarkable difference in the state of sedimentation of the Ishikari group between the two areas, north and south, of the Akabira-Ikushumbetsu upwarping zone.
The stratigraphic sequence of the Ishikari group and Poronai group is remarkably different between, the north side area and the south side area. The author indicated a possiblility that the Ikushumbetsu formation in the southern area is correlated with the upper three formation of the Ishikari group, that is the Ikushumbetsu formation, the Hiragishi formation, and the Ashibetsu formation in the northern area.