Studies on the Sedimentary Environment of the ishikari Group in the Ashibetsu District, Sorachi Coal Field, Hokkaido

Abstract

The writer studied the sedimentary environment of the Ishikari group in the Ashibetsu district, Sorachi Coal Field on the basis of the geological data obtained through surface and underground survey.
The crustal movements which continued since the latest Cretaceous period formed the Ishikari geosyncline, containing some geanticlinal upheavals, such as Bibai-Ashibetsu tectonic line named by Dr. S. TASHIRO and the Panke-Panke-Poronai upheaval zone suggested by the writer. During the deposition of the Palaeogene sediments in this geosyncline, there were intermittent subsiding movements, accompanied by the tilting of the ground.
The above-mentioned upheaval zones not only had a great influence on the deposition of the Palaeogene sediments, but also played an important role in the structural history of the Sorachi Coal Field.
The important points of the depositional process of the Ishikari group in the Ashibetsu River district are as follows :
1. The thickness of the members of the Ishikari group has a general tendency to decrease towards the Bibai-Ashibetsu tectonic line.
2. The sedimentary strata, ranging from the Noborikawa formation to the Yubari formation, have a tendency to decrease their thickness northward and southward from the Bibai-Tanzangawa line which is the sedimentational center.
3. From the last sedimentation period of the Yubari formation (accumulation period of No.8 coal seam) to the sedimentation period of the lower Wakkanabe formation, the sinking movement with northward tilting was inactive in the area between Sakipenbetsu and Right 6, a branch of the Hachigatsuzawa in the span of approximately 5.8 km, but the sinking started again with the sedimentation of the upper Wakkanabe formation which, as a consequence, directly overlies the Yubari formation.
4. During the period from the Wakkanabe formation to the Tanzangawa formation (Woodwardia formation), the sinking movement accompanyed by northward tilting became active, and the center of the subsidence in the Ashibetsu River district shifted from the Tanzangawa area to the Otanizawa area.
5. During the deposition period of the Bibai formation in the Ashibetsu River district, differential subsidence is observed. This is marked during the deposition of the Torakawa Bed, i.e., the upper portion of the Bibai formation, resulting in the locally variable thickness of the strata.
6. During the deposition period from the lower Raijo formation (Upper Corbicula formation) to the Ashibetsu formation, the center of development of the strata in the Ashibetsu River district from the Otanizawa area to the Tanzan-Jyugosenzawa area. Especially, during the deposition of the upper Raijo formation, the Jyugosenzawa area had the thickest strata.
7. There were occasional volcanic activities during the subsiding movements in the Sorachi basin, and it may be surmised from the following data that the volcanic activities had some relationship to the uplifting and subsiding crustal movements.