| Abstract |
Not with standing that the Tertiary System in the Chikuho Coalfield has been regarded as the one of the standard stratigraphical sequence in the Tertiary coalfield, North Kyushu, the Ideyama Formation..., the lower division of the Otsuji Group, has not always been studied sufficiently, owing to lacks of economical importance or paleontological interest compared with the other formations of the same Group. A more detailed study of the Ideyama Formation is therefore really necessary in stratigraphy, relationships with its over-and under-lying formations, and geological structure. As the first part of the study on them, this paper deals merely with the description, of the subdivision, and the geological structure of the Ideyama Formation distributing in the Tagawa district. The obtained results are as follows; 1) The Ideyama Formation is distributed in the east side of this district and unconformable to the underlying Uwaishi Formation of the Nogata Group, It is the Ideyama Formation could be subdivided into 5 members (from A to E members in ascending order) by cyclothem widely observed in the district. 2) In the district, the lower members (A to C) of the formation are predominantly composed of coarse-grained deposits such as sandstone or conglomerate, with intercalations of siltstone and coaly shale. But, the upper members are characterized by fine-grained sediments and coal seams. 3) The geological stucture is characterized by the "Chikuho Type" structure (MATSURHITA 1951). On the western boundary of the Tertiary area, the Nogata Group covers unconformably the basement rocks, and on the eastern border, the boundary between the Tertiary sediments and the basement complex is great faults (so-called "Fukuchiyama Fault" and "Tagawa Fault"), by which a synclinal structure observed in the Tertiary sediments along the faults was formed.show more
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