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The lithology and stratigraphy of the Ideyama Formation in the Iizuka district are described in this paper as the third report on the Ideyama Formation distributed in the Chikuho Coal-field. The probl...ems on the lower-limit of the formation and the relationships to the underlying Paleogene formations and to the Ideyama Formation distributed in the other districts of the coal-field are also discussed in this paper. The Ideyama Formation in this district develops in the Iwasaki and the Shonai-Yamada areas which are apart from each other and also separated far from the main distribution region extending over the Nogata and the Tagawa districts. By the lithological facies this formation is discriminated from the underlying Uwaishi Formation of the Nogata Group in most localities of this district, although the relationship between both the formations is presumed to be conformable. It is composed predominantly of coarse-grained sandstones and conglomerates, but in the upper half it rarely intercalates siltstones and a few coals. Based on the cycle of sedimentation the Ideyama Formation is divided into two members, the lower Iwasaki and the upper Takakura. Although it has a total thickness of about 300m in the Shonai-Yamada area, just the lower part of the formation, having about 90 m in thickness is distributed in the Iwasaki area. A fossil bed known well under the name of "the Iwasaki fossil bed" develops near the basal part and is quite well traceable throughout the field. Judging from the assemblage of fossils obtained, it may belong to the Funazu stage by A. Mizuno (1962), the lowermost Oligocene in age. The Paleogene including the Ideyama Formation discussed here dips gently to the east, but it forms a synclinal structure where it meets with a great fault on the eastern margin. The axis of the syncline generally runs in parallel with the great fault described above, along which two semi-basin structures are found at the north and the south of Takakura and due to slight undulation of the axis.show more
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