The area described is the city of Taira and the adjacent villages of the Middle Joban Coal Field, including Yoshima, Akai, and a few other smaller collieries.
This paper is divided into two parts for convenience. In Part I, the following subjects are discussed: (1) the changes in thickness of formations above and below the coal bearing member, (2) changes of sedimentary facies of the Shiramizu Group except the coal bearing member, (3) newly detectd formation in a borehole. In Part II, the writer interpreter and discusses the problems of the deposition and deterioration of the coal and the coal seams.
In Part I, here presented, an interpretation of the stratigraphy and tectonic geology of the area is followed by the three main subjects as below.
(1) Borehole sections reveal that the Goyasu Sandstone, the lowest member of Yunagaya Group, increases in thickness abruptly in the eastern, deeper part of the Taira District, probably with the boundary of narrower zone of NNE-SSW direction. In this deeper part, the thickened Go-yasu Member has thick vitric tuff beds in its lower part, underlain by a basal conglomerate bed which rests on the Shirasaka Shale. The vitric tuff beds are completely absent from the western part of the Taira District. Above the vitric tuff beds are coal-bearing beds having successions of remarkable cyclic sedimentation similar to those of the Iwaki Coal-bearing Member. This coal-bearing upper half of the Goyasu member is present in the western area where it is thinner than in the east. By these reasons, the depth of boring to reach Iwaki coal seams is increased. in eastern area, and the boring itself is obliged to stop frequently, jammed by the collapse of vitric tuff.
(2) The Shirasaka Shale, Asagai Sandstone, and Iwaki Sandstone, all of the Shiramizu Group, might be found in vertical succession in a borehole. However, they vary greatly in thickness, and in some places one or the other is absent, although the total thickness of the section penetrated is unchanged. These three members seem to be of simultaneous deposition only different in facies of sedimentation. Judging from an isopacous map of each member, the Iwaki Sandstone appears to be a subaqueous deltaic deposit. The deltas are believed to extend to the east or to the southeast. To the east, the direction of deepening, the Iwaki Sandstone is apparently replaced by the Asagai Sandstone which can be shown to be in the relation of interfingering with the Iwaki Sandstone in the known borehole sections.
(3) The Second Kaso, which is the lowest coal seam of the Taira District, has a thick clay bed under it, the so-called "gaerome clay". The clay bed rests directly on granite or metasediments of the basal complex, in the western half of the area, but in the eastern area it is underlain by sedimentary rocks. The latter resembles the Kasamatsu Shale of Cretaceous Age both in its lithology and in its aspects of sedimentation. From these data, the writer believes this member to be of Cretaceous age, beveloping from the north of Yotsukura, although in this area it lies completely underground with no outcrop.

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The Pre-Tertiary basement underlying the main coal bearing strata, Hanakawa Formation, developed in the Joban coal-fields, often produced buried hills in the overlying beds. The develop-ment of the workable coal seams has been influenced by the existence of the buried hills; namely, the main coal seams all tend to become thinner as they approach the hills and thin out abruptly or gradually at the tops of them. Accordingly, predicting the location of the hills is important to the coal mine engineers.
The author, co-operating with the field technicians, has recently discovered a technical wise saying "a discovery of grey-greenish schistose mudstone and angular schist fragments in the boring core give us a definite hint on the existence of the buried hills around the boring stations".
This is due to the development of the dark-grey-greenish schistose mudstone zone which proba-bly resulted from the direct transportation of eroded materials derived from the buried hills. If we recognize this characteristic facies in the boring core, irrespective of the distance to the Pre-Tertiary basemant, we can determine concretely the same zone and consequently discover the presence of a buried hill.

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The Iwaki Coal-bearing Beds of the Taira-Yoshima District contain the following six coal seams or groups of seams, named in ascending order : the second Kaso group, the first Kaso group, Honso group, Nakaso, Joso and Funtanso. This coal bearing formation repeats the cycles of sedimentation in a very regular order, beginning with the deposition of sandstone and conglomerate soon after subsidence of the coal area and finishing with the deposition of coal and shale after the silting up of the area. The cycles may be classified into tow types, i.e., the Kaso-type and. the Joso-type.
Coal seams above the Honso group belong to the Joso-type. This type of cycle (or cyclothem) begins with marine sedimentation, represented by sandstone and conglomerate or conglomeratic sandstone accompanying marine shell fossils and sometimes sand pipes, which was caused by a temporary marine invasion after sudden subsidence of the coal-forming field.
Coal seams below the Honso group consist of three groups, each group containing a few coal seams intercalating sandstone and shale, and between each group occurs a dominant conglomerate bed. The coal seams of each group have the tendency to converge into one seam in the western, shallower portion, and, on the contrary, split into several seams in the eastern, deeper region. These belong to the Kaso-type, and the roof of each coal seam is generally composed of shale or inudstone, or, rarely, of sandstone.
The sedimentation bf the Kaso-type must signify that of a lake basin. The transition to the Joso-type above the Honso group may imply the silting up of the basin and the direct connection of the coal-forming field with marine water. The cycles of the Joso-type may represent a sort of deltaic cyclothein, and also possibly a piedmont cyclothem, because of the narrowness of coal field along the coast line. The area of sedimentation in the Joso-type coal seams should have become narrower in each successive seam finally giving way to a purely marine facies without coal.
The roof of the Joso-type of coal seam such as the Honso and the Joso is composed mostly of coarse grained sandstone and conglomerate, and the coal itself has been washed out locally by the action of marine waves and currents, frequently forming a sort of intraformational conglomerate directly above the coal seam. This conglomerate has been named by the writer as "coaly simultaneous conglomerate".
The deterioration of the quality of coal of the Joso-type is caused, on one hand, by the thinning of the coal from above by erosion, or sometimes by the overlapping of the coal directly on the basement rock. On the other hand, the deterioration of quality by an increase in the ash content occurs in the eastern region. Among the Kaso-type of coal seams, the second Kaso group is thick and of good quality where it is deposited in the valley bottoms between hidden hills, which represent the true hills and valleys of coal forming age.
Away from the hidden hill region, however, the coal seam of the second Kaso shows a transition to the mere plant fossil bed with latifoliate tree leaves, and moreover was eroded away, in some places, probably by streams.

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The pre-Tertiary basement rocks underlying the coal-bearing strata in the Jôban Coalfield often form buried hills. The development of workable coal seams has been markedly influenced by the existence of the buried hills, in that, the main coal seams. tend to become less thick towards the hills and thin out over the hills. These geological features are important in economic prospecting.
The present writer has recently determined that the following two characteristic features are of outstanding importance.
(1) Many kinds of pre-Tertiary schists such as amphibolite and amphibolite schist, together with the associated hornblende biotite granite, biotite granite, etc., arranged subparallel to the strike of the schists, as observed at their outcrops form the high buried hills.
The birth of the buried hills seems to have no relation to any hidden fault scarp in the basement rocks, but seems to have resulted from the differential susceptibility to erosion among granitic rocks, schists and the metamorphic rocks of the contact zones.
(2) Near the buried hills, large major faults are often accompanied by various kinds of minor faults which were formed by the influence of the buried hills.
They are classified as follows:
(i) minor normal or reverse faults; (ii) minor faults not cutting the basement rocks; (iii) faults and slips along the bedding planes; (iv) minor faults disappearing in the beds; (v) faults with a varied throw along their trends; (vi) faults curved around the buried hills.

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The coal-bearing formations of Joban Coal-field are the heteropic facies of the correlative marine formations. The Palaeogene marine transgression of this coal-field shows Grabau's regular transgressive overlapping. The coal-bearing formations of the Kido, Tomioka areas, northern part of Futaba district are considered synchronous with the marine Asagai formation. The cyclothem of the coal-bearing bed is the smallest periodic transgressive unit. The coal seam taken as a whole indicates a layer of the last and stable conditions in which a coal swamp was formed through a relative long time.

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