常磐炭田北端部における深部の層序および構造

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In this article is treated the subsurface geology of the northern part (Tomioka district) of the Joban coalfield, chiefly based upon the data obtained from the drilling cores and seismic prospecting as well as from a resurvey of the surface geology for development of coal mines. The coal bearing Tertiary formations underlying the Pliocene Soma group extensively stretch along the low foreland of the Abukuma massif. The Shiramizu group and the Yunagaya group are variable in rock facies ; northwards from the environs of Kido station (Joban line) the sediments of the Shiramizu and Yunagaya groups become finer grained. The Iwaki formation of this area consists of sandstone, intercalating lenses of conglomerate, shale beds and coal seams. Marine shells similar to the Asagai fauna were collected from the upper parts (in borings at Takinosawa and Daizenbara, both in Tomioka-machi). The Asagai formation consists of fine grained sandstone with abundant marine shells, is relatively uniform in rock facies throughout the area, and is unconformably overlain by the Goyasu formation of the Yunagaya group. The Shirasaka formation, the uppermost part of the Shiramizu group is missing here. The lower member of the Goyasu formation consists mainly of conglomerate and coarse sandstone, with liparitic tuff and lignite seams. This member thins rapidly towards the dip side (eastwards) and the upper member directly overlies the Asagai formation. The upper member is massive, fine-grained sandstone with abundant marine shells, which is characterized by having similar aspect with the Asagai fauna. The Goyasu formation grades upwards into the Mizunoya formation and then to the Kamenoo formation, and both of the latter formations are composed of siltstone and shale, intercalating some sandstones. In the deeper part (eastern part of the area) the formations become entirely muddy in facies. These formations are in turn overlain by the Pliocene Soma group with gentle clino-unconformity. The Soma group has some workable lignite beds near the base. Striking movement along the Futaba fault and flexure zone might probably have occured at least at two different times ; the first is after the deposition of the Yunagaya group and before that of the Soma group, and the second is after the deposition of the Soma group. Natural gas is encountered at certain drilling wells of the areas.
東北大学の論文
1960-05-05

常磐炭田北端部における深部の層序および構造

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