岡山県成羽町北西地域の地質

This district is situated in the central part of the Kibi plateau which is considered as an elevated peneplain, and has been studied by a lot of geologists and palaeontologists because of abundant occurrences of animal and plant fossils and its complicated geologic structures.
The stratigraphy of the district is summarized as follows:
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The Palaeozoic rocks are composed of calcareous and non-calcareous groups. The latter consists of non-fossiliferous sandstone, shale and chert, and is named the Fuka formation. The former can be subdivided into 2 members by the lithofacies; the lower is the schalstein-bearing Nakamura limestone group, while the upper is called the Uji formation mainly composed of shale and schalstein sometimes with limestone nodules. From the evidence of fusulinids the calcareous formations are assigned to the Pennsylvanian-Middle Permian in age.
The "Inkstone" group, which forms a basin structure, can be subdivided into two parts by the presence of limestone conglomerate; i. e. the upper is composed of the thick red tuff beds, while the lower of the thick limestone conglomerate and thin red tuff layers. The limestone conglomerate is limited to occur in the vicinities of the huge limestone bodies. Judging from the lithofacies, the "Inkstone" group in this district might be the sediments mainly of pyroclastic materials and deposited in a small narrow basin which was due to the Ohga thrust movement.
The Palaeozoic Nakamura limestone group thrusts over the Norian Nariwa group, and both of them are covered unconformably by the "Inkstone" group and later extruded andesite and rhyolite masses successively. Therefore, the Ohga thrust movement might be happened in pre-"Inkstone" time.