吉備高原西部，神石・油木地方の古生界，とくに石灰岩層群と非石灰質岩層との関係について

As to the Carboniferous and Permian strata in the Inner Zone of Southwest Japan, there is a striking contrast between calcareous and non-calcareous facies. Surrounded by thick series of sandstone and shale (sometimes with chert), large masses of limestone with some schal-stein at the base are developed in several isolated areas. An example is in and around the Taishaku limestone plateau, a part of the Kibi plateau, Central Chugoku.
The non-calcareous group distributed to the south of the Taishaku plateau is a northwestern extension of the Yoshii group (YOSHIMURA, 1961), and is divided into the following two parts.
Lower formation (more than 800m. thick): Mainly composed of black shale, with rather small amount of sandstone and chert. Thin layers of schalstein are intercalated and a small lens of limestone is included.
Upper formation (1500-2000m. thick): Composed of sandstone and shale repeated in various thickness, containing conglomerate, schalstein and limestone as thin layers or lenses.
From the fossil evidence, which is however insufficient, this group may range in age from Middle Permian to Upper Permian.
The limestone groups ranging from upper Lower Carboniferous to Middle Permian thrust up the Yoshii group from both the northeastern and southwestern sides. The thrust-faults run in a direction of WNW-ESE or NW-SE, and dip generally with a high angle. Dikes of por-phyrite of Cretaceous age outcropping along the thrust planes are cut by the re-acted faults.