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As a result of the geological studies on the structure of the Akiyoshi Plateau in Minegun, Yamaguchi Prefecture, the rocks were classified into the units shown in Table 6, and each of them are described in detail in the text. From the fusulinids collected from the Akiyoshi limestone the following zones were recognized, from the lower to the upper; Eudothyra zone (C_<I>), Millerella zone (C_<II>), Profusulinella zone (C_<III>), Fusulinella-Fusulina zone (C_<IV>), Pseudoschwagerina zone (P_<I>), Parafusulina zone (P_<II>), Neoschwagerina zone (P_<III>), and the Yabeina zone (P_<IV>). Among these zones, the Pseudoschwagerina zone is subdivided into Triticites simplex subzone (P_<I-1>) in the lower and the Pseudoschwagerina-Pseudofusulina vulgaris subzone (P_<I-2>) in the upper. Between the Fusulinella-Fusulina zone and the Triticites simplex subzone the top of the Zone of Fusulina and the whole of the Zone of Triticites are missing. Although no unconformity could be observed in the field one is expected because the two fossil zones are missing. The upper part of the Neoschwagerina zone to the Yabeina zone changes to a muddy facies and breccia limestone becomes dominant, but no unconformity could be determined. The relationship between the Akiyoshi limestone and the non-calcareous rocks which are distinguished into groups and formations is shown in the correlation table (Table 6). The Katada formation, the lowest of the Beppu group corresponds to the Pseudoschwagerina zone and the Parafusulina zone, the former of non-calcareous and the latter of calcareous rocks. The Ryugenji formation (Beppu group) is equivalent to the lower part of the Neoschwagerina zone, the Kawarakami formation (Beppu group) corresponds to the upper part of the Neoschwagerina zone including the Yabeina zone. The Aigyo formation of the Ofuku group yielded no fossils and thus its age is not certain, but from its relation to the superjacent formation whose age is known, it is considered that it belongs to the Carboniferous in age. The Maki formation can be correlated paleontologically with the Pseudoschwagerina zone to the Parafusulina zone. The Maki formation contains in its lower part a facies of coarse grained sandstone and granule conglomerate, and thus it is considered that a slight unconformity may exist between it and the Aigyo formation. The Tsutsumi formation from paleontological evidence can be correlated with the Neoschwagerina zone to the Yabeina zone. The Futagami formation in the lower part of the Ota group is a chert facies with intercalated small limestone lenses which have yielded Endothyra, Plectogyra, and therefore, may be correlated with the Endothyra zone of the Akiyoshi limestone. The Yaegahara formation shows a slight parallel unconformity at its base and yields from its upper part fossils enabling correlation with the Yabeina zone. From the distribution of the fossils in the Akiyoshi limestone it is inferred that in the northeastern part bf the present area there is an anticline, and in its southeastern part there are three synclines almost with parallel lineation. The anticlines between the synclines are cut with thrust faults, riding over the synclinal structures and their original structures remain merely as relics. The synclinal axes trend NE-SW to NEE-SWW with the western side gently plunging and the axes-plane is inclined at 60°-80° towards the southeast. Previous authors have assumed a major recumbent fold of the present area, however, this may have been a misinterpretation of anticlinal structures thrust up on the synclines. The age of folding of the non-calcareous Ofuku group coincides with that of the Akiyoshi limestone and the age of faulting of both groups is in agreement. The axes of folding of the Akiyoshi limestone and Ofuku groups correspond with one another. These two groups were not deposited in different areas as previously thought, but are contemporaneous in deposition but of different facies, and therefore, were subjected to the same movements and at the same time. The Beppu and Ota groups have folding axes of NEE-SWW, being the same as the Akiyoshi and Ofuku groups. The latter rides over the former with thrust fault, and the .Ota is : thrust up on the Akiyoshi limestone and the Ofuku group. The relation between the Paleozic rocks and the Triassic Mine group is a distinct thrust plane, dipping towards the south. The Cretaceous Kwamon group covers the Paleozoic and Mine groups with distinct clino-unconformity, but at places the latter two, separately, are thrust up on the Cretaceous Kwamon group. In the south of Isa, the thrust plane is invaded with prophyrite. The geological age of diastrophism is post-Permian to pre-Triassic, because the significant folding structures observed in the Paleozoic rocks do not influence the Triassic deposits. The thrust faults were developed at the time of this magnificant folding. The next movement occurred after the deposition of the Mine group.

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