KJ00000197306.pdf 1.29 MB
Stratigraphy of the Koyama limestone and structural relation between the limestone and the surrounding strata in the Oga area, Okayama Prefecture, western Japan
The Koyama Limestone in the southwestern part of Okayama Prefecture of western Japan can be divided into five foraminiferal zones; namely, the Endothyra-Mediocris Zone, the Eostaffella-Millerella Zone, the Pseudostaffella-Profusulinella Zone, the Fusulinella Zone and the Pseudoschwagerina-Parafusulina Zone in ascending order. It is obvious that this limestone is lacking in the Upper Carboniferous Triticites Zone. This fact suggests that the depositional environment of limestone had changed in uppermost Carboniferous to early Permian time. Moreover, some part of the Koyama Limestone is lacking in some kind of foraminiferal zones, and the discontinuous deposition or partial unconformity is inferred. The distribution of the foraminiferal zones indicates that the Koyama Limestone has a large gentle synclinal structure, which is very different from the fold structure of the Hina Limestone. Judging from the distribution, lithology and geological age of the Koyama Limestone, Yoshii Group, Sugeno Chert Formation and Uji Formation, these Paleozoic strata in this area formed pile nappe structure : the Koyama Limestone thrust over the Middle to Upper Permian Yoshii Group; the Sugeno Chert Formation which is not the uppermost formation of the Koyama Limestone and the Uji Formation also thrust over the Koyama Limestone. These movements have occurred after the Late Permian and before the deposition of the Upper Triassic Nariwa Group. These strata are closely contacted without fault breccia and fault gouge. This fact suggests that pile nappe structure was probably formed by the submarine sliding.
広島大学総合科学部紀要. IV, 理系編