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廣島大學地學研究報告 10 号
1961-06-30 発行
中国地方中部大賀台地の古生層の層序と構造
Geological Studies of the Paleozoic Groups in the Oga Plateau, Central Chugoku, Japan
吉村 典久
Abstract
The Oga district and its adjacent areas are one of the most important fields for analysis of the tectonic history of the Late Paleozoic and Early Mesozoic age in the central Chugoku, because various kinds of strata are well developed in the narrow area and their structure are very complicated. Though a number of geologists and Paleontologists had engaged in studies of the district, numerous unsolved ploblems are suspended.
So, the writer has surveyed and studied the stratigraphy and the geologic structure of the Paleozoic groups in several important places of this region. As a result of these studies, the salient facts are given as follows:
1) The Paleozoic groups are composed of metamorphic and non-metamorphic facies; the latter is divided into two members, i.e. non-calcareous and calcareous members, based on the differences of rock-facies and the geologic structure. Non-calcareous members are separable into the upper Mihara
formation (sandstone and shale) and the lower Otake formation (chert), and calcareous members are separable into the upper Uji formation (mainly shale with limestone conglomerate) and the lower Koyama group (mainly limestone) by rock-facies.
The stratigraphy and the fossil zones of the Paleozoic groups in the surveyed areas of this region are summarized on the tentative correlation table (TABLE 1).
2) It is very interesting fact that the change of rock-facies from the Koyama group and its equivalents to the Uji formation and its similar formations is remarkable and rapid. This phenomena was probably caused by some tectonic movements in the middle stage of the Middle Permian age, and the stage is equivalent approximately to Imamura's Pre-Maki unconformity. In the Hane and Yotsumine area Yabeina shiraiwensis and its allies are very abundant in the basal part of the Uji formation and its equivalent, while Neoschwagerina douvillei and its associated forms were obtained from the Oga, Fukiya, and Hanagi area.
3) Generally speaking, the Koyama group and its equivalents consist of the upper limestone and the lower schalstein formations. Millerella sp., Clisaxophyllum awa etc. were scarcely found from the lowest part of the limestone formation (probably of the Millerella zone).
4) Because no fossils have been obtained from the Yoshii group which is cut off by many faults from the surrounding formations, its precise age is uncertain. The group is, however, tentatively referred to the Permian as can be judged from the litho-facies.
5) There are a number of limestone lenses in the phyllitic Yamano group, and fairly amounts of Fusulinids, such as Yabeina shiraiwensis, Y. columbiana, Neoschwagerina douvillei, Pseudofusulina sp., Triticites sp. were collected from various horizons. So, the age of the Yamano group may be the Lowest Permian to the Upper Permian. The group is possibly synchronous but heteropic with the greater part of the Koyama group plus the Uji formation.
6) The Shodera formation may be excluded better from the Paleozoic groups as can be judged from the rock-facies and the tectonic allignment, and it belongs possibly to the Triassic sediments in which a number of limestone pebbles are fairly found. This fact must indicate that the limestone of the Koyama group were distributed in the present position in the Late Triassic age.
So, the writer has surveyed and studied the stratigraphy and the geologic structure of the Paleozoic groups in several important places of this region. As a result of these studies, the salient facts are given as follows:
1) The Paleozoic groups are composed of metamorphic and non-metamorphic facies; the latter is divided into two members, i.e. non-calcareous and calcareous members, based on the differences of rock-facies and the geologic structure. Non-calcareous members are separable into the upper Mihara
formation (sandstone and shale) and the lower Otake formation (chert), and calcareous members are separable into the upper Uji formation (mainly shale with limestone conglomerate) and the lower Koyama group (mainly limestone) by rock-facies.
The stratigraphy and the fossil zones of the Paleozoic groups in the surveyed areas of this region are summarized on the tentative correlation table (TABLE 1).
2) It is very interesting fact that the change of rock-facies from the Koyama group and its equivalents to the Uji formation and its similar formations is remarkable and rapid. This phenomena was probably caused by some tectonic movements in the middle stage of the Middle Permian age, and the stage is equivalent approximately to Imamura's Pre-Maki unconformity. In the Hane and Yotsumine area Yabeina shiraiwensis and its allies are very abundant in the basal part of the Uji formation and its equivalent, while Neoschwagerina douvillei and its associated forms were obtained from the Oga, Fukiya, and Hanagi area.
3) Generally speaking, the Koyama group and its equivalents consist of the upper limestone and the lower schalstein formations. Millerella sp., Clisaxophyllum awa etc. were scarcely found from the lowest part of the limestone formation (probably of the Millerella zone).
4) Because no fossils have been obtained from the Yoshii group which is cut off by many faults from the surrounding formations, its precise age is uncertain. The group is, however, tentatively referred to the Permian as can be judged from the litho-facies.
5) There are a number of limestone lenses in the phyllitic Yamano group, and fairly amounts of Fusulinids, such as Yabeina shiraiwensis, Y. columbiana, Neoschwagerina douvillei, Pseudofusulina sp., Triticites sp. were collected from various horizons. So, the age of the Yamano group may be the Lowest Permian to the Upper Permian. The group is possibly synchronous but heteropic with the greater part of the Koyama group plus the Uji formation.
6) The Shodera formation may be excluded better from the Paleozoic groups as can be judged from the rock-facies and the tectonic allignment, and it belongs possibly to the Triassic sediments in which a number of limestone pebbles are fairly found. This fact must indicate that the limestone of the Koyama group were distributed in the present position in the Late Triassic age.
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