天草島における Nummulites 帯について
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概要
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Haha-jima in the Ogasawara group has long been known to be the only island where Nummulites are found in Japan. But in 1923, the late Dr. Takumi Nagao discovered that the Nummulites bed is also developed in Hongo of Amakusa-Shimo-shima and in the headland of Tagui situated at the northern extremity of Shishi-jima of Kagoshima Prefecture, and his discovery has aided in the decision of the geological age of the older Tertiary formation in Amakusa, and also contributed to the geological history of the Tertiary system in Japan. Dr. Nagao has reported that the Nummulites limestones are found intercalated in the lower Kyoragi shale, but according to the present writer's division, it is intercalated in the uppermost part of the Akashi-misaki formation which contain the Fukami formation and Fukuregi formation (of Matsushita, 1949). Drs. H. Yabe and S. Hanzawa described Nummulites amakusensis-subamakusensis Yabe et Hanzawa and Orthophragmina (Discocyclina) aff. pratti Michelin from the above-mentioned Nummulitic rocks of Amakusa-Shimo-shima and Shishi-jima and considered that the rocks are Ypresian or Lower Lutetian in age, and not younger. The writer has been able to trace Nummulitic rocks in the regions of Amakusa-Shimo-shima, Naga-shima and Shishi-jima of Kagoshima Prefecture, and also Take-shima and Maki-shima, both belonging to the Islands of Amakusa ; he still believes that it is possible to trace their northern part, and that the Nummulites Zone, with its advance to the north, gradually shifted to the Lower Orthaulax Zone of Amakusa-Kami-shima. The writer has described the Numulites Zone of the districts, which contains : Nummulites amakusensis-subamakusensis Yabe et Hanzawa (abundant) Discocyclina sp. (abundant) Turritella (Zaria) aff. merriami Dickerson Turritella okadai Nagao Orthaulax japonicus Nagao (rare) Flabellum sp. (rare) Platycyathus sp. (rare) Balanophyllia sp. (rare) Discocyclina sp. in the present paper is to be identified with Orthophragmina aff. pratti Michelin described by Yabe and Hanzawa, but the writer refrains from usage of the name of Orthophragmina aff. pratti Miehelin, because he followed Dr. Hanzawa's suggestion that, as there still remains room to study the species, the name had better be Discocyclina sp. for the present. Nummulitic rocks found in the southern extremity of Amakusa-Shimo-shima are intercalated in a black siltstone bed as small limestone lenses or calcareous sandstone lenses. The writer has tentatively named the above-mentioned one as the I-type of occurrence. In the northern adjacent district the Foraminifera are thickly accumulated in sandstone in band shape side by side with the bedding plane and Discocyclina is more abundant in individual number than Nummulites. The above is tentatively named as the II-type of occurrence. In the northern half of Amakusa-Shimo-shima there are sometimes found Discocyclina sp. sparsely mixed in the shell-bed intercalated in the sandstone. This is tentatively named as the III-type of occurrence. These changes of fossil facies not only indicate that the Hondo transgression or Nummulites transgression was from the south to the north, but also provide evidence for an ecological study of Nummulites and Discocyclina.
- 東北大学の論文
- 1960-05-05