中村石灰岩の堆積相
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概要
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The limestone formation distributed over northern part of Oga-Dai, one of the well-known limestone plateaus in central Chugoku, Southwest Japan, is called Nakamura Limestone and has been considered equivalent to Koyama Limestone at southern part of Oga-Dai. This limestone is composed mainly of massive limestone with schalstein at the base. It is overlain by the Upper Permian Uji Formation, which consists of black shale with some amount of limestone-conglomerate. Biostratigraphically, the Nakamura Limestone is divided, in ascending order, into seven zones of Endothyra (upper Lower Carboniferous), Millerella, Profusulinella, Fusulinella-Fusulina (Upper Carboniferous), Pseudoschwagerina (Lower Permian), Parafusulina and Neoschwagerina (Middle Permian), while the Uji Formation is represented by the Yabeina zone. By the microscopic petrographical examination of thin sections of limestone from more than 500 localities and from subsurface handspecimens obtained from 17 boring cores, the following eleven rock types are discriminated ; 1) oolitic grainstone, 2) algal bioclastic grainstone, 3) fusuline bioclastic grainstone, 4) oolitic packstone, 5) algal bioclastic packstone, 6) fusuline bioclastic packstone, 7) crinoid-bryozoan bioclastic packstone, 8) bioclastic wackestone, 9) mudstone, 10) algal boundstone, 11) limestone-conglomerate. They are distributed to constitute four distinct facies I, II, III and IV : Facies I is characterized by the predominance of oolitic grainstone, Facies II by crinoid-bryozoan bioclastic packstone, Facies III by algal fusuline bioclastic grainstone, and Facies IV by mudstone and wackestone. Facies I and II develop in the Carboniferous ; the former occupies the greater part, while the latter is restricted narrowly. Facies III and IV develop in the Permian; the former occupies the main part and is distributed over southern part, while the latter over northern part. Judging from the distribution of facies mentioned above, the formation of the main part of Nakamura Limestone had originated from the development of oolite shoal under high energy condition on the bank of Visean submarine volcanic mound. A great deal of oolite was spread around and accumlated to form the oolitic grainstone of Facies I in the Carboniferous. On the shallowest part of the mound of the Permian under high energy condition, the algal-fusuline bioclastic grainstone was deposited. Relatively northern part in the Permian, there had been existed lagoon. Compared with Nakamura Limestone herein described, Koyama Limestone at southern part of Oga-Dai may have been formed in a fore-reef environment which is under conditions of open marine circulation.
- 地学団体研究会の論文
- 1980-11-25