000-000 口絵-長谷川欧文 2009.11.25 9:33 AM ページ XV

地質学雑誌 Vol. 115, No. 11, 2009

Coal-bearing succession of the middle Eocene Ishikari Group in Sanbi Coal Mine, central Hitoshi Hasegawa＊, Noriyuki Suzuki＊† and Hiroyuki Saito＊

Received August 6, 2009 Accepted October 1, 2009 ＊ Research Division of JAPEX Earth Energy Frontier, Creative Research Institution Sousei (CRIS), Hokkaido University, 001-0021, † Division of Earth and Planetary System Sciences, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan Corresponding author; H. Hasagawa, [email protected]

Coal-bearing succession of the Formation in the middle Eocene Ishikari Group is well exposed in the Sanbi Coal Mine, Bibai city, central Hokkaido（Figs. 1, 2; Tanai, 1990; Takano and Waseda, 2003）. The Bibai Formation is composed of alternating beds of sandstone, siltstone, mudstone and coal. Coal seams in this area are developed at 6 major horizons and they are numbered as No. 1 Seam to No. 6 Seam, in ascending order（Figs. 3-5）. No. 1-3 and 6 seams are intercalat- ed with thick sandstone beds of fluvial channel（Figs. 4, 5.A, 5.D）, indicating the deposition in floodplain environments of meandering river system. On the other hand, No. 4-5 seams are interbedded with siltstone and mudstone beds of horizontal lamination and ripple cross-lamination, and interbedded with several siderite nodules layers（Figs. 3, 4, 5.B）, indicating the deposition of low-energy floodplain and swampy area. Coal is generally massive or thinly laminated with micro-alter- nations of vitrinite and degradenite macerals（Fig. 6）. Eocene coals in Hokkaido are characterized by high hydrogen content and rich in degradenite, showing strongly oil-prone features. Most of oil and natural gas resources in East Asian mid- to high-latitude are also derived from coal and organic-rich lacustrine sediments of the Eocene age. The extremely warm and humid “greenhouse” climate of the Eocene period might play an important role for the deposition of these organic-rich sediments on land, although their possible link- age has not yet been clarified. Coal-bearing succession of the Ishikari Group represents a rare glimpse for evaluating the linkage of source rock deposition and Earth’s climate change during the Eocene “greenhouse” world.

References Takano, O. and Waseda, A., 2003, Sequence stratigraphic architecture of a differentially subsiding bay to fluvial basin: the Eocene Ishikari Group, Ishikari Coal Field, Hokkai- do, Japan. Sediment. Geol., 160, 131-158. Tanai, T., 1990, Euphorbiaceae and Icacinaceae from the Paleogene of Hokkaido, Japan. Bull. Nat. Sci. Mus., Tokyo, Ser. C 18, 91-118.

Ashibetsu

Fig. 1. （A）Distribution map of Eocene sedimentary basins and coal-bearing strata, and location of Sanbi Fig. 2. General lithostratigrapic chart of the Ishikari Coal Mine in Bibai and Yuhutsu Oil and Gas Field, central Hokkaido.（B）Geological map of the Ishikari Group, modified after Takano and Waseda（2003）. Group, and location of Sanbi Coal Mine, Bibai city, central Hokkaido, shown in part of 1/25,000 topographic Fission track age data（FT）are from Tanai（1990）. map“Bibai”by Geographical Survey Institute of Japan.

Fig. 3. Outcrop photo- graph of several horizons of coal seams and inter- calated mudstone, silt- stone and channel sand- stone（ss）beds（No. 4 Seam to No. 6 Seam）of the Bibai Formation and the overlying Formation in Sanbi Coal Mine. XV 000-000 口絵-長谷川欧文 2009.11.25 9:33 AM ページ XVI

Fig. 5. Close-up photographs of the coal-bearing succession in Sanbi Coal Mine.（A）Outcrop of No. 5U ① and No. 6L seams and intercalated horizontal and lenticular channel sandstone（ss）which show lateral accretion struc- ture.;（B）Outcrop of No. 5L Seam and underlying siderite nodules, which are composed mainly of siderite and dolomite.;（C）Outcrop of No. 4U ① and No. 4U ② seams and intercalated tuff.; （D）Outcrop of No. 2 Seam and overlying several lenticular channel sandstones（ss）with floodplain coaly mudstone.

Fig. 6. Microphotographs of the thinly laminated coal（No. 3 Seam）:（A）stereomicroscope and（B）reflected-light microscope. These analyses, together with reflected-light fluorescence microscope observations（excitation wave- length: 400−500 nm）, demonstrate that coal seam consists of micro-alternations（mm- to several hundred µm- scale）of vitrinite and degradinite macerals. Degradinite layers partly show ripple cross-lamination structure（A）and contain significant amounts of alumino-silicate clay minerals, suggesting the deposition by low-energy current flows. Deg: Degradinite, Vit: Vitrinite.

Fig. 4（←）. Columnar section of the coal-bearing succession of the Bibai Formation in Sanbi Coal Mine. In Sanbi Coal Mine, coal seams are numbered from No. 1L（No. 1 Lower）to No. 6U（No. 6 Upper）, in ascending order.

Acknowledgements: We thank Sanbi Mining Company for giving opportunity to study in the Sanbi Coal Mine and their kind help during the field survey.

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