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Coal-bearing succession of the Bibai 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 intercalated 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-alternations 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 midto 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 linkage 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.

Coal-bearing succession of the middle Eocene Ishikari Group in Sanbi Coal Mine, central Hokkaido