Abstract:We report on Eocene ostracode species from Hokkaido, northern Japan for the first time and discuss their paleobiogeographic implications. Five species were found in the lower part of the Sankebetsu Formation in the Haboro area, and the Akabira and Ashibetsu Formations in the Yubari area, central Hokkaido. The ostracode assemblages are characterized by Robertsonites, a circumpolar Arctic genus of the modern fauna. This finding marks the southernmost occurrence of the Eocene record of the genus, as well as the oldest occurrence. As the characteristics of the Eocene Hokkaido fauna are similar to those of the modern-day fauna in the Seto Inland Sea, a contrast in species composition is observed between the Seto Inland Sea and Kyushu. This contrast is attributable to differences in sea temperature. Five species are described, including Robertsonites ashibetsuensis sp. nov.Keywords: Eocene, Hokkaido, Ostracoda, paleobiogeography, Robertsonites 1 Course of Earth Sciences, School of Natural System, College of Science and Engineering, Kanazawa University, Kakumamachi, Kanazawa City, 920-1192, Japan 2 Department of Geology, Faculty of Science, Niigata University, 8050 Ikarashi-2nocho, Niigata City, 950-2181, Japan Bulletin of the Geological Survey of Japan, vol.59 (7/8), 2008 Fig. 1 A: localities of Eocene marine ostracodes around the Japanese Islands. 1, Middle-Upper Eocene Iwaya Formation (Yamaguchi et al., 2005). 2, Upper Eocene-lowermost Oligocene sequence (Yamaguchi et al., in press). 3, uppermost Eocene-lowermost Oligocene Kishima Formation . 4, Middle Eocene-Lower Oligocene Iojima Group (Yamaguchi, 2006). 5, Lower Eocene Oujiang and Middle Eocene Wenzhou Formations (Yang et al., 1990). 6, Lower Eocene Oujiang Formation (Liu, 1989). B: distribution of Paleogene strata and localities of examined samples in Hokkaido. Geology after Editioral Committee of Hokkaido (1990).Eocene ostracodes from Hokkaido, northern Japan (Yamaguchi and Kurita) 2.1 Ashibetsu and Akabira Formations (Middle Eocene) These formations consist of shale beds and alternating beds of shale and sandstone that represent marine intervals in the coal-bearing deposits of the Ishikari Group in the Sorachi area. These strata yield the Ishikarian molluscan fauna, indicating a Middle Eocene age (Mizuno, 1964). This age assignment is supported by fission track dating of the underlying Yubari and Wakkanabe Formations (ca. 44-42 Ma;Tanai, 1986) and the ages of calcareous nannofossils in the overlying Poronai Formation (late Middle Eocene; Okada and Kaiho, 1992).
Poronai Formation (Middle-Upper Eocene)The Poronai Formation, which overlies the Ishikari Group in the Sorachi and Ishikari areas, is dominantly mudstone, and has a thickness of 600 m. It is correlated with the calcareous nannofossil Zone CP14-CP15 of Okada and Bukry (1980) (Okada and Kaiho, 1992) and the dinoflagellate cyst Bellatudinium hokkaidoanum and Trinovantedinium boreale Zones (Kurita, 2004). Fossil planktic foraminifers from the formation indicate a Middle-Late Eocene age (~...