Paleontological Research ISSN 1342-8144 Formerly Transactions and Proceedings of the Palaeontological Society of Japan Vol. 5 No.3 September 2001 The Palaeontological Society of Japan Co-Editors Kazushige Tanabe and Tomoki Kase Language Editor Martin Janal (New Yorl<, USA) Associate Editors Alan G. Beu (Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand), Satoshi Chiba (Tohoku University, Sendai , Japan), Yoichi Ezaki (Osaka City University, Osaka, Japan), James C. lngle, Jr. (Stanford University, Stanford, USA), Kunio Kaiho (Tohoku University, Sendai, Japan), Susan M. Kidwell (University of Chicago, Chicago, USA), Hiroshi Kitazato (Shizuoka University, Shizuoka, Japan), Naoki Kohno (National Science Museum, Tokyo, Japan), Neil H. 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Phone: (978)750- 8400, Fax: (978)750- 4744, www.copyright. com Cover: Idealized sketch of Nipponites mirabilis Yabe, a Late Cretaceous (Turonian) nostoceratid ammonite. Vari ous reconstructions of the mode of life of this species have been proposed, because of its curiously meandering shell form (after T. Okamoto, 1988). All communication relating to this journal should be addressed to the PALAEONTOLOGICAL SOCIETY OF JAPAN c/o Business Center for Academic Societies, Honkomagome 5-16- 9, Bunkyo- ku, Tokyo 113- 8622, Japan Vi sit our society website at http://ammo.kueps.kyoto-u.ac.jp/palaeont! Paleontological Resear~h, vol. 5, no. 3, pp. 143-162, September 28, 2001 © by the Palaeontological Society of Japan Ostracode biostratigraphy of the Lower Cretaceous Wakino Subgroup in northern Kyushu, Japan KEIICHI HAYASHI Faculty of Science and Engineering, Konan University, Kobe, 658-8501, Japan (e-mail: [email protected]) Received 3 March 2000; Revised manuscript accepted 6 November 2000 Abstract. Nonmarine ostracodes from the Lower Cretaceous Wakino Subgroup in northern Kyushu were studied biostratigraphically. Analysis of abundant fossil ostracodes has led to rec­ ognition of 11 assemblages and subassemblages. Vertical changes of these assemblages have made possible biostratigraphical zonation by ostracodes as follows: the Darwinula a. z. (assem­ blage zone), the transitional a. z. and the large Cypridacean a. z. in ascending order. Using this zonation, the formations of the Wakino Subgroup in the eastern area are correlated with those in the western type area. Key words: Biostratigraphy, Lower Cretaceous, Northern Kyushu, ostracodes, southwest Japan, Wakino Subgroup Introduction propose a correlation scheme. The correlation of Lower Cretaceous nonmarine sedi­ Geologic settings ments is controversial in Eastern Asia, because their rela­ tions with marine sediments are known only in restricted Geologic settings of the study area were already reported regions (Matsukawa and Obata, 1992, 1994). The same is in Hayashi (1998). Only the outlines of the geology are de­ true in Japan. Molluscan fossils have been traditionally scribed here to the extent necessary for later description and used in the correlation of Cretaceous strata of Japan by discussion in this paper. Early Cretaceous nonmarine sedi­ many investigators (Kobayashi and Suzuki, 1936; Ota, 1960; ments are scattered in the Inner Zone of Southwest Japan. Hase, 1960; etc.). This approach is not reliable due to the Especially in northern Kyushu and western Chugoku situ­ fact that the same molluscan assemblages and characteris­ ated in the western areas of the Zone, they are assigned as tic species occur in several different horizons showing simi­ the Kanmon Group (Matsumoto, 1951). The group is com­ lar depositional environments (Matsukawa and Ito, 1995). posed of the Wakino Subgroup in the lower part and the Thus, other methods have recently been introduced for the Shimonoseki Subgroup in the upper part. The Shimonoseki correlation of Cretaceous nonmarine sediments; for exam­ Subgroup overlies disconformably the Wakino Subgroup ples by using other fossil groups such as fish (Yabumoto, and oversteps the older basement in places. 1994) and sedimentary facies suites (Sakai et al., 1992; Seo In northern Kyushu, the group is distributed in two major et al., 1992, 1994). One potential approach is to use the areas (Figure 1). To the west of Nakama and Nogata cities, ostracode fossils that are abundantly found in Early the upper part is generally observed sequecially from south Cretaceous nonmarine sediments. Cao (1996) described to north, though it is discontinuously crop out into several fossil ostracodes from the Cretaceous in Japan and com­ isolated areas by some folding and faulting; to the east of pared them with those in China. However, her correlation Nakama and Nogata cities, obscure upper sequences are is still insufficient because she discussed faunal assem­ seen from south to north, but it is divided into many isolated blages based on the samples from restricted localities. For blocks by much folding'and faulting; hence the stratigraphic estimating the stratigraphical significance of each species positioning of these delimited exposures is difficult. and genus, a more detailed study of ostracode Among many areas occupied by the Wakino Subgroup in biostratigraphy is necessary. Recently, Hayashi (1998) re­ Figure 1, three areas, i. e. Wakino, Yurino and southern ported 74 ostracode species belonging to 17 or more genera Kokura areas, were selected for this study, because from Cretaceous nonmarine sediments. This study aims to ostracode fossils have been known to occur only in these establish a biozonation by using fossil ostracodes from the areas. As the Wakino and Yurino areas are relatively close Lower Cretaceous Wakino Subgroup in North Kyushu and to to each other, those two are described together. 144 Keiichi Hayashi f '1' , Intrusive rocks Shimonoseki Subgroup Wakino Subgroup o 5 10 km , ~ Sangun Metamorphic ~ rocks Figure 1. Geological map showing the distribution of the Kanmon Group in northern Kyushu (modified from Seo et al., 1992). A: Wakino area, B: Yurino area, C: southern Kokura area. '"~ Wakino area Yurino area Southern Kokura area <{ I III Uc Kobayashi & I. Ota Hase <1 958, Ota Ota <1955, Ota et al ~ III Ota <1 953) Hase <1958, 1960) "- (1936) 1960) (1960) 1957, 1960) (1979) c:::III ILI ULli WULi Upper Upper J - Miyata shale ~f The Fourth Fm. Wakamiya Fm. Wakamiya Fm. rmrnrm the Upper Uppermost Fm. Wakamiya Fm. =W4 ~~ ~ 0. :::l e Lower Lower Equivalent of the Lower C) Kinsho sandstone W3 Upper Fm. =W3 The Third Fm. .D Wakamiya Fm. Wakamiya Fm. Wakamiya Fm . :::l (f) /"">. "-""' 0 c Sengoku Equivalent of the Nyoraida :52 Nyoraida Fm. Nyoraida Fm. Wz Middle Fm. =Wz The Second Fm. 3:'" conglomerate Fm. ~ ,,-.r....... -../'-'"' ~'-"'"' Equivalent of the 5engoku Wakino Fm. 5engoku Fm. Sengoku Fm. Wl Lower Fm. =Wl The First Fm. Fm. Figure 2. Comparison of stratigraphy of the Wakino Subgroup by previous studies. Ostracode biostratigraphy of the Lower Cretaceous Wakino Subgroup 145 Wakino-Yurino area debris flow deposits (Seo et al., 1992). In North Kyushu, the stratigraphical study of Mesozoic The Yurino area is situated 7 km northeast of the Wakino