Full length article Calcareous nannofossil assemblages of the upper Miocene to Pliocene Shimajiri Group on Okinawa-jima, Ryukyu Islands, southwestern Japan Ryo Imai a,⇑ , Tokiyuki Sato b , Yasufumi Iryu c a SOHKEN CONSULTANT Co., Ltd., Sanno 1-9-22, Akita 010-0951, Japan b Institute of Applied Earth Sciences, Faculty of Engineering and Resource Science, Akita University, Tegata Gakuencho, Akita 010-8502, Japan c Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Aramaki-aza-aoba 6-3, Sendai 980-8578, Japan article info Article history: Received 3 August 2015 Received in revised form 6 December 2016 Accepted 9 December 2016 Available online 10 December 2016 Keywords: Biostratigraphy Paleoceanography Calcareous nannofossil Miocene Pliocene Ryukyu Islands Shimajiri Group abstract We investigated the calcareous nannofossil biostratigraphy and paleoceanography of the Shimajiri Group in two exploratory wells (depths, 1243 and 1800 m) drilled on southern Okinawa-jima, Ryukyu Islands, southwestern Japan. Three late Miocene to Pliocene datum planes were identified: the first occurrence of Amaurolithus spp. (7.42 Ma), the last occurrence of Discoaster quinqueramus (5.59 Ma), and the first occur- rence of Ceratolithus rugosus (5.12 Ma). These datum planes were also identified in two other deep wells on southern Okinawa-jima. Nutrient levels (reconstructed from the relative abundances of small Reticulofenestra spp., and the relative abundances of Discoaster spp. plus Sphenolithus abies) suggest that deposition of the Shimajiri Group (<8.29–3.50 Ma) occurred under conditions of increasing eutrophica- tion. The eutrophication was likely caused by shallowing of the sedimentary basin, but a stepwise shift was associated with an abrupt eutrophication event that occurred in the northwestern Pacific Ocean at 5 Ma. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction The Ryukyu Islands to the southwest of Japan form an island arc and are ideally placed to help understand paleoceanographic evo- lution of northwestern Pacific margin. The upper Miocene to Pleis- tocene Shimajiri Group, which is composed mainly of mudstone and sandstone (thought to have been deposited in a shelf slope to fore-arc basin setting), occurs on some of the Ryukyu Islands, southwestern Japan (Fig. 1). The Shimajiri Group underlies the Ryukyu Group, which consists of Pleistocene coral reef and shelf carbonates. Between these a transitional lithofacies the Chinen Formation is locally developed. The appearance of coral reefs in the Ryukyu Islands is related to the opening of the Okinawa Trough, a back-arc basin of the Ryukyu Island Arc. Although timing of the opening is controversial, it is considered likely that the Trough was opened during the late Miocene (Gungor et al., 2012) to early Pleistocene (Park et al., 1998). The subsequent influx of the Kuroshio Current into the back-arc basin resulted in decreased sediment transport to the islands and the development of clear- water coastal environments that promoted coral growth (Ujiié, 1994; Iryu et al., 2006). Therefore, the Shimajiri Group is important for interpretations of paleoceanographic change that occurred before and during the opening of the Okinawa Trough. Moreover, such interpretations are essential for understanding the evolution of the Kuroshio Current, the western boundary current in the northwestern Pacific (Gallagher et al., 2015). Biostratigraphic studies of the Shimajiri Group have been con- ducted using planktonic foraminifers and calcareous nannofossils (Ibaraki and Tsuchi, 1975; Natori, 1976; Nishida, 1980; Tanaka and Ujiié, 1984; Ujiié, 1994; Imai et al., 2013). However, the group is complexly folded and faulted, and most of the group is located in the subsurface; thus, studies based exclusively on samples from surface outcrops provide only a partial record of the group, and are less reliable than studies utilizing well data. Based on their studies on benthic and planktonic foraminifers from the Shimajiri Group in wells as well as at surface outcrops, LeRoy (1964) and Fukuta et al. (1970) reconstructed sedimentary environments dur- ing deposition of the Group. The reconstructions, however, were different between the two studies, which was attributed to differ- entiation of sedimentary basin (Fukuta et al., 1970). Natori (1976) identified nine planktonic foraminiferal datum planes in the Shi- majiri Group and the Chinen Formation in wells and at surface out- crops and correlated them to zones N16 to N22 of Blow (1969) (upper Miocene to Pleistocene). This chronological framework has been followed by many workers. Based on the results of Mishima and Ujiié (1983), Tanaka and Ujiié (1984) developed cal- careous nannofossil and planktonic foraminiferal biostratigraphies http://dx.doi.org/10.1016/j.jseaes.2016.12.011 1367-9120/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: imai@sohken-c.co.jp (R. Imai). Journal of Asian Earth Sciences 135 (2017) 16–24 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes