Early Oligocene–Pleistocene Calcareous Nannofossil Biostratigraphy of the Northern South China Sea (Leg 184, Sites 1146–1148)

Sites 1146 and 1148 of Ocean Drilling Program Leg 184, in the South China Sea (SCS), comprise long sediment sections with a time span from the early Oligocene to the Pleistocene. Calcareous nannofossils from these two sites were biostratigraphically studied. We recognized 53 early Oligocene to Pleistocene events that are commonly found in open sea areas and can therefore be correlated within a large geographic range. This study also revealed that a few conventionally used nannofossil events are not suitable for the SCS, and further evaluation is needed. The lower Oligocene to Pleistocene sequences recovered at Sites 1146 and 1148 were subdivided into the 4 Paleogene zones and 21 Neogene to Quaternary zones of Martini, in correlation with the Paleogene to Quaternary zones of Okada and Bukry. This provided a lower Oligocene through Pleistocene nannofossil biostratigraphic framework. A significant unconformity was recognized in the Oligocene–Miocene transition, in which the upper part of Oligocene Zone NP25 and lower part of Miocene Zone NN1 were missing. The time span of the unconformity was estimated to be ~1 m.y. Very high sedimentation rates were seen in the Oligocene, relative low values were seen in the Miocene, and the highest values were seen in the Pleistocene, which was believed to be the result of tectonic and sedimentation history of the SCS. 1Su, X., Xu, Y., and Tu, Q., 2004. Early Oligocene–Pleistocene calcareous nannofossil biostratigraphy of the northern South China Sea (Leg 184, Sites 1146–1148). In Prell, W.L., Wang, P., Blum, P., Rea, D.K., and Clemens, S.C. (Eds.), Proc. ODP, Sci. Results, 184, 1–24 [Online]. Available from World Wide Web: . [Cited YYYY-MM-DD] 2China University of Geosciences (Beijing), Xueyuan Road 29, Beijing, 100083, People’s Republic of China. xsu@cugb.edu.cn Initial receipt: 30 January 2004 Acceptance: 9 August 2004 Web publication: 19 November 2004 Ms 184SR-224 X. SU ET AL. EARLY OLIGOCENE–PLEISTOCENE CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY 2 INTRODUCTION The South China Sea (SCS) is the largest marginal sea in the West Pacific. Previous biostratigraphic studies on calcareous nannofossils in the SCS were limited to incomplete Oligocene to Pleistocene marine sequences from exploration wells in offshore basins (Duan and Huang, 1991; Huang and Zhong, 1992; Xu, 1996; Huang, 1997) or late Quaternary materials from shallow piston cores in the deep-sea area (Duan, 1985; Su, 1989). During Ocean Drilling Program (ODP) Leg 184 (Fremantle to Hong Kong, February–April 1999), the JOIDES Resolution drilled six sites in the deep-sea area of the SCS. This was the first time that sediment cores deeper than 20 m were sampled from the continental slope in the SCS. Of these six sites, Sites 1146 and 1148 were the source of the two longest sections from the SCS: a 642-m-long composite section spanning the last 18 m.y. at Site 1146 and an 861-m-long composite section spanning the last 32 m.y. from Sites 1147 and 1148. Sediments recovered at these two sites yield abundant and well-preserved calcareous nannofossils. The objective of this study was to establish an early Oligocene to Pleistocene calcareous nannofossil biostratigraphic framework for the northern SCS. MATERIAL AND METHODS Materials Sites and Samples Early Oligocene to Pleistocene calcareous nannofossils at Sites 1146, 1147, and 1148 in the northern SCS (Fig. F1; Table T1) were analyzed. Sample spacing is ~100 cm for Site 1146 and 150 cm for Sites 1147 and 1148. A total of 1200 samples were examined. Site 1146 is located at a water depth of 2092 m within a small rift basin on the mid-continental slope of the northern SCS (Fig. F1). Coring at Site 1146 recovered a 600-m-long, lower Miocene through Pleistocene section of relatively carbonate rich, hemipelagic nannofossil clays with a basal age of ~19 Ma. Site 1146 provides one of the most continuous Neogene sections ever recovered by ODP. Sediments recovered at Site 1146 yield abundant nannofossils that are generally well preserved above 531.2 mcd but are moderately overgrown below that level. Many samples exhibited some degree of reworking. For this reason, a number of last occurrence (LO) events at this site were determined based on semiquantitative methods. Site 1148 is located at a water depth of ~3294 m on the lowermost continental slope off southern China near the continent/ocean crust boundary and is the most offshore site drilled during Leg 184 (Fig. F1). Site 1147 is located at a water depth of ~3245 m ~0.45 nmi upslope from Site 1148 (Fig. F1) and was designed to recover the uppermost section that appeared to be missing on seismic profiles from Site 1148 (Wang, Prell, Blum, et al., 2000). Sediments at Site 148 yielded abundant nannofossils with preservation that varied downhole. Reworked nannofossils are commonly seen at this site, particularly in early Miocene to late Oligocene Cores 184-1148A-44X, 48X, and 49X. Pro100°E 110° 120° 130° 20 0 10 00 20 00 30 00 40 00 Site 1147-1148 Site 1146