Oligocene–Miocene Planktonic Foraminifer Biostratigraphy, Site 1148, Northern South China Sea

Abstract

Over 30 first and last occurrence (FO and LO, respectively) planktonic foraminifer datums were recognized from the Oligocene–Miocene section of Ocean Drilling Program (ODP) Site 1148. Most datum levels occur in similar order as, and are by correlation as probably synchronous with, their open-ocean records. Several datum levels represent local bioevents resulting from dissolution and Site 1148’s unique paleoceanographic setting in the northern South China Sea. An age of 9.5– 9.8 Ma is estimated for the local LO of Globoquadrina dehiscens (257 meters composite depth [mcd]), whereas the local LO of Globorotalia fohsi s.l. (301 mcd) is projected to be at ~13.0 Ma and the local FO of Globigerinatella insueta (367 mcd) is projected to be at ~18.0 Ma. The combined planktonic foraminifer and nannofossil results indicate that the Oligocene–Miocene section at Site 1148 is not complete. Unconformities up to 2–3 m.y. in duration, occurring at and before the Oligocene/Miocene boundary (OHS1, OHS2, OHS3, and OHS4 = MHS1), are associated with slump deposits between 457 and 495 mcd that signal tectonic instability during the transition from rifting to spreading in the South China Sea. Shorter unconformities of . [Cited YYYYMM-DD] 2Key Laboratory of Marine Geology DoE, Tongji University, Shanghai 200092, People’s Republic of China. qli01@mail.tongji.edu.cn 3School of Earth and Environmental Sciences, The University of Adelaide, Adelaide SA 5005, Australia. Correspondence author: qianyu.li@adelaide.edu.au 4Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 210008, People’s Republic of China. Initial receipt: 19 December 2002 Acceptance: 9 August 2004 Web publication: 15 December 2004 Ms 184SR-220 Q. LI ET AL. OLIGOCENE–MIOCENE PLANKTONIC FORAMINIFER BIOSTRATIGRAPHY 2 366 mcd (MHS4 = Mi-1b), 358 mcd (MHS5 = MLi-1), 333 mcd (MHS6 = Mi-2), 318 mcd (MHS7 = MSi-1), 308 mcd (MHS8 = Mi-3), 295 mcd (MHS9 = Mi-4), 288 mcd (MHS10 = Mi-5), 256 mcd (MHS11 = Mi-6), and 250 mcd (MHS12 = Mi-7). The correlation of these unconformities with Mi events indicates that some related driving mechanisms have been operating, causing deepwater circulation changes concomitantly in world oceans and in the marginal South China Sea. INTRODUCTION Site 1148, the deepest site drilled during Ocean Drilling Program (ODP) Leg 184, is located at 18°50.169′N, 116°33.939′E. At a water depth of ~3294 m, it lies on the lower continental slope of China, close to the continent/ocean crust boundary (Fig. F1). One of the objectives of Site 1148 was to recover a continuous Oligocene–Miocene sequence of hemipelagic sediments for studying the early paleoclimate history of the South China Sea (SCS) as related to Himalayan–Tibetan uplift and sea level change (Wang, Prell, Blum, et al., 2000). Drilling at Site 1148 recovered a 857-m-long Cenozoic sediment sequence comprising two main sections: a much expanded (mainly lower) Oligocene and the relatively slower accumulated Miocene to Pleistocene. The lower section represents synrift sediments filled during the later Paleogene rifting in half grabens, whereas the upper section corresponds to wider sedimentation during the broad subsidence in the Neogene SCS (Ru et al., 1994; Wang, Prell, Blum, et al., 2000). These two sections are separated by a slumped interval between 457 and 495 meters composite depth (mcd). Imaged as a double reflector in seismic profiles (Wang, Prell, Blum, et al., 2000), the slump may be related to a change in the pattern and direction of seafloor spreading, or “ridge jump” (Briais et al., 1993), which marks the onset of a new tectonic and sedimentological regime beginning in the early Miocene in the SCS region (Wang, Prell, Blum, et al., 2000). On the basis of initial shipboard data and postcruise results, this study aimed to recover more biostratigraphically useful datums of planktonic foraminifers and to provide a more accurate stratigraphic framework for better dating of Oligocene and Miocene climatic and tectonic events in the region. SEDIMENT CHARACTERISTICS The dominant lithology at Site 1148 is clay with variable concentrations of nannofossils. Seven lithostratigraphic units were identified on the basis of composition, depositional facies, and, especially, color variations (Wang, Prell, Blum, et al., 2000). The Pliocene–Holocene is represented by Unit I, the Miocene by Units II–V, and the Oligocene by Unit VI. The main sedimentary characteristics of the Oligocene–Miocene lithostratigraphic units (Units II–VII) are summarized as follows (Wang, Prell, Blum, et al., 2000): Unit II (194.02–328.82 mcd) is dominated by clay with nannofossils. Light-colored, carbonate-rich layers are frequent, but quartz grains and siliceous microfossils are practically absent. Unit II is defined and subdivided on the basis of color. Olive-gray sediment is found in the upper part of the unit (Subunit IIA, upper F1. Site 1148 location, p. 13. 100°E Asia So ut h Ch in a Se a 0° 10° 20° N 110° 120°