Site 1148 is the deepest site for Ocean Drilling Program (ODP) Leg 184, at a water depth of 3294 m. It is located on the lowermost continental slope off southern China at 18°50.17´N, 116°33.94´E (Fig. F1). Lying near the continental/oceanic crust boundary, Site 1148 recovered a rather thin, but continuous sequence of hemipelagic sediment, from which the early evolution and paleoclimate history (Oligocene–Miocene) of the South China Sea may be reconstructed (Wang, Prell, Blum, et al., 2000). To achieve this objective, it is necessary to precisely calibrate the seismic stratigraphic section using paleontological and other chronostratigraphic methods. The stratigraphic value of dinoflagellate fossils has been proven in Deep Sea Drilling Project (DSDP)/ODP history for 30 yr (Habib, 1972; Poulsen et al., 1996). Dinoflagellate stratigraphy represents one of the most important paleontological tools because of their planktonic habit and the good preservation potential of organic cycsts. Dinoflagellates are used in this paper, along with data from nannofossils and other microfossils, to achieve an integrated biostratigraphy for Site 1148.
Dinoflagellate stratigraphy of the Oligocene and Miocene, mainly from the North Atlantic and Western Europe, has made great progress over the last 30 yr (Williams, 1975; Biffi and Manum, 1988; Köthe, 1990; Brinkhuis et al., 1992; de Verteuil and Norris, 1996). The accumulated data proved useful for stratigraphic correlation in the current study. In the last decade, Williams et al. (1993, unpubl. data [N1, N2]) compiled worldwide chronological ranges of Mesozoic–Cenozoic (Upper Cretaceous–Neogene) key species, which has helped considerably in studies of dinoflagellate stratigraphy. However, little published information on Oligocene and Miocene dinoflagellates exists from low-latitude areas of the Pacific region, such as the South China Sea (Mao and Lei, 1996). Fortunately, Site 1148 yielded a continuously cored section of Oligocene–Pleistocene strata (Wang, Prell, Blum, et al., 2000), which provided us with a rare opportunity to study Oligocene–Miocene dinoflagellates from the South China Sea.
Two holes were drilled at Site 1148, ~20 m apart from each other, in an attempt to recover coring gaps in Hole 1148A in the cores drilled in Hole 1148B (Wang, Prell, Blum, et al., 2000). Hole 1148A was drilled to a total depth of 704 meters below seafloor (mbsf), and Hole 1148B was drilled to 853 mbsf. The nominal depths of cores reported in mbsf have uncertainties of up to a few meters as a result of operational and environmental conditions, and the mbsf depth of stratigraphic horizons may therefore vary by a few meters across the two holes. These depth uncertainties are reflected in the different depths of lithologic unit boundaries in Holes 1148A and 1148B given below. A meters composite depth (mcd) scale was constructed for Site 1148 during Leg 184 that correlates stratigraphic features between cores from the two holes. Tables T1 and T2 list the mcd depths along with the mbsf depths for all samples analyzed, but the text and Figure F2 refer to hole-specific mbsf depths only.
The dominant lithology at Site 1148 is clay containing variable amounts of nannofossils. Seven lithologic units were identified on the basis of lithologic composition, depositional facies, and, especially, color variations. A detailed description of the upper three lithologic units, ranging from Pleistocene through middle Miocene (Cores 184-1148A-1H through 38X) can be found in Wang, Prell, Blum, et al. (2000). Samples for the present study are from the lower four Units IV, V, VI, and VII (Sections 184-1148A-40X-1 through 76X-6 and 184-1148B-39X-CC through 56X-1) (Fig. F2).
Unit IV (348–400 mbsf; Cores 184-1148A-38X through 43X) is composed of brownish nannofossil clay with minor intercalations of greenish gray nannofossil clay. Generally, the sediments appear reddish and oxidized. Bioturbation is intense throughout this unit.
Unit V (400–445 mbsf; Cores 184-1148A-43X through 48X and 439.9–449.5 mbsf; Core 184-1148B-17X) comprises greenish gray nannofossil clay interbedded with minor amounts of clay with nannofossils. This unit is distinguished from the overlying Unit IV by its greenish rather than reddish brown appearance. Trace fossils are often recognized throughout the unit, mostly those characteristic of deepwater (bathyal) forms. Evidence for redeposition is sparse in the unit.
Unit VI (445–482.7 mbsf; Cores 184-1148A-48X through 53X and 449.5–487.9 mbsf; Cores 184-1148B-17X through 21X) differs from the overlying Unit V in facies and color. The light greenish gray color at the top of Unit V rapidly changes to tan at the boundary between Units V and VI; greenish clay layers, common in Unit V, are almost absent throughout this unit. Though similar in composition to Unit V, this unit shows evidence of episodic gravitational redeposition, including mass flows and slumping, and therefore does not represent continuous hemipelagic sedimentation. In addition, Unit VI shows the first clear evidence in the cored stratigraphy, going downsection, of brittle faulting. All these features are indicative of tectonic activity associated with the formation of the South China Sea.
Unit VII (482.7–704.4 mbsf; Cores 184-1148A-54X through 77X and 487.9–853.2 mbsf; Cores 184-1148B-22X through 56X) is composed of an intensely bioturbated sequence of grayish olive-green nannofossil clay. The whole section is monotonous with only minor lithologic variation. The abundant bioturbation traces are strongly compacted and give the sediment a laminated appearance. Toward the base of the unit, the general picture of hemipelagic sedimentation is disrupted by occasional flaser sandstone laminae. The sandstones are predominantly composed of quartz and lithic fragments, with small quantities of mica, glauconite, and foraminifer fragments (Wang, Prell, Blum, et al., 2000).