Lithostratigraphic and Sedimentological Overview

The South China Sea sediments recovered on Leg 184 represent the simple mixing of nannofossil ooze and detrital clays derived from Asia, via the Pearl and Red Rivers on the northern margin and the Mekong and Molengraaff Rivers, and directly from Borneo on the southern margin. These sediments appear to represent deep-water deposition throughout the Neogene and back into the lower Oligocene. Although the dominant hemipelagic sedimentation is expected for a mature passive margin, the lack of coarse clastic material in the syn-spreading sequences cored at the base of Site 1148 (Fig. 21, Units VI and VII) is surprising given the expectation of a rugged and readily erodable syn-rift topography. Also, lower Oligocene coal-bearing swamp and littoral plain sediments are known to occur in wells of this age farther north within the Pearl River Mouth Basin (e.g., Su et al., 1989). The lack of similar coarse clastic material suggests that the Pearl River Mouth Basin and shallower slope basins were acting as efficient sediment traps. In addition, the presence of a deep-water, bathyal facies during the initial seafloor spreading period does not readily fit with simple rift models of the South China Margin. These facies may indicate that the margin extension proceeded very rapidly during the initial rifting phase in the middle Eocene (e.g., Taylor and Hayes, 1980; P. Clift and J. Lin, 1999). We note that the slumping, brittle faulting, and mineralization observed in Units VI and VII at Site 1148 (Fig. 21) indicate that the sediments do not completely postdate active tectonism.

Volcanic Ash

Although the volume of volcanic ash found in Leg 184 sediments is not large, it does provide some constraints on the activity of the volcanic arcs of the region. All recovered ashes are thin, generally <5 cm, and are light colored in the Pleistocene, reflecting a dominant dacitic-rhyolitic composition of the arc's explosive fraction. Most of the ashes were deposited since 1 Ma on the northern margin and since 2 Ma in the south. This trend is similar to the global pattern noted by Kennett and Thunell (1975), although it has been disputed by several authors (e.g., Ninkovich and Donn, 1976). The uphole increase in volcanic ash may reflect either more volcanic eruptions during the Pleistocene or the diagenetic alteration of chemically unstable volcanic glass during burial. This latter explanation may account for much of the pattern on the northern margin, since older ashes in this area tend to be devoid of glass and are simply composed of angular quartz, mica, and other accessory mineral grains. In contrast, at Site 1143, fresh glass is found in Miocene-aged ash beds. Certainly the Philippine Arc is not a recent feature; unless wind directions have radically changed, ashes from this arc were probably deposited over the entire cored interval.

Green Layers and Iron Sulfides

Green clay layers are a common, yet volumetrically small, part of the sequence at most of the Leg 184 drill sites. They occur as discrete layers as thick as 3 cm and even more commonly as disrupted layers, patches, or mottles. XRD analyses show that they are not glauconite, and no clear relationship is observed between green clay layers and depth of burial. Most of the layers are confined to the Pliocene-Pleistocene (Fig. 21) except for a lower Miocene set recovered at Site 1148. Their common association with burrows and patches caused by burrowing suggests that they may be linked to the presence of organic matter. Certainly their green color is suggestive of reducing conditions, which are linked to organic matter alteration. They do not seem to be equivalent to green layers found by Lind et al. (1993) on the Ontong Java Plateau and by Gardner et al. (1986) from the Lord Howe Rise, which were interpreted as altered volcanic ash. In the case of the South China Sea, the green clay layers are interbedded with clear tephra-bearing unaltered volcanic glass. No appreciable change in the background sediment is noted over these intervals; thus, the diagenetic environment seems uniform between beds. Other diagenetic minerals noted in the Leg 184 sediments are iron sulfide minerals (well-crystallized golden-colored pyrite often present as nodules, concretions, and replacement burrows) and fine-grained, black sulfide dispersed in the sediment. The latter style is described as FeS in the cores but as this is chemically unstable, this material must also be pyrite in mineralogy. The lack of a clear regional pattern either in depth or age in the distribution of pyrite or FeS suggests that the minerals' development reflects only local variations in sediment composition and burial.

Synthesis-Environmental History of the South China Sea

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