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 Miocene), and reddish brown sediment composes the lower part of the unit (Subunit IIB, middle to upper Miocene).

Unit III (328.82–360.22 mcd) consists of a grayish green clayey nannofossil ooze with 10- to 50-cm-thick intercalations of dark reddish brown clayey nannofossil ooze or clay with nannofossils. A sudden jump in the chromaticity a* value helps separate Unit III from the underlying unit. This unit is lower middle Miocene.

Unit IV (360.22–412.22 mcd) comprises brownish nannofossil clay mixed sediment with a minor amount of greenish gray nannofossil clay intercalations. Again, the transition across the lithostratigraphic Unit III/IV boundary is marked by a significant increase in the a* value. This unit is lower to lower middle Miocene.

Unit V (412.22–457.22 mcd) mainly consists of greenish gray nannofossil clay mixed sediment interbedded with nannofossil clay and very minor amounts of clay with nannofossils. This unit is lower Miocene.

Unit VI (457.22–494.92 mcd) is the slump deposit, dominantly clay nannofossil mixed sediment and nannofossil clay that resulted from episodic gravitational redeposition, including mass flows and slumping. This unit is upper Oligocene to lower Miocene. Note that the present study identifies the typical slumping interval from 457 to 472 mcd.

Unit VII (494.92–859.45 mcd) is an intensely bioturbated sequence of monotonous grayish olive-green nannofossil clay. It represents the first continuous hemipelagic sedimentation in the SCS during the early and late Oligocene (Wang, Prell, Blum, et al., 2000).