Seven lithologic units were identified at Site 1148 on the basis of composition, depositional facies, and especially color variations (Fig. F5). The sediment at Site 1148 is more variable than the other sites drilled on Leg 184. The dominant lithology is clay with variable concentrations of nannofossils. Unit I is composed of grayish green clay with quartz and nannofossils, Unit II contains olive-gray and reddish brown clay with nannofossils, Unit III contains light grayish green clayey nannofossil ooze, Unit IV contains brown nannofossil clay with intervals and patches of reduced, green nannofossil ooze, and Unit V is composed of greenish gray nannofossil clay mixed sediment and nannofossil clay. None of these units shows any evidence of large-scale, episodic sediment redeposition and represents continuous hemipelagic sedimentation. Units VI and VII, although still composed of nannofossil clay and nannofossil clay mixed sediment, are unique because they contain slumped and faulted intervals. A 2.5-m.y. hiatus marks the base of Unit VI. Thus, sediments of Units VI and VII were affected by the tectonic activity associated with the opening of the South China Sea.
Unit I is composed of clay with quartz and nannofossils and is divided into two subunits on the basis of nannofossil content. The upper Subunit IA, above Section 1148A-16H-1 (133.8 mbsf; 146.02 mcd), is more clay-rich than the lower part of the unit. Green clay layers and irregular green clay mottles, characterized by a clay that is stiffer than the normal background sediment, occur frequently and are well developed above ~110 mcd in Unit I (Fig. F6). These features decrease in frequency until they become absent in the lower part of the unit (Subunit IB). Lighter intervals with increased nannofossil content are observed below 85 mcd; color fluctuations above this depth are only detectable with the spectrophotometer. The light intervals in the lower subunit correspond to increases in nannofossil abundance as observed in the smear slides. The downward increase of lighter intervals commences in the lower part of Subunit IA and continues throughout Subunit IB (Table T5). The light layers are generally thinner and less pronounced than those recovered at Sites 1144, 1145, and 1146. Many of the light layers exhibit a characteristic "lightening-upward" signature, consisting of a very gradational lower contact with a continuous lightening over several tens of centimeters and a comparatively sharp but bioturbated upper contact with the overlying dark interval. The abundance of light layers increases further downsection into Unit II. The boundary between Unit I and Unit II is drawn where the light carbonate-rich nannofossil clay layers dominate, as shown clearly by an increase in the a* parameter of the spectrophotometer data (Fig. F5). Units I and II are also distinguished by the frequency of pyrite concretions that are abundant within Unit I and are almost absent from Unit II. The common occurrence of "iron sulfide" in the middle part of Unit I decreases to a minimum at the unit base (Fig. F6). Features observed within the light layers include abundant visible foraminifers on the cut core surface, common green clay layers (typically 1-3 cm thick), well-defined bioturbation, and slightly yellowish gray patches or mottles, which probably represent traces of bioturbation. Smear-slide estimates indicate calcareous nannofossil contents of more than 10%, compared to ~0%-5% in the background clay.
Bioturbation is intense throughout Unit I. The sediment is generally completely homogenized, and individual burrows are only rarely observed. Pyrite-filled burrows are the exception to this pattern. They occur at a frequency of about one to two burrows per core section (1.5 m) within the lower part of Unit I. Large pyrite-filled burrows reach several centimeters in length and up to 2 cm in diameter. Downsection, pyrite-filled burrows first appear at Sample 184-1148A-8H-4, 136 cm (63.66 mbsf; 71.03 mcd), although disseminated pyrite occurs above this interval. Significant numbers of siliceous microfossils are only observed in the upper part of Unit I; their abundance, as measured by smear-slide analysis, gradually decreases downsection and remains <5% below Subunit IA (Fig. F5).
We observed vitric ash infilling burrows, finely dispersed within the sediment, and as isolated pumice clasts in Subunit IA (Table T6). Most ashes are a light color, reflecting the high silica content of the constituent grains. Continuous dark gray (presumably more mafic) ash layers occur at intervals 184-1148A-11H-2, 133-136 cm (97.85-97.88 mcd) and 12H-4, 39-45 cm (110.36-110.42 mcd).
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), and reddish brown sediment comprises the lower part of the unit (Subunit IIB), which is clearly reflected in the a* values (Fig. F5). The dominant lithology in both subunits is clay with nannofossils. The total carbonate content is higher than in Unit I, mainly as a result of a significantly higher frequency of light-colored, carbonate-rich layers (Fig. F7). Quartz grains and siliceous microfossils are practically absent from smear slides taken in Unit II, whereas the content of calcareous nannofossils may reach 50%. This contrasts with maximum nannofossil contents of 20% in Unit I. "Iron sulfide" occurrence is moderate; pyrite concretions and green clay layers are almost absent throughout the Unit II (Fig. F6).
Volcanic ash layers are absent in Unit II with the exception of a strongly bioturbated dark ash layer at interval 184-1148A-33X-3, 114-121 cm (313.67 mcd), and a graded black ash layer with a sharp base at 184-1148A-33X-5, 95-101 cm (316.47 mcd) (Fig. F8). Both ash layers are characterized by unusually low magnetic susceptibility.
Unit III consists of a grayish green clayey nannofossil ooze with intercalations of dark reddish brown clayey nannofossil ooze or clay with nannofossils, typically 10-50 cm thick. These intercalations compose <10% of the total sediment. Color changes are gradational over lengths of a few centimeters. Unit III is defined on the basis of its dominantly greenish color and the disappearance of a distinguishable contrast between lighter carbonate-rich and darker clay-rich layers below Section 184-1148A-34X-CC, 30 cm (327.8 mcd). This change is seen as a significant decrease in the a* value collected by the spectrophotometer (Fig. F5). The top of the unit can be defined in Section 184-1148A-34X-CC, where a distinct color change from brown (Unit II) to greenish gray (Unit III) sediment color is observed. Green layers rarely occur, "iron sulfide" stains decrease, and no pyrite concretions are observed within this unit (Fig. F6).
Unit IV comprises brownish nannofossil clay mixed sediment with a minor amount of greenish gray nannofossil clay intercalations. Again, the transition across the lithologic Unit III/IV boundary is illustrated best by a significant increase in the a* value (Fig. F5). The sediment generally appears reddish and oxidized, although thin reduced greenish intervals and irregular greenish reduction zones, several centimeters in diameter, occur throughout the unit. Well-defined green clay layers and "iron sulfide" stains occur infrequently in the upper part of the unit, and their abundance increases downward. Few pyrite concretions are found in Unit IV (Fig. F6).
Bioturbation is intense throughout Unit IV. The sediment is generally completely homogenized, and individual burrows are observed only in exceptional cases in the upper part of the unit. Below Core 184-1148A-40X (386.42 mcd), single burrows become more evident, with the bathyal assemblage of Zoophycos, Chondrites, and Planolites trace fossils dominating.
A distinct single turbidite layer composed of foraminiferal tests and a small number of quartz grains is observed in the upper part of Unit IV (Section 184-1148A-38X-5, 82-90 cm; 364.3 mcd) (Fig. F9). This deposit shows a graded, parallel-laminated lower portion corresponding to the B division of a classic Bouma sequence (Bouma, 1962). A convolute upper part with mud clasts represents the C division.
Unit V consists of greenish gray nannofossil clay mixed sediment interbedded with nannofossil clay and very minor amounts of clay with nannofossils. The unit is distinguished from the overlying Unit IV on the basis of its greenish rather than reddish brown appearance, illustrated by a decrease in the a* value (Fig. F5). The total carbonate content of the sediment measured by coulometer is 40%-50% (see "Organic Geochemistry"). The variation in lithology is a result of fluctuations in the relative proportions of clay and carbonate, variations that show up in color changes traced by the spectrophotometer, usually over intervals of 50-300 cm. Color variations are usually gradational, because of the strong homogenization of the sediment by bioturbation (Fig. F10), but, occasionally, they are sharp.
The upper part of this unit is characterized by the abundant occurrence of green clay layers. Unit V also shows relatively high concentrations of diagenetically precipitated "iron sulfide," seen as black, fine-grained material (Fig. F6). Diagenesis has caused a common light green staining around pieces of "iron sulfide," associated with local reduction and probably linked to the presence of organic material in the original sediment. In addition, concentrations of "iron sulfide" surrounded by green mottling is frequently noted in the background sediment. Trace fossils are often recognized throughout the length of the unit, most notably Zoophycos and Chondrites, both characteristic deep-water (bathyal) forms, although numerous deformed, typically unlined horizontal circular burrows, up to 2 cm across, are also noted. Chondrites fossils are often found within the fill of larger burrows.
Evidence for redeposition is sparse in Unit V, although at the top of the sequence, two thin carbonate sand turbidites were noted at intervals 184-1148A-43X-3, 73-76 cm (397.23-397.26 mbsf; 409.45-409.48 mcd), and 43X-5, 103-105 cm (400.53-400.55 mbsf; 412.75-412.77 mcd). Sedimentation through the rest of Unit V is typically deep-water hemipelagic.
This unit differs from the overlying Unit V on the basis of facies and color. Unit V is light greenish gray at the top but rapidly changes to tan at the Unit V/VI boundary, making it easy to spot using the a* value from the spectrophotometer data (Fig. F5). The distinction is more pronounced in the sediment facies and structure, which, despite being similar in composition (i.e., dominantly clay nannofossil mixed sediment and nannofossil clay), do not represent continuous hemipelagic sedimentation. Instead, they signify episodic gravitational redeposition, including mass flows and slumping. As evidence of this origin, the interval contains convolute bedding, soft sediment plastic deformation, and light-colored carbonate mud clasts within a massive bed of light gray to grayish brown nannofossil clay such as those at intervals 184-1148A-48X-4, 57-70 cm (441.87-441.90 mbsf; 454.09-454.12 mcd) (Fig. F11), 50X-1, 144-150 cm (462.44-462.50 mbsf; 474.66-474.72 mcd), and 50X-2, 140-150 cm (463.90-464.00 mbsf; 476.12-476.22 mcd). The matrix sediments often contain a deep-water trace fossil assemblage of Zoophycos and Chondrites. The sediment composition and texture suggest that the water depths did not differ significantly from those of Units I-V. Both "iron sulfide" stains and pyrite concretions are rarely observed (Fig. F6). The green clay layers that were so common in Unit V are almost absent throughout this unit. Unit VI is the first part of the cored stratigraphy, going downsection, to show clear evidence of brittle faulting in the form of small normal microfaults that cannot be attributed to drilling disturbance (Fig. F12). Despite its modest thickness, the unit is quite different in character from all sediment deposited since that time, which suggests that this period was one of tectonic activity in the formation of the South China Margin.
Unit VII is composed of an intensely bioturbated sequence of grayish olive-green nannofossil clay. The whole sequence is extremely monotonous and comprises only very minor lithologic variation from Cores 184-1148A-53X and 184-1148B-21X to the bottom of the hole (492.22-859.76 mcd). The abundant bioturbation traces are strongly compacted and give the sediment a laminated appearance. Visual description reveals two intervals (one at ~492.2-552.2 mcd and another at 652.2-682.2 mcd) in which relatively abundant occurrences of pyrite concretions are observed. Between these two intervals, stains of "iron sulfides" are relatively common (Fig. F6). Ichnofossils include Chondrites, Zoophycos, and Planolites. Most of the burrows are filled with dark brownish sediment, and Chondrites often is observed within fillings of larger burrows. The occasional occurrence of pyrite concretions indicates reducing conditions in the subsurface, although the abundance of trace fossils indicates that the bottom water itself could not have been anoxic. Authigenic dolomite rhombohedral crystals are observed in smear slides, usually from the dark brown sediment found in burrow fills or rare layers (Fig. F5). Dolomite occurrence is higher in the lower part of the unit.
Toward the base of the cored section, the general picture of hemipelagic sedimentation is disrupted by the identification of occasional flaser sandstone laminae, the first occurrence being in Core 184-1148B-48X (784.4 mbsf; 790.65 mcd) (Fig. F13). These compose a tiny fraction (<1%) of the total sediment over these intervals. Compositionally, the sandstones are dominated by quartz and lithic fragments, with mica, glauconite, and foraminifer fragments in smaller quantities. The nannofossil claystone closest to the sandstone often contains parallel laminations suggesting current-related deposition. In spite of this, little sedimentological evidence suggests that Unit VII was deposited in substantially shallower water than the younger units. In addition, slumping and other forms of soft sediment deformation affect normal hemipelagic sediment below interval 184-1148B-51X-2, 81 cm (811.0 mcd).
As in Unit VI, we found frequent evidence of fracturing and faulting. Quartz and calcite-filled fractures are noted but are not abundant. Faulted zones can be discrete but often range up to 30 cm in length, forming an anastomosing network of small faults. Faulting is often found in association with slumped sediment sequences.
The major mineral phases determined by bulk X-ray diffraction (XRD) are illustrated in Figure F14. Most notable is the variation in quartz concentration, which is relatively high in the deepest part of the recovered section and decreases to the lowest concentrations ~100 m below the slumped sediments of Unit VI. Above Unit VI, the quartz concentration monotonically increases to the base of Unit I, where it remains high until the top of the sediment column. Plagioclase is present in relatively high concentrations in Unit I. Calcite concentration determined by XRD analysis shows the same pattern as the lightness and coulometer-determined carbonate contents (i.e., low values in Unit I and higher concentrations in the older sediments).
The clay minerals demonstrate fairly unusual behavior, in that ~20 m below the base of Unit VI they are no longer detected by bulk XRD analysis. The clays are replaced by a single unidentified mixed layer phase that exists in the upper 120 m of Unit VII. It is unlikely that all of these clay phases are absent from the sediments, but the concentration has dropped below the detection limit for bulk XRD analysis. Dolomite was only detected in trace amounts in a few samples.
The stratigraphy at Site 1148 spans most of the postrift history of the South China Sea, including the entire duration of active seafloor spreading (Briais et al., 1993). Despite this, beyond a series of sharp color changes, very little lithologic variation occurs since the early Oligocene, beyond a general trend toward decreasing carbonate content since Unit VI times. Although the lithology does not change, the mineral variation in the post-Unit VI sediments demonstrates a tendency toward a more strongly physically weathered mineral assemblage, based on the increasing quartz, plagioclase, and chlorite concentrations. This could represent changing sea level, accelerated uplift in the source regions, and/or aridification of the source areas. The upsection trend of decreasing quartz in Unit VII may reflect the progressive degradation of a rift-generated topography. Importantly, we found no apparent deepening or shallowing of the water depth of sedimentation, remaining hemipelagic and probably bathyal throughout. This is surprising given the anticipated thermal subsidence following mid-Eocene rifting (e.g., Taylor and Hayes, 1980) and may lend support to models that propose a deep-water basin in the area during the early Paleogene and even the Late Cretaceous (e.g, Wissmann et al., 1996).
The most noteworthy sedimentary unit is the mass-flow sequences of Unit VI, which also marks the point in the core above which no significant normal faulting is noted. Given the seismic constraints and the observations laid out here, the top of Unit VI may be interpreted as marking the end of active extension deformation, assuming a simple mid-Eocene rifting of the type proposed by most workers. Certainly, the association of slumping and faulting at the base of Unit VII suggests sedimentation in a tectonically active environment. In effect, this implies that the Unit VI/Unit V boundary is equivalent to the breakup unconformity of Falvey (1974). This layer also marks an important change in the mineralogy. The mixed-layer clay phase found beneath the slumps of Unit VI may be produced by diagenesis induced by slumping.