Sites 1147 and 1148

Sites 1147 (proposed site SCS-5E) and 1148 (proposed site SCS-5C) are located on the lowermost continental slope off southern China, near the continent/ocean crust boundary, and are the most offshore Leg 184 sites on the Chinese margin (Fig. 18). Site 1147 (18°50.11'N, 116°33.28'E, at a water depth of 3246 m) recovered the uppermost section that appeared to be missing on seismic profiles from Site 1148 (18°50.17'N, 116°33.94'E, at a water depth of 3294 m) (Table 1). The greater water depth and distance from terrigenous sources common to these sites combine to produce lower sedimentation rates at this location, at least within the late Neogene. The reason to locate Site 1148 on the lower continental slope was to take advantage of the thinner sediment thickness in order to recover the Oligocene and Miocene hemipelagic sediments that record the evolution and early paleoclimate history of the South China Sea. Elsewhere on the Chinese margin, the Oligocene sediments are too deep or are hydrocarbon bearing and thus cannot be cored to establish onset of monsoonal climates and variability in the SCS. We hoped that the combination of Sites 1148 and 1146 would provide a continuous history of accumulation rates that could be used to evaluate models of the SCS continental margin evolution, sea-level influence on deposition, and the impact of Himalayan-Tibetan uplift on monsoon onset and intensification. Site 1147 is located upslope ~0.45 nmi west of Site 1148 and was proposed during Leg 184 when Site 1148 was moved to a location within a surface slump scar. Hence, Site 1147 was designed to recover the continuous sequence of the uppermost hemipelagic sediments thought to be lost to slumping or channeling at Site 1148.

We cored three APC holes at Site 1147. Holes 1147A, 1147B, and 1147C were cored to depths of 81, 86, and 79 mbsf, respectively, with an average recovery of 99% (Table 1). At Site 1148, we cored two APC/XCB holes. Hole 1148A was continuously cored to 704 mbsf and then wireline logged with the full complement of logging tools: triple combo (111-711 mbsf), FMS-sonic (201-711 mbsf), and GHMT (201-711 mbsf). During logging operations, after we had confirmed that there were no significant hydrocarbon concentrations in the entire interval recovered in Hole 1148A, we requested approval to deepen Hole1148B beyond the originally approved 700 m penetration to extend the paleoenvironmental and tectonic record in time. Hole 1148B was APC cored to 152 mbsf; then, due to time constraints, drilled down from 152 to 450 mbsf, XCB cored to 650 mbsf, drilled down from 650 to 700 mbsf, and cored to 850 mbsf. The drilled-down intervals had excellent recovery in Hole 1148A. Downhole and bottom-water temperature measurements at Site 1148 yielded a thermal gradient of 83°C/km, which is consistent with the location and water depth.

The sediments at Sites 1147 and 1148 reflect a complex sequence of hemipelagic deposition, the beginning of which is coeval with the initiation of seafloor spreading in the South China Sea at ~32 Ma (Figs. 19, 20). The dominant lithologies are grayish green clay with quartz and nannofossils, olive gray and reddish brown clay with nannofossils, light grayish green clayey nannofossil ooze, brown nannofossil clay, and greenish gray nannofossil clay mixed sediment. Lower sections have the same composition but contain slumped and faulted intervals. The mcd scale and splice at Sites 1147 and 1148 are based on the stratigraphic correlation of whole-core multisensor track and split-core color spectral reflectance data (lightness, L*) collected at 5-cm intervals. Magnetic susceptibility data were the most useful stratigraphic tool for correlation at these sites. Natural gamma radiation and CR data were helpful in intervals where structure in the MS profile was ambiguous. At Site 1147, a composite spliced section was constructed from the three holes that spans the entire interval from 0 to 90.73 mcd. At Site 1148, an mcd scale was constructed over the entire cored sequence, 0 to 852 mcd. However, because of core recovery gaps throughout the sequence, the scale is discontinuous ("floating") rather than linked to the sediment-water interface. A floating splice that extends from 46.57 to 155.34 mcd can be combined with data from nearby Site 1147 to construct a continuous mcd and splice, extending from 0 to 155.34 mcd.

Sediments of Pliocene-Pleistocene age at Sites 1147 and 1148 (0 to ~190 mcd) are composed of intensely bioturbated clay with quartz and nannofossils; the upper part is more clay rich and the lower part more nannofossil rich. Green clay layers and irregular green clay mottles decrease downsection, whereas lighter intervals with increased nannofossil content increase slightly downhole as do L*, MS, and bulk density. Siliceous microfossils are abundant in the upper Pleistocene and decrease rapidly to zero in the Pliocene. The core and wireline logging data show that the Pleistocene part of the section (to ~160 mcd) is characterized by a downhole increase in MS, which decreases during the Pliocene interval. A number of properties, including bulk density, porosity, and P-wave velocity, show typical downhole patterns related to compaction and dewatering. Natural gamma radiation and CR display more complex patterns related in part to the carbonate content. Almost all properties show the cyclic fluctuations that are associated with orbital-scale climate changes. The Miocene/Pliocene boundary is marked by an increase in the light carbonate-rich nannofossil clay layers and the disappearance of pyrite concretions. Total organic carbon decreases systematically from a maximum of 0.8% at the top of the hole to <0.2% by 130 mcd and remains at this level throughout the Miocene (to ~485 mcd). Based on C/N values, a purely marine organic source for organic matter is suggested for the upper 130 m of Site 1148. The variation in sulfur abundance follows that of TOC in the top 130 m of the hole, decreasing slowly with depth but exhibiting a normal marine S/C ratio (0.4). Interstitial water profiles at Site 1148 are dominated by sediment water exchanges driven by sulfate reduction in the upper 110 mcd. The upper Pleistocene interval has high H4SiO4 concentration indicative of the higher production of silica-bearing organisms. Sulfate values never reach zero, indicating that sulfate reduction is incomplete and methanogenesis is limited.

Calcareous nannofossils and planktonic foraminifers are abundant and well preserved in Pliocene/Pleistocene sediments. At Site 1148 (1147), the Brunhes/Matuyama boundary can be tentatively placed at 55.2 (58) mcd, the upper Jaramillo Subchron at 69.1 (71 ) mcd, the lower Jaramillo at 73 (76) mcd, the upper Olduvai Event at 111.4 mcd, and the lower Olduvai (tentatively) at 118.5 mcd. The age of the oldest sediments recovered at Site 1147 is estimated at 1.22-1.47 Ma. The combined biostratigraphy placed the Pleistocene/Pliocene boundary between 125.8 and 135.5 mcd and the Pliocene/Miocene boundary between 184.5 and 193.8 mcd.

The Miocene-age sediments at Site 1148 (~190-475 mcd) are a mixture of olive gray and reddish brown clay with nannofossils, light grayish green clayey nannofossil ooze, brown nannofossil clay with intervals and patches of reduced green ooze, and greenish gray nannofossil clay mixed sediment and nannofossil clay. The wireline logs of this interval reveal downhole increases in bulk density, electric resistivity, P-wave velocity, and photoelectric effect (PEF) but decreases in neutron porosity. Similar to the core-log data, MS has several local maxima but decreases downhole. Natural gamma radiation is variable but has no long-term trend until the lowermost Miocene, when it rapidly decreases at the slumped section. None of these units shows any evidence of sediment redeposition; they are representative of continuous hemipelagic sedimentation. These lithologic changes are reflected in the carbonate content and especially the CR a* and L* variations. The lowermost sediments have relatively high concentrations of diagenetically precipitated iron sulfide, seen as black, fine-grained material. Trace fossils are common, most notably Zoophycos and Chondrites, both characteristic deep-water (bathyal) forms. Evidence for redeposition in the lower Miocene sediments is sparse, although a few thin carbonate sand turbidites do occur.

Interstitial water profiles in the Miocene and Oligocene interval are dominated by sediment water exchanges driven by volcanic alteration, clay mineral diagenesis, and calcite recrystallization at depth. Sulfate values never reach zero, indicating that sulfate reduction is incomplete and methanogenesis is not an important process in these sediments. As a result, the high methane values at depth are related to thermogenic production of hydrocarbons.

The Oligocene-age sediments at Site 1148 (475-852 [mcd]) represent a major change in deposition. The uppermost Oligocene sediments are light tan in color, which reflects a distinct increase in carbonate content and the associated a* and L* CR values. This interval is also marked by a sharp increase in P-wave velocity, L* and a*/b* CR parameters, PEF, bulk density, and low porosity. These properties most likely reflect the increased carbonate content in this interval (50%-75%) and are probably responsible for prominent double reflectors seen in seismic reflection profiles. The sonic P-wave velocity of this interval is 2.3 km/s, which is substantially greater than the value of 2.1 and 1.9 km/s at the top and bottom of this interval, respectively. Although similar in composition to overlying sediments (i.e., dominantly clay nannofossil mixed sediments and nannofossil clays), this layer represents gravitational redeposition by mass flows and slumping as evidenced by convolute bedding, soft-sediment plastic deformation, and the occurrence of light-colored carbonate mud clasts within a massive bed of light gray to grayish brown nannofossil clay. These sediments also show clear evidence of brittle faulting in the form of small normal microfaults and thus are likely related to tectonic activity in the formation of the South China Margin. However, the matrix sediments often contain a deep-water trace fossil assemblage of Zoophycos and Chondrites and provide no evidence that water depths differed significantly from the overlying Miocene sediments. Iron sulfides, pyrite concretions, and green clay layers are rarely observed. A sudden increase in TOC is noted below 485 mcd (>0.4%), and the concentration of TOC remains in this range downhole (0.2%-0.5%). The higher C/N values in the lower section may indicate significant terrestrial input. Just below the slumped interval, salinity and chlorinity values become more variable, and ammonium and silica values increase. Changes in ammonium (as well as chloride and salinity) below 470 mcd can also be related to the dehydration reaction of clay minerals. In this interval XRD data show that below 470 mcd, smectite, illite, and kaolinite are absent and mixed-layer clays increase. The slump and the underlying chalk layer may act as a barrier to diffusion of gas and possibly to some elements. The microenvironments within fractures may also lead to variable interstitial water concentrations in this lower interval.

The bulk of the Oligocene sediment is an intensely bioturbated sequence of quartz-rich, grayish olive green nannofossil clay. The whole sequence is extremely monotonous, with little lithologic variation, and is characterized by low values of MS, bulk density (although disturbed by the XCB coring), NGR, and PEF and by decreased L* but increasing a*/b* parameter of CR. These trends generally reflect the decreased carbonate and increased clay content in the rapidly accumulating Oligocene section. The abundant bioturbation traces are strongly compacted and give the sediment a laminated appearance. High H4SiO4 concentration in the mid-Oligocene interval is likely associated with intervals of higher biogenic silica content. Toward the base of the section, evidence of current activity is found in the form of occasional flaser sandstone laminae that are dominated by quartz and lithic fragments as well as mica, glauconite, and foraminifer fragments. As with younger sediments, little evidence suggests that these early Oligocene sediments were deposited in substantially shallower water.

In the Oligocene sediments, total S concentration increases, following TOC. However, the S/C ratio is anomalously high for normal marine sediments (>1), suggesting the addition of S from another source. An increase in methane to the bottom of the hole (711 mcd) is accompanied by the presence of ethane and propane as well as heavier hydrocarbons downhole. Maximum methane and ethane concentrations were detected at 593 mcd (569 and 25, ppmv respectively). From the first detection of ethane at 480 mcd, the C1/C2 ratio declined rapidly from 99 to approach a minimum of 15 at the bottom of Hole 1148A. Between 715 and 851 mcd, methane concentrations remained low (<200 ppm) and decreased with depth downhole. The C1/C2 ratio decreased to as low as 4; this is expected, however, for the small amounts of organic matter in these poor source rocks as they enter the zone of petroleum maturation. As much as 50 ppm of C5 and lesser amounts of other light hydrocarbons were detected.

Thirty-nine nannofossil and 29 planktonic foraminifer biostratigraphic datums were recognized from the lower Oligocene to Pleistocene sediments at Site 1148. A gap in the nannofossil and foraminifer datums indicates that sediments between the lowermost part of Zone NP25 (Zone N4 ) and Zone NN2 (Zone P22 ) are missing and that the base of Hole 1148B is still within Zone NP23 (P19 ) (<32.3 Ma). Site 1148 yields few to abundant deep-sea benthic foraminifers, and the ratio of benthic to planktonic foraminifers is high because of strong carbonate dissolution. The benthic foraminifers (e.g., Heterolepa, Gavelinopsis, Globocassidulina, Martinottiela, Sigmoilopsis, Textularia, and Uvigerina) in the lower part of Hole 1148A (>~510 mcd) are comparable to those observed at 1000-2000 m in the modern South China Sea. However, no clear evidence exists for reworked benthic foraminifers from the shelf and upper slope. An increase in the abundance of Globobulimina and Chilostomella (indicative of high productivity) was observed in the upper (above ~50 mcd) and lower (below ~500 mcd) sections of Hole 1148A. This corresponds to the higher organic carbon content and abundant siliceous fossil content (radiolarians and diatoms) found in the two intervals. Below the middle Miocene, nannofossils are moderately to well preserved, and planktonic foraminifers are poor to moderately preserved.

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, apart from a series of sharp color changes and associated differences in physical properties, very little lithologic variation has occurred since the early Oligocene, beyond a general trend toward decreasing carbonate content. 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 middle Eocene rifting (e.g., Taylor and Hayes, 1980). The most noteworthy sedimentary feature at Site 1148 is the mass-flow sequence, which is also responsible for the prominent reflector at the base of the Miocene section. Ironically, the strong basement reflector at ~800 mbsf is within the gray-green Oligocene clays and does not show any distinct lithologic change (although recovery in this section was poor).

Synthesis-Lithostratigraphic and Sedimentological Overview

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