Oligocene: the Seafloor-Spreading Phase (Site 1148)
The lower and mid-Oligocene (Site 1148; 32-27 Ma, 850-480 mcd) are composed of laminated claystone with brownish gray and greenish gray stringers (Figs. 20, 21) and have high LSRs comparable to those in the Pleistocene (Figs. 22, 23). The average carbonate MAR from 480 to700 mcd (23.7-31 Ma) averages 3.45 g/cm2/k.y. (Fig. 23), which is by far the highest observed over the past 32 m.y. at Site 1148. At the same time, the relatively high noncarbonate MAR implies a significant supply of terrigenous fine-grained clasts (Fig. 23). Because the ratio of carbonate to noncarbonate MAR is relatively constant, the high carbonate accumulation rate may reflect sediment focusing of upslope sediments as well as a higher surface productivity. The relatively high organic carbon content, the presence of siliceous microfossils, and the composition of benthic foraminiferal fauna also suggest higher productivity.
On the basis of magnetic anomaly (C11-C5c) patterns, the seafloor-spreading phase of the SCS took place between 32 and 16 Ma (Briais et al., 1993; Taylor and Hayes, 1983). Before 27 Ma, the newly opened deep-sea basin was oriented east to west and connected with the Pacific in the East. At that time, the South China Sea was a narrow basin between the Asian continent and a number of terranes that were eventually rifted to the southern part of the SCS (North Palawan and Reed Bank, among others). The high sedimentation and accumulation rates recorded at Site 1148 may be related to the incipient spreading phase, which might focus sediment input into the newly opened basin. Judging from the absence of marine sediments of this age on the northern shelf and upper slope, the ocean waters were restricted in the area. The proximity of continental runoff and sediment transport may be one of factors responsible for the higher rates of deposition in this interval. The poor core recovery in the upper part of this sequence (470-555 mbsf) suggests some lithologic changes in the latter part of the time interval. The scarcity of core samples, however, precludes any conclusion about the nature of the possible events.
A distinct and regionally consistent seismic reflector was observed to occur in the lower section of Site 1148. The JOIDES Resolution seismic reflection data indicated that the reflector was ~0.86 s below seafloor; the P-wave velocity data from downhole logging in Hole 1148A suggested that the reflector was ~800 mbsf. Although the drillers reported a distinct decrease in penetration rate at 800 mbsf, we did not recover any material that was indicative of a strong reflector. All cores in this interval were greenish gray claystone. The lack of coarse sediments, a weathering zone, or other features indicative of erosion or shallower water signify that this site was relatively deep throughout the deposition of the Oligocene sequence. The true nature of the acoustical basement reflector is unclear.
Latest Oligocene: a Turning Point (Site 1148)
The latest stage of the late Oligocene witnessed drastic changes in deposition at Site 1148. Carbonate content abruptly increased (~50%-70%) as chalk replaced clay over the depth interval of 457-478 mcd (Fig. 24). Almost all physical parameters reflect this rapid change, including sharp increases in bulk density, P-wave velocity, photoelectric effect, and CR parameters, and decreases in porosity, NGR, and MS values (Figs. 20, 25). However, the microfossil assemblages show the same fossil zone (P21b and NP24) as the underlying deposits. The chalk section is overlain by slump deposits of chalk and clay with a mixed nannofossil assemblage, and the absence of nannofossil zone NP25 indicates a deposition hiatus of ~1-3 m.y. at the end of Oligocene (Figs. 20, 22, 23).
The Oligocene/Miocene boundary represents one of the most significant changes in the tectonic and environmental history of the South China Sea during the Cenozoic (Wang, 1990). During this interval, the sedimentary basins of the northern SCS shelf are thought to have experienced a transition from the rifting stage to one of broad subsidence (Ru et al., 1994). According to the age model for Site 1148 (Fig. 22), the Oligocene/Miocene boundary events started ~27 Ma, at the same time that the spreading ridge of the SCS basin is thought to have jumped southward (Briais et al., 1993). Because of its position close to the boundary between continental and ocean crust, Site 1148 should have been sensitive to any tectonic episode associated with changes in the spreading of the SCS basin. Postcruise research will show whether the drastic changes in deposition regime at the site is related to the spreading event.
Early Miocene: Carbonate Deposition and Transgression (Sites 1146 and 1148)
The early Miocene (16.5-23.7 Ma) at Sites 1146 and 1148 is represented by a chalky clay with an average carbonate content of ~ 35% (Fig. 24). The total MARs for this interval average 1.13 g/cm2/k.y. at both Sites 1146 and 1148, or about three times lower than in the Oligocene (Fig. 23). These lower rates are more representative of pelagic sedimentation and may also imply that the previously high rates were affected by sediment focusing on the margin. During the second stage of seafloor spreading (27-16 Ma), the South China Sea basin became much broader than in the Oligocene (Briais et al., 1993; Lee and Lawver, 1994). This early Miocene interval was distinguished by an expansion of reef facies in the shallow waters of the western Pacific including the Pearl River Mouth Basin. The relatively low carbonate accumulation rate but high carbonate content may be attributed to the more pelagic environment of the larger SCS basin, the lack of sediment focusing, and the wide distribution of reef facies on the shelves.
On the northern shelf, the early Miocene was a time of marine transgression. The marine intercalations in nearshore facies are first observed in the upper Oligocene (NP25) on the upper slope and shelf break in industrial wells (BY7-1-1, at a water depth of 499 m; and PY33-1-1, at a water depth of 188 m; Huang, 1997), and in the lower Miocene on the shelf (Wang, 1990). Further studies are needed to determine whether the local rise in sea level resulted from global eustatic changes or from tectonic subsidence of the basins and how this shelf transgression phase affected sedimentation at the deeper water Sites 1146 and 1148.
Middle Miocene: after the Spreading (Sites 1146 and 1148)
The seafloor-spreading phase of the SCS basin stopped at magnetic anomaly C5c or ~16 Ma, which is close to the boundary between early and middle Miocene. The middle Miocene section (~16-11 Ma) from the northern continental margin (Sites 1146 and 1148) has relatively high carbonate content (>30%), only slightly lower than the early Miocene but much higher than the modern values (Fig. 23). Carbonate increases at the shallower Site 1146 but remains high at Site 1148 despite its greater water depth, which is below the modern lysocline (~3000 m). Total accumulation rates of the early Miocene were 1.91 g/cm2/k.y. at Site 1148, slightly lower than in the early Miocene and much lower than at Site 1146 (~4.34 g/cm2/k.y.) (Fig. 24). Additional postcruise biostratigraphic control will be needed to establish whether the slower accumulation rates are related to a change in tectonics or to depositional hiatuses observed on the northern shelf.
Late Miocene: High Carbonate Supply (Sites 1143, 1146, and 1148)
Upper Miocene sediments were recovered in the northern SCS at Sites 1146 and 1148 and in the southern SCS at Site 1143. About half of the mass of sediments from both northern and southern sites above the modern lysocline (Sites 1146 and 1143) is composed of carbonate (Figs. 23A, 24). At Site 1143, the carbonate and noncarbonate accumulation remains high (Fig. 23A); Site 1146 shows a drastic increase in carbonate percentage (from ~40% in the middle Miocene to ~55% in the upper Miocene), but the increase in carbonate accumulation rate was not significant (from 1.46 g/cm2/k.y. in the middle Miocene to 1.55 g/cm2/k.y. in the upper Miocene) (Figs. 23A, 24). At Site 1148, the average carbonate percentage still exceeds 30% (Fig. 24), but the poor preservation of planktonic foraminifers shows enhanced carbonate dissolution. During this late Miocene interval, differences begin to develop between the shallower Site 1146 and the deeper Site 1148. In the shallow site, NGR values are lower and decreasing whereas PEF values are increasing; in the deeper site, the NGR values are higher and much more variable, as are the PEF values. These changes suggest more clay-carbonate contrasts in the deeper Site 1148 and are generally consistent with the higher carbonate in Site 1146.
Despite the similar carbonate concentrations in upper Miocene sediments, the accumulation rate at Site 1143 is about two times higher (~3 g/cm2/k.y.) then at Site 1146 (~1.5 g/cm2/k.y.) (Figs. 23, 24). The high carbonate accumulation at tropical Site 1143 might be related to the late Miocene to early Pliocene "biogenic bloom" in the equatorial Pacific but also seems partly related to redeposition of adjacent sediments as evidenced by the frequent turbidites and slumped sediments in the lower section. The preservation of siliceous microfossils in the lower section may indicate higher productivity at that time. The high carbonate percentages in the Miocene deposits from the northern sites imply a low supply of terrigenous clasts from the land that may be related to rising sea levels during this interval (Prell and Kutzbach, 1997). In general, the high carbonate sediments throughout the Miocene and the similarity between the northern and southern sites suggest a much more uniform environment than during the Pliocene-Pleistocene.
Pliocene and Pleistocene: Increase of Terrigenous Input (Sites 1143, 1144, 1145, 1146, 1147, and 1148)
Leg 184 recovered Pliocene deposits at four sites (1143, 1145, 1146, and 1148) and Pleistocene sediments at all six sites, although with substantially different accumulation rates (Fig. 23B). Despite the different LSRs, all sites exhibit a significant increase in silica at ~1 Ma, which is ascribed to increased productivity. Differences between the southern and northern SCS also begin to emerge during this interval.
At the southern Site 1143, both carbonate and noncarbonate accumulation rates decrease from the upper Miocene to Pliocene (Fig. 23B): carbonate from 2 to 4 g/cm2/k.y. to ~1 g/cm2/k.y., and noncarbonate from 3 to 4 g/cm2/k.y. to ~2 g/cm2/k.y. The decreasing trend continues to the Pleistocene for carbonate, whereas the noncarbonate rate rises up again after ~3 Ma, indicating some increased supply of terrigenous material. However, unlike the northern sites, the MAR at Site 1143 decreases toward the present despite the increase in LSR, a result of decreased bulk density within that most recent interval.
On the northern continental margin, Pliocene accumulation rates remain at the late Miocene level at Sites 1146 and 1148 but with slightly lower rates in the deeper Site 1148. Both sites exhibit the rapid increase in LSR and noncarbonate deposition that starts at ~3 Ma. Site 1145 records only the past 3 m.y., but the noncarbonate accumulation increases after ~ 2.5 Ma. This apparently regional increase in noncarbonate accumulation may be evidence for an intensification of erosion that is related to climatic/sea-level and/or tectonic events. For example, Chinese geologists report evidence for significant uplift of the Tibetan Plateau at ~3 Ma (e.g., Li et al., 1996), and the widespread accumulation of loess in central China started ~2.4 Ma. However, sea-level changes associated with increased global glaciation may have also contributed to transporting sediments to these continental margin sites.
In contrast to the southern site, all the northern sites show an increase in MAR in the late Pleistocene, especially the last 0.25 m.y. (Fig. 23B). The higher MAR is mainly the result of increased supply of terrigenous material, as the carbonate contribution is insignificant for that time interval. The trend is most prominent for Sites 1144, 1145, and 1146. The cause of this most recent increase is not clear at this stage. The relatively high content of biogenic silica and organic carbon indicate an enhanced productivity for the latter part of the Pleistocene.
The general decrease in carbonate concentration upsection, particularly since the mid- Pliocene (Figs. 21, 23, 24), could also reflect decreasing carbonate productivity as well as increasing clastic input, both of which might be expected from an intensification of glaciation at that time. Glaciation is known to have produced arid conditions in Asia and reduced runoff into the South China Sea, although windblown material such as loess may be more important at these times. However, glaciation in the Himalayas (and possibly Tibet) would be expected to produce more clastic debris that would then be available for redeposition when rainfall increased again at the start of deglaciation. The increasing strength of glacial cycles, especially since ~2.5 Ma, correlates well with the general increase in the detrital component since that time. The variation from darker to lighter sediment intervals noted at a number of the northern margin sites--more precisely represented as cyclic variations in core-logging data such as CR, NGR, and MS--most likely reflects such glacial-interglacial variation in clay and carbonate accumulation.
In summary, the sequence of hemipelagic sediments over 30 m.y. is rich in calcareous microfossils and yields almost continuous records of the environmental history of the South China Sea. The depositional history of the northern slope had three important stages: the Oligocene, with extremely high sedimentation rates; the Miocene and early Pliocene, with a low sedimentation rate and high carbonate content; and the last 3 m.y., with high clastic sediment accumulation rates. A different trend of depositional history is indicated at the southern Site 1143: the carbonate accumulation rate decreases from the late Miocene toward the late Pleistocene, and the noncarbonate rate rises again after 3 Ma. However, the upper Miocene sediments were similar in composition between the northern and southern sites, containing more than 50% of carbonate.
Synthesis-Geothermal Gradients: Tectonics and Hydrocarbon
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