RESULTS

As the Site 1148 locality lies between the modern lysocline (~3000 m) and the carbonate compensation depth (CCD; ~3500 m) (Wang, Prell, Blum, et al., 2000), all calcareous components in the sediment have been affected at least partially by dissolution. The <150-µm residue contains more fragmented planktonic foraminifer tests than the coarser fraction. When few tests are left in the >150-µm residue, the entire finer fraction often contains 100% finely fragmented pieces or is barren. The most severely affected intervals are found at 272–295, 347–349, and 360 mcd, in the middle and upper Miocene. Therefore, the composition as well as species abundance of planktonic foraminifers could have been altered.

The biofacies is dominated by dissolution-resistant species, especially Sphaeroidinellopsis spp. and Globoquadrina spp. Frequent to abundant taxa include Globigerinoides, Globoturborotalita, Paragloborotalia, Neogloboquadrina, and Catapsydrax. Others are rare, except in a few short intervals.

Five assemblages can be recognized on the basis of dominant species and species having a shorter range. Because many dissolution-resistant species range through several time periods, these assemblages are only loosely defined for characterizing the altered planktonic foraminifer fauna found at Site 1148. Their main features are briefly described below.

1. The Sphaeroidinellopsis seminulina–Globoturborotalita nepenthes assemblage characterizes the upper Miocene Zones N14–N18 between 190 and 275 mcd. Dominated by S. seminulina, this assemblage includes many Neogene globigerinid forms, especially Globigerinoides obliquus, Globigerinoides sacculifer s.l., Globigerinoides ruber, Globigerinoides conglobatus, G. nepenthes, Globoturborotalita druryi, Globoquadrina altispira, Globoquadrina globosa, Neogloboquadrina acostaensis, Globorotalia menardii, and Orbulina.
2. The G. altispira–S. seminulina–Orbulina assemblage between 275 and 358 mcd is from the middle Miocene Zones N8–N12. Numerous Globoquadrina, Sphaeroidinellopsis, and Paragloborotalia mayeri–Paragloborotalia siakensis characterize this assemblage, together with the Praeorbulina–Orbulina bioseries. Several globorotaliid lineages are also confined to this assemblage interval, including the Globorotalia peripheroronda–Globorotalia fohsi and Globorotalia praescitula–Globorotalia praemenardii lineages.
3. The G. globosa–Sphaeroidinellopsis kochi assemblage characterizes the lower Miocene Zones N4–N11 between 358 and 454 mcd. Apart from these two species, several related species are also common, including Globoquadrina dehiscens, Globoquadrina venezuelana, G. altispira, and Sphaeroidinellopsis disjuncta. Species of Globigerinoides, Paragloborotalia, Globorotaloides, Globoturborotalita, and Catapsydrax are few to abundant, depending on their detailed stratigraphic position. Numerous phosphate or algal flakes are present in some samples in which planktonic foraminifers are rare.
4. The G. venezuelana–Paragloborotalia pseudokugleri assemblage characterizes the upper Oligocene Subzone P21b and Zone P22 from 454 to 488 mcd. Also frequently occurring are such species as Catapsydrax dissimilis, Globorotaloides suteri, Globoquadrina sellii, Globigerina ciperoensis, Paragloborotalia nana, and Paragloborotalia opima (P21b).
5. The lower Oligocene Chiloguembelina cubensis assemblage below 488 mcd is characterized by C. cubensis, P. nana–P. opima, Globoquadrina tripartita, G. ciperoensis, Catapsydrax, G. suteri and many tenuitellid forms. High sedimentation rate in this interval diluted the abundance of planktonic foraminifers in most samples.

Figures F4 and F5 illustrate the stratigraphic range of most planktonic foraminifer species in the Oligocene–Miocene section of Site 1148 on the basis of observation results (Table T1; Fig. F3). More than 30 datum levels were selected to define the stratigraphy (Tables T2, T3. The consistent sequential pattern demonstrates that these datum levels are in similar order as from the open ocean and presumably synchronous with the age estimates by Berggren et al. (1995), Chaisson and Pearson (1997), and Pearson and Chaisson (1997). Supporting evidence comes from correlation with nannofossil events identified by Xin Su (pers. comm., 2002) (Table T4). At least three planktonic foraminifer datums, however, appear to represent local bioevents, and they are discussed briefly below.

G. dehiscens is found in and below Sample 184-1148A-27X-4, 90–92 cm (257.23 mcd), although the related species G. altispira and G. venezuelana range throughout the studied interval. The last occurrence (LO) of G. dehiscens has been reported to fall between 5.6 and 6.6 Ma from tropical regions (Spencer-Cervato et al., 1994; Berggren et al., 1995; Chaisson and Pearson, 1997). At Site 1148, it lies ~2.4 m above the first occurrence (FO) of N. acostaensis (259.63 mcd) and 14 m below the FO of Globigerinoides extremus (244.18 mcd). Respectively, the latter two datums have been estimated to be ~10 Ma (Chaisson and Pearson, 1997) and 8.3 Ma (Berggren et al., 1995). Several other planktonic foraminifer and nannofossil datums also occur within the interval between 240 and 280 mcd (Table T3). The most important are the LOs of Discoaster hamatus at 256.37 mcd (9.40 Ma) and Catinaster calyculus at 261.81 mcd (9.64 Ma). Therefore, the LO of G. dehiscens at 257.23 mcd may bear an age not younger than 9.5 Ma. It could be slightly older, up to ~10 Ma, if an older age of 10.3 Ma (Chaisson and Pearson, 1997) for the FO of G. extremus was followed, but the existence of several younger planktonic foraminifer and nannofossil datums below the G. extremus level indicates that the age given by Chaisson and Pearson (1997) may not be valid, at least for Site 1148. The LO of G. dehiscens at ~10 Ma in planktonic foraminifer Zone N16 has also been recorded from other SCS localities including Sites 1143 and 1146 (Wang, 2001) and many offshore industrial wells (Qin, 1996), indicating a regional event.

The LO of G. fohsi at 301.03 mcd marks a sudden disappearance of all related species, and it is apparently truncated by dissolution-related unconformity at ~13 Ma. At Site 1146, this datum appears to be synchronous with its open-ocean record of 11.68 Ma (Wang, 2001).

The FO of Globigerinatella insueta at 367.30 mcd is projected to be at ~18.0 Ma, between the much older LO of Globoquadrina binaiensis (19.1 Ma; 377 mcd) and slightly younger LO of C. dissimilis (17.3 Ma; 365.03 mcd). Pearson and Chaisson (1997) found the FO of G. insueta s.s. at 17.4 Ma in cores from the Ceara Rise. The occurrence of these three datums from 19.1 to 17.3 Ma in an interval of 12 m (365–377 mcd) indicates a condensed section that is probably caused by unconformities.

On the basis of the planktonic foraminifer datum levels given in Table T2, the Oligocene Zones P18–P22 and Miocene Zones N4–N18 can be recognized (Figs. F6, F7). We used the zonation scheme and age estimates by Berggren et al. (1995) for the Oligocene–Miocene section. Age adjustment on the basis of Chaisson and Pearson (1997) has been made for several datums that define the upper middle and upper Miocene zones. As indicated in Figure F2 and Table T3, 9 of the 15 Miocene zonal boundaries are affected by the adjustment, with the Zone N10/N11 boundary registering the biggest difference of 1.3 Ma (12.7–14.0 Ma) (Table T3). Zones N12 and N13 cannot be divided at Site 1148 because the nominated datum, the LO of G. fohsi, is truncated by an unconformity.

Therefore, the Pliocene/Miocene boundary is now placed close to 188 mcd on the basis of the FO of Sphaeroidinella dehiscens (5.54 Ma), very close to the lithostratigraphic Unit I/II boundary. The upper/middle Miocene boundary falls close to 275 mcd where the FO of Globoturborotalita nepenthes (11.19 Ma) also occurs, about 23 m below the Subunit IIA/IIB boundary. The middle/lower Miocene boundary at 355 mcd is defined by the FO of Praeorbulina sicana (16.4 Ma), close to the Unit III/IV contact (360 mcd). The Miocene/Oligocene boundary lying at ~460 mcd within the slump is indicated by the FO of Paragloborotalia kugleri, only ~3 m below the Unit V/VI boundary. The upper/lower Oligocene boundary is placed at 488 mcd by the LO of C. cubensis (28.5 Ma), ~7 m above the lithostratigraphic Unit VI/VII contact. The LO of Pseudohastigerina (32 Ma) at ~697 mcd is the oldest planktonic foraminifer datum found at Site 1148, marking the top of Zone P18. This taxon, however, occurs sporadically in the much expanded lower Oligocene section from the bottom part of the core. Nannofossil results indicate that the oldest sediment recovered at 857 mcd of Site 1148 could bear an age of ~33 Ma because the LOs of Reticulofenestra umbilicus and Reticulofenestra hillae were found at ~730 mcd, both indicating 32.3 Ma (Xin Su, pers. comm., 2002).

An Oligocene–Miocene succession similar to that described here for Site 1148 also occurs in other parts of the SCS. For example, Wang et al. (1981) and Qin (1996) reported Oligocene–Pliocene planktonic foraminifers in several northern SCS basins. Based on sidewall cores and cuttings, however, these studies generate very limited biostratigraphic and paleoceanographic information. A high-resolution biostratigraphy for sediment sequences from the northern continental shelf of the SCS is still lacking.

Miocene and younger sediments were also recovered at Site 1146 during Leg 184. Lying above the lysocline, in a water depth of 2092 m, Site 1146 provides much better preserved material with considerably less dissolution than at Site 1148 (Wang, 2001). Figure F7 attempts a correlation between these two sites based on planktonic foraminifer zones identified. Clearly, the middle and upper Miocene sections at these two sites are relatively complete, as represented by Zones N8–N18. A thinner middle to upper Miocene section at Site 1148, ~160 m, compared to 270 m at Site 1146, is interpreted mainly due to its remote locality and a greater water depth where sedimentation rates were lower and dissolution was stronger (Fig. F7).

Abundant reddish planktonic foraminifers were preserved between 314 and 316 mcd at Site 1148. Together with some volcanic remnants, they probably indicate a period of strong local volcanism at ~14.8 Ma. Further studies are needed to clarify this.