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doi:10.2204/iodp.proc.319.103.2010

Biostratigraphy

Calcareous nannofossils

The biostratigraphic framework of the sequence in Hole C0009A was established exclusively based on calcareous nannofossils in cuttings and cores from 715.7 to 1603.7 m MSF. Cuttings were collected at 5 m intervals for the entire riser drilled sequence (715.7–1603.7 m MSF), whereas only the lowermost section was cored from 1509.7 to 1593.9 m CSF. The uppermost part of the hole had been cemented prior to drilling the sampled sequence, stratigraphically isolating the upper 715.7 m interval and therefore preventing downhole contamination from within this interval. A total of 75 samples were examined at 5–30 m spacing, with closer spacing at important lithologic boundaries (see "Lithology") or where the presence of nannofossil datums and/or zonal boundaries was anticipated. Calcareous nannofossils are generally moderately preserved throughout the sequence, although there is a trend toward poorer preservation in the lower section, especially below 1292.7 m MSF (Sample 319-C0009A-127-SMW). Except for a few samples from the top sand-rich interval in lithologic Unit II (see "Lithology"), most samples yielded abundant nannofossils and the major age-diagnostic taxa appear reasonably continuous throughout their ranges. Therefore, most of the important Neogene and Quaternary datums summarized by Raffi et al. (2006) (see "Biostratigraphy" in the "Methods" chapter) were recognized and, consequently, the drilled sequence was zoned using Martini's (1971) nannofossil scheme (Tables T8, T9; Fig. F46). However, frequent downhole contamination prevented the use of some important nannofossil datums based on first (or first consistent) occurrence.

Pleistocene

The first cuttings Sample 319-C0009A-2-SMW (712.7 m MSF) is composed of cement with muddy water and yielded a few nannofossils including the age-diagnostic large- and medium-sized Gephyrocapsa specimens (Table T8). The presence of this genus and co-occurrence of Pseudoemiliania lacunosa indicate that this sample lies between the last occurrence (LO) of P. lacunosa (0.436 Ma) and the first occurrence (FO) of Gephyrocapsa. The latter datum is considered most probably equivalent to the reentrance (RE) of medium Gephyrocapsa (≥4 µm) (1.04 Ma; see "Biostratigraphy" in the "Methods" chapter), considering its stratigraphic position relative to the underlying, more reliable datum. The first reliable datum is the last consistent occurrence (LCO) of Reticulofenestra asanoi (0.90 Ma), which was observed between 777.7 and 787.7 m MSF (between Samples 319-C0009A-19-SMW and 21-SMW) (Tables T8, T9; Fig. F46).

We observed the LCO of large Gephyrocapsa (>5.5 µm) (1.24 Ma) between 842.7 and 852.7 m MSF (between Samples 319-C0009A-33-SMW and 35-SMW). The LO of Helicosphaera sellii (1.34 Ma) is recorded between 897.7 and 902.7 m MSF (between Samples 319-C0009A-44-SMW and 45-SMW). Directly below the latter sample, the LO of Calcidiscus macintyrei (1.60 Ma) was observed between 902.7 and 912.7 m MSF (between Samples 319-C0009A-45-SMW and 47-SMW). The single fragmented specimen of C. Macintyrei in Sample 319-C0009A-45-SMW is considered reworked. The close stratigraphic positioning of the LOs of H. sellii and C. macintyrei could be attributed to the reworking of C. macintyrei into younger sediments, a common phenomenon in a tectonically active region, or the possible existence of a hiatus or a condensed interval. Further study is needed to help address this problem.

Other important Pleistocene datums based on first occurrence are not applicable, considering the stratigraphic context of more reliable LOs in cuttings, likely because of caving and mixing during mud circulation (Table T8). Examples include the first consistent occurrences (FCOs) of large Gephyrocapsa (>5.5 µm) and R. asanoi, RE of medium Gephyrocapsa (≥4 µm), and FOs of Gephyrocapsa size groups (see "Biostratigraphy" in the "Methods" chapter). Therefore, the Pliocene/Pleistocene boundary, defined by the FO of medium Gephyrocapsa (>3.5 µm) (1.67 Ma), is not identified in the hole.

Pliocene

As an alternative datum to the normally used FO of medium Gephyrocapsa, the Pliocene/Pleistocene boundary is approximated by the LO of Discoaster brouweri (2.06 Ma), which defines the base of Zone NN19 and is recorded between 927.7 and 932.7 m MSF (between Samples 319-C0009A-51-SMW and 52-SMW) with an increase in the number of discoasters (Tables T8, T9). The base of Zone NN18 is placed between 952.7 and 957.7 m MSF (between Samples 319-C0009A-56-SMW and 57-SMW), which is defined by the LO of Discoaster pentaradiatus (2.393 Ma). The rare occurrence of this species in the overlying Samples 319-C0009A-51-SMW and 47-SMW is considered as a result of reworking (Table T8).

The Zone NN17/NN16 boundary, defined by the LO of Discoaster surculus (2.52 Ma), is observed between 977.7 and 987.7 m MSF (between Samples 319-C0009A-61-SMW and 63-SMW). Slow sedimentation rates are indicated for the upper part of the Pliocene based on the closely spaced four datums (1.6–2.52 Ma) below 842.7 m MSF (Figs. F46, F47).

In Zone NN16, the LOs of Discoaster tamalis (2.87 Ma) and Sphenolithus spp. (3.65 Ma) are also recognized, and both taxa were continuously observed below 1062.7 and 1242.7 m MSF (Samples 319-C0009A-79-SMW and 117-SMW), respectively (Table T9). The LO of Reticulofenestra pseudoumbilicus (>7 μm) (3.79 Ma) recorded between 1272.7 and 1282.7 m MSF (between Samples 319-C0009A-123-SMW and 125-SMW) corresponds to the base of Zone NN16.

Miocene

Directly below the Zone NN16/NN15 boundary, Discoaster quinqueramus (an important index species in the upper Miocene) was identified at 1292.7 m MSF (Sample 319-C0009A-127-SMW) and consistently occurs downhole to TD (Table T8). The LO of D. quinqueramus (5.59 Ma) is placed between 1282.7 and 1292.7 m MSF (between Samples 319-C0009A-125-SMW and 127-SMW). Sample 319-C0009A-127-SMW is assigned to the upper part of Zone NN11 because of the abundant occurrence of R. pseudoumbilicus (>7 µm) and the presence of Amaurolithus specimens. Therefore, most of Zone NN15, Zones NN14–NN12, and very possibly part of Zone NN11 are missing, indicating the presence of a major unconformity (Figs. F46, F47). The missing interval is ~1.8 m.y. but needs to be refined in further study.

The presence of D. quinqueramus and the co-occurrence of Discoaster berggrenii down to the base of the hole indicate that the interval below Sample 319-C0009A-127-SMW falls within Zone NN11. The paracme end (end of temporarily absent and/or present with scattered specimens; Raffi et al., 2006) of R. pseudoumbilicus (>7 µm) (7.077 Ma) is tentatively placed between 1482.7 and 1492.7 m MSF (between Samples 319-C0009A-167-SMW and 169-SMW). Moreover, D. surculus continuously occurs below the upper part of the Pliocene down to the base of the drilled sequence, and thus its FCO (7.88 Ma) must be below TD. The base of the hole is therefore <7.88 Ma.

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