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doi:10.2204/iodp.proc.338.107.2014

Biostratigraphy

Preliminary biostratigraphy for Hole C0022B is based on shore-based examination of calcareous nannofossils and planktonic foraminifers. Ages constrained by calcareous nannofossils and planktonic foraminifers are consistent overall. Both nannofossils and foraminifers indicate an age of ~2.0 Ma for the sediment at the bottom of the hole and a stratigraphic reversal or reworking of older sediment at ~130 mbsf. Calcareous nannofossils indicate additional stratigraphic reversals or reworking of older sediment at ~80 and ~145 mbsf. Stratigraphic reversals are likely associated with megasplay faulting, whereas reworking of older sediment could also be explained by mass-movement processes.

Calcareous nannofossils

A total of 38 core catcher samples recovered from Samples 338-C0022B-1H-CC (6.98 mbsf) to 41X-CC (415.88 mbsf) were examined (Table T8). Well-preserved nannofossils occur abundantly throughout the sequence in Hole C0022B. Although reworked specimens are frequently observed in examined samples, a good biostratigraphic framework was established for the entire succession, although some stratigraphic reversals, discontinuities, or sediment reworking are observed.

The uppermost sample from Hole C0022B (Sample 338-C0022B-1H-CC) contains dominant Emiliania huxleyi (Table T8). Therefore, this sample is younger than 0.291 Ma and the interval above this sample corresponds to Zone NN21. The last occurrence (LO) of Pseudoemiliania lacunosa (0.436 Ma) is placed between Samples 1H-CC and 2H-CC (28.98 mbsf) because of the consistent occurrence of P. lacunosa below the latter sample. This event marks the base of Zone NN20. Both Samples 4H-CC (47.98 mbsf) and 5H-CC (57.48 mbsf) contain Reticulofenestra asanoi. Thus, the LO and first occurrence (FO) of this species are clearly recognized. The presence of R. asanoi along with common medium Gephyrocapsa spp. (≥4 µm), which has a bar parallel to the short axis of a specimen, provides a maximum age of 1.04 Ma between Samples 4H-CC and 5H-CC. The LO of the large form of Gephyrocapsa spp. (>5.5 µm) (1.24 Ma) is placed between Samples 6H-CC (66.98 mbsf) and 7H-CC (76.48 mbsf).

A stratigraphic reversal is observed between Samples 338-C0022B-7H-CC (76.48 mbsf) and 8H-CC (84.48 mbsf) because the latter sample can be correlated with the last consistent occurrence of R. asanoi and the reentrance of medium Gephyrocapsa spp. (≥4 µm) (0.903–1.04 Ma) (Table T8). Larger forms of Gephyrocapsa spp. (>5.5 µm) are consistently observed from Samples 9T-CC (89.48 mbsf) to 21X-CC (209.31 mbsf). The LO of Helicosphaera sellii (1.34 Ma) is recognized between Samples 21X-CC and 22X-CC (211.18 mbsf). Thus, the interval between 89.48 and 209.31 mbsf is younger than 1.34 Ma. However, there are some discontinuities in this interval, because Sample 16X-CC (132.38 mbsf) contains abundant Calcidiscus macintyrei (age of the LO is 1.6 Ma) and other older nannofossil species. Sample 17X-CC (142.70 mbsf) (1.34–1.589 Ma) is slightly older than samples below Sample 18X-CC (148.46 mbsf) because it only contains H. sellii, whose last occurrence corresponds to 1.34 Ma.

Gephyrocapsa spp. (≥4 µm) first appeared in Sample 338-C0022B-28X-CC (300.48 mbsf) (Table T8), and the FO of this species (1.67 Ma) is recognized in this horizon. The base of nannofossil Zone CN13b (NN19) is also found between Samples 28X-CC and 29X-CC (305.42 mbsf) (Fig. F15 in the “Methods” chapter [Strasser et al., 2014a]; Table T8). The interval below Sample 38X-CC (390.98 mbsf) is characterized by abundant occurrences of discoasters (Table T8). The LO of Discoaster brouweri is placed between Samples 37X-CC (374.68 mbsf) and 38X-CC; thus, this horizon (2.06 Ma) corresponds to the base of nannofossil Zone NN19. Discoaster pentaradiatus is only found in Sample 41X-CC (415.88 mbsf). Therefore, the Zone NN18/NN17 boundary is placed between Samples 40X-CC (406.83 mbsf) and 41X-CC.

Planktonic foraminifers

Globigerinoides ruber (pink) is found in Sample 338-C0022B-1H-CC (6.98 mbsf) (Table T9), so this sample is older than 0.12 Ma, corresponding to its LO. In addition, the LOs of Truncorotalia tosaensis (0.61 Ma) and Globoturborotalita obliquus (1.30 Ma) are recognized between Samples 1H-CC and 2H-CC (28.98 mbsf) and between Samples 8H-CC (84.48 mbsf) and 9H-CC (89.48 mbsf), respectively.

The change in the dominant coiling direction of Pulleniatina spp. from sinistral to dextral (1.7–1.8 Ma), occurring between Samples 338-C0022B-26X-CC (290.48 mbsf) and 28X-CC (300.48 mbsf), is consistent with the LO of Neogloboquadrina asanoi (1.8 Ma) between Samples 27X-CC (295.48 mbsf) and 28X-CC (Table T9). However, the change in coiling direction of Pulleniatina spp. is also recognized at an interval between Samples 15X-CC (123.60 mbsf) and 16X-CC (132.38 mbsf), indicating a stratigraphic reversal as recognized by calcareous nannofossils around this interval.

The FO of Truncorotalia truncatulinoides (1.93 Ma) is placed between Samples 338-C0022B-34X-CC (352.50 mbsf) and 35X-CC (362.48 mbsf) (Table T9).

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