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doi:10.2204/iodp.proc.338.103.2014

Biostratigraphy

Preliminary biostratigraphy for Hole C0002F is based on shore-based examination of calcareous nannofossils and radiolarians, whereas that for Holes C0002J–C0002L is exclusively based on calcareous nannofossils.

Calcareous nannofossils from Hole C0002F suggest that cuttings samples from 935.5 and 985.5 mbsf are early to middle Pliocene and late Miocene in age, respectively. These nannofossil ages likely reflect that the majority of cuttings at 935.5 mbsf are derived from Unit III, whereas those at 985.5 mbsf are derived from Unit IV. Radiolarian ages, which are less precise, are overall consistent with this interpretation. In this hole, a discrepancy between the logging Unit III/IV boundary (918.5 mbsf) and the lithologic Unit III/IV boundary (1025.5 mbsf) is considered to be due to mixing of cuttings over an interval of as much as ~100 m (see the “Methods” chapter [Strasser et al., 2014a]). Mixing of nannofossils occurs accordingly.

Calcareous nannofossils from Hole C0002J suggest that sediment above 925.5 mbsf is middle to late Pliocene in age, whereas sediment below 926.7 mbsf contains rare nannofossils. This supports the lithologic Unit III/IV boundary at 926.7 mbsf in this hole, below which sediment of Unit IV is noncalcareous and supposed to have been deposited below the carbonate compensation depth (see “Lithology”).

The age range of the Kumano Basin section between 200 and 500 mbsf in Holes C0002K and C0002L was constrained from biostratigraphy and magnetostratigraphy data from Expedition 315 to be older than 1.04 Ma but younger than 1.34 Ma (Expedition 315 Scientists, 2009b). Calcareous nannofossils from Hole C0002L confirmed that the base of this hole (502.74 mbsf) is older than 1.34 Ma. The nannofossil event of 1.04 Ma, however, was found at ~250 mbsf in Hole C0002K, so an interval of normal polarity paleomagnetism between 240.72 and 299.37 mbsf (see “Paleomagnetism”) may rather be correlated with the Jaramillo Subchron of 0.988–1.07 Ma. However, this nannofossil event and the top of the Jaramillo Subchron were also encountered at 137.46 and 119.58 mbsf, respectively, in Hole C0002D. The duplicate occurrence of the nannofossil event and the Jaramillo Subchron is possibly due to the presence of a normal fault between Holes C0002D and C0002K, where the former hole penetrated the footwall and the latter hole penetrated the hanging wall.

Calcareous nannofossils

Calcareous nannofossils of 17 cuttings samples (338-C0002F-22-SMW [935.5 mbsf] to 284-SMW [1985.5 mbsf]) from Hole C0002F, 9 core samples (338-C0002J-1R-CC, 0–5 cm, to 7R-CC, 19.5–24.5 cm) from Hole C0002J, 8 core samples (338-C0002K-1H-CC, 36.0–41.0 cm, to 11X-CC, 20.0–25.0 cm) from Hole C0002K, and 14 core samples (338-C0002L-1X-CC, 36.0–41.0 cm, to 24X-CC, 33.5–38.5 cm) from Hole C0002L were examined. Well to poorly preserved, abundant nannofossil specimens are found in these holes.

Hole C0002F

The uppermost sample (338-C0002F-22-SMW; 935.5 mbsf) examined contains Reticulofenestra pseudoumbilicus and Sphenolithus spp. without a typical form of Discoaster quinqueramus (Table T19). This together with other accompanying nannofossil species indicate that this sample may be assigned a Pliocene age, corresponding to calcareous nannofossil Zones NN15–NN12 (Table T19; see also Table T11 in the “Methods” chapter [Strasser et al., 2014a]). D. quinqueramus and/or Discoaster berggrenii, which characterize the Miocene nannofossil Zone CN9 (NN11) (Table T11 in the “Methods” chapter [Strasser et al., 2014a]), consistently occur below Sample 338-C0002F-34-SMW (985.5 mbsf) (Table T19). This may indicate that the entire section to 1986 mbsf is younger than 5.59 Ma. However, the occurrence of nannofossils becomes sporadic in the lower part of the hole and species composition is incomplete. Therefore, downhole contamination by those younger species cannot be excluded.

Hole C0002J

The presence of Discoaster tamalis and the absence of Sphenolithus spp. indicate that Sample 338-C0002J-1R-CC (906.085 mbsf) is clearly correlated with nannofossil Zone NN16 and corresponds to 2.87–3.65 Ma (Table T20). Moreover, the last occurrence (LO) of Sphenolithus spp. is placed between Samples 338-C0002J-1R-CC and 2R-CC (907.85 mbsf). The interval between Samples 338-C0002J-4R-CC (921.78 mbsf) and 5R-7 (925.481 mbsf) may coincide with nannofossil Zone NN14–NN15 because of the consistent occurrence of middle Pliocene species. No age indication is obtained below Sample 338-C0002J-5R-8 (926.7 mbsf) because these samples are barren of nannofossils.

Hole C0002K

The uppermost sample (338-C0002K-1H-CC; 204.48 mbsf) contains dominant Reticulofenestra asanoi along with common occurrence of medium Gephyrocapsa spp. (≥4 µm) (Table T27). The first occurrence of medium Gephyrocapsa spp. (≥4 µm) (= Gephyrocapsa sp. 3) is placed between Samples 338-C0002K-7X-CC (244.58 mbsf) and 8X-CC (254.83 mbsf), which provides an age of 1.04 Ma. R. asanoi is continuously observed to the lowermost sample, and thus, this hole is entirely correlated with the interval above the first consistent occurrence (FCO) of this species (i.e., younger than 1.078–1.136 Ma) (note that according to Raffi [2002] this event is diachronous in the world’s oceans; we therefore assigned the medium age of 1.107 Ma).

Hole C0002L

The FCO of R. asanoi, which occurs at 1.107 Ma, is placed between Samples 338-C0002L-4H-CC (314.49 mbsf) and 5H-CC (324.23 mbsf) (Table T28). The LO of Gephyrocapsa spp. (>5.5 µm), corresponding to 1.24 Ma, is found between Samples 338-C0002L-5H-CC and 6H-CC (333.98 mbsf). Samples from 338-C0002L-6H-CC to 24X-CC (502.74 mbsf) are correlated with the interval between the LO of Gephyrocapsa spp. (>5.5 µm) and that of Helicosphaera sellii, which corresponds to 1.24–1.34 Ma.

Radiolarians

Radiolarians in Hole C0002F are present in 4 samples and absent from the other 19 samples examined. In the four samples, radiolarians are rare to very rare and show signs of dissolution (moderate preservation) (Table T21). The occurrences of Stichocorys delmontensis from Sample 338-C0002F-22-SMW (935.5 mbsf) and of Stichocorys peregrina from Sample 34-SMW (985.5 mbsf) indicate that the two samples can be correlated to the Lychnodictyum audax Zone (RN11) or older zones (i.e., 2.7 Ma or older [Pliocene–Miocene]). No age-diagnostic radiolarian species were found from Samples 46-SMW and 56-SMW.

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