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doi:10.2204/iodp.proc.314315316.135.2009

Biostratigraphy

The biostratigraphy determined for Site C0007 was based on examination of calcareous nannofossils, radiolarians, and foraminifers from Holes C0007C and C0007D.

Calcareous nannofossils

Sediments recovered from Holes C0007C and C0007D are dominated by interbedded sand layers and volcanic ash layers (see “Lithology”), yielding few to rare and moderate to poorly preserved nannofossils in most samples from core catchers and cores. This deficiency results in difficulties establishing the biostratigraphy, including few nannofossils, poor preservation, and reworking. To solve these problems, we used the same methods as those used at Site C0006 (see “Biostratigraphy” in the “Expedition 316 Site C0006” chapter), allowing us to obtain common to abundant nannofossils from a number of samples (Tables T9, T10).

For Hole C0007C, two nannofossil biostratigraphic events were recognized (Table T11). The uppermost Sample 316-C0007C-1H-CC, 7.0–12.0 cm, contains common Pseudoemiliania lacunosa; therefore, this sample was assigned to Zone NN19. Sample 316-C0007C-14X-CC, 24 cm, contains dominant Gephyrocapsa spp. medium I and medium II but rare P. lacunosa. This case has been commonly observed in Zone NN20 nannofossil assemblages in the Pacific warm regions where rare P. lacunosa is present as reworked in Zone NN20 assemblages. This sample was therefore assigned to Zone NN20. Common P. lacunosa occurs again in Sample 316-C0007C-15X-CC, marking its “repeated” last occurrence (LO) in this horizon (Fig. F33). The Zone NN20 sediments and the repeated LO of P. lacunosa suggest a possible disturbance of the sediment sequence. Core 316-C0007C-17H contains gravels in which no nannofossils were available.

In Hole C0007D, a total of 17 nannofossil biostratigraphic events were recognized: 7 Pleistocene Zone NN19 events and 10 Pliocene Zone NN16–NN12 events, including 3 possible repeated events (Table T11). Based on these events, three repeated intervals and one age gap were recognized (Fig. F33).

The uppermost repeated event is the LO of P. lacunosa at 138 m CSF. The normal LO of P. lacunosa was observed in Sample 316-C0007C-1H-CC, 7.0–12.0 cm. However, a repeated LO of P. lacunosa was seen in Sample 316-C0007C-15X-CC. In Hole C0007D, a few P. lacunosa specimens were found in Sample 316-C0007D-1R-CC, 0–5 cm, but they are common in Sample 316-C0007D-1R-CC, 10 cm. These samples contain abundant Gephyrocapsa spp. medium I and medium II, which are the dominant species in Zone NN20 and common in the uppermost part of Zone NN19. It is reasonable to consider the common occurrence of P. lacunosa in Sample 316-C0007D-1R-CC, 0–5 cm, as the repeated LO, in correlation with the repeated LO of P. lacunosa at 138 m CSF in Hole C0007C. The repeated LO of P. lacunosa indicates an age reversal, probably caused by faulting.

Determination of the reentrance (RE) of Gephyrocapsa spp. (≥4 µm) was somewhat difficult because the record of the RE of this group was interrupted by samples lacking fossils or poor preservation and by faulting (Table T10). However, the records of this Gephyrocapsa group suggest one “normal” sequence to 249.46 m CSF and another sequence from 266.57 to 305.82 m CSF. Therefore, we placed the normal RE of Gephyrocapsa spp. (≥4 µm) at 249.46 m CSF and considered the lowest RE of Gephyrocapsa spp. (≥4 µm) at 305.82 m CSF as repeated. This conclusion is in reasonable accord with the results from seismic and structural studies. The first fault zone (fault Zone 1 in Fig. F33) in Hole C0007D was observed between 237.50 and 259.30 m CSF (see “Structural geology”). The repetition of the RE of Gephyrocapsa spp. (≥4 µm) might be caused by this fault.

The lower part of Zone NN19, marked by the base of a middle Zone NN19 event, the first consistent occurrence (FCO) of Gephyrocapsa spp. large (>5.5 µm) was truncated by a number of late Pliocene events: the LOs of Discoaster brouweri (marker of Zone NN18), Discoaster pentaradiatus (marker of Zone NN17), and Discoaster surculus (marker of Zone NN16) at 361.79 m CSF (Tables T10, T11; Fig. F33), indicating the absence of the lower part of lower Pleistocene Zone NN19 and upper Pliocene Zones NN18 and NN17. We noted a similar truncation in the same time interval at Sites C0004 and C0006 (see “Biostratigraphy” in the “Expedition 316 Site C0004” chapter and “Biostratigraphy” in the “Expedition 316 Site C0006” chapter) but lack an explanation at the present time. For Site C0007, this age gap is correlated with a second fault zone (fault Zone 2 in Fig. F33) between 341.50 and 362.30 m CSF in Hole C0007D (see “Structural geology”).

The interval from 362.15 to 376.17 m CSF in Hole C0007D was assigned to middle Pliocene calcareous nannofossil Zone NN16, based on the co-occurrence of D. surculus (marker of the top of Zone NN16) and Discoaster tamalis, as well as the absence of Reticulofenestra pseudoumbilicus (>7 µm) (marker of top of Zones NN15–NN14) (Table T10). Recognition of the LO of Sphenolithus spp. at 370.88 m CSF provides a subdivision of this zone. The moderate numbers of R. pseudoumbilicus at this horizon were considered to be reworked, as this species occurs commonly as reworked in the sequence above. The interval from 370.88 to 390.35 m CSF was assigned to lower Pliocene Zones NN15–NN14 with its top marked by the LO of R. pseudoumbilicus (>7 µm) and its bottom marked by the presence of D. asymmetricus (marker of base of Zones NN15–NN14). The rare presence of Ceratolithus acutus at 409.93 m CSF was taken as the reliable LO of this species, whereas its trace and sporadic presence in the upper sequence of Hole C0007D was considered to be due to reworking. This phenomenon subdivides Zone NN13. The lowest occurrence of the Ceratolithus rugosus (marker of Zone NN13/NN12 boundary) at 418.38 m CSF was considered as its FO, marking the boundary between Zones NN13 and NN12. The sequence from 418.78 to 438.13 m CSF was assigned to Zone NN12, according to continuous occurrence of C. acutus (FO = 5.32 Ma) in this interval and the absence of D. quinqueramus (marker of top of Zone NN11; LO = 5.59 Ma).

An age reversal was seen in the interval from 438.24 to 438.61 m CSF in Core 316-C0007D-29R (Fig. F34), where sediments were fractured and brecciated by faulting (fault Zone 3 in Fig. F33) (see “Structural geology”). The occurrence of C. acutus, together with the common presence of R. pseudoumbilicus and Sphenolithus spp. in Sample 316-C0007D-29R-2, 33 cm, and in samples above (e.g., Sample 316-C0007D-29R-1, 16 cm), suggests a Zone NN12 age (or ~5.32 Ma) for this horizon. On the other hand, the nannofossil assemblage in Sample 316-C0007D-29R-2, 70 cm, and the two samples below suggest a Zone NN16 assemblage, characterized by the common presence of P. lacunosa and Sphenolithus spp., trace but continuous D. tamalis, and lack of a reliable presence of R. pseudoumbilicus. Thus, the sediment ages change from Zones NN12 to NN16 in this 37 cm interval. This age reversal in coordination with the results of structural studies implies a significant sequence disturbance by faulting. To estimate the time difference, we took the occurrence of Sphenolithus spp. in this horizon as its repeated LO (3.65 Ma) in the top of the reversed sequence, giving an estimate of ~1.67 m.y.

Age assignment was not possible for samples below 439.39 m CSF because of lack of fossils (Sample 316-C0007-34R-CC) or uncertainty whether samples were in situ or fallen pieces (Samples 316-C0007-30R-CC, 31R-CC, and 35R-CC; see “Lithology”).

Other microfossil groups

All core catcher samples were processed for radiolarian analysis, but most of them were barren. Sample 316-C0007A-1H-CC revealed a moderately preserved assemblage of radiolarians, some foraminifers, and a few broken diatom frustules. In Sample 316-C0007-15X-CC, abundant diatom frustule fragments and a few broken radiolarian shells and foraminifers were observed. Discrete horizons within cores were also sampled for diatoms and radiolarians, but those that were not barren revealed only very rare broken diatom frustules.

Summary

A total of 19 nannofossil biostratigraphic events were recognized in sediments recovered from Site C0007, which were then assigned an age of Pliocene Zone NN12–NN20. The uppermost sediments at this site might contain a thin Holocene deposit; however, it is not possible to determine the age of this interval by means of nannofossil biostratigraphy. Plots of age versus depth for Holes C0007C and C0007D were combined with magnetostratigraphic results from the same hole (Fig. F33).

Three age reversals and an age gap were suggested by the nannofossil record and may be caused by faulting.

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