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doi:10.2204/iodp.proc.314315316.134.2009

Biostratigraphy

Four holes were drilled at Site C0006 into the toe of the Nankai accretionary prism during Expedition 316. Only one core each was retrieved in Holes C0006C and C0006D. Hole C0006E was cored from the seafloor to 406.97 m CSF through slope sediments and into the sediment inferred to have been the marginal trench wedge deposits. Hole C0006F was cored from 390 to 603 m CSF in the lower part of the thrusted trench deposits into the upper part of Shikoku Basin sediments.

The biostratigraphy determined for Site C0006 is based on examination of calcareous nannofossils, radiolarians, and foraminifers from Holes C0006E and C0006F. In addition, a few samples from C0006F were analyzed for diatoms.

Calcareous nannofossils

Sediments recovered from Holes C0006E and C0006F are dominated by interbedded sand layers and volcanic ash layers in turbidite sequences (see “Lithology”), yielding few to rare and moderately to poorly preserved nannofossils in most samples from core catchers and within cores. This lack of nannofossils results in difficulties establishing a biostratigraphy. At least three difficult issues are merged: scarcity of nannofossils, poor preservation, and reworking.

A number of nannofossil samples were carefully selected from thin clay or particular ash layers within cores that contain relatively abundant and moderately preserved nannofossils (Tables T7, T8).

Problems due to poor preservation still exist. A case commonly seen in many samples is that only large-sized nannofossils were preserved in sediments, whereas most small-sized species were lost. That is, the nannofossil assemblages found in sediments were changed by various processes, probably by dilution during turbidite flow, breakage by stress of deformation, and dissolution. These changes result in difficulties in finding and identifying event markers or in distinguishing them from species that are morphologically similar. Uncertain determination of species is indicated with a “?” in Tables T7 and T8. These uncertain data will be improved through shore-based study.

Reworking is very common throughout the sediment sequence recovered in Holes C0006E and C0006F, leading to confusion in the determination of a number of last occurrence (LO) events. To solve this problem, the co-occurrences of other age-diagnostic specimens or the nannofossil assemblages were examined in detail to make a reliable determination. For example, a few Reticulofenestra asanoi and Calcidiscus macintyrei were observed in the uppermost two cores of Hole C0006E (Table T7). Observations of nannofossil assemblages in this interval suggest that the assemblages are dominated by Gephyrocapsa carribeanica in the medium-sized Gephyrocapsa groups (Gephyrocapsa spp. medium I and II). The first occurrence (FO) of this species was estimated at 0.76 Ma (Su, 1996). Therefore, the presence of R. asanoi and C. macintyrei in samples from these two cores was considered to be due to reworking.

A total of 17 nannofossil biostratigraphic events were recognized (Table T9). The presence of Emiliania huxleyi in Samples 316-C0006E-1H-CC and 2H-1, 34 cm, was not determined with high confidence because of the small size of this species and poor preservation. However, correlation with geomagnetic data in the same hole suggests that these samples are younger than 0.29 Ma. Therefore, the transition (X) from Gephyrocapsa spp. (>3.5 µm) to E. huxleyi was placed at 5.16 m CSF and the FO of E. huxleyi was placed at 5.19 m CSF, marking the bottom of Zone NN21 at this horizon. Common Pseudoemiliania lacunosa occurs in Sample 316-C0006E-3H-4, 79 cm, which was taken as its LO to mark the boundary between nannofossil Zones NN20 and NN19 (0.436 Ma), whereas its rare presence in samples above 15.22 m CSF is attributed to reworking.

The subdivision of Zone NN19 was difficult in Holes C0006E and C0006F, as nannofossil assemblages in most samples were significantly affected by the presence of large amounts of reworked fossils younger than Zone NN19, dilution by turbidite flow, and dissolution. Two separate intervals (from 73.35 to 85.54 m CSF and from 192.45 m CSF to the bottom of the hole) in Hole C0006E contain R. asanoi (LO = 0.9 Ma and FO = 1.078 Ma); however, the interval between 85.54 and 192.45 m CSF contains frequent Gephyrocapsa spp. large (>5.5 µm) and common C. macintyrei with co-occurrence of Helicosphaera sellii, suggesting an earlier Zone NN19 age for this interval (Table T7; Fig. F31). Common medium-sized Gephyrocapsa lumina (a typical early Pleistocene species) are also present, indicating an early Pleistocene assemblage. The nannofossil data obtained during shipboard analyses support the assumption that the sedimentary sequence within the occurrence range of R. asanoi contains at least one age reversal and possibly one or more minor or poorly defined age reversals. With this assumption, we propose a preliminary subdivision of Zone NN19 into three parts for Site C0006:

1. Normal Zone NN19 from 17.22 to 72.90 m CSF in Hole C0006E.
2. A possible repeated Zone NN19 in the middle part of Hole C0006E.
3. Normal Zone NN19 from 192.45 to 406.92 m CSF in Hole C0006E and from 396 to 496 m CSF in Hole C0006F (Table T7; Fig. F31).

Subsequently, a number of nannofossil events were determined for further subdivision of these parts of Zone NN19 (Table T9). Within the repeated part of Zone NN19, a short younger interval marked by the presence of R. asanoi in Sample 316-C0006E-24X-1, 96 cm (164.29 m CSF), is noted (Tables T7, T9). This sample was collected from Section 316-C0006E-24X (not a core catcher sample) and contains rare to frequent R. asanoi, which was confirmed by careful examination of several slides from the same sample. This sample contains a mix of younger and older species. Common to frequent Gephyrocapsa spp. (≥4 µm) (reentrance [RE] = 1.04 Ma) and common C. macintyrei (1.6 Ma) together with a few Gephyrocapsa spp. large (>5.5 µm; 1.617 Ma) were observed, implying a possible contact boundary of older and younger sediments, possibly due to faulting. This sample (164.29 m CSF) is located near a change in lithology. Therefore, we consider the occurrence of R. asanoi to be an indication of a short age reversal (>0.9 to <1.04 Ma) in the repeated Zone NN19 sequence in Hole C0006E. At the bottom of Hole C0006E, the RE of Gephyrocapsa spp. (≥4 µm) at 391 m CSF is comparable to the RE of Gephyrocapsa spp. (≥4 µm) at 396 m CSF in Hole C0006F, providing consistent evidence from both holes. Four events, including the RE of Gephyrocapsa spp. (≥4 µm), were available for subdivision of Zone NN19 sediments in Hole C0006F (Table T9).

Nannofossils below 434.71 m CSF were very poorly preserved, and a number of samples were barren of nannofossils (Table T8). Most nannofossil records were obtained from carefully selected thin clay intervals, ash layers, or ash pods within cores. These records allow identification of event markers and subdivision of the lower part of Hole C0006F into a sequence from middle Pliocene Zone NN16 to upper Miocene Zone NN12 (Table T9). The bottom of the repeated Zone NN19 interval, marked by the bottom of the first consistent occurrence (FCO) of Gephyrocapsa spp., 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 459.01 m CSF (Tables T8, T9), indicating the absence of the lower part of lower Pleistocene Zone NN19, upper Pliocene Zone NN18, and Zone NN17. A truncation in the same time interval was also observed at Site C0004 (see “Biostratigraphy” in the “Expedition 316 Site C0004” chapter); however, we are unable to explain the possible correlation at the present time. The sediment sequence from 459.01 to 485.88 m CSF was assigned to middle Pliocene calcareous nannofossil Zone NN16 based on the co-occurrence of D. surculus (marker of top of Zone NN16) and D. tamalis, as well as on the absence of Reticulofenestra pseudoumbilicus (>7 µm) (marker of top of Zones NN15–NN14) (Table T8). The recognition of the LO of Sphenolithus spp. at 476.98 m CSF provides subdivision of this zone. The interval from 487.67 to 505.42 m CSF was assigned to lower Pliocene Zones NN15–NN13 with its top marked by the LO of R. pseudoumbilicus (>7 µm) and its bottom marked by the FO of Ceratolithus rugosus (marker of boundary of Zone NN13/NN12). Zones NN15–NN13 could not be further subdivided in Hole C0006F because of the trace or sporadic presence of Discoaster asymmetricus (marker of base of Zones NN15–NN14) in Hole C0006F. The absence of Discoaster quinqueramus (marker of top of Zone NN11; LO = 5.59 Ma) from the whole sequence of Hole C0006F suggests that the interval from 507.53 m CSF to the bottom of the hole is within Zone NN12 and younger than 5.59 Ma (Table T8). The lowest occurrence of Ceratolithus acutus at 580 m CSF (FO = 5.32 Ma) was the only possible event marker identified for the long interval of Zone NN12.

Other microfossil groups

All core catcher samples were processed for radiolarian analysis, but most of them were barren. Sample 316-C0006E-1H-CC revealed a poorly preserved assemblage with apparent breakage. A few undiagnostic or nonidentifiable diatom frustules and fragments were found within the section to Core 316-C0006E-13H.

Planktonic foraminifers occur sporadically to Core 316-C0006E-11H (71.38 m CSF). However, abundant Pulleniata finalis were found in a horizon in Section 316-C0006F-6R-1, 100 cm (439 m CSF), indicating an age younger than 2.06 Ma.

A few discrete horizons within cores were also sampled for diatoms and radiolarians, but those that were not barren revealed only very rare broken specimens.

Summary and correlation to faults

Based on the nannofossil record at Site C0006, 17 nannofossil biostratigraphic events were recognized and assigned an age of upper Miocene Zone NN12 to Pleistocene. The uppermost part of Zone NN21 might include a thin sediment layer of Holocene age. This is undeterminable based on nannofossil biostratigraphy. Plots of age versus depth in Holes C0006E and C0006F are combined with magnetostratigraphic results in Figure F31.

An age repetition interval was identified within Zone NN19; the occurrence range of R. asanoi in Zone NN19 may contain one or more repetitions within the sequence from 85.54 to 185.39 m CSF in Hole C0006E. There is also some inconclusive evidence for either a hiatus or an erosive event within this interval; however, shipboard data were insufficient to clarify nannofossil stratigraphy at this level of detail. This repetition is probably caused by faulting, in agreement with the lithologic Subunit IIB/IIC boundary. It is also possible that the “older” age assignment was affected by the presence of a large number of reworked older fossils and a lack of sufficient nannofossil data because of poor preservation. This will be improved through postcruise study. A short time gap of ~0.17 m.y. was found between 434.71 and 439.50 m CSF (Table T8; Fig. F31).

The bottom of repeated Zone NN19 sediments is truncated by middle Pliocene Zone NN16 sediments at 449.70 m CSF in Hole C0006F, indicating a time gap of ~1 m.y. Abundant planktonic foraminifers P. finalis (<2.06 Ma) found in Sample 316-C0006F-6R-1, 100 cm (439 m CSF), provide evidence for this age estimate. The time gap in Hole C0006F is consistent with the lithologic Unit II/III boundary. Unit III sediments were subdivided into middle–lower Pliocene Zone NN16, Zones NN15–NN13, and upper Miocene–lower Pliocene Zone NN12.

Results from seismic and structural studies at Site C0006 suggest several faults below 230 m CSF (see “Structural geology”). More detailed examination of nannofossil records from samples within these fault zones reveals poor preservation, indicating mechanical reworking and/or chemical degradation. For example, more nannofossil debris was observed in Samples 316-C0006E-45X-CC, 316-C0006F-6R-CC, and 17R-CC, whereas nannofossils have overgrown features in Samples 316-C0006F-31X-CC and 36X-CC (see the five zones of deformation in “Structural geology”). The short time gap between 434.71 and 439.50 m CSF (Table T8; Fig. F31) can be correlated to the possible fourth fault zone (see “Structural geology”). In addition, a difference between the upper and lower parts of the repeated interval of R. asanoi (192.48–434.71 m CSF; >0.9 to <1.078 Ma) was noted: nannofossils are moderately preserved above 358 m CSF, whereas they are poorly preserved and sediments contained more common reworked nannofossils below that depth (Tables T7, T8).

Furthermore, two very long sediment intervals covering such a short time span seem unlikely to be solely due to high sedimentation rates: the interval of R. asanoi (0.9–1.078 Ma) between 192.45 and 434.71 m CSF and the interval of Zone NN12 between 505.42 and 591.45 m CSF. Considering faults observed in these two intervals, it is reasonable to infer that their thickness might be partially caused by duplication due to thrust faulting.

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