Previous   |    Next

doi:10.2204/iodp.proc.348.103.2015

Biostratigraphy and paleomagnetism

Biostratigraphy

Preliminary biostratigraphy for Holes C0002M, C0002N, and C0002P is based exclusively on the examination of calcareous nannofossils. Abundance and preservation of calcareous nannofossils varies throughout the section. A general preservational pattern of well- to moderately preserved nannofossils was observed in the upper part of the site (475.09–985.50 mbsf), with moderate to poor preservation observed below 985.50 mbsf. Assemblages recovered from Hole C0002M (475.09–506.68 mbsf) are Pleistocene in age, and assemblages from Holes C0002N and C0002P (875.50 and 3058.50 mbsf) indicate late Pliocene to late Miocene age.

Calcareous nannofossils

Shipboard nannofossil stratigraphy for this site is based on the recognition of the events reported in Table T8 in the “Methods” chapter (Tobin et al., 2015). Nannofossils are continuously present throughout the sequence. Their abundance decreases significantly downhole; however, a good biostratigraphic framework was established for the entire succession. A total of 25 samples containing calcareous nannofossils from Sections 348-C0002M-1R-1, 9 cm, to 4R-3, 86 cm, and 287 cuttings samples from 348-C0002N-3-SMW to 327-SMW and 348-C0002P-9-SMW to 300-SMW were examined. In addition, 25 samples from Sections 348-C0002P-1R-CC, 20 cm, to 6R-CC, 10 cm, were examined.

Hole C0002M

Cores collected in Hole C0002M from 475.00 to 512.50 mbsf yield well-preserved and abundant calcareous nannofossils that exhibit Pleistocene age for the upper part of this interval. Table T13 summarizes the calcareous nannofossil assemblages found in Hole C0002M. Section 348-C0002M-1R-1, 9 cm, from 475.09 mbsf contains Pseudoemiliania lacunosa, which indicates a minimum age of 0.44 Ma based on its last occurrence (LO). The presence of P. lacunosa indicates nannofossil Zone NN20. The LO of Helicosphaera sellii was detected in Section 348-C0002M-1R-3, 64 cm, at 478.47 mbsf. Gephyrocapsa spp. (>3.5 µm) was found at 506.68 mbsf, indicating a maximum age of 1.67 Ma for Sample 4R-3, 86 cm, based on the LO of Gephyrocapsa spp. (>3.5 µm). Comparison of the ages found in Hole C0002M can be made with Hole C0002B. The LO of H. sellii was found in Hole C0002B between 485.98 and 495.37 mbsf (Expedition Scientists 315, 2009), whereas this nannofossil event occurs at a shallower depth of 478.47 mbsf in Hole C0002M. In Hole C0002L, the LO of H. sellii was found at 502.74 mbsf (Strasser et al., 2014b).

Holes C0002N and C0002P

Cuttings and core samples from Holes C0002N and C0002P from 875.50 to 3058.5 mbsf contain assemblages ranging in age from the late Pliocene to late Miocene (Table T14). There is a difference in both preservation and abundance of calcareous nannofossils between Holes C0002N and C0002P. Generally, abundance of calcareous nannofossils is lower in Hole C0002P, and assemblages are also less well preserved. Samples collected from the same depths have different preservation and abundances in Holes C0002N and C0002P. In Hole C0002P, abundance of calcareous nannofossils tends to be lower where there is a greater percentage of sand in the sample. Preservation of calcareous nannofossils is also generally poorer where the samples have a higher percentage of sand. Specimens of the genus Discoaster are poorly to moderately preserved, commonly with broken rays, making identification to the species level difficult.

Preliminary examination of the cuttings and core samples from Holes C0002N and C0002P revealed good to poor preservation of calcareous nannofossils, with assemblages having relatively low species diversity. Dissolution or deformation occurred, leading to barren or poor occurrence of nannofossils in certain intervals. Several of the zonal markers of the zonation by Raffi et al. (2006) were identified in the sedimentary sequence. The nannofossils identified in Holes C0002N and C0002P are listed in Table T14, and biostratigraphy is summarized in Table T15. Biostratigraphic events observed in Holes C0002N and C0002P are presented in Figure F49.

The uppermost sample from Holes C0002N and C0002P contains P. lacunosa, assigning Sample 348-C0002N-3-SMW at 875.50 mbsf to a maximum age of 3.92 Ma. At 905.50 mbsf, Discoaster brouweri is present, therefore indicating an age of older than 2.06 Ma and the top of the late Pliocene Zone NN19 for Sample 9-SMW based on the age of its LO. Discoaster pentaradiatus is present in Sample 14-SMW, indicating the top of Zone NN18 and an age range of 2.393–2.512 Ma at 925.50 mbsf. Sample 24-SMW (975.50 mbsf) contains specimens of Sphenolithus spp., which has a LO of 3.6 Ma and indicates the top of Zone NN17. An age of older than 3.79 Ma can be assigned to Sample 34-SMW (1025.50 mbsf) because it contains Reticulofenestra pseudoumbilicus (LO at 3.79 Ma). The presence of R. pseudoumbilicus also indicates the bottom of Zone NN16. Discoaster asymmetricus is present in Sample 46-SMW, designating Zones NN14–NN15 and an age of older than 4.13 Ma at 1085.50 mbsf. Sample 85-SMW (1245.50 mbsf) contains Amaurolithus primus (LO at 4.5 Ma), indicating an age of older than 4.5 Ma. An age of older than 5.59 Ma is indicated by the presence of Discoaster quinqueramus in Sample 125-SMW (1436.50 mbsf) because of its LO, marking the top of Zone NN12. The first occurrence of A. primus was identified in Sample 192-SMW (1735.50 mbsf), giving an age range of at least 7.362–7.424 Ma and indicating late Miocene Subzone NN11b. Sample 287-SMW (2145.50 mbsf) contains the LO of Discoaster hamatus, indicating Subzone NN10a and an age of 9.56 Ma. In Sample 348-C0002P-111-SMW (2245.5 mbsf), D. hamatus was last observed downhole, therefore indicating its first occurrence (FO), dated at 10.541 Ma and marking the top of Zone NN9. In Sample 348-C0002P-273-SMW (2945.5 mbsf), D. brouweri is present, therefore indicating a maximum age range of 10.734–10.764 Ma at this depth because its FO is at that age range. The presence of D. brouweri indicates Zone NN9. Samples analyzed between 2955.5 and 3055.5 mbsf did not yield any calcareous nannofossil zonal marker species; therefore, the deepest section of Holes C0002N and C0002P cannot be dated.

In Hole C0002F, nannofossil events was observed at shallower depths than in Holes C0002N and C0002P (Strasser et al., 2014b). For example, R. pseudoumbilicus and Sphenolithus spp. was observed at 935.5 mbsf in Hole C0002F (Strasser et al., 2014b), whereas Sphenolithus spp. was first found at 975.5 mbsf and R. pseudoumbilicus was observed at 1025.5 mbsf. Sphenolithus spp. was observed at a shallower depth of 906.85 mbsf in Hole C0002J (Strasser et al., 2014b).

Calcareous nannofossil assemblages can provide some insight into paleoceanographic conditions. Species abundance is generally common to rare. In the Pliocene and late Miocene, sediment contains warm-water genera such as Discoaster and Sphenolithus.

Paleomagnetism

Hole C0002P

Remanent magnetization of archive-half sections from Hole C0002P were measured at demagnetization levels of 0, 5, 10, 15, and 20 mT peak fields to identify characteristic directions. Demagnetizations of 10–15 mT successfully removed low-coercivity components, and magnetic directions after the demagnetizations indicate stable constant directions (Fig. F50). Declination, inclination, and intensity profiles with depth after 20 mT demagnetization are shown in Figure F51. The declination profile represents widely scattered directions, which is indicative of “biscuiting” of cores created during RCB coring operations. The inclination profile reveals that data are of dominantly positive inclination, and the degrees of the positive inclination in each interval are not constant. For example, the calculated mean inclinations using the method proposed by Arason and Levi (2010) are 34.22° for the interval between 2172.450 and 2174.955 mbsf, 61.83° for the interval between 2194.005 and 2196.985 mbsf, and 36.50° for the interval between 2210.0 and 2215.0 mbsf. Some intervals show steep negative inclinations, which occur in relatively short intervals. Interestingly, the interval between 2205.195 and 2205.515 mbsf, which shows a clear negative inclination, corresponds to the brittle fault zone (see “Structural geology”). This suggests a different timing of magnetization for this interval from that of the intervals lying above and below. In order to elucidate the lock-in timing of those magnetizations, careful evaluation referencing structural analysis (e.g., bedding) results is required for postcruise study.

Top of page   |    Previous   |    Next