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doi:10.2204/iodp.proc.346.106.2015

Stratigraphic correlation and sedimentation rates

A composite section and splice (as defined in “Stratigraphic correlation and sedimentation rates” in the “Methods” chapter [Tada et al., 2015b]) were constructed for Site U1425 in an effort to establish a continuous sediment sequence using Holes U1425B and U1425D. Hole U1425A recovered only one core and was dedicated to training. Three cores were recovered from Hole U1425C, which were dedicated to future optically stimulated luminescence measurement without any onboard use. Holes U1425B and U1425D were cored, switching back and forth between the standard APC, XCB, and half APC to 407.2 m CSF-A at the base of Core 346-U1425B-61X and to 431.0 m CSF-A at the base of Core 346-U1425D-72H. Switching among coring devices allowed penetration of alternating hard layers and soft-sediment layers. Hole U1425E was cored to 113.1 m CSF-A at the base of Core 346-U1425E-13H in order to obtain additional core material. Splicing between Holes U1425B and U1425D enabled us to construct a continuous stratigraphic sequence, with the exception of three potential gaps, from the seafloor to the bottom of Core 346-U1425B-51H (336.91 m CSF-A). Deeper than 336.9 m CSF-A, poor recovery of sediment in both Holes U1425B and U1425D prohibited us from constructing a composite section.

Construction of CCSF-A scale

Definition of top (0 m CCSF-A)

The first cores in Holes U1425B, U1425D, and U1425E all recovered the mudline. We selected Core 346-U1425D-1H as the anchor core in order to avoid a whole-round sampling interval at Section 346-U1425B-1H-1 and defined the top as 0 m CCSF-A (as defined in “Stratigraphic correlation and sedimentation rates” in the “Methods” chapter [Tada et al., 2015b]).

Compositing of cores

The CCSF-A scale for Site U1425 is based on correlation of magnetic susceptibility and GRA data from the WRMSL and the Special Task Multisensor Logger, as well as RGB blue (B) data extracted from images acquired by the Section Half Imaging Logger (see “Physical properties” in the “Methods” chapter [Tada et al., 2015b] for details). Magnetic susceptibility and GRA bulk density were measured at 2.5 cm intervals for Holes U1425A and U1425B and at 5 cm intervals for Hole U1425C, whereas B was recovered at 0.5 cm intervals. Correlative horizons are most easily identified in the magnetic susceptibility and B data with the aid of GRA data when identical features of the variations were difficult to identify. Extremely fine scale correlations are best achieved using the 0.5 cm B data.

Core gaps

A partial stroke of the standard APC occurred at 104.6 m CSF-A during coring of Core 346-U1425B-12H because of an indurated layer (e.g., dolomite, see “Lithostratigraphy”). The following core (13X) successfully penetrated this hard layer, although the top of the core included fall-in. Although careful operations using a combination of the half APC (Core 346-U1425D-12H) and XCB (Core 346-U1425D-13X) in the next hole successfully cored this interval, the thickness of the hard layer between Cores 346-U1425B-12H and 13X and Cores 346-U1425D-12H and 13X remained unclear. Even during the third attempt (Hole U1425E), the recovery of Core 346-U1425E-12H was not sufficient, and the following core (13H) did not recover the interval. This difficult interval separates the splice segments above (Segment 1) and below (Segment 2) (Table T20; Fig. F65B).

Partial strokes of the standard APC also occurred at 211.63 and 210.89 m CSF-A during coring of Cores 346-U1425B-28H and 346-U1425D-25H, respectively, where a >40 cm thick ash layer was eventually recovered (see “Lithostratigraphy”). Because the tops of the following cores (346-U1425B-29H and 346-U1425D-26X) were not identical to one other, a potential gap at the bottom of the ash layer and below it remains, which separates the splice segments above (Segment 2) and below (Segment 3) (Table T20; Fig. F65E).

Core gaps aligned between Cores 346-U1425B-39H and 40X and between Cores 346-U1425D-35H and 36H because of the relatively large heave during coring operations at this site. Physical properties such as magnetic susceptibility, GRA density, NGR, and B values were not continuous beyond these boundaries (Fig. F65E). Therefore, the intervals between Cores 346-U1425B-39H and 40X and between Cores 346-U1425D-35H and 36H were potentially uncovered, which separates the splice segments above (Segment 3) and below (Segment 4) (Table T20; Fig. F65E).

Poor recovery and frequent coring disturbance (e.g., flow-in) made the sedimentary sequence significantly fragmented below Cores 346-U1425B-51H and 346-U1425D-45H (see “Lithostratigraphy”). Because this site was logged, the original stratigraphic relationships might be assigned to even the highly fragmented section recovered in these intervals using core-log comparison, as described below.

Summary

Based on three core gaps and the fragmented nature of the intervals from Cores 346-U1425B-52H through 61X and Cores 346-U1425D-46H through 72H, the CCSF-A scale given to Site U1425 cores is divided into five segments (Table T20; Fig. F65):

• Segment 1 consists of Cores 346-U1425A-1H, 346-U1425B-1H through 12H, and 346-U1425D-1H through 12H, which extends from 0 to ~110 m CCSF-A.

• Segment 2 consists of Cores 346-U1425B-13X through 28H and 346-U1425D-13X through 25H, which extends from ~110 to ~221 m CCSF-A.

• Segment 3 consists of Cores 346-U1425B-29H through 39H and 346-U1425D-26X through 35H, which extends from ~221 to ~292 m CCSF-A.

• Segment 4 consists of Cores 346-U1425B-40X through 51H and 346-U1425D-36H through 45H, which extends from ~292 to ~350 m CCSF-A.

• Segment 5 consists of Cores 346-U1425B-52H through 61X and 346-U1425D-46H through 72H, which extends from ~350 to ~446 m CCSF-A, where common vertical offsets of 15.53 and 15.40 m were assumed for the cores in Holes U1425B and U1425D, respectively.

Construction of CCSF-D scale

A combination of Holes U1425B and U1425D can cover all the stratigraphic intervals from Segments 1–4 to 336.91 m CSF-A (352.44 m CCSF-A). Because Hole U1425E was drilled 22 days after the drilling of Holes U1425B and U1425D, cores from Hole U1425E were not used for the splice, although a composite depth scale (CCSF-A) is supplied for Hole U1425E cores. Selected splice intervals are listed in Table T21.

We tried to construct a splice, avoiding whole-round sampling intervals and minimizing inclusion of disturbed intervals as much as possible. All the cores in Segment 1 were aligned properly and spliced completely.

For Segment 2, the 1.73 m thick interval (splice Segment 2-6 in Table T21) between 133.10 and 134.83 m CCSF-D (as defined in “Stratigraphic correlation and sedimentation rates” in the “Methods” chapter [Tada et al., 2015b]) was taken from Core 346-U1425B-19H, which showed a very homogeneous appearance with suspected drilling disturbance. We correlated this interval, however, to the top part of Core 346-U1425D-16H based on a resemblance in the magnetic susceptibility profiles.

In Segment 3, splicing between Cores 346-U1425B-29H and 346-U1425D-27H (splice Intervals 3-1 and 3-2 in Table T21) at 230.17 m CCSF-D was performed without pattern matching of physical properties; instead, splicing was only based on the fact that no jumps in magnetic susceptibility, GRA density, and B values were observed (Fig. F65D). A similar situation occurred for splicing between Cores 346-U1425D-33H and 346-U1425B-38H (splice Intervals 3-14 and 3-15 in Table T21) at 281.75 m CCSF-D (Fig, F65E).

In Segment 4, a 2.56 m thick homogeneous interval (splice Interval 4-5 in Table T21) from Core 346-U1425B-43H was spliced into the composite section between Cores 346-U1425D-38H and 39H. Although the homogeneous appearance of Core 346-U1425B-43H may reflect coring disturbance, we adopted it as a splice interval because all the physical properties such as magnetic susceptibility, GRA density, and B were mostly the same as the intervals above (Core 346-U1425D-38H) and below (Core 346-U1425D-39H). Cores 346-U1425D-40H and 346-U1425B-46H (splice Intervals 4-7 and 4-8 in Table T21) were also spliced at 320.65 m CCSF-D based only on the similarity of magnetic susceptibility, GRA density, and B values (Fig. F65F).

Estimation of gaps between cores based on core-log comparison

Because NGR was measured by sensors on the FMS-sonic and paleo combo tool strings during downhole logging in Hole U1425B (see “Downhole measurements”), we can align the spliced NGR profile with the wireline log depth below seafloor (WSF). The correlation between core NGR and total spectral gamma ray (HSGR) from downhole logging was good even at <0.1 m scale, which also enabled us to estimate the size of potential gaps between segments.

Figure F66 shows the comparison between the corresponding intervals for NGR core splice and HSGR logging around Segment 1/2, 2/3, and 3/4 boundaries. The distance between correlative horizons above and below the Segment 1/2 boundary (Fig. F66A) was 9.02 m CCSF-D, whereas those for logging were 8.97 m WSF from the paleo combo tool string and 9.33 m WSF from the FMS-sonic tool string (Fig. F66B). Because both tool strings were separately run, there is an uncertainty in the WSF depth scale due to cable stretching and/or ship heave (see “Downhole measurements”), here estimated to ~30 cm. Therefore, no significant gap could be confidently recognized between Segments 1 and 2.

In the case of the Segment 2/3 boundary, the distance between correlative horizons above and below the boundary was 5.68 m CCSF-D for the core splice (Fig. F66C), whereas it was 5.73–5.83 m WSF for downhole logging (Fig. F66D). The uncertainty in wireline depth is small enough (<10 cm), and a gap of 10–20 cm between Segments 2 and 3 could explain the smaller distance in the NGR splice.

The distance of 6.43 m CCSF-D between correlative horizons above and below the Segment 3/4 boundary (Fig. F66E) is even larger than the 5.43–5.47 m WSF for logging, which suggests that there is no significant gap here. The larger distance in the core splice might be due to expansion of sediment during the coring process.

Although the sediment sequence below Cores 346-U1425B-51H and 346-U1425D-45H were fragmented by poor recovery and frequent flow-in disturbance, the NGR profiles from Holes U1425B and U1425D seem identical if the data taken from the flow-in intervals were removed (Fig. F66G). When these core data are compared to the HSGR log from the paleo combo tool string (see “Downhole measurements”), major fluctuation patterns are well matched throughout the interval between 340 and 410 m CCSF-A (Fig. F66G). Therefore, any data taken from the nonflow-in intervals plotted on the CCSF-A scale could be regarded as a reasonable property profile of Segment 5. It is also clear that a significant coring gap exists between 383 and 390 m CCSF-A.

Sedimentation rates

All age control datums, including biostratigraphic markers, paleomagnetic events, and tentatively dated tephra, were plotted on Figure F67A and listed in Table T22. Paleomagnetic datums constrained the sediment ages well for Subunits IA and IB. Therefore, ages of the Subunit IA/IB and IB/IIA boundaries were determined exclusively based on linear fits to paleomagetic datums. An apparent outlier in Subunit IB is the LO of H. parviakitaensis that was determined by “rare” occurrence of this species, which might suggest reworking (see Table T22). Within Subunit IIA, the depth-age relationship was narrowed by the lower limits defined by the LOs of N. kamtschatica and D. bullatus and the upper limits defined by the FOs of G. praeinflata and H. parviakitaensis (Fig. F67A; Table T22). The age of the Subunit IIA/IIB boundary was set to keep the sedimentation rate constant within Subunit IIA, thus satisfying these lower and upper limits. Because the LO and FO of G. ikebei were defined by rare occurrence from only one sample, they were considered reworked (see Table T8). Because an assumption of a single constant sedimentation rate within Subunit IIB disagrees with the constraints given by the FO of L. pylomaticus at 187.7 m CCSF-A and the LO of L. parallelipes at 198 m CCSF-A, we adopted these two datums as inflection points of depth-age lines. Between 137 and 187.7 m CCSF-A, the lower limit defined by the LO of L. redondoensis and the LO of Thalassiosira jacksonii and the upper limit defined by the FO of D. bullatus and the FO of L. pylomaticus narrowed a possible depth-age line. At 259 m CCSF-A, the LO of C. nakasekoi and the FO of L. parallelipes define the age of the Subunit IIB/IIIA boundary. One of two outliers in Subunit IIB is the LO of L. redondoensis that was determined by rare occurrence of this species, which might suggest reworking (see Table T7). Another apparent inconsistency in the LO of Thalassiosira schraderi in Subunit IIB might be due to uncertainty in age assignment (7.6 Ma; Yanagisawa and Akiba, 1998), which was estimated as 7.0 Ma from the marginal sea by Koizumi (1992). For Subunits IIIA and IIIB, because the constraints were sparse after the removal of data taken from the intervals of flow-in disturbance, the age of the Subunit IIIA/IIIB boundary was determined to minimize the inflection point within the lower limit defined by the last common occurrence (LCO) of Lychnocanoma magnacornuta and the upper limit defined by the FO of C. nakasekoi (Fig. F67A; Table T22).

Sedimentation rates at Site U1425 range from 25.7 to 64.8 m/m.y. and are lower in Subunits IB, IIA, and the middle of Subunit IIB; moderate in Subunit IA; and higher in the upper and lower parts of Subunit IIB and Unit III (Fig. F67B). Low sedimentation rates in Subunit IIA and the middle of Subunit IIB are associated with very low GRA density, which suggests a decrease of detrital flux and relative increase of diatom fraction. Relatively high sedimentation rates in Subunits IIB and IIIA are associated with lower GRA density, which suggests that the diatom flux was higher during these periods. The lower sedimentation rate in Subunits IB and IIA, which is associated with an increase in GRA density, may indicate a decrease in diatom productivity and relative increase of the detrital fraction. An ensuing increase in sedimentation rate in Subunit IA keeping GRA density high suggests increased flux of detrital material during this period. Although possible compaction because of the dissolution of porous diatom frustules and the precipitation of opal-CT is suggested from the higher GRA density in Subunit IIIB (see “Lithostratigraphy” and “Biostratigraphy”), the sedimentation rate is rather higher here than in Subunit IIIA. Subunit IIIB is also characterized by higher NGR (Fig. F66G), which suggests a more enhanced detrital flux at that time.

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