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doi:10.2204/iodp.proc.346.108.2015

Stratigraphic correlation and sedimentation rates

A composite section and splice “Stratigraphic correlation and sedimentation rates” in the “Methods” chapter [Tada et al., 2015b]) were constructed for Site U1427 to establish a continuous sediment sequence using Holes U1427A, U1427B, and U1427C, which were cored to 548.6, 405.6, and 351.79 m CSF-A, respectively. Splicing among these three holes enabled us to construct a continuous stratigraphic sequence for the interval from the seafloor to the bottom of Core 346-U1427A-63H (407.3 m CSF-A). From that depth downhole, only sediment from Hole U1427A was recovered.

Construction of CCSF-A scale

Definition of top (0 m CCSF-A)

Holes U1427A–U1427C all recovered the mudline. We selected the longer Core 346-U1427C-1H, where no whole-round sampling was conducted, as the anchor core and defined the top as 0 m core composite depth below seafloor, Method A (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 U1427 is based on correlation of magnetic susceptibility and GRA density data from the WRMSL and the Special Task Multisensor Logger (STMSL), as well as RGB blue (B) data extracted from images acquired by the Section Half Imaging Logger (SHIL) (see “Physical properties” in the “Methods” chapter [Tada et al., 2015b] for details). Magnetic susceptibility and GRA density were measured at 5 cm intervals for Hole U1427A and at 10 cm intervals for Holes U1427B and U1427C, whereas B was recovered at 0.5 cm intervals.

Correlative horizons were primarily identified using the B data, and confirmed with magnetic susceptibility data. The cores at Site U1427 were frequently fractured because of gas expansion; therefore, GRA density data were difficult to use for precise pattern matching. The typical spatial scale of the pattern matching of B data was several tens of centimeters because of a relatively homogeneous lithology. Fine tuning of correlation was possible using ash-bearing horizons and ash layers correlative among Holes U1427A–U1427C (see Table T3).

All the cores to 407.3 m CSF-A (bottom of Core 346-U1427A-63H) were well aligned and prepared to construct a splice record (Fig. F56A, F56B, F56C). Because only Hole U1427A was drilled deeper than 407.3 m CSF-A, the CCSF-A depth scale was provided to Cores 346-U1427A-64H through 87X, applying a constant offset of 22.45 m. The vertical depth offsets used to create the CCSF-A scale are given in the affine table (Table T21).

Construction of CCSF-D scale

A combination of Holes U1427A–U1427C cover the complete stratigraphic section to 407.3 m CSF-A (430.34 m CCSF-A). We constructed a splice primarily using Holes U1427B and U1427C, avoiding whole-round sampling in Hole U1427A intervals and minimizing inclusion of disturbed intervals as much as possible. Because Hole U1427C was only recovered to 365.41 m CSF-A (370.80 m CCSF-A), Holes U1427A and U1427B were used for splicing from 364.33 to 430.34 m CCSF-A. Selected splice intervals are listed in Table T22, and a sample splice of NGR data is shown in Figure F56A, F56B, and F56C.

Although the cores deeper than Core 346-U1427A-63H were not included in the splice, the NGR profiles from Cores 346-U1427A-64H through 87X were compared to the total spectral gamma ray (HSGR) profile from paleo combo logging (Fig. F56D; see “Downhole measurements”). Even the fine fluctuation patterns are well correlated downhole to 512.88 m CCSF-A (bottom of Core 346-U1427A-81H). We infer that no significant core gaps exist beyond the general size of gaps identified in the spliced section above (Fig. F56D).

Age model and sedimentation rates

A preliminary age model was established on the basis of all available biostratigraphic and paleomagnetic age control points. All the identified age datums were plotted on Figure F57A and listed in Table T23. The Brunhes/Matuyama (B/M) boundary is situated nearly in the middle of the whole sequence, helping to define the average sedimentation rate. Above the B/M boundary, age-depth lines are set between the constraints defined by the LOs of G. ruber (pink) and N. kagaensis group and the FO of E. huxleyi. In order to minimize the number of inflection points within a lithologic subunit, we defined the age of Subunit A1/A2 boundary as 0.5 Ma. Although this age-depth relationship might violate the constraint given by the LO of P. lacunosa, this event was regarded as reworked and could be set at a deeper horizon (see “Biostratigraphy”). Below the B/M boundary, the Bc of R. asanoi and the LO of Gephyrocapsa (>5.5 µm) narrowed a possible depth-age line. Although the LO of H. sellii was an apparent outlier, the age of disappearance of this species was diachronous and might be younger by ~1.2 Ma in this marginal sea (Muza, 1992). The resulting age of the bottom of the hole is 1.4 Ma (Table T23).

Sedimentation rates at Site U1427 range from 264 to 630 m/m.y. and are lower in Subunit A1, moderate in lower Subunit A2, and higher in upper Subunit A2 (Fig. F57B). Higher sedimentation rates tend to be associated with higher GRA density, which suggests the higher detrital flux made the sedimentation rate and GRA density increase.

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