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doi:10.2204/iodp.proc.346.110.2015

Stratigraphic correlation and sedimentation rates

During drilling operations, real-time tracking of the relative positions of core gaps in the three holes at Site U1430 was accomplished using magnetic susceptibility and GRA data from the WRMSL and Special Task Multisensor Logger. Data were collected at a resolution of 5 cm, which was sufficient to keep up with core recovery rates. At this site, the sea state was relatively calm and gas expansion was minimal. Detailed (centimeter scale) compositing and splicing (see “Stratigraphic correlation and sedimentation rates” in the “Methods” chapter [Tada et al., 2015b]) are based on the high-resolution RGB color data (blue) recovered from the Section Half Imaging Logger at 0.5 cm resolution. For detailed discussion of these data sets, see “Physical properties” in the “Methods” chapter (Tada et al., 2015b).

Three holes were drilled at this site: two holes (U1430A and U1430B) to ~275 m CSF-A and a third hole (U1430C) to ~250 m CSF-A. After all cores were composited (Table T18), a splice (Table T19) was constructed primarily using Hole U1430B with shorter sections from Hole U1430A utilized to span the Hole U1430B core gaps. The splice spans the entire length of the overlapped intervals among the three holes from the mudline to Section 346-U1430A-28H-3, 76 cm (243.9 m CSF-A; 259.1 m CCSF-D, as defined in the “Methods” chapter [Tada et al., 2015b]) (Fig. F53). Based on the difference between the CCSF-D and CSF-A depths at the bottom of the spliced interval, expansion at this site was ~6%.

A CCSF-C scale (as defined in “Stratigraphic correlation and sedimentation rates” in the “Methods” chapter [Tada et al., 2015b]) was created for cores from Hole U1430C by mapping them into the splice using the core-log integration functionality in Correlator. Cores 346-U1430C-4H and 5H are not well correlated to cores at equivalent depths in the other two holes because of the presence of slump structures (see “Lithostratigraphy”). Construction of the CCSF-C scale is based on correlation of structure in the RGB (blue) data (Table T20).

Age model and sedimentation rates

A preliminary age model was established on the basis of all available biostratigraphic and paleomagnetic age control points (Fig. F54A; Table T21). The B/M boundary is taken as an age-depth inflection point at 34.45 m CCSF-A (0.781 Ma) because the average slope in the interval between 50 and 80 m CCSF-A is well constrained by biostratigraphic events and appears to be smaller than the slope between the seafloor and the B/M boundary.

Because the LO of L. redondoensis (5.06 Ma), the FOs of L. pylomaticus (5.30 Ma) and Thalassiosira oestrupii (5.56 Ma), and the LO of C. nakasekoi (7.40 Ma) all occurred at the same horizon (Section 346-U1430A-9H-CC; 83.45 m CCSF-A), we infer a hiatus lasting at least from 7.40 to 5.06 Ma at this horizon. The line connecting the B/M boundary and the depth-age of the LO of L. redondoensis defines a second segment of depth-age relationship, which defines the ages of the Subunit IA/IB and IB/IIA boundaries. In order to minimize the number of inflection points in the depth-age relationship below the inferred hiatus, we connected the LO of C. nakasekoi and the LO of L. renzae. The depth-age line connecting these two points defines the ages of the Subunit IIB/IIIA and IIIA/IIIB boundaries. The depth-age line that defined the sedimentation rates of Subunit IIIA was also well constrained between the LOs of D. katayamae, Cyrtocapsella japonica, and D. hustedtii and the FOs of C. nakasekoi and Lychnocanoma magnacornuta. A depth-age line connecting the LO of L. renzae and the LO of Dendrospyris sakii defines the sedimentation rates of Subunit IIIB and Unit IV. The resulting ages of the lithologic boundaries are provided in Table T21.

Sedimentation rates at Site U1430 range from 3.3 to 44 m/m.y. with an inferred hiatus at 83.45 m CCSF-A. Sedimentation rates are relatively high in Subunits IA and IIIA, moderate in Subunits IB and IIA, and low in Subunit IIIB and Unit IV (Fig. F54B). Higher sedimentation rates tend to be associated with higher GRA density, which suggests higher detrital flux. Increased detrital flux results in increased sedimentation rates and GRA density because of reduced dilution of low-density biogenic material. Although clear lithologic evidence of a hiatus was not found in Cores 346-U1430A-9H and 10H, low sedimentation rates are suggested by the glauconite-bearing condensed section found in interval 346-U1430A-9H-4, 70–150 cm, and in the corresponding intervals 346-U1430B-9H-1, 105–147 cm, and 346-U1430C-9H-2, 120 cm, to 9H-2, 25 cm (see “Lithostratigraphy”). Glauconite sand–rich intervals of Unit IV also indicate low sedimentation rates at 3.3 m/m.y.

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