Previous   |    Next

doi:10.2204/iodp.proc.346.107.2015

Stratigraphic correlation and sedimentation rates

During drilling operations, real-time tracking of the relative positions of core gaps in Hole U1426C relative to those in Hole U1426A was accomplished using magnetic susceptibility and GRA density data from the WRMSL and Special Task Multisensor Logger (STMSL). Hole U1426B (four APC cores) was dedicated to geochemical measurements, whereas Hole U1426D (11 APC cores) was drilled rapidly without regard to gap locations in order to attain additional sediment for the anticipated volume of sampling requests. Data were collected at a resolution of 5 cm, sufficient to keep up with core recovery rates. At this site, the sea state was relatively calm but gas expansion was strong, resulting in extensive voids within the liners and extrusion of sediment out the top of the core barrel on the drill floor. Although these extruded sediments were recovered and curated as part of Section 1, they were very often strongly disturbed. Some of the voids were eliminated on the catwalk by drilling numerous holes along the liner and using a plunger to close the voids by sliding sediment up or down the liner. The remaining voids were left intact, whereas the whole-round sections were run through the WRMSL and STMSL. After splitting, the smaller voids within each section were further consolidated into large voids and filled with styrofoam for long-term curation. Because of these protocols, the data collected by the WRMSL and STMSL will plot differently than data collected by the remaining logging instruments. 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 (SHIL) at 0.5 cm resolution. For detailed discussion of these data sets, see “Physical properties” in the “Methods” chapter (Tada et al., 2015b).

After all cores were composited (Table T21), a splice was constructed using intervals from three of the four holes (U1426A, U1426C, and U1426D), excluding the hole dedicated to geochemistry (Hole U1426B) (Table T22). The splice (Fig. F50) spans the upper 236.23 m CCSF-D (as defined in “Stratigraphic correlation and sedimentation rates” in the “Methods” chapter [Tada et al., 2015b]) of Site U1426 (210.49 m CSF-A). The only tie point in the splice that may require further evaluation is the tie between Sections 346-U1426C-15H-4, 146.7 cm, and 346-U1426A-15H-2, 66.9 cm (141.77 m CCSF-D). Structure in the vicinity of this tie is not sufficient for an exact (centimeter scale) correlation. Based on the difference between the CCSF-D and CSF-A depths at the bottom of the spliced interval, expansion at this site was ~12%.

A core composite depth below seafloor, Method C (CCSF-C) scale (as defined in “Stratigraphic correlation and sedimentation rates” in the “Methods” chapter [Tada et al., 2015b]) was created for cores that were not selected for use in the splice. This differs slightly from the approach at previous sites (e.g., Site U1423) where if part of a core was used in the splice, the remaining parts were assigned CCSF-C depths as well. The strong expansion (voids and Section 1 core disturbance) at this site preclude accurate assignment of CCSF-C depths to most intervals not used in the splice. Construction of the CCSF-C scale, where applicable, is based on centimeter-scale correlation of structure in the RGB (blue) data. These CCSF-C depth maps (CSF-A to CCSF-D) are provided in Table T23.

Age model and sedimentation rates

A preliminary age model (Fig. F51) was established on the basis of all available biostratigraphic and paleomagnetic age control points. For details, see “Biostratigraphy” and “Paleomagnetism.” At this site, line segments connecting all the paleomagnetic events, the FO of H. parviakitaensis, and the rapid increase of S. arachnea group yield a reasonable depth-age relationship (Fig. F51A) with three exceptional outliers including the LO of N. koizumii, the FO of N. seminae, and the LO of T. temperei (Table T4). Sedimentation rates in lithologic Subunit IA (~1.3–0 Ma, 125–0 m CSF-A) are ~110 m/m.y. with a high (~195 m/m.y.) interval in the lower part, whereas those in lithologic Subunit IB (~2.9–1.3 Ma, 283–125 m CSF-A) are less variable, from ~90 to ~140 m/m.y. Sedimentation rates in Unit II (~4.5–2.9 Ma, 396–283 m CSF-A) are ~90 m/m.y. in the upper part to the middle and decreased (at ~15 m/m.y.) in the lower part (Fig. F51B).

Top of page   |    Previous   |    Next