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doi:10.2204/iodp.proc.346.103.2015

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 U1422 in an effort to establish a continuous sediment sequence. Only one core each was recovered from Holes U1422A and U1422B. Hole U1422C was cored to APC refusal, which occurred at 205.2 m CSF-A at the base of Core 346-U1422C-31H. Cores 346-U1422C-15H through 31H were cored using the half APC coring system, which uses a 4.7 m long core barrel. Holes U1422D and U1422E were cored using the standard APC system to refusal at 141.8 (base of Core 346-U1422D-16H) and 111.6 m CSF-A (base of Core 346-U1422E-14H), respectively. Splicing among these holes enabled us to construct a continuous stratigraphic sequence from the seafloor to the bottom of Core 346-U1422C-18H (144.1 m CSF-A), with the exception of two gaps. Deeper than 144.4 m CSF-A, only sediment from Hole U1422C was recovered.

Construction of CCSF-A scale

Definition of top (0 m CCSF-A)

Holes U1422C and U1422D both recovered the mudline. We selected the longer Core 346-U1422D-1H as the anchor core 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 U1422 is based on the correlation of magnetic susceptibility and GRA data from the WRMSL, 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]). Magnetic susceptibility and GRA bulk density were measured at 2.5 cm intervals, whereas B was calculated at 0.5 cm intervals.

Correlative horizons are most easily identified in the magnetic susceptibility and B data (Fig. F39). Extremely fine scale correlations are best achieved using the 0.5 cm B data at this site. The depth offsets used to create the CCSF-A scale are given in the affine table (Table T20).

An ~5 m section within Hole U1422D was replicated as the result of raising the bit between APC strokes at Cores 346-U1422D-1H and 2H, likely resulting in recovery of the borehole sidewall. As a result, interval 346-U1422D-1H-3, 44 cm (3.44 m CSF-A), to 1H-6, 45 cm (7.95 m CSF-A), is equivalent to interval 346-U1422D-2H-1, 0 cm (8.1 m CSF-A), to 2H-4, 40 cm (13 m CSF-A).

Core gaps

Partial strokes of the standard APC occurred at 77.8 and 78.3 m CSF-A during coring of Cores 346-U1422C-9H and 346-U1422D-9H, respectively, which prevented full recovery at these depths. The third attempt, using the half APC in Hole U1422E, recovered only disturbed sediment in Core 346-U1422E-9H, which includes flow-in structure below 1.8 m. Because the top 0.95 m of Core 346-U1422D-10H and the bottom 3 m of Core 346-U1422E-9H were disturbed with fall-in and flow-in, respectively, at least 0.95 m of sediment below ~84 m CCSF-A was not recovered (Table T20; Fig. F40B).

Drilling disturbance (flow-in) in Core 346-U1422D-13H at 113.3 m CSF-A and APC refusal of Core 346-U1422E-14H at 111.6 m CSF-A, as well as its poor recovery, prevents us from estimating the size of the core gap between Cores 346-U1422C-12H and 13H at 106.3 m CSF-A (Table T20; Fig. F40C).

Hole U1422C was the only hole at this site drilled deeper than 144.1 m CSF-A (bottom of Core 346-U1422C-18H). Because this site was not logged, no composite depth could be established for the interval from Cores 346-U1422C-19H through 31H (Fig. F40C, F40D).

Summary

Based on two core gaps and single hole coring of the interval from Cores 346-U1422C-19H through 31H, the CCSF-A scale given to Site U1422 cores is divided into four segments (Table T20; Fig. F40):

• Segment 1 consists of Cores 346-U1422A-1H, 346-U1422B-1H, 346-U1422C-1H through 9H, 346-U1422D-1H through 9H, and 346-U1422E-1H through 9H and extends from 0 to ~84 m CCSF-A.

• Segment 2 consists of Cores 346-U1422C-10H through 12H, 346-U1422D-10H through 12H, and 346-U1422E-10H through 13H and extends from ~84 to ~116 m CCSF-A.

• Segment 3 consists of Cores 346-U1422C-13H through 18H, 346-U1422D-13H through 16H, and 346-U1422E-14H and extends from ~115 to ~156 m CCSF-A.

• Segment 4 consists of Cores 346-U1422C-19H through 31H and extends from ~156 to 217 m CCSF-A.

Construction of CCSF-D scale

As for Segments 1 and 2, either a combination of Holes U1422C and U1422D or of Holes U1422D and U1422E covered all the stratigraphic intervals of each segment independently. In order to avoid whole-round sampling intervals and minimize inclusion of disturbed intervals, we selected Holes U1422D and U1422E to construct the primary splice for Segments 1 and 2. However, a flow-in disturbance in Core 346-U1422D-7H was aligned to the core gaps between Cores 346-U1422E-6H and 346-U1422E-7H, which forces us to use Core 346-U1422C-7H for this interval (Table T21). The splice for Segment 3 is constructed from Holes U1422C and U1422D, and the spice intervals are listed in Table T22. A splice for Segment 4 cannot be constructed and the depth scale is only provided as CCSF-A, assuming the same offset to all the cores from Cores 346-U1422C-19H through 31H (Table T20).

Sedimentation rates

Sedimentation rates at Site U1422 were estimated based on datums provided in biostratigraphy and paleomagnetism (see “Biostratigraphy” and “Paleomagnetism”). In order to integrate the data taken from Holes U1422C–U1422E, the composite depth scale (CCSF-A) was used here. Datums used in the estimation of sedimentation rates are plotted on Figure F41A and listed in Table T22.

Paleomagnetic datums constrained the sediment ages well for the lower part of Subunit IA and Subunit IB. Between 19.9 and 23.7 m CCSF-A, the lower and upper limits defined by the LO of Spongodiscus sp. and the FO of E. huxleyi narrow a possible range of depth-age relationships, which was selected as an inflection point (Table T22). Between 158.3 and 163.0 m CCSF-A, the lower and upper limits defined by the LO of N. kamtschatica and the FO of C. davisiana narrow a possible range of depth-age relationships, which was selected as a tie point of depth-age lines in Unit II. Thus, the most likely depth-age relation was determined, which also gave the ages of lithologic unit boundaries such as 1.4 and 2.1 Ma for the Subunit IA/IB and Subunit IB/Unit II boundaries, respectively.

Assuming constant sedimentation rates between selected tie points of depth-age lines, the calculated sedimentation rate of Site U1422 ranges from 32.0 to 95.7 m/m.y. (lower to moderate in Unit I and higher in Unit II) (Fig. F41B). The higher sedimentation rate in Unit II may result from the frequent turbidite layers. A slightly lower GRA density in Unit II also suggests relatively higher diatom content during these periods. Higher diatom flux may also contribute to the higher sedimentation rate in Unit II (see “Lithostratigraphy” and “Biostratigraphy”).

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