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doi:10.2204/iodp.proc.346.105.2015

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 U1424 to establish a continuous sediment sequence utilizing Holes U1424A–U1424C, which were cored to 158.8, 154.7, and 63.9 m CSF-A, respectively. Because Cores 346-U1424C-1H through 3H were dedicated to OSL dating, no onboard measurements were conducted. Only Cores 346-U1424C-4H through 7H were subject to stratigraphic correlation. Splicing among all holes enabled us to construct a continuous stratigraphic sequence for the entire drilled interval.

Construction of CCSF-A scale

Definition of top (0 m CCSF-A)

Holes U1424A and U1424B both recovered the mudline. We selected the longer Core 346-U1424A-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 U1424 is based on correlation of magnetic susceptibility and GRA density 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 density were measured at 2.5 and 5 cm intervals for Holes U1424A and U1424B, respectively, whereas B was calculated at 0.5 cm intervals. Correlative horizons are most easily identified in the magnetic susceptibility and B data. Extremely fine scale correlations are best achieved using the 0.5 cm B data.

For the interval between ~125 and ~150 m CSF-A, including Cores 346-U1424A-15H and 16H and 346-U1424B-15H and 16H, physical properties such as magnetic susceptibility, GRA density, and B data show no definitive correlative patterns. Thus, we visually identified unique lithologic features correlative between the holes. An ash layer found at Sections 346-U1424A-15H-6, 65 cm (138.45 m CSF-A), and 346-U1424B-15H, 103 cm (133.23 m CSF-A), were judged to be identical based on their common white color, sharp base, gradational upward with thickness of 3–5 cm, and association with a thin ash layer ~30 cm shallower (Fig. F42A). The size and shape of volcanic glass particles are similar between the two ashes. Thus, this ash layer is composited to 142.7 m CCSF-A.

Dispersed ash in mottled diatomaceous mud shown in Figure F42B was commonly found at Sections 346-U1424A-15H-6, 110 cm (138.9 m CSF-A), and 346-U1424B-16H-1, 35 cm (136.05 m CSF-A). Though this feature is not unique and further sedimentological examination is necessary, this horizon is composited to 143.2 m CCSF-A for Holes U1424A and U1424B.

In order to tie Core 346-U1424A-16H to Core 346-U1424B-16H, we matched an ash layer (Fig. F42C) at Sections 346-U1424A-16H-1, 99 cm (140.71 m CSF-A), and 346-U1424B-16H-5, 28 cm (141.98 m CSF-A). This dark gray ash layer has a sharp base grading upward for ~3 cm above the base. This feature is composited to 149 m CCSF-A in both holes.

Cores 346-U1424C-4H through 7H are tied to cores from Holes U1424A and U1424B using the B data (Fig. F43A, F43B). The vertical offsets used to create the CCSF-A scale are tabulated in Table T17. The correlative features discussed above yield reasonable alignment of the cores. However, it implies an ~4 m large coring gap between Cores 346-U1424B-1H and 2H (Fig. F43A) and between Cores 346-U1424A-15H and 16H (Fig. F43C). An apparent ~2 m overlap is also implied at the bottom of Core 346-U1424A-14H relative to the top of 15H and at the bottom of Core 346-U1424B-15H relative to the top of 16H (Fig. F43C). These overlaps might be due to stretching of sediment during APC coring, which is consistent with a vertical linear disturbance observed in the upper 1 m of Section 346-U1424A-15H-5.

Construction of CCSF-D scale

The combination of Holes U1424A and U1424B covers the complete stratigraphic section to 158.87 m CSF-A (167.75 m CCSF-A). We constructed a splice, avoiding whole-round sampling intervals and minimizing inclusion of disturbed intervals as much as possible. Selected splice intervals are listed in Table T18. Data sets used to choose correlative features are identified in the last column.

Sedimentation rates

All age control datums, including biostratigraphic markers, paleomagnetic events, and tentatively dated tephra from Holes U1424A–U1424C, were plotted on Figure F44A and listed in Table T19. Depth-age fits determined using only paleomagnetic datums fall within the upper and lower limits defined by the FO and LO of biostratigraphic markers, except for the interval between 115 and 144 m CCSF-A. In this interval, the FO of N. koizumii strongly constrains the upper limit of the depth-age line and we set it as an inflection point, which yields the slow sedimentation rate between 137.87 and 143.55 m CCSF-A. This apparent slow sedimentation rate could be due to a hiatus as suggested by a clear shift in physical properties at around this interval (see “Physical properties”). Relatively large depth uncertainty in the identification of the Cochiti paleomagnetic event might be due to this slow sedimentation rate. The tentative identification Znp-Ohta tephra at 142.86 m CCSF-A (see “Lithostratigraphy”) also departs from the most likely depth-age relation probably due to this potential hiatus. The rapid decrease of S. arachnea group at 132.84 m CCSF-A is apparently an outlier, which may be due to the uncertainty in the age of this event (see “Biostratigraphy” in the “Methods” chapter [Tada et al., 2015b]). The resulting ages of the lithologic Subunit IA/IB, Subunit IB/IIA, and Subunit IIA/IIB boundaries and the hole bottom are at 1.21, 2.14, 2.69, and 5.12 Ma, respectively.

Sedimentation rates at Site U1424 range from 14.3 to 72.3 m/m.y. and are lower in Subunit IB and the lower middle of Subunit IIB; moderate in the upper Subunit IA, Subunit IIA, and the upper and lower parts of Subunit IIB; and higher in the lower Subunit IA and the middle of Subunit IIB (Fig. F44B). The moderate to higher sedimentation rates are associated with lower GRA density, which suggests that the diatom flux was higher during these periods. Lower sedimentation rates in Subunit IB and the lower middle of Subunit IIB are associated with maxima in GRA density, which suggests a decrease in the diatom flux and relative increase of detrital fraction (see “Lithostratigraphy” and “Biostratigraphy”).

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