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doi:10.2204/iodp.proc.341.107.2014

Stratigraphic correlation

The composite depth scale at Site U1421 was constructed from 0.0 to 695.72 m core composite depth below seafloor (CCSF-A). The splice consists of one complete and continuous interval from the mudline to 33.21 m core composite depth below seafloor (CCSF-D). The CCSF-A and CCSF-D depth scales are defined in “Stratigraphic correlation” in the “Methods” chapter (Jaeger et al., 2014).

The splice ranges from the top of Core 341-U1421C-1H (the mudline) to the base of Core 5H (Tables T9, T10). Because only three holes were cored at Site U1421 (and Hole U1421B consisted of a single disturbed core), the splice only includes cores from Holes U1421A and U1421C and contains two “appended” intervals that are not strictly continuous with adjacent intervals.

Weather was calm and ship heave was negligible while coring Site U1421, but the lonestone- and sand-rich lithologies proved challenging for core recovery and for interhole correlation. Coring was terminated when the APC system was damaged by hitting an impenetrable layer; Core 341-U1421C-6H was recovered by tripping the pipe. Intervals of core disturbance, incomplete recovery, and use of the half APC coring system all presented complications in the development of composite depths and a spliced record. In particular, we observed many instances of flow-in and other coring disturbance. We urge caution in using the splice from Site U1421.

Correlations between holes were accomplished using Integrated Ocean Drilling Program Correlator software (version 1.695), and all the splice tie points were checked with digital line-scan images using Corelyzer (version 2.0.2), linked to Correlator. During coring, real-time development of composite depths and guidance for coring operations relied on Special Task Multisensor Logger (STMSL) GRA bulk density and magnetic susceptibility data. The final composite depth scale (CCSF-A) and the splice scale (CCSF-D) are based primarily on the stratigraphic correlation of magnetic susceptibility and GRA density from the Whole-Round Multisensor Logger (WRMSL) (Figs. F15, F16), as well as whole-round natural gamma radiation (NGR) and reflectance spectroscopy color data from the Section Half Multisensor Logger (SHMSL). Of these variables, magnetic susceptibility offered the most reliable tool for correlation at Site U1421; the other variables served primarily as verification data and were generally much noisier. Anomalously low GRA density was used primarily as an indicator of core disturbance. Magnetic susceptibility from the loop sensors was checked with point-sensor data, although the presence of small lonestones in intervals of diamict made those data relatively noisy.

The CCSF-A and CCSF-D depth scales were constructed by assuming that the uppermost sediment (the “mudline”) in Core 341-U1421C-1H represented the sediment/water interface. An approximate mudline was also recovered in Core 341-U1421A-1H, confirming the fidelity of the top of the recovered interval. Core 341-U1421C-1H serves as the “anchor” in the composite depth scale and is the only core with depths that are the same on the CCSF-A and CCSF-D scales. From this anchor we worked downhole, matching the variations in core logging data on a core-by-core basis using Correlator.

The splice includes two append points. The base of Section 341-U1421A-2H-7 was appended to the top of Section 341-U1421A-3H-1 at 15.9 m CCSF-A. Additionally, the base of Section 341-U1421C-3H-7 was appended to Section 341-U1421A-4H-1 at 27.44 m CCSF-A. The splice is not strictly continuous at append points. Some disturbed material exists in the splice in Sections 341-U1421A-3H-1 and 341-U1421C-4H-1; nevertheless, we considered these intervals sufficiently continuous to warrant inclusion in the splice.

Within the splice, the CCSF-A depth scale is (by definition) identical to the CCSF-D depth scale. Note that CCSF-D rigorously applies only to the spliced interval. Intervals outside the splice, although available with CCSF-A composite depth assignments, should not be expected to correlate precisely with fine-scale details within the splice or with other holes because of normal variation in the relative spacing of features in the recovered intervals from different holes. Such apparent depth differences may reflect coring artifacts or fine-scale spatial variations in sediment accumulation and preservation at and below the seafloor.

Given the short length of the splice and difficulties during coring, we did not calculate an affine growth factor at Site U1421. We also did not calculate a compressed depth scale (CCSF-B, see “Stratigraphic correlation” in the “Methods” chapter (Jaeger et al., 2014).

Initial age model

Shipboard paleomagnetic and biostratigraphic age datums are insufficient for the construction of a detailed initial shipboard age model (see “Paleomagnetism” and “Paleontology and Biostratigraphy”). Based on radiolarian and diatom biostratigraphy, the bottom of the hole (near 700 m CSF-A) is younger than 300 ka and older than 30 ka. Paleomagnetic measurements were all representative of normal magnetic polarity. We can conclude that the entire sedimentary sequence recovered at Site U1421 is younger than 0.781 Ma and likely less than 0.3 Ma.

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