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doi:10.2204/iodp.proc.303306.111.2006

Stratigraphic correlation

All cores recovered from Hole U1312A were measured for magnetic susceptibility at 5 cm resolution using the “Fast Track” magnetic susceptibility core logger (MSCL) soon after recovery. Below ~80 mbsf, magnetic susceptibility decreased to noise level because of the high carbonate content of the sediments. Therefore, only the uppermost 14 cores were measured with the MSCL in Hole U1312B. Initial correlation based on these data did not influence coring operations because so much of the upper portion of Hole U1312A was highly disturbed. In essence, any core recovered from Hole U1312B was important for filling both gaps between cores and long intervals from Hole U1312A that contained deformed core, mainly in the form of flow-in. Later GRA density, NGR, magnetic susceptibility, magnetic intensity, and color reflectance data became available from the various multisensor tracks and was included in building the composite section. Magnetic intensity of the NRM after AF demagnetization in peak fields of 20 mT (see “Paleomagnetism”) measured at 5 cm interval and color reflectance parameter L* measured at 2 cm intervals proved most useful for correlating between holes at Site U1312.

As noted above, coring deformation was severe in much of the upper portion of Hole U1312A, particularly in Cores 306-U1312A-1H through 5H, but also in other intervals downhole. The deformation was mainly the result of large ship heave experienced during operations in Hole U1312A. Weather conditions were more favorable during coring in Hole U1312B, which resulted in excellent core quality with the exception of Core 306-U1312B-8H and a few other core sections from deeper in the hole. As at other Ocean Drilling Program and IODP drill sites, the tops of cores were also often disturbed. A list of the disturbed intervals is provided in Table T18. For between-hole correlation and building a spliced section, we removed the data from these disturbed intervals.

Because of the core disturbance in the uppermost cores from Hole U1312A, it is not possible to construct a complete splice for the entire section, and potential breaks can occur where cores had to be appended (see Table T18). In the 0–32.5 meters composite depth (mcd) interval, the splice is built from Cores 306-U1312B-1H through 3H, and so the mcd scale for this interval is equivalent to the mbsf scale of Hole U1312B (Fig. F19). A comparison with Core 306-U1312A-2H revealed that there is probably little or no gap or overlap between Cores 306-U1312B-2H and 3H. Interestingly, significant relative compression and expansion of the sedimentary section occurs between the holes, which is particularly evident in the interval from 12 to 18 mbsf (Fig. F19). Because coring deformation is low for both holes in this interval and because the holes are only separated by 28 m, the different relative compression and expansion is surprising. Flow-in appears absent in this interval, so possibly there are real lateral variations in sedimentation rates or deformation that is not apparent to the eye.

The core break between Cores 306-U1312B-4H and 5H was filled by splicing with Core 306-U1312A-4H (Figs. F20A, F21A), but at 42 mcd Core 306-U1312B-6H had to be appended to Core 306-U1312B-5H. Another potential gap occurs at 68 mcd because the break between Cores 306-U1312A-7H and 8H could not be closed by an equivalent section of Core 306-U1312B-8H, all of which is disturbed. From 68.05 to 158.89 mcd, NRM intensity and color patterns match relatively well between Holes U1312A and U1312B, and all core breaks could be filled, resulting in a complete splice for this interval (Figs. F20B, F21B). A possible exception occurs where the basal portion of Core 306-U1312A-8H is tied to the top portion of Core 306-U1312B-9H because the between-hole correlations are poor. Directly below this, however, the basal portion of Core 306-U1312B-9H has several distinct lightness anomalies that correlate very well with Core 306-U1312A-9H.

Below 158.89 mcd, which corresponds to the bottom of Core 306-U1312B-16H, stratigraphic correlation was difficult because the high-resolution records show only minor amplitude changes because of a very uniform sediment composition. Because of this, we did not continue to build a spliced stratigraphic section, as it might mislead rather than assist future studies. Possibly other data sets collected postcruise will assist in between-hole correlation of this very homogeneous interval. In our composite section, we are able to correlate Cores 306-U1312B-24H and 25H to the long-wavelength increase in magnetic intensity evident, and more completely recovered, in Hole U1312A (Fig. F22). The correlation is crude, however, as it is only based on long-wavelength features in the signal, which probably constrain the correlation to no better than ±0.5 m.

Core offsets and composite depths are listed in Table T19, whereas the sections of core used for the splice are identified in Table F20. Further lithologic features and magnetic reversal boundaries have been identified in both holes drilled at Site U1312 and were used as constraints in the construction of the composite depth scale. These are listed in Table T21.

We noticed that marine oxygen isotope stages of the last 1.5 m.y. as displayed in the global benthic oxygen isotope stack by Lisiecki and Raymo (2005) are mirrored in the lightness parameter L*, which mainly reflects the carbonate content at Site U1312. Despite the fact that the L* splice is not complete, its stratigraphic potential is demonstrated by the preliminary correlation shown in Figure F23. Sedimentation rates derived from this correlation vary between 0.5 and 3.5 cm over the past 1.5 m.y.

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