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doi:10.2204/iodp.proc.339.105.2013

Stratigraphic correlation

The meters composite depth (mcd) scale at Site U1387 was based primarily on correlation of magnetic susceptibility between holes in the upper 153 mcd and on a combination of NGR and magnetic susceptibility data below 153 mcd. GRA density data provided confirmation of the correlation in some intervals and was used as a constraint in other intervals where susceptibility and NGR were low or lacked correlative features.

The correlation between Holes U1387A and U1387B is relatively straightforward downhole to 153 mcd, with a few exceptions (Figs. F47, F48). Two of these exceptions are the short Cores 339-U1387A-6H and 339-U1387B-9X, neither of which have susceptibility, NGR, or density anomalies that correlate particularly well with the other hole. Also, the continuity of correlation from one core to the next is problematic because the gaps between Cores 339-U1387A-7X and 16X are aligned with those between Cores 339-U1387B-6X and 15X, respectively. This unfortunate coincidence resulted even though Hole U1387B was initially spudded 4 m deeper than Hole U1387A, which was sufficient enough offset gaps in Hole U1387A that were filled by cores from Hole U1387B and vice versa. This situation changed below ~55 mcd. Drilling difficulties in Hole U1387A led to coring several short intervals rather than the typical 9.6 m core. These changes in the drilling offset between holes in combination with tidal variations resulted in the cored intervals in Holes U1387A and U1387B virtually aligning from ~55 to 140 mcd. Once physical property data had been acquired from the STMSL, short Core 339-U1387B-16X was drilled to rectify this situation. Given the calm sea conditions and relatively shallow water depth, the amount of material lost between cores is relatively small (roughly a few tens of centimeters), as can be noted in the correlation and splice (Figs. F47, F48).

Gaps are very short or nonexistent in the composite stratigraphic record from ~155 to 395 mcd, which spans from the top of Core 339-U1387B-16X downhole to the base of Core 339-U1387C-8R. Below this, the section is only single-cored, so some material is lost between each core.

Although the exact amount of material missing cannot be precisely determined for those cores where the gaps align between holes or for the gaps between cores in single-cored intervals, estimates can be made in two ways. First, prior construction of mcd scales have shown repeatedly that expansion between mcd and mbsf scales is ~7%–15%. Hence, a similar expansion should be expected. In fact, we observed that a moderately accurate mcd scale could be constructed by merely expanding the mbsf depth scale by ~10%. Ultimately, detailed correlation is necessary, but, where correlative features do not exist, using a ~10% expansion factor serves as a useful guide and provides rough estimates for the amount of material that might have been missed in the coring gaps. Also, the Site U1387 susceptibility, density, and NGR records correlate well with the Site U1386 splice, particularly in the upper 150 mbsf. Furthermore, logging NGR data (standard [total] gamma ray [HSGR]) in Hole U1387C correlates well with NGR data from the cores. Correlation with both Site U1386 and with the Site U1387 HSGR data allows us to examine the relative amount of material missing within the coring gaps and to adjust the mcd scale by an appropriate amount. Below ~150 mbsf, the Site U1386 splice is less certain (see “Stratigraphic correlation” in the “Site U1386” chapter [Expedition 339 Scientists, 2013d]), and we do not use it as a guide, although the Site U1387 HSGR data retain good correlation over the entire interval that was multi-cored.

Correlation between some cores is obvious, but expansion of one core relative to another requires that aligning one anomaly can misalign other anomalies. This is particularly evident for the correlation of Core 339-U1387A-13X with 339-U1387B-12X. The interval recovered in these two cores overlaps nearly completely, but Core 339-U1387B-12X is expanded by ~12% relative to Core 339-U1387A-13X. Below Core 339-U1387B-16X, gaps between cores in one hole are mostly filled by cores from other holes. Even so, several of the links between cores are uncertain because the susceptibility signal is relatively weak and coring disturbance (biscuiting) is pervasive. This weak susceptibility signal appears to be anomalous in several cores, possibly from minor drilling-related contamination. This appears to be the case for the base of several cores, from Core 339-U1387A-20X to the base of 25X. Hence, we rely heavily on NGR data to link the top of Core 339-U1387A-21X to the base of 339-U1387B-20X and a turbidite (occurring in intervals 339-U1387A-24X-7, 43–70 cm, and 339-U1387B-24X-4, 48–75 cm; see “Lithostratigraphy”) to link Core 339-U1387A-24X to 339-U1387B-24X. Links that rely heavily on NGR data are generally not as well constrained as when susceptibility anomalies are used because NGR data are collected at lower resolution and NGR anomalies have a relatively long wavelength (3–15 m). NGR data do, however, have roughly cyclic alternations that can be correlated between holes as well as to HSGR data. HSGR data were thus used to aid in correlating some of the more complicated intervals, such as that between 300 and 340 mcd.

The offsets and composite depths are listed in Table T21. A growth factor of 1.105 is calculated by linear regression for the multicored interval at Site U1387, indicating a 10.5% increase in mcd values relative to mbsf values (Fig. F49). Because of the very linear nature of the expansion, the mcd scale can readily be compressed by dividing by 1.105 to produce a modified mcd that corresponds more closely to true coring depths while retaining the between-hole correlations.

The three holes cored at Site U1387 provide enough material to produce a splice with relatively few gaps within the upper 395 mcd of the section, except in those intervals where coring gaps in Holes U1387A and U1387B aligned, as noted above (Figs. F47, F48; Table T22). Below this, most of the section is single-cored, and the spliced section is created by merely appending the remaining cores using a 10.4% expansion, which corresponds to adding a 1 m gap between each 9.6 m core. Spliced records are provided for susceptibility (Table T23) and NGR (Table T24), which were first cleaned by removing data affected by section- and core-edge artifacts or that occurred in disturbed intervals or gaps, as listed in Table T13.

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