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doi:10.2204/iodp.proc.339.107.2013

Stratigraphic correlation

The meters composite depth (mcd) scale for Site U1389 was based primarily on correlation of magnetic susceptibility between holes in the upper 190 mcd and on a combination of NGR and susceptibility below this depth. Logging NGR data (referred to as HSGR) in Hole U1389A, lithostratigraphic logs, and correlative features in susceptibility and NGR data from Sites U1385–U1387 were used to help guide the correlation in more complex intervals.

The correlation between Holes U1389A–U1389D is relatively straightforward downhole to 240 mcd (Figs. F49, F50). Correlative features are sparse from 240 to 320 mcd, and the resulting mcd scale is more uncertain in this interval. Distinct susceptibility and NGR anomalies can be confidently correlated between Holes U1389A and U1389C in the interval from 320 to 385 mcd. Correlation from 385 mcd to the base of the multi-cored section at 404 mcd is poor because of poor core recovery in this interval in Holes U1389A, U1389C, and U1389E.

Composite section completeness is largely a function of the coring system used and the number of times an interval was cored. The upper 10 m of the section was quadruple APC cored and is complete. The uppermost cores from Holes U1389A, U1389C, and U1389D recovered the mudline, although invariably a few centimeters of the uppermost and soupiest material may be missing or disturbed in any of these cores. We shift Cores 339-U1389A-1H and 339-U1389D-1H downward 14 and 20 cm, respectively, relative to Hole U1389C to align several prominent susceptibility anomalies below 2.5 mcd. This should not be used to infer that the upper portions of the cores are missing the mudline; rather it is more likely an indication of differential expansion.

Triple APC coring in Holes U1389A, U1389C, and U1389D extends downhole to APC refusal depth of ~107 mcd (~97 mbsf). A virtually complete composite section was constructed in this APC-cored interval, with the exception of three possible small gaps at about 58, 68, and 78 mcd, where the base of Cores 339-U1389A-7H through 9H, 339-U1389C-7H through 9H, and 339-U1389D-8H through 10H are roughly aligned.

Below 107 mcd, the stratigraphic section was double-cored with the XCB system downhole to 355 mbsf (~393 mcd) in Holes U1389A and U1389C, with RCB coring in Hole U1389E overlapping the XCB-cored interval from 335 to 355 mbsf. Several relatively short gaps (~2 m to a few centimeters) exist in the composite section below the base of Cores 339-U1389A-13X and 339-U1389C-12X (~120 mcd) as a result of incomplete recovery in some cores, some unfortunate aligning of coring gaps between the holes, and relatively ubiquitous biscuit-type coring disturbance. The most notable of these short gaps are at about 122, 145, 164, 212, 225, 235, 243, 254, 335, 347, and 357 mcd. A gap of several meters likely exists in the composite section at ~372–380 mcd, which is the coring break between the base of Cores 339-U1389A-36X and 339-U1389C-36X and the tops of 339-U1389A-37X and 339-U1389C-37X. Part of gap may be filled by core from Hole U1389E, but no confident correlations can be established between any of the cores from Hole U1389E and those from Holes U1389A or U1389C. We therefore did not include data from Hole U1389E for stratigraphic correlation purposes (see “Lithostratigraphy”). The last, clearly correlative susceptibility and NGR anomalies occur in Cores 339-U1389A-37X and 339-U1389C-37X. Thus, below the base of these cores (~385 mcd), the sizes of the gaps that occur in the composite section are very uncertain. Below 404 mcd (~355 mbsf), the section is only single-cored, so some material is not recovered between each core.

Although the exact amount of material missing cannot be determined precisely 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 has shown that expansion between mcd and mbsf scales is ~7%–15%. Hence, a similar expansion should be expected at Site U1389. 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. Second, susceptibility and NGR records from Site U1389 correlate with those records from Sites U1385–U1387. Furthermore, logging NGR data from Holes U1389A and U1389E correlate well with NGR data from Site U1389 cores (Fig. F41). Using these correlations, we can estimate the relative amount of material missing within the coring gaps and adjust the mcd scale by an appropriate amount.

The offsets and composite depths in Holes U1389A–U1389D are listed in Table T27. A growth factor of 1.12–1.13 is calculated by linear regression for the multicored interval at Site U1389, indicating a 12%–13% increase in mcd values relative to mbsf values (Fig. F51). Because of the very linear nature of the expansion, the mcd scale can readily be compressed by a factor 0.887 to produce a modified mbsf* scale that corresponds more closely to true coring depths while retaining the between hole correlations.

The five holes cored at Site U1389 provide enough material to produce a splice that is virtually complete in the upper 120 mcd of the section, contains only relatively short gaps from 120 to 372 mcd, has one large gap from 372 to 380 mcd, and is poorly constrained from 385 to 404 mcd (Figs. F49, F50). Below 404 mcd, the section is single-cored in Hole U1389E. The spliced record is provided for the susceptibility (Table T28), which was first cleaned by removing data affected by section- and core-edge artifacts or that occurred in disturbed intervals or gaps, as listed in Tables T14 and T29.

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