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doi:10.2204/iodp.proc.339.109.2013

Stratigraphic correlation

The meters composite depth (mcd) scale for Site U1391 was based on correlation of magnetic susceptibility and NGR data between Holes U1391A–U1391C. Both Holes U1391A and U1391B were cored to APC refusal, which occurred at ~171 mbsf at the base of Cores 339-U1391A-19H and 339-U1391B-18H, and then XCB cored to ~353 mbsf. Hole U1391C was RCB cored downhole from 340 mbsf, overlapping only the very basal part cored in the other two holes.

The correlation between Holes U1391A and U1391B is relatively straightforward downhole to 169 mcd (base of Core 339-U1391A-16H), except that the correlation of Core 339-U1391A-14H to 339-U1391B-13H is poor because susceptibility and NGR data lack distinctive features across this connection, which spans ~128–131 mcd (Fig. F40). From 169 mcd downhole to ~310 mcd, susceptibility is very low and has few features that can be correlated with confidence. Hence, NGR data are the primary constraint through this interval and generally provide distinct anomalies that can be correlated to within ~20 cm. An exception to this occurs from ~250 to 262 mcd, where the NGR signal is relatively constant, making correlation between holes very uncertain.

No gaps occur in the stratigraphic section until the base of Cores 339-U1391A-19H and 339-U1391B-18H, where both cores were unable to penetrate deeper than ~171 mbsf (i.e., the APC refusal depth), corresponding to ~196 mcd. Thus, a small gap occurs at this depth. Another small gap occurs at ~243 mcd (between Cores 339-U1391B-23X and 24X) that is not filled by core from Hole U1391A because Core 339-U1391A-24X had no recovery. Other very small gaps (less than ~30 cm) may occur at approximately 272, 283, 292, and 336 mcd.

NGR and susceptibility data for Cores 339-U1391C-2R and 3R correlate well with Cores 339-U1391A-38X and 339-U1391B-37X and 38X and those of Core 339-U1391C-4R with the base of 339-U1391B-38X. Hence, the base of Core 339-U1391C-4R marks the base of the multi-cored interval.

Overall, the three holes cored at Site U1391 provide enough material to produce a composite stratigraphic section that is complete, with few or no gaps from seafloor downhole to the base of Core 339-U1391C-4R at 417.66 mcd (358 mbsf). The section below this depth is cored only in Hole U1391C downhole to a total depth of 671.5 mbsf, with short gaps inevitably occurring between cores and larger gaps occurring wherever core recovery is low.

The offsets and composite depths are listed in Table T19. A growth factor of 1.144 is calculated by linear regression for the multicored interval in Holes U1391A and U1391B, indicating a 14.4% expansion of mcd values relative to mbsf values (Fig. F41). Because of the very linear nature of the expansion, the mcd scale can readily be compressed by dividing by 1.144 to produce a modified depth scale (mbsf*) that corresponds more closely to true coring depths while retaining the between-hole correlations. This is illustrated for NGR data in Holes U1391A–U1391C in Figure F42. For the single-cored interval, the relative gaps between cores can be estimated by using a similar growth factor (Table T19).

Splice tie points for the composite section are given in Table T20. Spliced records are provided for susceptibility (Table T21) and NGR (Table T22).