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doi:10.2204/iodp.proc.339.108.2013

Stratigraphic correlation

The meters composite depth (mcd) scale for Site U1390 was based on correlation of magnetic susceptibility and NGR data between Holes U1390A–U1390C. The correlation is relatively straightforward downhole to 195 mcd (~175 mbsf, which is the base of Cores 339-U1390B-19H and 339-U1390C-19H), with only a few exceptions where the gaps between cores from all three holes nearly align (Fig. F46). These gaps occur at approximately 92, 102, and 142 mcd. In all three cases, it is likely that little (a few tens of centimeters) or no material is missing. Even if no gap exists, the overlap of underlying cores with those above is too short and/or the susceptibility and NGR anomalies too indistinct to establish a firm correlation. Similarly, the quality of the correlations from 195 mcd to the base of the multicored interval at ~210 mcd is relatively poor, mainly because of lower core recovery and higher coring disturbance within this interval.

Overall, the three holes cored at Site U1390 provide enough material to produce a composite stratigraphic section that is complete with little or no gaps from seafloor downhole to the base of Core 339-U1390A-22X at 214.87 mcd (198.4 mbsf) (Table T20). The section below this depth is cored only in Hole U1390A to a total depth of 350 mbsf, with short gaps inevitably occurring between cores and larger gaps occurring where core recovery is low.

Although the exact amount of material missing cannot be determined precisely 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. Second, Site U1390 susceptibility, density, and NGR records can be correlated to other nearby Expedition 339 sites, particularly in the upper 150 mbsf. Furthermore, logging NGR data (HSGR) in Holes U1390A and U1390C correlate well with NGR data from the cores (Fig. F39).

Offsets and composite depths are listed in Table T21. A growth factor of 1.099 is calculated by linear regression for the multicored intervals in all three holes, indicating a 9.9% expansion of mcd values relative to mbsf values (Fig. F47). Because of the very linear nature of the expansion, the mcd scale can readily be compressed by dividing by 1.099 to produce a modified depth scale (mbsf*) that corresponds more closely to true coring depths while retaining the between-hole correlations. For the single-cored interval, we retained roughly the same growth factor by adding 1 m to the total offset at the top of each core from Core 339-U1390A-23X downhole.

Spliced records are provided for magnetic susceptibility (Table T22) and NGR (Table T23) data, 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 T12. Additional anomalous magnetic susceptibility data were excluded from the splice, as listed in Table T24.

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