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doi:10.2204/iodp.proc.320321.207.2013

Results

Correlations within Sites U1337 and U1338

The first step is to correlate core splices and downhole logs at Sites U1337 and U1338 so that both records can be placed on the same depth scale (Fig. F1). The Monte Carlo sampling started from eight nodes at Site U1337 and four nodes at Site U1338. These starting nodes were chosen to match major features in the two records; in particular, they correlate a thin 16–40 cm chert interval that was imaged in the downhole logs and that had poor core recovery. This distinctive “baby chert” was located at 240 m WMSF at Site U1337 and 281.6 m WMSF at Site U1338 (see Fig. F47 in the “Expedition 320/321 summary” chapter [Pälike et al., 2010]).

The estimated mapping functions are close to a constant expansion of the composite core depth scale that is ~12% at Site U1337 and ~11% at Site U1338 (Figs. F3, F4). These values are consistent with the 9%–16% composite depth scale expansions determined for Expedition 320 Sites U1331–U1334 by Westerhold et al. (2012). However, differences between the estimated mapping functions and constant core expansion reach about ±5 m at Site U1337 and ±2 m at Site U1338 (Figs. F3, F4). Although the magnitude of these differences is not the same, the overall pattern is similar at the two sites. A possible explanation may be lithology variations. The shallower half of the interval drilled at both sites contains a sizable siliceous component, whereas the deeper half is dominated by calcareous nannofossil ooze grading downward to chalk. These different lithologies may result in different core expansions. On the other hand, these differences are relative to the downhole log depth scale, which may itself be affected by errors (e.g., due to stretching of the wireline cable). Whatever the cause, results in Figures F3 and F4 show that a detailed correlation that goes beyond a constant core expansion is necessary to match small-scale features in the core and downhole log data.

Figures F5 and F6 show the close match of core splice and downhole log data that is achieved when both records are put on the same depth scale. The match is generally excellent, with the exception of the deepest intervals below ~350 m WMSF or ~400 m CCSF. Density variations are smaller in these deep intervals, making correlations more ambiguous. Also, in the deeper hole intervals, core recovery was lower and core conditions were poorer, making the splice data less robust. Outside of these deep intervals, the uncertainties of the mapping functions are generally <1 m (1 standard deviation; Figs. F3, F4).

Correlations between Sites U1337 and U1338

Figures F7 and F8 show the results of correlating core splice data and downhole log data between Sites U1337 and U1338. To obtain correlations that could be directly compared, the downhole log data were placed on the composite core depth scale (CCSF) using the mapping functions obtained previously (Figs. F3, F4). The Monte Carlo sampling started from six nodes for the core splice correlation and four nodes for the downhole log correlation. The site-to-site mapping functions obtained for core splice and downhole log data are illustrated in Figure F9. Once the two data sets are placed on the same depth scale, the site-to-site correlations are entirely consistent.

These lithostratigraphic correlations have the smallest uncertainties in the intervals 130–330 m CCSF at Site U1337 and 140–380 m CCSF at Site U1338. In these intervals, the match of small-scale features in the two records is excellent (Figs. F7, F8). The uncertainties of the mapping function outside this well-correlated interval reach several meters (1 standard deviation; Fig. F9). These uncertainties quantify the confidence of correlations based on lithostratigraphy. For example, a stratigraphic event observed at 400 m CCSF at Site U1338 can be correlated to a depth of 346 m CCSF at Site U1337, but this correlation has an inherent uncertainty of about ±5 m (1 standard deviation). To assist in building a composite sedimentary record in the equatorial Pacific Ocean, the mapping functions that correlate Sites U1337 and U1338 (Fig. F9) are provided in Table T1 (core splice data) and Table T2 (downhole log data).

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