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Stratigraphic correlation

Hole U1352D, located ~20 m from Hole U1352B, was cored as a dedicated APC hole to ~128 m and no whole-round samples were taken. This hole was cored specifically to provide an independent record of Site U1352 that could be correlated with Holes U1352A and U1352B. These three overlapping records provide an opportunity to construct a spliced stratigraphic record against a common core composite depth below seafloor (CCSF) depth scale (see "Stratigraphic correlation" in the "Methods" chapter). MSL and NGR data were used to facilitate correlation, and further appraisal of correlations was carried out using color and GRA bulk density records. Because of whole-round sampling of Hole U1352A, coupled with its relatively short length and an analysis of its correlation potential in the software Correlator, the stratigraphic record from this hole was not required for correlation purposes. NGR and MSL data from Holes U1352B and U1352D share a number of key features, allowing correlative ties between the holes to be made (Fig. F75). The depths of these correlative features (in CSF-A) often differ between the two holes, requiring depth shifts of individual and/or multiple cores of as much as 7.63 m. Cumulative depth adjustments made to cores from Site U1352 are provided in Table T29. Unless they are erroneous, these large adjustments strongly suggest that the stratigraphic record of both holes is characterized by localized differences in the sedimentation records of the two localities. Depth adjustments of this size are much larger than is typical for correlation of pelagic sediments, reflecting the fact that the sedimentation and/or postsedimentation history of the Site U1352 continental slope environment was extremely dynamic. Equally, however, the validity of the composite depth–adjusted records for each hole needs to be treated with caution because of these unusually large depth shifts. Additionally, correlation tie points are generally ambiguous in one way or another, making the correlation fundamentally uncertain. The lack of unambiguous correlation ties between the two holes may stem from a variety of sources, including genuine inconsistencies in the physical properties of contemporaneous strata, inaccuracies/imprecision in NGR and MSL data, and drilling disturbances that affect the fidelity of the data (see "Stratigraphic correlation" in the "Methods" chapter).

The spliced record presented in Figure F75 was constructed using Hole U1352B as the basis of the record because it was deemed to be the most complete. The splicing in of parts of Hole U1352D was performed only over limited intervals as close as possible to correlative ties. For the final spliced record, only four relatively short intervals were spliced in from Hole U1352D to cover either recovery gaps or suspected intervals of relative incompleteness in Hole U1352B.

Correlation between Holes U1352B and U1352D in the uppermost ~90 m was relatively straightforward and required only minimum adjustments of core depths in each hole. Two tie points in the uppermost 10 m suggest that ~4 m of strata may be missing in Hole U1352D between two peaks in the NGR record; the MSL data also support this interpretation. Because the standard shipboard protocol permits depth adjustments only to entire cores, only one point per core can be tied to a point in another core (see "Stratigraphic correlation" in the "Methods" section). This means that the depth adjustment of Hole U1352D at the ~7 m tie does not align the prominent low in NGR values at ~11 m in both holes because this feature is in the same core as the tie point (at ~7 m). Between ~64 and ~67 m, the stepwise shift to lower NGR values and the trough in MSL data suggest that Hole U1352B is missing ~2 m of strata relative to Hole U1352D. Edge effects and shell-hash cave-in complicate the records farther downhole, and care was therefore taken that these artifacts were not used to aid in correlation.

A tie at ~102 m was made at the base of a prominent trough in the NGR values of both records, but the known edge effects associated with these measurements on the Whole-Round Multisensor Logger means that this tie also takes into account the overall trend to lower values in the two data sets between ~67 and ~100 m. A smaller depth adjustment was made in Hole U1352B at ~89 m to correlate to a prominent peak in the NGR data of Hole U1352D (Fig. F75).

Note that the depth shift in Hole U1352D that was needed to create the tie made at ~102 m is potentially erroneous because it is particularly large (~8.3 m) and a good match between the MSL records of the two holes could not be ascertained. In contrast, depth shifts made to both Holes U1352B and U1352D above 70 m resulted in a very good match between the MSL data of both holes (particularly between 50 and 65 m). The lack of correlation between MSL records below ~70 m potentially represents an important caveat to the veracity of the correlations below this depth. The use of additional data such as color and GRA bulk density below ~102 m to aid in correlation between the two holes did not provide any additional support for the ties made using NGR, and the potential correlation of these data sets is similarly equivocal. GRA bulk density measurements within individual APC cores are sometimes inconsistent because of the progressive compaction of sediment downcore and the sensitivity of this physical property to coring disturbance; hence, these data were omitted from the correlation exercise.

An additional tie was made at ~122 m between NGR peaks present in both Holes U1352B and U1352D (Fig. F75). Another peak in both data sets at ~118 m also ties as a consequence of this depth adjustment. A prominent low in the NGR record of Hole U1352D at ~135 m (which is not considered to represent an edge effect) can be spliced into the Hole U1352B record to cover a recovery gap in Hole U1352B based on the correlation of a peak in NGR at ~131 m. As with the depth shift made to create the tie at 102 m, these correlations provide little correlation in the MSL records, and thus these ties should be treated with caution.

Overall, the correlation between Holes U1352B and U1352D emphasizes the difficulty of correlating continental slope sediments within the constraints of the correlation approach used (see "Stratigraphic correlation" in the "Methods" chapter) and the fact that both records are potentially stratigraphically incomplete relative to each other.