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doi:10.2204/iodp.proc.317.105.2011

Stratigraphic correlation

Holes U1353A and U1353B were drilled ~20 m from each other, with Hole U1353A being dedicated to whole-round sampling for geochemical analyses. Drilling two holes at Site U1353 provides an opportunity for stratigraphic correlation and possible construction of a spliced stratigraphic record with 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 the correlation of cores at Site U1353. L* colorimetric values and GRA bulk density measurements were also used to cross-check the validity of depth adjustments. A number of key features in the analyzed data are shared by both holes, allowing correlative ties to be made (Fig. F52). The depths of these features (in CSF-A) are often different between the two holes and require depth shifts of individual and/or multiple cores by as much as 1.36 m. Depth adjustments made to cores at Site U1353 are provided in Table T22. The amount of depth adjustment required demonstrates that the stratigraphic record of both holes is characterized by localized differences in the sedimentation history of the two localities (Fig. F52), although we note that depth shifts made to cores at Site U1353 are not as large as those necessary for correlation at Site U1352.

Two sandy units between 13 and 18 m and 28 and 35 m are the key correlatable features in both holes (Fig. F52). The base of the lower sandy unit at ~35 m CCSF coincides with the base of a core in both holes, and the cores immediately below this depth are associated with shell-hash cave-in. The thickness of the sandy units in both holes is different, and it is possible that ~1.9 m of this unit is missing in Hole U1353A. A tentative correlation for the base of the deeper sand unit was thus made at the base of the cores in both holes, but this required that a correlation tie and corresponding depth adjustment be made in the underlying unit close to the shell-hash drilling disturbance. This correlation is marked with a dashed line in Figure F52.

Other anomalous values in both MSL and NGR data (abrupt increases in MSL values and decreases in NGR values), particularly below 37 m, occur as a result of drilling disturbance (i.e., shell-hash cave-in) and likely edge effects, and these features were not used to aid in correlation. The necessary depth adjustments made to the cores in both holes meant that the core ends and associated artifacts in each hole aligned with each other, and thus it was not possible to remove these intervals during the creation of a spliced record. Indeed, analysis of the composite depth records of both NGR and MSL data for both holes demonstrates that Hole U1353B represents the most relatively complete record and that no part of Hole U1353A is suitable for splicing into the Hole U1353B record to create a composite spliced record for Site U1353.

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