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doi:10.2204/iodp.proc.317.106.2011

Stratigraphic correlation

Holes U1354A, U1354B, and U1354C were drilled ~20 m from each other, with Hole U1354A being dedicated to whole-round sampling for geochemical analyses. The drilling of three holes at Site U1354 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). Magnetic susceptibility (MSL) and NGR data were used to facilitate the correlation of cores at Site U1354. L* reflectance 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 all holes, allowing correlative ties to be made (Fig. F43). The depths of these features (expressed in m CSF-A) are often different between the holes and require depth shifts of individual and/or multiple cores by as much as 2.18 m. This demonstrates that the stratigraphic record for all holes is characterized by localized differences in sedimentation history, as was also found at Sites U1352 and U1353. Because Site U1354 was drilled on the continental shelf, this observation fits with the interpretation that the sedimentation and/or postsedimentation history of this environment was extremely dynamic.

The correlation presented here suggests that Hole U1354B is incomplete relative to Hole U1354A, and depth shifts of cores from Hole U1354B are hence necessary to align tie points between each hole. The correlated holes with depth shifts are plotted in Figure F43, and depth adjustments made to cores are provided in Table T22. A key correlative feature of Holes U1354A and U1354B is the geomagnetic polarity reversal interpreted to include the Brunhes/Matuyama boundary, which occurs at ~71.3 m CCSF (see "Paleomagnetism"). This reversal and its associated lithologic boundary have a distinctive expression in the NGR and MSL data, allowing the event to be unambiguously correlated between the two holes (Fig. F43). Cores from Hole U1354C are below the depth of the Brunhes/Matuyama boundary based on the correlation presented here, and this inference is supported by paleomagnetic polarity data. Below the paleomagnetic reversal, correlation between Holes U1354A, U1354B, and U1354C is complicated by artifacts in the physical property records arising from drilling disturbance such as shell-hash cave-in. Care, therefore, was taken that these artifacts were not used in correlation. One tie point at 82 m CCSF was identified in Holes U1354B and U1354C after these artifacts were taken into account.

The presence of drilling disturbance in all holes hindered the creation of a spliced composite record for Site U1354 that includes only the parts of each hole deemed to be best representative of a given depth interval. Indeed, the exclusion of clear artifacts from a spliced record could not be avoided, and only limited increased stratigraphic coverage can be gained by splicing. Furthermore, the fact that Hole U1354B is likely incomplete relative to Hole U1354A is problematic for creating a spliced record because the record of Hole U1354A is itself incomplete because of extensive whole-round sampling.

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