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doi:10.2204/iodp.proc.323.104.2011

Stratigraphic correlation

The composite depth scale and splice at Site U1340 is complete from 0.0 to 59.14 m CCSF-A (as defined in "Stratigraphic correlation" in the "Methods" chapter). This splice is tentative because of some evidence for soft-sediment deformation in some of the cores, including features that do not appear to correlate between holes and, in some cases, appear to be repeated within a single hole. Some of the depth transfers implied by the affine table cannot be explained with plausible amounts of heave and likely represent real thickness variations of strata on a small spatial scale. We emphasize that this splice should be checked with more detailed shore-based data, and, because we adjusted depths for all intervals regardless of whether deformation was observed, the composite depths do not represent an estimate of depths in an undeformed sediment column and cannot easily be converted back to drilling depths. The splice ranges from the top of Core 323-U1340D-1H to the bottom of Section 323-U1340B-6H-7 (Tables T16, T17). Cores below the splice are not continuous but are appended with a constant depth adjustment. Correlations were accomplished using IODP Correlator software (version 1.655) linked with the image display software Correlyzer (version 1.3.2).

The composite (CCSF-A) and splice (CCSF-D) depth scales are based primarily on the stratigraphic correlation of WRMSL GRA bulk density and magnetic susceptibility data collected at 5 cm intervals (Figs. F35, F36). These primary correlations were assisted and verified throughout the cored interval with split-core color reflectance parameter b* (Fig. F37). We further improved the ties between holes by visual inspection of the cores and digital line-scan images (Fig. F38), which revealed marker beds such as a distinctive pink ash layers (Fig. F38C).

The CCSF-A and CCSF-D scales were constructed by assuming that the uppermost sediment (the mudline) in Core 323-U1340D-1H was the sediment/water interface; this is the longest of three mudline cores collected (also Cores 323-U1340A-1H and 323-U1340B-1H), and these three cores confirmed the fidelity of the top of the recovered section (Fig. F39). Note that in the CCSF-A scale, the tops of Cores 323-U1340A-1H and 323-U1340B-1H have negative depths at the mudline because their depths were adjusted upward to improve their correlation to Core 323-U1340D-1H, which was selected as the anchor in the composite depth scale because it is longer than the others. Core 323-U1340D-1H is the only core with depths that are the same on the mbsf and CCSF-A scales. From this anchor we worked downhole, correlating the stratigraphy on a core-by-core basis using Correlator; however, we also considered digital line-scan images, especially in complex intervals with nonhorizontal beds or where we suspected that intervals may have been repeated between nonconformable surfaces (Fig. F38).

The match between Cores 323-U1340D-1H and 323-U1340A-2H is not well constrained because of minimal overlap between these two cores (Fig. F39). The match between Cores 323-U1340B-3H and 323-U1340A-3H appears convincing (based on WRMSL GRA bulk density data), but at equivalent drilling depths this interval appears to be missing from Holes U1340C and U1340D. We infer that the same pink ash is present in Cores 323-U1340A-5H, 323-U1340B-4H, and 323-U1340D-3H (Fig. F39). The presence of similar pink ash was observed three times in Hole U1340C (twice in Core 323-U1340C-2H and once in Core 3H) in structures that appear similar in their pattern of magnetic susceptibility variations (Fig. F39). We infer that these are repeated sections caused by soft-sediment deformation. This is consistent with other evidence for tilted beds and nonconformable surfaces (see "Lithostratigraphy"). It is likely that deformation of the sediment column has produced thickened and noncorrelative intervals in the depth interval from ~20 to ~45 m CCSF-A in Holes U1340B and U1340C and perhaps also in Holes U1340A and U1340D. The full sedimentary sequence may not be represented in the composite splice for these intervals. Cores 323-U1340B-3H, 323-U1340A-4H, and 323-U1340B-4H are tentatively spliced and include the least disturbed intervals through the interval of inferred deformation (Figs. F39, F40).

The cumulative offset between the mbsf and CCSF-A depth scales is not straightforward (Fig. F41), in part because of the possibility of slumped and repeated sections and also because Hole U1340C was started 10.13 m below the mudline. Affine growth factors (a measure of the fractional stretching of the composite section relative to the drilled interval; see "Stratigraphic correlation" in the "Methods" chapter) in the spliced interval are 1.14 in Hole U1340A and 1.18 in Hole U1340B. These values are within the normal range typical of many drill sites. Affine growth factors for Holes U1340C and U1340D are not calculated given the apparent dominance of slumping at these locations.

The remaining cores in Hole U1340A (Cores 323-U1340A-7H through 71X) are not tied to the splice but are appended to the bottom of the splice with a constant affine value of 7.31 m.

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