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doi:10.2204/iodp.proc.323.105.2011

Stratigraphic correlation

The composite depth scale and splice at Site U1341 was constructed from 0.0 to 374.40 m CCSF-A (as defined in "Stratigraphic correlation" in the "Methods" chapter). The splice consists of one continuous splice from the mudline to 141.32 m CCSF-A and two appended "floating" splices from 141.3 to 326.43 m CCSF-A and from 326.44 to 374.40 m CCSF-A.

The continuous splice ranges from the top of Core 323-U1341A-1H to interval 323-U1341B-14H-7, 88 cm (Tables T17, T18). The first floating splice ranges from interval 323-U1341B-15H-1, 0 cm, to 323-U1341A-34H-7, 64 cm. The second floating splice ranges from interval 323-U1341A-35H-1, 0 cm, to the base of Section 323-U1341B-39H-7. These appended intervals are supported by wireline logging data compared to core logging of NGR (see "Downhole measurements"). Additional cores below the splice are included in the composite depth framework by appending them with a constant affine value specific to each hole.

Correlations were accomplished using IODP Correlator software (version 1.655, which was replaced by version 1.656 during site operations), and some intervals were checked with digital line-scan images with Correlyzer (version 1.3.2, which was replaced by version 1.3.3 during site operations).

The composite (CCSF-A) and splice (CCSF-D) depth scales are based primarily on the stratigraphic correlation of WRMSL magnetic susceptibility data (Fig. F31), augmented with STMSL GRA bulk density data (Fig. F32) and whole-round NGR data (Fig. F33). P-wave velocity data were also collected for part of the cores but are not illustrated here.

The CCSF-A and CCSF-D scales were constructed by assuming that the uppermost sediment (the mudline) in Core 323-U1341A-1H was the sediment/water interface. A mudline was also recovered in Cores 323-U1341B-1H, 323-U1341C-1H, and 323-U1341D-1H, confirming the fidelity of the top of the recovered section. Core 323-U1341A-1H serves as the anchor in the composite depth scale and is the only core with depths that are the same on the mbsf, CCSF-A, and CCSF-D scales. From this anchor the splice was built downhole, correlating the variations in core logging data on a core-by-core basis using Correlator.

At two depth horizons there was no clear correlation between holes, and it appeared that the gaps between cores in all holes were aligned here. In the first horizon, the top of Section 323-U1341B-15H-1 (132.58 mbsf; 141.32 m CCSF-A) is appended below the bottom of Section 323-U1341B-14H-7 (132.2 mbsf; 140.97 m CCSF-A). In the second horizon, the top of Section 323-U1341A-35H-1 (309.20 mbsf; 326.43 m CCSF-A) is appended below the bottom of Section 323-U1341A-34H-7 (309.34 mbsf; 326.43 m CCSF-A).

In addition to these appended intervals, the splice includes several suspect tie points where overlap between holes was short or uncertain: the tie point at 35.91 m CCSF-A from interval 323-U1341C-4H-7, 78.15 cm (34.28 mbsf), to 323-U1341A-5H-2, 99.5 cm (33.99 mbsf), is uncertain because it occurs within an interval of sediment deformation. We selected Core 323-U1341A-5H for this interval of the splice because it was visually less deformed than cores at equivalent CCSF-A depths in other holes and its base provided good overlap with Core 323-U1341B-5H. The tie points at 48.81 m CCSF-A between interval 323-U1341B-5H-7, 18.10 cm (46.38 mbsf), and 323-U1341C-6H-1, 140.2 cm (44.9 mbsf), and at 122.69 m CCSF-A between interval 323-U1341C-13H-7, 20.4 cm (114.20 mbsf), and 323-U1341A-14H-2, 65.76 cm (119.16 mbsf), is uncertain for similar reasons. All splice tie points at composite depths below 224 m CCSF-A are tentative because the amplitude of variation in magnetic susceptibility is very low in this interval. The section below 246.87 m CCSF-A (at the bottom of Hole U1341C) was only cored in Holes U1341A and U1341B, and the lack of a third hole made it impossible to check the affine values or splice tie points. In these intervals the splice is based on NGR data rather than magnetic susceptibility data.

Although several weak points were identified in the development of composite depths and a splice at Site U1341 and this model should be tested with more detailed shore-based data, the spliced section in these intervals is supported by comparison of NGR core logging data (Fig. F34) with wireline logging data (see "Downhole measurements").

Within the splice, the composite CCSF-A depth scale is defined as the CCSF-D depth scale. Note that CCSF-D rigorously applies only to the spliced interval. Intervals outside the splice, although available with CCSF-A composite depth assignments, should not be expected to correlate precisely with fine-scale details within the splice or with other holes because of normal variation in the relative spacing of features in different holes. Such apparent depth differences may reflect coring artifacts or fine-scale variations in sediment accumulation and preservation at and below the seafloor.

The cumulative offset between mbsf and CCSF-A depth scales is approximately linear (Fig. F35). The affine growth factor (a measure of the fractional stretching of the composite section relative to the drilled interval; see "Stratigraphic correlation" in the "Methods" chapter) at Site U1341 is 1.06 between 0 and 374.4 mbsf. A few significant anomalies around this relatively uniform affine growth relationship are too large to be caused by ship heave and are unexplained. Calculation of MARs based on the CCSF-A or CCSF-D scales should account for this affine growth factor by dividing apparent depth intervals by the appropriate growth factor. After it is divided by the growth factor (accounting for the different depth intervals), this scaled depth scale should be referred to as CCSF-B.

Cores deeper than the spliced interval (i.e., Cores 323-U1341A-41H and 323-U1341B-40H through 71X) are not tied to the splice but are appended with a constant affine value of 19.95 m. MARs in this interval should not be divided by the affine growth factor because their depths are a linear transformation of drilling depths.

Although Hole U1341B was used for wireline logging, we were unable to provide detailed integration of the core data with the wireline logs because of software errors.

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