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doi:10.2204/iodp.proc.342.111.2014

Stratigraphic correlation

Sampling splice

We constructed a sampling splice for Site U1410 that is continuous to ~226 m core composite depth below seafloor (CCSF) based on unambiguous signals in magnetic susceptibility. Physical property data at Site U1410 are very similar to those from Site U1408—orbital cyclicity superimposed on clear long-term trends provided useful patterns to correlate among the three holes. Clear trends and patterns in NGR also aided verification of the splice (Fig. F31). Hole U1410A is the deepest hole drilled at this site, with a maximum depth of ~260 mbsf (~280 m CCSF). Holes U1410B and U1410C extended to 245.2 m mbsf (265.37 m CCSF) and 243.8 m mbsf (258.51 m CCSF), respectively. We are most confident about the correlation among the three holes for the upper ~170 m CCSF. Below this depth, some tie points are more tentative because of ambiguous patterns in physical properties. Across the middle to early Eocene transition, a distinct lithostratigraphic change from green clay-rich sediments to white calcium carbonate–rich strata is accompanied by a sharp downhole decrease in magnetic susceptibility values to almost zero. As a result, correlations are tentative in the carbonate-rich interval. These tentative tie points are denoted in the offset and tie point tables (Tables T22, T23). Additional shore-based data will be used to verify the accuracy of the splice in this interval. Our correlation results in a growth rate of 6% for Holes U1410A–U1410C (Fig. F32).

Correlation during drilling operations

Based on correlation between WRMSL magnetic susceptibility data generated on cores from Hole U1410A and STMSL magnetic susceptibility data generated on cores from Holes U1410B and U1410C, we were able to guide drilling operations. Good estimates of seafloor depth enabled drilling operations to achieve the targeted lengths of mudline cores in Holes U1410B and U1410C, which were 3.8 and 6.8 m, respectively. Below the relatively thick (~35 m) Pleistocene–Pliocene cover, magnetic susceptibility values dropped; however, the signal was sufficient to guide drilling. Orbital cycles in both the Pleistocene and Eocene intervals yielded good depth control. Offsets among cores from adjacent holes decreased incrementally with depth because of varying resistance of the lithology while drilling. However, we were able to secure a continuous splice downhole to ~226 m CCSF, with only a few depth adjustments made while drilling in Holes U1410B and U1410C.

Correlation and splice construction

Orbital cyclicity in the WRMSL magnetic susceptibility data, in combination with similar, but lower resolution patterns and trends in NGR (see “Physical properties”) (Fig. F31) formed the primary signal for constructing the composite depth scale and splice. Core 342-U1410A-1H is defined as the anchor in the splice because it is the longest of the three mudline cores recovered. The clear cyclicity present in the uppermost, relatively thick (compared to other sites at J-Anomaly Ridge and South East Newfoundland Ridge) Pleistocene–Pliocene cover disappears in the very condensed Pliocene to late Eocene interval between ~35 and ~65 m CCSF. Throughout the Eocene drift deposit, between ~65 and ~228 m CCSF and corresponding to lithostratigraphic Unit III (see “Lithostratigraphy”), persistent orbital cyclicity is again present. Cycle amplitude is largest in the upper part of the sequence and drops to lower amplitudes below ~145 m CCSF. Tie points in the lower portion of the record are therefore more tentative and require shore-based verification.

At ~228 m CCSF, the transition from the middle to early Eocene is marked by different lithologies recovered in Hole U1410A with respect to Holes U1410B and U1410C. In Hole U1410A, a gradual transition in magnetic susceptibility and NGR values correspond to clear cycles in the lithology from greenish clay to white carbonate, whereas in Holes U1410B and U1410C, these cycles are absent. We interpret a gap in Hole U1410A and append Core 342-U1410A-23X to the bottom of Core 342-U1410C-23X. Below, we interpret gaps between Cores 342-U1410B-25X and 26X and between Cores 342-U1410C-23X and 24X. No operational explanation is apparent for such gaps; rather, we consider the gaps in the composite depth scale reflective of lateral variability within the formation at the middle to early Eocene transition. Relatively strong APC coring disturbance, with variable expansion-compression profiles along the lengths of the cores, is recognizable in cores across all three holes at Site U1410. Cores 342-U1410A-27X and 28X were appended because this interval was not recovered in Holes U1410B and U1410C.

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