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doi:10.2204/iodp.proc.342.109.2014

Stratigraphic correlation

Sampling splice

Clear signals in shipboard magnetic susceptibility and shore-based X-ray fluorescence (XRF) core scanning measurements of the ratio of calcium to iron resulted in a continuous splice at Site U1408 downhole to ~234 m core composite depth below seafloor (CCSF). The expression of orbital obliquity (and/or precession) cycles between ~35 and ~220 m CCSF aided correlation and splice construction. Hole U1408A is the deepest hole drilled at this site and reaches a maximum depth of ~247 mbsf (~274 m CCSF). Holes U1408B and U1408C extend to ~212 mbsf (~233 m CCSF) and ~187 mbsf (~204 m CCSF), respectively. The overall trends and patterns in physical properties are very similar among the three holes; however, tentative tie points still exist in the splice below ~175 m CCSF, where features in the data sets analyzed are less distinctive and show lower amplitude variability. We denote tentative offsets and tie points in the affine and splice tables. We are most confident in the upper ~175 m CCSF of the correlation and consider this interval of the splice most appropriate for high-resolution sampling (Fig. F34). Our correlation yields a growth rate of 13% for Hole U1408A, 6% for Hole U1408B, and 5% for Hole U1408C. We mainly attribute the larger growth rate for Hole U1408A to core expansion and the slightly thicker sedimentary sequence compared to Holes U1408B and U1408C (Fig. F35).

Correlation during drilling operations

Using WRMSL magnetic susceptibility data generated on cores from Hole U1408A and Special Task Multisensor Logger (STMSL) magnetic susceptibility data generated on cores from Holes U1408B and U1408C, we were able to correlate among holes during drilling operations. The WRMSL data were generated on cores equilibrated to room temperature, whereas the STMSL data were generated on cores immediately after curation. We guided drilling operations to make depth adjustments while drilling Holes U1408B and U1408C to correct for short cores and missed intervals in Holes U1408A and U1408B. Clear cycles in sediment color and composition are present in the magnetic susceptibility data of the Pleistocene cover (0–15 m CCSF) and the middle Eocene drift deposits (35–225 m CCSF). These cycles, together with large step changes and trends in the physical property records, made correlation during drilling operations possible.

During drilling operations at Site U1408, weather conditions were favorable and ship heave was insignificant. A clear mudline and a Pleistocene cover of even thickness were recovered in all three holes, indicating a uniform seabed with minimal lateral variability in the uppermost part of the sequence. These clear tie points allowed for quick depth adjustments in Holes U1408B and U1408C in order to offset core gaps among the three holes. In Hole U1408B, we advanced without coring by 3 m at ~28 m CCSF, and in Hole U1408C we advanced without coring by 3 m at 9 m CCSF. Prior to shooting Core 342-U1408C-12H, we pulled up by 2.5 m to prevent alignment of core gaps with Holes U1408A and U1408B; we regard the upper part of this core (the excess recovery) as fall-in.

Correlation and splice construction

The shipboard composite depth scale and splice for Site U1408 were primarily based on WRMSL magnetic susceptibility data (see “Physical properties”). We also analyzed core photographs, NGR, and GRA bulk density data as additional lines of evidence for a few tie points. Cyclic variations were recognized in magnetic susceptibility and color reflectance data as well as by visual analysis of split core sections. Given heavy high-resolution sampling requests for Eocene material from Site U1408, we also collected shore-based Ca/Fe measurements using the Avaatech XRF core scanner at Scripps Institution of Oceanography in order to verify and/or adjust splice tie points. We scanned the archive halves of cores along the splice from ~17 to ~234 m CCSF. In order to check the splice, we also scanned additional intervals adjacent to tie points. Where tie points were tentative based on our analysis of shipboard data or fell in disturbed cores, we scanned additional material. XRF core scanning allowed for a few modifications to the originally selected tie points and also allowed us to verify tie points previously considered tentative. All remaining tentative tie points (Tables T22, T23) are the result of inconclusive XRF core scanner data as well as shipboard physical properties.

We defined Core 342-U1408B-1H as the anchor in the splice because it is the most representative mudline core recovered (Fig. F34). Pleistocene cover with very clear cyclic variability in lithostratigraphy and magnetic susceptibility is present between 0 and 15 m CCSF. Between ~15 and ~35 m CCSF, a very condensed Miocene through late Eocene sequence is present, with much lower magnetic susceptibility values compared to the Pleistocene cover. However, values are still above the noise level and sufficiently recognizable patterns enable correlation in this interval. The correlation between ~35 and ~175 m CCSF is also robust because color and lithologic cycles provide clear tie points and depth control across this interval of high sedimentation rates (see “Age-depth model and mass accumulation rates”). We consider the splice for this interval of the middle Eocene drift deposit sufficient for high-resolution sampling. The interval below ~175 m CCSF to the bottom of the Site U1408 splice at ~234 m CCSF has a large number of tentative ties. In this interval, the magnetic susceptibility pattern is different among the three holes, particularly in Hole U1408B between ~150 and ~205 m CCSF as compared to Holes U1408A and U1408C. However, we were able to use XRF Ca/Fe measurements to verify tie points downhole to ~175 m CCSF, and in general Ca/Fe are more similar between holes below this interval compared to magnetic susceptibility data. The stratigraphic interval recovered by Cores 342-U1408A-24X through 27X was not recovered in Holes U1408B and U1408C. These cores are therefore appended.

APC disturbance of cores across holes at Site U1408 results in relatively strong variability in compression and extension along the length of the cores. This is apparent in the varying spacing between, or thickness of, equivalent lithologic cycles. GRA density data often show an increase along the length of cores, indicating that the bottoms of cores are more compressed compared to the relatively expanded core tops.

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