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doi:10.2204/iodp.proc.342.105.2014

Stratigraphic correlation

Sampling splice

Achieving an accurate splice using shipboard data at Site U1404 was challenging because of weak signals in the physical property data throughout the upper ~200 m of the sediment column. In order to verify the splice across the EOT, we conducted postcruise X-ray fluorescence (XRF) core scanning of the archive halves of sections from ~162 to ~313 m core composite depth below seafloor (CCSF) using the Avaatech XRF core scanner at Texas A&M University (USA). The sampling splice is based primarily on Fe/Ca ratio. Our correlation yields a growth rate of 3%–4% (Fig. F36), which represents the average increase of the CCSF depth scale relative to each hole’s mbsf depth scale. The affine table (Table T22) summarizes the individual offsets for each core drilled.

Correlation during drilling operations

We attempted real-time correlation between Holes U1404A, U1404B, and U1404C using magnetic susceptibility and GRA bulk density data collected at 2.5 cm resolution on the STMSL before allowing cores to equilibrate to room temperature; however, the quickly generated records were inadequate for guiding drilling operations because the magnetic susceptibility signal rarely exceeded the noise level. GRA bulk density data were difficult to interpret on a core-by-core basis for the majority of the drilled strata, perhaps as a result of core deformation during the drilling process. Another possible explanation for the between-hole discrepancy in GRA density data is lateral variation in the drift deposits. Despite the difficulty in correlating features between individual cores, overall GRA density increases with depth. In combination with the top-core depths reported on the drill floor, this trend provided a rough estimate of the depth relationships between the three holes while drilling. Hole U1404A spans the thickest sediment column recovered at this site, with a maximum depth for the bottom of Core 342-U1404A-36X of 299.82 mbsf. Hole U1404B extended only to 228.75 mbsf because of difficulty with APC drilling. Hole U1404C was drilled to target a shallow low-density zone thought to indicate the presence of gas hydrates and ended after three cores at 45.01 mbsf.

Tides were insignificant (<0.8 m) while drilling and the sea state was calm, ensuring comparatively low ship heave, but strong bottom currents may have added a bend to the drill string. Drilling recovered mudlines in Holes U1404A and U1404B, but the physical properties of Cores 342-U1404A-1H and 342-U1404B-1H revealed no identifiable tie point. As a result, we were unable to determine the relative offset between Holes U1404A and U1404B based on the mudline cores. The first clear correlation occurred between Cores 342-U1404A-2H and 342-U1404B-2H, indicated by an abrupt drop in magnetic susceptibility. These data demonstrated that coring gaps were roughly aligned, so we directed drilling operations to pull-up prior to shooting Core 342-U1404B-4H. This was the only adjustment to drilling operations made for the next ~150 mbsf in Hole U1404B.

The overall downhole increase in GRA bulk density allowed us to estimate the offset between cores leading up to the EOT, thought to occur between Cores 342-U1404A-23H and 24H. To bridge this critical gap, we directed drilling operations to pull-up prior to shooting Core 342-U1404B-18H. On both occasions that we pulled up to shoot a short core (342-U1404B-4H and 18H), recovery exceeded the coring advance significantly (by 2.41 m and 3.04 m, respectively). The upper portions of these cores are highly disturbed. We interpreted the extra recovery as indicative of either fall-in at the top of the core and/or doubling of the recovered sediment, which can occur when piston coring soft sediments. We discarded data collected on all clearly disturbed intervals (Table T22).

Correlation and splice construction

For stratigraphic correlation and splice construction, we used physical property data (primarily color reflectance b*) from 0 to ~162 m CCSF (Fig. F37) and XRF core scanner Fe/Ca ratio measurements from ~162 to 313 m CCSF (Fig. F38). XRF Fe/Ca ratio data shows significantly clearer features than shipboard physical properties, which are generally ambiguous (see “Physical properties”). WRMSL magnetic susceptibly shows no significant signal (<20 IU) or variability from ~20 to 200 m CCSF. The WRMSL GRA bulk density estimates show informative overall trends, but features of individual cores are difficult to correlate. We infer that drilling disturbance and deformation had a strong overprint on the GRA density signal, especially at the tops and bottoms of cores. Many individual cores show a GRA density record with an inverted U shape, which is due to differential compression and stretching rather than sedimentary features. Paleomagnetic and biostratigraphic results (see “Paleomagnetism” and “Biostratigraphy”) aided the stratigraphic interpretation of the EOT interval between ~200 m and 230 m CCSF.

We defined Core 342-U1404A-1H as the anchor in our splice. We interpret the clear difference between mudline cores as evidence of local erosion/deposition of sediment over short distances on the seafloor as a result of strong bottom currents (see “Lithostratigraphy”). A number of appended cores exist in the upper ~30 m CCSF of the splice, and we regard all the offsets applied to cores in the upper ~162 m CCSF of the stratigraphic column as tentative because they are based on ambiguous shipboard physical properties.

We included one of the cores drilled in Hole U1404C in the splice, though we added a large (15.5 m) offset to Core 342-U1404C-2H from the nominal mbsf top depth. We are confident that this offset is justified because Core 342-U1404C-2H has the low GRA density, low magnetic susceptibility, and low NGR characteristics of Core 342-U1404B-4H and below. In addition, the clear uncertainty in the seafloor depth demonstrated by the operational difficulty in achieving mudline cores for Holes U1404A and U1404B means that the assignment of the seafloor depth for Hole U1404C was likely inaccurate, especially because we did not recover a mudline core.

Below the prominent change in physical properties between Cores 342-U1404A-2H and 3H and between Cores 342-U1404B-2H and 3H, most shipboard physical properties show weak signals between ~18 and 160 m CCSF. Moreover, some apparently prominent features occur in only one hole. For instance, Cores 342-U1404A-6H through 9H show relatively large amplitude changes in color reflectance b* that do not appear in the equivalent cores in Holes U1404B or U1404C (Fig. F37). In the splice table (Table T23) we labeled tie points as tentative if

1. They occur near the top or bottom ~50 cm of a core,

2. They are based on general trends rather than clearly correlative features, and

3. he tie yields a poor correlation in at least one of the available physical property data sets.

We have more confidence in the splice below ~160 m CCSF as a result of XRF core scanning measurements. As a result of these data sets, combined with supporting biostratigraphic and paleomagnetic evidence (see “Biostratigraphy” and “Paleomagnetism”) we are confident that Core 342-U1404B-24H bridges the gap between Cores 342-U1404A-23H and 24H, suggesting complete recovery across the Eocene/Oligocene boundary. Following Core 342-U1404A-24H, we appended Core 342-U1404B-26H.

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