Previous   |    Next

doi:10.2204/iodp.proc.342.104.2014

Stratigraphic correlation

Sampling splice

We constructed a sampling splice for Site U1403 using shipboard physical property data that is continuous from 0 to ~160 m core composite depth below seafloor (CCSF) and consists of a series of floating splices from ~169 to 274 m CCSF. The splice is based on GRA bulk density data in the upper ~160 m CCSF, where magnetic susceptibility is low, and on WRMSL magnetic susceptibility from ~160 to ~274 m CCSF. As a result of more ambiguous physical property data in the upper ~160 m CCSF, a number of tie points have been tentatively placed where features in the physical properties are inconclusive or the overlap between successive cores is small. Below ~160 m CCSF, several intervals were not recovered in either Hole U1403A or U1403B as a result of the presence of chert. Hole U1403B spans the thickest sediment column recovered at this site, with a maximum depth for the bottom of Core 342-U1403B-32X of 255.40 mbsf (290.23 m CCSF). Hole U1403A extends to 233.9 mbsf (262.69 m CCSF). As a result, we append the last two cores from Hole U1403B at the bottom of the splice. Our correlation yields a growth rate of 10% in CCSF relative to mbsf depth for Hole U1403A and 13% for Hole U1403B Site U1403 (Fig. F32). The affine table (Table T23) summarizes the individual offsets for each core drilled.

Correlation during drilling operations

In order to provide real-time assessment of the composite section to monitor and direct drilling, we collected magnetic susceptibility and GRA bulk density data at 2.5 cm resolution on the Special Task Multisensor Logger) soon after recovery (before allowing cores to equilibrate to room temperature). We added preliminary depth shifts to cores from both holes on the basis of these data. Despite good weather conditions and minimal tides (<0.8 m), an inconsistency in the apparent seafloor depth between the two holes led to the inadvertent alignment of core gaps between Holes U1403A and U1403B. Our attempts to achieve overlap resulted in an apparent double recovery of the same interval between Cores 342-U1403B-2H and 3H and between Cores 3H and 4H. The doubling of recovered sequences is evident through comparison of intervals 342-U1403B-2H-6, 25 cm, and 3H-1, 20 cm, in the core photographs.

Following early corrections, we were able to maintain a relatively constant offset between each hole downhole to the chert layer at ~160 m CCSF. We consider the gap between ~160 and ~169 m CCSF to be unbridgeable because of the presence of chert. Intervening chert intervals from ~160 m CCSF to the bottom of Hole U1403A at ~274 m CCSF prevented the offset of all coring gaps, but a series of prominent features in physical properties allowed reliable correlation (especially across the end of ETM2 to the PETM in Cores 342-U1403B-19H to 342-U1403A-21X and the K/Pg boundary in Cores 342-U1403A-26X and 342-U1403B-28X).

Correlation and splice construction

Stratigraphic correlation and splice construction were based on WRMSL magnetic susceptibility and GRA density data collected after equilibration on cores from both holes. We also assessed color reflectance data available from the SHMSL, NGR data, core photographs (see “Physical properties”), and paleomagnetism reversal data (see “Paleomagnetism”). Magnetic susceptibility is low (<50 IU) from ~10 to 160 m CCSF, and a few intervals show very low magnetic susceptibility (<15 IU) (e.g., ~15–18 and ~23–38 m CCSF) (Fig. F33). As a result, GRA density data generally proved most useful for the correlation of the upper ~160 m CCSF, whereas magnetic susceptibility proved most useful for correlating below this depth. The majority of tentative ties in the splice occur in the interval of lowest magnetic susceptibility, from ~25–140 m CCSF.

Table T24 lists sections of core used for the splice. We defined the mudline (where 0 mbsf equals 0 m CCSF) of Site U1403 at the top of Core 342-U1403B-1H. Although Core 342-U1403A-1H seemed to recover a clear mudline, the drill floor reported that the drill string had touched the seabed prior to the first firing of the piston. Therefore, we could not rule out a possible doubling of recovered material in Core 342-U1403A-1H, making Hole U1403A less suitable for defining the mudline. Using Core 342-U1403B-1H for the mudline definition yields negative CCSF depths for the top of Core 342-U1403A-1H.

A tentative tie occurs between Cores 342-U1403B-19H and 20H at ~173 m CCSF, where it appears that the same interval was recovered by each core. The intervals 342-U1403B-19H-4, 15 cm, and 20H-1, 10 cm, show the same clear dark brown to light yellow transition visible in core line scan images. Without an obvious tie between Cores 342-U1403A-19X and 342-U1403B-19H, we chose to tie Cores 342-U1403B-19H and 20H in the splice in order to achieve a continuous record of the ETM2. However, this tie point is tentative given the unusual practice of selecting a tie point from two cores in the same hole.

We applied a large offset (5.86 m) to Core 342-U1403A-22X, which advanced 9.7 m yet only recovered ~3 m of sediment, and we correlated Core 342-U1403A-22X to Core 342-U1403B-24X on the basis of magnetic susceptibility. We applied another large offset (4.06 m) to the top of Core 342-U1403A-26X, where despite a nominal advance of 4.8 m during the drilling of Core 25X, 8.65 m of core was recovered, so that the bottom of Core 25X had an mbsf depth of almost 4 m greater than the mbsf depth for the top of Core 26X. However, the physical property data of Cores 25X and 26X do not visually resemble each other, suggesting no overlap occurred. We added an offset to Core 26X such that there is no overlap with Core 25X (Fig. F32). Unfortunately, this gap was not bridged in Hole U1403B.

The Site U1403 splice can be used as a sampling guide to recover a single sedimentary sequence, though it is advisable to overlap splice intervals at the ties by a few decimeters when sampling to accommodate anticipated ongoing development of the depth scale. Although we did not stretch or compress cores in our splice, clear distortion of sedimentary features (especially at the top and bottom of cores) indicates additional adjustment is warranted. Much of the distortion occurs within individual cores, particularly XCB cores, so it was not possible to align every single feature in the magnetic susceptibility, GRA density, NGR, and color reflectance records. For instance, there is a strong tie between Cores 342-U1403A-19X and 342-U1403B-20H, but the top of Core 342-U1403A-19X is clearly compressed, such that not all features between the overlapping intervals from each hole align (Fig. F34). The distortion in Hole U1403A in this interval is probably a result of XCB drilling compared to the APC recovery in Hole U1403B. Given the number of significant events recovered at Site U1403, we took care to avoid the use of event beds as tie points, so that these can be sampled in one section.

Top of page   |    Previous   |    Next