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doi:10.2204/iodp.proc.342.110.2014

Stratigraphic correlation

Sampling splice

We constructed a splice for Site U1409 that is stratigraphically continuous from ~0 to ~130 m core composite depth below seafloor (CCSF) and ~150 to 190 m CCSF (Fig. F41). From ~130 to 150 m CCSF, poor recovery associated with the change to XCB coring prevented the generation of a continuous splice. Hole U1409A spans the thickest sediment column recovered at this site, with a maximum depth for the bottom of Core 342-U1409A-26X of 200.1 mbsf (231.63 m CCSF). Holes U1409B and U1409C extend to 170.5 m mbsf (192.32 m CCSF) and 160.8 m mbsf (179.85 m CCSF), respectively. We based our correlation and splice construction on magnetic susceptibility and NGR measured on whole-round sections. Our correlation yields a growth rate of 16% for Hole U1409A, 14% for Hole U1409B, and 13% for U1409C (Fig. F42), which represents the average increase of the CCSF depth scale relative to each hole’s mbsf depth scale. The affine table (Table T23) summarizes the individual offsets for each core drilled.

Correlation during drilling operations

To aid real-time correlation at Site U1409, we assessed magnetic susceptibility and GRA bulk density data collected at 2.5 cm resolution on the Special Task Multisensor Logger before allowing cores to equilibrate to room temperature. A clear signal in magnetic susceptibility allowed reliable correlation during drilling operations downhole to ~120 m CCSF.

After partial strokes for Cores 342-U1409A-15H and 16H at ~143 m CCSF, with recovery of 1.5 and 0.9 m, respectively, drilling operations switched to XCB coring for Core 17X, which was a short (4.6 m) core. After 77% nominal recovery for Core 18X, Core 19X had <1% nominal recovery from a 9.6 m advance. Core 20X achieved >100% nominal recovery, whereas nominal recovery was <100% for Cores 21X through 24X, and the hole was ended after two consecutive advances of <2 m for Cores 25X and 26X.

Based on the poor recovery of Core 342-U1409A-19X and identification of the Paleocene/Eocene boundary associated with a chert layer in Core 20X, the strategy for Holes U1409B and U1409C was to core until just below the Paleocene/Eocene boundary. In Hole U1409B, we switched to XCB coring after a 0.2 m partial stroke for Core 342-U1409B-14H. The Paleocene/Eocene boundary interval was recovered in Core 18X. In Hole U1409C, we directed drilling operations to switch to XCB coring for Core 342-U1409C-15X (~143 m CCSF). Cores 15X through 19X were all short cores of ~6 m or less. Hole U1409C was terminated after recovering Core 21X, which had a 3.5 m advance and recovered nannofossil Subzones NP9a and NP9b, associated with the Paleocene/Eocene boundary, in the core catcher.

Correlation and splice construction

For stratigraphic correlation and splice construction, we used magnetic susceptibility and NGR data. These two data series showed clear, correlatable features throughout the sediment column (see “Physical properties”). Magnetic susceptibility proved to be the most useful data set for correlation from 0 to ~130 m CCSF, and NGR was most useful from ~130 to 180 m CCSF (Figs. F41, F43). We also considered line-scan core images of section halves for cores associated with the Paleocene/Eocene boundary interval from ~170 to 185 m CCSF. Our correlation is consistent with biostratigraphic and paleomagnetic results (see “Biostratigraphy” and “Paleomagnetism”).

We defined Core 342-U1409C-1H as the anchor in our splice, though a clear agreement exists between mudline cores from all three holes. Clearly correlatable, large-amplitude cycles in magnetic susceptibility are present in the upper ~25 m CCSF of the sediment column, corresponding to Pleistocene sediment (Fig. F44). Magnetic susceptibility decreases significantly below ~25 m CCSF, but clear cycles in physical properties are recognizable from ~25 to ~100 m CCSF, allowing the identification of mostly reliable tie points. The color change from pale brown to white sediment at ~115 m CCSF corresponds to a pronounced decrease in magnetic susceptibility, so the splice tie point at ~120 m CCSF is tentative. Below ~135 m CCSF, magnetic susceptibility values begin to increase, but the data are noisy because of the numerous chert layers in this interval, which are responsible for magnetic susceptibility peaks that are not easily correlatable between holes. As a result, NGR was more useful for the identification of splice tie points below ~135 m CCSF (Fig. F43). Tie points are labeled as tentative in Table T24 if they occur in sections where physical properties do not show highly distinctive features.

Poor recovery following the transition from APC to XCB coring at the first hard layer (~140 m CCSF) impeded construction of a continuous splice from ~130 to 150 m CCSF. We were able to correlate individual cores from the three holes in this interval, but insufficient coverage across coring gaps prevented us from building a splice (Fig. F43).

The splice tie point between Cores 342-U1409B-17X and 342-U1409C-20X is reliable, but the splice tie point between Cores 342-U1409C-20X and 342-U1409B-18X is tentative because both magnetic susceptibility and NGR of Sections 342-U1409C-20X-5 and 20X-6 are different from the records for Core 342-U1409B-18X. We interpreted an overlap between Cores 342-U1409C-20X and 342-U1409B-18X because

• Biostratigraphic datums suggest that these two cores are equivalent in age;

• Analysis of line-scan core images demonstrates that the lithologies of the two cores are similar; and

• The Paleocene/Eocene boundary chert layer in Core 342-U1409B-18X correlates to the chert layer in the core catcher of Core 342-U1409C-21X, suggesting that it would be inappropriate to add a large offset to Core 342-U1409B-18X in order to eliminate any overlap between Core 342-U1409B-18X and Core 342-U1409C-20X.

The Site U1409 splice (Table T24) can be used as a sampling guide, but we suggest using caution when using the splice for sampling associated with the Paleocene/Eocene boundary interval in Cores 342-U1409C-20X through 342-U1409B-18X, given the uncertain splice tie point between Holes U1409C and U1409B.

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