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doi:10.2204/iodp.proc.342.107.2014

Stratigraphic correlation

Sampling splice

We constructed a sampling splice for Site U1406 that is continuous downhole to ~235 m core composite depth below seafloor (CCSF). Below ~235 m CCSF, there are two floating splices from ~235 to ~247 m CCSF and ~248 to ~284 m CCSF. Shipboard physical property analysis led to an ambiguous correlation from ~20 to ~175 m CCSF (Fig. F31), and we anticipated heavy sampling requests across the EOT from ~220 to ~230 m CCSF, so we scanned archive-half sections from all three holes between ~5 and ~283 m CCSF using the Avaatech X-ray fluorescence (XRF) core scanner at Texas A&M University (Fig. F32). The sampling splice is based primarily on XRF core scanning measurements of Ca/Fe, but other elements and physical properties were occasionally used. Tie points in the upper ~20 m CCSF of the splice are tentative and based on shipboard NGR, WRMSL magnetic susceptibility, and color changes present in the core image composites. Future interpretation of XRF core scanning data may lead to adjustments in these ties. XRF Ca/Fe measurements were crucial for determining tie points below ~20 m CCSF. Real-time correlation was difficult between 20 and 200 m CCSF, where a low Special Task Multisensor Logger (STMSL) magnetic susceptibility signal and ambiguous GRA bulk density data were insufficient to determine the offset between the three holes. Hole U1406A is the deepest hole drilled at this site, with a maximum depth of 281.05 mbsf (313 m CCSF). Holes U1406B and U1406C extend to 254 mbsf (283 m CCSF) and 241 mbsf (268 m CCSF), respectively. Our correlation yields a growth rate of 11% for Hole U1406A and 12% for Holes U1406B and U1406C (Fig. F33), which represent the average increase of the CCSF depth scale relative to each hole’s mbsf depth scale. The affine table (Table T21) summarizes the individual offsets assigned to each core drilled.

Correlation during drilling operations

In the upper ~20 m CCSF of the drilled sediment column at Site U1406, distinct changes in sediment color and physical properties allowed the offset of coring gaps between holes. Below this depth, we attempted correlation during drilling operations 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. These quickly generated records were difficult to interpret because the magnetic susceptibility signal rarely exceeded the noise level, and GRA bulk density showed few distinctive features. Our experience at previous sites indicated that GRA bulk density might be unreliable on a core-by-core basis as a result of core deformation during drilling and curation. However, an overall increase in GRA bulk density with depth, combined with the top depths reported on the drill floor, provided an indication of the relative offset between holes.

Drilling switched from APC to XCB coring with Core 342-U1406A-26X after zero recovery from the previous two consecutive cores (Core 342-U1406A-24H was a full stroke, and Core 25H was a partial stroke). As a result, the strategy for Hole U1406B was to switch to XCB drilling earlier in an attempt to recover some of the missing ~10 m interval corresponding to Cores 342-U1406A-24H and 25H. Unfortunately, the final APC attempt for Core 342-U1406B-22H was also a partial stroke, and the first XCB coring attempt resulted in poor recovery (13%). In order to recover this interval in Hole U1406C, drilling switched to XCB coring for Core 342-U1406C-20X, and Cores 20X through 24X had excellent recovery with no obviously disturbed intervals (>100% nominal recovery), thereby successfully bridging the unrecovered interval in Holes U1406A and U1406B.

Correlation and splice construction

For stratigraphic correlation and splice construction, we primarily used XRF core scanner Ca/Fe data, which showed significantly clearer features than the available shipboard physical properties. Our correlation and splice is generally consistent with biostratigraphic and paleomagnetic results (see “Biostratigraphy” and “Paleomagnetism”).

In the splice table (Table T22), tie points are labeled as tentative if they occur near the top or bottom ~50 cm of a core and/or are not associated with a prominent feature in the available data. These tentative tie points fall in the upper ~20 m CCSF (where we used only physical property data to assign ties) and below ~200 m CCSF. However, we are confident that we have recovered a continuous splice across the Oligocene–Miocene transition from ~67 to 111 m CCSF and a continuous EOT in the interval spanned by Cores 342-U1406A-22H to 324-U1406C-24H from ~215 to ~235 m CCSF.

There is a large disagreement (~15 m) between our correlation and paleomagnetic chron identification for Holes U1406A and U1406C from ~67 to 82 m CCSF. Our correlation in this interval is based on XRF Ca/Fe measurements, which suggests correlation between the bottom of Core 342-U1406A-7H and the top of Core 342-U1406C-7H (Fig. F33A). In contrast, the Chron C6Bn.2n/C6Br reversal is assigned to the bottom of Cores 342-U1406A-7H and 342-U1406C-8H (Table T13). The spacing between chron boundaries in Hole U1406C mbsf depths is consistently smaller than spacing between the same chron boundaries in Hole U1406A and U1406B mbsf depths, suggesting possible differences in the thickness of the same stratigraphic intervals between Hole U1406C compared to Holes U1406A and U1406B. Postcruise XRF core scanning revealed large differences between cores from Holes U1406A and U1406B compared to cores from Hole U1406C between ~65 and ~110 m CCSF and between ~150 and ~190 m CCSF. XRF data indicate the presence of hiatuses in Hole U1406C that do not appear in Holes U1406A and U1406B. These occur between Cores 342-U1406C-6H and 7H and within Core 17H. In order to correct for the latter, we applied a correction to Core 18H rather than break up Core 17H. A number of large overlaps are also apparent between successive cores from Hole U1406C, which we interpret as relatively expanded intervals in Hole U1406C compared to Holes U1406A and U1406B (Fig. F33). The overlapping cores in Hole U1406C on the CCSF depth scale are solely the result of correlating more expanded cores in Hole U1406C to a splice that is mostly based on more compressed cores from Holes U1406A and U1406B. The drift origin of the sediment package drilled in the upper ~200 m CCSF of this site is likely responsible for the large lateral variability between holes at Site U1406.

As a result of poor recovery for Cores 342-U1406A-24H and 25H and 342-U1406B-25X, only Cores 342-U1406C-24X and 25X recover the interval from ~233 to 241 m CCSF. Core 342-U1406C-24X had ~140% recovery but showed no evidence of disturbance. As a result, we appended Core 342-U1406C-25X below 24X. Based on XRF core scanning, only a small overlap exists between Cores 342-U1406C-25X, 342-U1406B-26X, and 342-U1406A-26X; as a result, the tie point at ~243 m CCSF is tentative (Fig. F31). This correlation is also consistent with biostratigraphy, although the core catcher of Core 342-U1406C-25X belongs to nannofossil Zone NP19/NP20, the core catchers of both Cores 342-U1406A-26X and 342-U1406B-26X belong to Zone NP18 (see “Biostratigraphy”). Finally, we appended Cores 342-U1406A-31X through 34X, which have no equivalent in either Hole U1406B or U1406C, using offsets consistent with the growth rate.

The Site U1406 splice can be used as a sampling guide to recover a single sedimentary sequence, though it is advisable to overlap splice intervals at the tie points by a few decimeters (or more, where splice ties are labeled as tentative).

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