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doi:10.2204/iodp.proc.342.106.2014

Stratigraphic correlation

Sampling splice

Achieving an accurate splice using shipboard data at Site U1405 was challenging because of weak signals in the physical property data throughout most of the sediment column. As a result, we conducted extensive postcruise X-ray fluorescence (XRF) core scanning of the archive halves of sections from 39.9 to 300.84 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 the ratio of iron to calcium in this interval (Fig. F37), with other elemental data used as necessary to select tie points. For the remainder of the correlation and splice construction (0–39.9 and 300.93–366.01 m CCSF), we relied on shipboard physical property data (Fig. F38). The correlation between Holes U1405B and U1405C is more reliable than either hole’s correlation with Hole U1405A. In order to account for clear hiatuses in all three holes, we inserted some very large gaps (>10 m). Hole U1405A is the deepest hole drilled, with a maximum depth of ~299 mbsf; Hole U1405B extends to ~214 mbsf and Hole U1405C to ~223 mbsf. Our correlation yields a growth rate of 27% for Hole U1405A, 28% for Hole U1405B, and 29% for Hole U1405C (Fig. F39), which represents 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 for each core drilled.

Correlation during drilling operations

We attempted real-time correlation between Holes U1405A, U1405B, and U1405C using magnetic susceptibility and GRA bulk density data collected at 2.5 cm resolution on the Special Task Multisensor Logger (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 (<15 IU) below ~20 mbsf. Based on our experience with similar lithologies at Site U1404, we were skeptical of relying on GRA bulk density data alone for real-time correlation, particularly because the density of individual cores may be altered during drilling and curation. A clear color change in Cores 342-U1405A-3H and 342-U1405B-3H allowed us to obtain an initial offset in coring gaps by directing drilling operations to advance ~5 m without recovery (Core 342-U1405B-41) before shooting Core 342-U1405B-5H. This was the only adjustment to drilling operations made while drilling Holes U1405B and U1405C. Using the range of S. delphix (see “Biostratigraphy”) and GRA density data, we were able to verify complete stratigraphic coverage of the Oligocene–Miocene transition between Cores 342-U1405A-20H and 342-U1405B-20H. A final complication for real-time correlation was large ship heave during Tropical Storm Debbie, which may have contributed to apparently large coring gaps.

Correlation and splice construction

For stratigraphic correlation and splice construction, we primarily used XRF core scanner Fe/Ca measurements, which showed significantly clearer features than shipboard physical properties (see “Physical properties”). We assessed additional XRF elemental data as needed. Core photographs did not aid correlation because of the homogeneity of the recovered sediment.

Measured from the individual mudlines, the change from Pleistocene to Miocene sediment (from lithostratigraphic Unit I to II; see “Lithostratigraphy”) in Cores 342-U1405A-3H, 342-U1405B-3H, and 342-U1405C-2H is present at ~18, ~21, and ~14 mbsf, respectively. The large variation in the thickness of Unit I between holes probably indicates the presence of dunelike features (mudwaves or sandwaves) in the Pleistocene surface sediment. Correlation between Holes U1405B and U1405C was more straightforward than for Hole U1405A. The two clearest tie points (the Pleistocene–Miocene hiatus and the Miocene–Oligocene transition) constrained the correlation of Hole U1405A to the other holes above ~50 m CCSF and below ~150 m CCSF. Between these depths, the stratigraphy was complicated by hiatuses in Cores 342-U1405A-7H and 20H, 342-U1405B-16H, and 342-U1405C-16H. These hiatuses complicate straightforward correlation because they are present at different stratigraphic levels in each hole. To enable correlation of the same stratigraphic intervals present in the three holes despite these hiatuses, we apply gaps in the affine table. First, in Hole U1405A we insert a 13 m gap at ~80 m CCSF and a 10.7 m gap at ~233 m CCSF. The offsets applied to account for hiatuses in Holes U1405B and U1405C are 17.5 m at ~182 m CCSF and 25.5 m at 192 m CCSF, respectively. These large offsets have a significant influence on the mbsf-to-CCSF growth rate, which is ~28% (Fig. F39). Therefore, the growth rate no longer reflects sediment expansion and differences between the advance of the drill string and curated core length alone but reflects the difference in the stratigraphy present among the three holes. Our resulting composite depth scale and splice agrees with most paleomagnetism, biostratigraphy, and physical property data (Fig. F40). However, a number of tie points and depth offsets are tentative (Tables T21, T22), especially where only physical property data are available.

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