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doi:10.2204/iodp.proc.342.108.2014

Stratigraphic correlation

Sampling splice

We constructed a splice for Site U1407 that is stratigraphically continuous from ~28 to ~116 m core composite depth below seafloor (CCSF) and from ~156 to 317 m CCSF (Fig. F37). A thick unrecovered interval from ~116 to 137 m CCSF is associated with a drilling advance through unrecoverable chert. Large differences in the thickness of distinctive lithostratigraphic units in the lower portion of the sediment column complicate the choice of tie points from ~156 to 317 m CCSF, where it is impossible to align all distinctive features in physical property records. We based our correlation and splice construction primarily on NGR and magnetic susceptibility measured on whole-round sections. Line-scan core images of section halves were particularly useful for correlation from ~260 to 312 m CCSF. Our correlation yields a growth rate of 17% for Holes U1407A and U1407B and 18% for Hole U1407C (Fig. F38), which represents the average increase of the CCSF depth scale relative to each hole’s mbsf depth scale. We interpret these large growth rates to be a function of thickness variations in stratigraphy across the site at the base of the sediment column (~156–317 m CCSF), which require the application of relatively large differential offsets. The affine table (Table T21) summarizes the individual offsets for each core drilled.

Correlation during drilling operations

To aid real-time correlation at Site U1407, 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. Distinct changes in lithology and physical properties aided real-time correlation in the upper ~91 m CCSF of the drilled sediment column at Site U1407. As a result, we were able to offset our coring gaps in this interval. In particular, three step changes in magnetic susceptibility associated with color changes provided reliable ties between the three holes. These occur at the transition from the Pleistocene in lithostratigraphic Unit I to the Oligocene in Unit II at ~8 m CCSF (Fig. F39), the color change from tan to green in Unit III at ~46 m CCSF, and the color change from green to white at the boundary between Units III and IV at ~94 m CCSF. The tan to green color transition occurs at similar mbsf depths in all three holes. In contrast, the distinct green-to-white color change occurs ~10 m shallower in mbsf depth in Hole U1407C compared to Holes U1407A and U1407B. Based on correlation using physical property data between the two ties provided by the color changes, it appears that there is a gap approximately equal to the length of one core between Cores 342-U1407C-6H and 7H. No obvious justification is evident for such a large lateral discontinuity in the sediment column penetrated at Hole U1407C compared to Holes U1407A and U1407B, so we consider an unrecorded drilling advance the most likely explanation for the large mbsf depth difference in Hole U1407C.

After a partial stroke for Core 342-U1407A-12H (~116 m CCSF), Cores 13H and 14H had zero recovery and Core 15H was a 0.1 m advance. As a result, drilling operations switched to XCB coring for Core 16X. In general, recovery in the XCB interval of Hole U1407A was poor, with a low recovery of 14% for Core 17X and only Core 20X achieving 100% nominal recovery.

As a result of poor recovery, the strategy for Holes U1407B and U1407C was to advance without recovery through the chert interval from ~112 to ~136 m CCSF. In Holes U1407B and U1407C, we directed drilling operations to advance without recovery until the drillers detected that they had broken through the chert-rich interval. In Hole U1407C, we began successfully recovering XCB cores at a shallower depth, corresponding to Core 342-U1407A-16H. XCB coring yielded better recovery in Holes U1407B and U1407C compared to Hole U1407A; many cores had >100% nominal recovery.

Hole U1407A spans the thickest sediment column recovered at this site, with a maximum depth for the bottom of Core 342-U1407A-35X of 308.7 mbsf (355.7 m CCSF). However, Cores 32X through 35X, corresponding to the interval below ~317 m CCSF, all had <2% recovery. The recovered material indicated drilling had reached Albian reef deposits. For this reason, drilling of Holes U1407B and U1407C ended at 276.3 mbsf (323.01 m CCSF) and 261.6 mbsf (309.69 m CCSF), respectively.

Correlation and splice construction

For stratigraphic correlation and splice construction, we used NGR and magnetic susceptibility data. These two data series showed clear, correlatable features throughout the sediment column (see “Physical properties”). NGR was the most useful data set for correlation from 0 to ~116 m CCSF, and magnetic susceptibility was most useful from ~137 to 200 m CCSF. Both data series showed distinctive features from ~200 to 317 m CCSF (Figs. F37, F40). We also considered additional data to ensure that there were no large discrepancies between data sets, including GRA bulk density collected on the whole-round sections after cores equilibrated to room temperature and color reflectance (see “Physical properties”). From ~200 to 312 m CCSF, line-scan core images of section halves aided interpretation of the physical property data. Our correlation is consistent with both biostratigraphic and paleomagnetism results (see “Biostratigraphy” and “Paleomagnetism”).

We defined Core 342-U1407A-1H as the anchor in our splice, though there was a clear agreement between the mudline cores recovered in Holes U1407A and U1407B. As a result of the large degree of overlap between Cores 342-U1407A-2H, 342-U1407B-2H, and 342-U1407C-3H and between Cores 342-U1407A-3H, 342-U1407B-3H, and 342-U1407C-4H, Cores 342-U1407C-4H and 342-U1407A-4H are appended. In the upper ~30 m, a number of tentative offsets are associated with these appended cores. We applied a very large offset (12.39 m) to Core 342-U1407C-7H because this core follows the hypothesized unrecorded advance between Cores 6H and 7H. Consistent with this large offset, Cores 342-U1407A-6H and 342-U1407B-7H both contain a distinctive, sawtooth-shaped peak in magnetic susceptibility that does not appear in any of the cores in Hole U1407C (Fig. F37B). We also evaluated line-scan core images, which support our correlation of Core 342-U1407C-7H with 342-U1407A-7H and 8H and 342-U1407B-8H. This offset also aligns the green-to-white color change associated with the boundary between lithostratigraphic Units III and IV between the three holes. Finally, paleomagnetism chron identifications also show a large, ~10 m discrepancy in Hole U1407C at ~75 mbsf compared to Holes U1407A and U1407B (Table T20).

We applied another very large offset (14.98 m) to Core 342-U1407B-13X, the first recovered core in Hole U1407B following the drilled advance through the chert layer. The top of Core 13X belongs to nannofossil Subzone NP9a and the core catcher to Zone NP8, whereas the first cores below the chert-rich interval in Holes U1407A and U1407C are entirely within Zone NP9. Cores 342-U1407A-18X, 342-U1407B-13X, and 342-U1407C-15X all show distinctive magnetic susceptibility peaks. Another reliable tie occurs between Cores 342-U1407B-14X and 342-U1407C-16X associated with a prominent NGR peak. We interpret the large offset for Core 342-U1407B-13X as indicative of differences in the thickness of the chert interval between the holes. Lithostratigraphic Unit IV, which terminates at the top of the chert (defined operationally by the first partial APC stroke), was ~8 m thinner in Hole U1407B compared to Holes U1407A and U1407C, indicating variations in the top depth of the chert between holes, as well.

Below ~190 m CCSF, splice construction was more difficult than in the upper sediment column above the chert-rich interval because of clear variations in stratigraphic thickness of similar units in the different holes. The first example occurs in Cores 342-U1407A-24X, 342-U1407B-20X, and 342-U1407C-21X and 22X, in which it is impossible to align all features in both NGR and magnetic susceptibility without large overlaps between successive cores at the same hole and/or squeezing and stretching of individual cores. Our correlation represents a compromise between these two data sets, but we cannot align all distinctive features.

A second example of variations in the thickness of similar stratigraphic units between holes is associated with the interval recording OAE 2 from ~265 to 275 m CCSF. In Cores 342-U1407A-28X, 342-U1407B-24X, and 342-U1407C-26X, the thickest black shale corresponds to a prominent NGR peak. Using this NGR peak as a correlation tie point means that it is impossible to align features in either NGR or magnetic susceptibility below the peak from ~270 to 275 m CCSF. These differences in physical properties between the three holes are also obvious in a bed-by-bed comparison of the lithostratigraphy (Fig. F13). Next, from the black shale to the bottom of the recovered sediment column in each hole, a series of distinctive color changes of varying thickness exists between holes. In this interval, from ~275 to 317 m CCSF, we used line-scan core images of section halves to interpret the possible overlap among cores from each hole. For instance, we applied a large offset (10.43 m) to the top of Core 342-U1407C-28X based on the very different colors of sediment between Cores 342-U1407B-25X (whitish gray to grayish pink) and 342-U1407C-28X (tan). Similarly, we do not allow a greater overlap between Core 342-U1407A-29X and 342-U1407B-25X because Core 342-U1407A-29X is grayish pink to tan, with none of the overlying whitish gray sediment present in Core 342-U1407B-25X. We suggest that these large offsets do not imply large coring gaps; rather, they are indicative of the varying thickness of sedimentary units in the infill over the Albian reefal sediment below the sediment column recovered at Site U1407.

The Site U1407 splice can be used as a sampling guide, but we suggest using caution when using the splice for sampling below ~225 m CCSF, where clear variations in the stratigraphy exist between the three holes. In the splice table (Table T22), we labeled tie points 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 physical properties. We recommend overlapping splice intervals at the tie points by a few decimeters (or more, where splice ties are labeled as tentative) to accommodate anticipated development of the depth scale.

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