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doi:10.2204/iodp.proc.346.104.2015

Stratigraphic correlation and sedimentation rates

Real-time tracking of the relative positions of core gaps among the various holes at Site U1423 was accomplished using magnetic susceptibility and GRA data from the WRMSL and STMSL. Data were collected at resolutions of either 2.5 or 5 cm, depending on the capacity to keep up with core recovery rates. Detailed (centimeter scale) compositing and splicing (see “Stratigraphic correlation and sedimentation rates” in the “Methods” chapter [Tada et al., 2015b]) are based on the high-resolution RGB (blue) color data recovered from the Section Half Imaging Logger at 0.5 cm resolution. For detailed discussion of these data sets, see “Physical properties” in the “Methods” chapter (Tada et al., 2015b).

After all cores were composited, a continuous splice was constructed spanning 0–218.8 m CCSF-D (as defined in the “Methods” chapter [Tada et al., 2015b]). All three holes were required to construct the splice; a combination of drilling disturbance and poor recovery within the interval from ~144 to ~184 m CSF-A prevented construction of a complete splice from Holes U1423A and U1423B alone. In addition, at three depths, between-core gaps in Holes U1423A and U1423B were aligned. Finally, a 6.5 m interval was also missing because of 0% recovery of Core 346-U1423B-18H. Consequently, a detailed drilling plan was constructed to recover these missing intervals in Hole U1423C, as described below. The resulting composite and splice ranges from the top of Core 346-U1423A-1H to the bottom of Core 346-U1423B-24H at 218.8 m CCSF-D (211.2 m CSF-A) (Tables T18, T19; Fig. F43). Only Hole U1423B extends deeper than this depth, to Core 346-U1423B-28H at 256.9 m CCSF-A (249.3 m CSF-A). The length of the splice (218.8 m CCSF-D) relative to the length of the uncomposited section (211.2 m CSF-A) indicates that expansion at this site is minimal (3.8%).

A CCSF-C scale (as defined in “Stratigraphic correlation and sedimentation rates” in the “Methods” chapter [Tada et al., 2015b]) was created for this site, extending to ~90 m CSF-A, covering Subunit IA as defined in “Lithostratigraphy.” Construction of the CCSF-C scale is based on centimeter-scale correlation of structure in the RGB (blue) data. These CCSF-C depth maps (CSF-A to CCSF-D) are provided for Holes U1423A and U1423B in Table T20.

Detailed drilling at Hole U1423C to recover between-core gaps

Hole U1423C was selectively cored to recover sections not recovered in Holes U1423A and U1423B. Hole U1423C was drilled without recovery to ~114 m CSF-A. This 114 m washed interval is designated in the coring sequence as Core 346-U1423C-11, as required by IODP terminology. The suffix “11” refers to Core “1” drilled Interval “1.” The wash interval was followed by two APC cores (2H and 3H) to establish the stratigraphic relationship relative to Holes U1423A and U1423B. The next APC core (4H) successfully recovered the first of the three targeted gaps. While the stratigraphic correlation was taking place, the bit was held at the top of the previously cored interval in order to minimize disturbance at the top of the following core. Following stratigraphic correlation, the bit was advanced the length of the previously recovered core (referred to on the rig floor as “cleaning out the rathole”) and the hole was washed an additional 7.5 m (Core 52 [drilled Interval 2 in Core 5]) in order to place the APC such that it would recover the missing 6.5 m interval in the middle of the following 9.6 m stroke. Following recovery of this interval, the hole was washed another 2 m (Core 73 [drilled Interval 3 in Core 7]) in order to place the last two gaps in the middle of the next two APC strokes. After successful recovery of these last two remaining gaps, the hole was terminated. The success of this detailed drilling effort was greatly aided by ideal conditions: near zero heave and a sediment lithology conducive to full stroke recovery with little to no expansion. This targeted approach resulted in considerable time savings, compared to coring the entire hole top to bottom.

Age model and sedimentation rates

A preliminary age model (Fig. F44) was established on the basis of all available biostratigraphic and paleomagnetic age control points. For details, see “Biostratigraphy,” “Paleomagnetism,” and “Lithostratigraphy.” At this site, no datums were excluded from the assessment, although three datums defined as the LOs of Proboscia curvirostris and Actinocyclus oculatus and the rapid decrease of Siphocampe arachnea group could be affected by reworking. Eight linear depth-age segments utilizing all the paleomagenetic events as well as the FO of Neodenticula koizumii, and the LO of Thalassiosira jacksonii provide a well-constrained preliminary age model (Fig. F44A). Sedimentation rate (Fig. F44B) ranges from 21 to 121 m/m.y., moderate in the upper Subunit IA, low in the lower Subunit IA and Subunit IIA, and high in Subunits IB and IIB, possibly reflecting the change in siliceous productivity. Estimated ages of subunit boundaries are 1.8, 2.2, and 3.0 Ma for the Subunit IA/IB, IB/IIA, and IIA/IIB boundaries, respectively.

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