Previous   |    Next

doi:10.2204/iodp.proc.346.109.2015

Stratigraphic correlation and sedimentation rates

Real-time tracking of the relative positions of core gaps among holes at Sites U1428 and U1429 was accomplished using magnetic susceptibility and GRA density data from the WRMSL and STMSL. Data were collected at a resolution of 5 cm, which was sufficient to keep up with core recovery rates. Compositing and splicing (see “Stratigraphic correlation and sedimentation rates” in the “Methods” chapter [Tada et al., 2015b]) are based on the high-resolution RGB color data (blue) recovered from the Section Half Imaging Logger at 0.5 cm resolution as well as the magnetic susceptibility and GRA density data. For detailed discussion of these data sets, see “Physical properties” in the “Methods” chapter (Tada et al., 2015b).

At Sites U1428 and U1429, sea state was relatively calm and gas expansion was limited compared to some of the previous sites. Voids within the liners and extrusion of sediment out the top of the core barrel were minimal to nonexistent. Although only 7.5 km apart and at similar water depths, there are differences in sedimentation rates and interbedded sands. For these reasons, separate splices were produced for each site.

Two holes were drilled at Site U1428. These two holes were composited (Table T23) and spliced (Table T24) from the mudline to ~145 m CCSF-D (as defined in the “Methods” chapter [Tada et al., 2015b]) near the top of the massive unconsolidated sand layers encountered near the bottom of the hole (Fig. F59). A gap in the splice occurs from 88.71 to 93.82 m CCSF-D, where Section 346-U1428B-10H-4, 130 cm, is appended to 346-U1428B-11H-1, 0 cm. This gap reflects strongly disturbed intervals identified in both Holes U1428A and U1428B. Sedimentation rate analysis (discussed below) suggests that the gap spans ~12 k.y. Preliminary correlation of the spliced magnetic susceptibility data from the two sites (Fig. F60) indicates that some or all of this gap may have been successfully recovered at Site U1429; the resolution of this question awaits detailed correlation using additional data. Comparison of the CCSF-D and CSF-A depths at the bottom of the spliced interval indicates expansion of ~7% at this site.

Three holes were drilled at Site U1429. These holes were composited (Table T25) and spliced (Table T26) from the mudline to ~191 m CCSF-D near the top of the sand layers encountered near the bottom of the hole (Fig. F61). Two gaps occur in this splice. The first gap occurs from 49.99 to 53.11 m CCSF-D, where Section 346-U1429B-6H-4, 149 cm, is appended to 346-U1429C-8H-1, 0 cm. This gap reflects strongly disturbed intervals identified in both holes and the 0% recovery in Core 346-U1429C-7H. Sedimentation rate assessment suggests that this gap spans ~10 k.y. However, correlation of the spliced magnetic susceptibility data from Sites U1428 and U1429 (Fig. F60) indicates that all of this gap was successfully recovered at Site U1428; a more precise definition of the correlative interval awaits detailed analysis. The second gap in the Site U1429 splice occurs from 123 to 125.76 m CCSF-D, where Section 346-U1429A-13H-5, 71 cm, is appended to 346-U1429A-15H-1, 0 cm. This gap reflects strong coring disturbance and the 0% recovery for Core 346-U1429A-13H. Sedimentation rate assessment suggests that this gap spans ~8.5 k.y. Preliminary correlation of the spliced magnetic susceptibility data from Sites U1428 and U1429 indicates that some or all of this gap may have been successfully recovered in Site U1428; the resolution of this question also awaits detailed correlation using additional data. Comparison of the CCSF-D and CSF-A depths at the bottom of the spliced interval indicates expansion of ~7% at this site.

Neither Site U1428 nor Site U1429 was logged. Thus, the length of the gaps was estimated using drillers depth (CSF-A) offsets for the appended cores. Experience during this expedition at sites drilled during calm conditions has been that the CSF-A scale is remarkably accurate. Our requests for mudline cores of specific lengths were consistently met with accuracies of ±1 m or better once the drillers had established the relationship between recovery length of the first mudline core and bit depth at which the core was taken.

Age model and sedimentation rates

Preliminary age models (Fig. F62) were established for both sites on the basis of a very limited number of biostratigraphic age control points. For details, see “Biostratigraphy.” No datums were excluded from the assessment. At Site U1428, a linear fit to the two datums (and constrained to pass through the origin) indicates an average sedimentation rate of ~42 cm/k.y. and an age of ~350 ka at the bottom of the spliced interval (~145 m CCSF-D). A linear fit to the three datums at Site U1429 indicates an overall average sedimentation rate of ~32 cm/k.y. but does not pass through the origin, suggesting a somewhat higher sedimentation rate over the upper ~50 m of the section (~76 cm/k.y.). Extrapolation to the bottom of the splice yields an age of ~500 ka at 190 m CCSF-D. This is inconsistent with the spliced magnetic susceptibility correlation between the two sites (Fig. F60). Alternatively, placing highest confidence in the LO of G. ruber (pink) datum and forcing a fit through the origin yields a sedimentation rate of 56 cm/k.y. and an age of ~340 ka at the bottom of the spliced interval. This scenario is consistent with the magnetic susceptibility–based correlation between the two sites.

Top of page   |    Previous   |    Next