Previous   |    Next

doi:10.2204/iodp.proc.303306.108.2006

Composite section

Six holes were drilled at Site U1308 to ensure complete recovery of the stratigraphic section to 239 mcd. Many holes were required because of disturbance in cores caused by excessive heave, poor recovery, and problems with damaged core liners (implosion). Several adjustments were made while drilling Holes U1308E and U1308F to correct offsets among holes by either drilling ahead or advancing by <9.5 m. The latter resulted in recored sections that overlapped the previous cores but was effective at avoiding lost sections that resulted when drilling ahead.

Because of the lower sedimentation rates at Site U1308, MST data were collected for most cores at a higher spatial resolution (every 2.5 cm) than other Expedition 303 sites. This proved to be beneficial especially for NGR because the signal bears a close resemblance to the marine oxygen isotope record (see “Physical properties”). Cores were depth-shifted on the basis of magnetic susceptibility data collected with the “Fast Track” magnetic susceptibility core logger soon after recovery. Features are generally well aligned among holes, although some sections are stretched or compressed relative to others (Fig. F17). It is therefore impossible to align every feature in cores across all holes, but we plan to apply corrections for stretching and squeezing postcruise. The offsets and composite depths are listed in Table T28. Because of the highly disturbed nature of Cores 303-U1308A-18H, 20H, and 24H through 26H, it was not possible to correlate these cores to the other holes. Similarly, Cores 303-U1308B-21H and 22H were not correlated because they represent short disturbed sections.

The sections of core used for the splice are identified in Table T29. When constructing the splice, we plotted the occurrence of all disturbed sections and excluded them from the splice. The cores from Site U1308 provide a continuous stratigraphic sequence to ~239 mcd with a single problematic interval between ~186 and 196 mcd where sediment deformation was apparent, including inclined bedding and sharp contacts (see “Lithostratigraphy”). Although it is possible to construct a continuous stratigraphic section through this interval, it may not be continuous in time. Below Core 303-U1308C-24H, cores were appended to the base of the splice.

A growth factor (GF) of 1.07 is calculated by linear regression for all holes at Site U1308, indicating a 7% increase in mcd relative to mbsf (Fig. F18). We used this value of GF to calculate corrected meters composite depth (cmcd) presented in Table T28 to aid in the calculation of mass accumulation rates.

We calculated sedimentation rates using paleomagnetic datums (Table T30). Linear regression provides a mean sedimentation rate of 8.3 cm/k.y. for the composite section down to 262 mcd (Fig. F19). The interval sedimentation rates vary slightly from 5.6 to 11.0 cm/k.y. Sedimentation rates for Subunit IIB (262.14–355.49 mcd) below the spliced sections were lower, averaging ~3 cm/k.y.

Top of page   |    Previous   |    Next