Coring at Site 1258 extended to a total depth of 485 mbsf, and a composite section was established for much of the cored interval. Magnetic susceptibility, gamma ray attenuation (GRA) bulk density, and noncontact resistivity (NCR) data were collected with the multisensor track (MST) at 2.5-cm intervals on all whole-core sections. Natural gamma ray (NGR) data were collected at 7.5-cm intervals on all whole cores at Site 1258. In addition, color spectral reflectance data were collected at 2.5-cm intervals on all split cores. The magnetic susceptibility data provided the primary data set used to correlate between holes above the Cretaceous black shale sequences. In the black shales, NGR data provided better core-to-core comparisons.
The depth offsets that define the composite section for Holes 1258A, 1258B, and 1258C are given in Table T11. Excellent RCB recovery in the upper portions of Holes 1258A and 1258B provided continuous core overlap from 35 mcd (top of Core 207-1258A-4R) to ~210 mcd (Section 207-1258A-20R-5, 122 cm), ~10 m below the P/E boundary. Data used to construct the composite section over this interval are presented in Figures F13 and F14. Faulting and/or slumping in Hole 1258A in the lower Eocene sequence resulted in 22 m not recovered in Hole 1258A (~130–150 mcd) that was recovered in Holes 1258B (Cores 207-1258B-14R, 15R, and top of 16R) and 1258C (from the lower part of Core 207-1258C-1R to the middle of Core 4R). Similar faulting/slumping resulted in ~11 m of missing section in Hole 1258B (between Cores 207-1258B-21R and 22R or possibly in Core 22R) (~205–215 mcd) that were recovered in Cores 207-1258A-20R and/or 21R from Hole 1258A.
Moderate core recovery and further faulting/slumping below the P/E boundary made it impossible to construct a continuous composite depth scale for the remainder of Site 1258. In some intervals below the P/E boundary, reliable correlations were still possible among the holes at Site 1258 (i.e., features on the MST profiles could be matched between holes) (Fig. F15). In other intervals, distinct core-to-core tie points across the holes were not obvious and correlations were based on similarities in overall trends rather than by matching unique structures in the MST profiles. Furthermore, below the P/E boundary, many coring gaps were not spanned and the absolute depth of the cores in this interval cannot be accurately determined (e.g., see alignment of the Cretaceous black shales in Fig. F16). Rather, the cores "hang" in the composite depth scale linked to correlative cores in other holes but not to those above or below. Improvements in the biostratigraphic and magnetostratigraphic record postcruise should result in refinements to the composite depth scale in many intervals, including the Cretaceous black shales.
The periodic variability in the Eocene magnetic susceptibility data at Site 1258 will provide a good basis for postcruise cyclostratigraphic studies. Age control is excellent, with well-defined paleomagnetic datums in the section (e.g., the top of Chron C21n and the base of Chron C22r) (see "Paleomagnetism"). Preliminary investigation suggests the dominant periodicities of the magnetic susceptibility data are Milankovitch in nature.
Following construction of the composite depth section at Site 1258, a single spliced record was assembled for the aligned cores in the interval from 35 to 210 mcd. Intervals having significant disturbance or distortion were avoided where possible when making the splice. As explained above, moderate to poor recovery below the P/E boundary prevented us from continuing the splice record below ~210 mcd. When utilizing this splice as a sampling guide, it is advisable to overlap a few decimeters from different holes during sampling to accommodate anticipated ongoing development of the depth scale. The reason for this approach is that distortion of the cored sequence can lead to stretching or compression of sedimentary features. However, at crossover points along the splice (Table T12), care was taken to align highly identifiable features from cores in each hole. Postcruise work will establish a detailed correlation between holes by establishing a revised meters composite depth (rmcd) scale that allows differential stretching and squeezing in cores.