Coring at Site 1261 extended to a total depth of 674 mbsf. The interval from just above the K/T boundary through the Cretaceous black shales (below ~525 mcd) was the only interval targeted for overlap and composite section construction. 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 from Holes 1261A and 1261B. NGR data were collected at 7.5-cm intervals on all whole cores from Holes 1261A and 1261B. Spectral reflectance data were collected at 2.5-cm intervals on all split cores. GRA bulk density data provided the most reliable core-to-core comparisons and were supplemented by NGR and magnetic susceptibility data.
The depth offsets that compose the composite section for Holes 1261A and 1261B are given in Table T9. The quasi-periodic variability of the claystone and chalks/limestone composing the black shales resulted in strong signal-to-noise ratios in both the GRA bulk density (Fig. F11) and NGR data sets. These data sets, combined with good RCB recovery over a significant portion of the black shale interval, allowed for the construction of a semicontinuous composite section from ~550 to 650 mcd.
Following construction of the composite depth section at Site 1261, a splice record was assembled for the aligned cores in the interval from 550 to 650 mcd (Table T10; Fig. F11). Hole 1261A cores were used as the "backbone" for each splice, with cores from Hole 1261B used to span core gaps between cores in Hole 1261A. Gaps exist in the splice at ~591–594 mcd (between Cores 207-1261A-43R and 44R), ~609–610 mcd (between Cores 207-1261B-10R and 11R) and ~617–619 mcd (between Cores 207-1261A-46R and 47R). The true magnitude of these core gaps, however, is not known. In each case, the core below the gap is appended to the core above the gap, using their respective mbsf depths.
When utilizing these splices as sampling guides, it is advisable to overlap a few decimeters from different holes to accommodate anticipated postcruise revisions to the composite 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, 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 within cores.