Coring at Site 1260 extended to a total depth of 507 mbsf. Moderate to poor core recovery in specific intervals between Holes 1260A and 1260B resulted in three discrete intervals suitable for constructing composite sections. These intervals cover 65% of the cored interval at Site 1260. Magnetic susceptibility and gamma ray attenuation (GRA) bulk density were collected with the multisensor track (MST) at 2.5-cm intervals on all whole-core sections from Hole 1260A and 1260B. Natural gamma ray (NGR) data were collected at 7.5-cm intervals on all whole cores from Hole 1260A and from Cores 207-1260B-1R and 2R and from Section 12R-3 to the base of Hole 1260B. To maintain a reasonable processing rate in the core laboratory, NGR data were collected at 15-cm resolution from the top of Core 207-1260B-3R to the base of Section 12R-2, an interval that showed little structure in the NGR record in Hole 1260A. Noncontact resistivity (NCR) data were collected at 2.5-cm intervals from the top of Core 207-1260A-1R through Section 27R-3 and from the top of Core 207-1260B-1R through Section 12R-2 (see "Physical Properties" in the "Explanatory Notes" chapter for a discussion of problems associated with the calibration and accuracy of NCR data). Spectral reflectance data were collected at 2.5-cm intervals on all split cores. Spectral reflectance and GRA bulk density were the primary data sets used to correlate between holes in the middle Eocene section at Site 1260 (~38–133 mcd). From the lower Eocene to the top of the Cretaceous black shale sequence (~230–395 mcd), magnetic susceptibility was the primary data set used to correlate between holes. Within the black shales (~418–484 mcd), NGR and bulk density data provided the most reliable core-to-core comparisons.
The depth offsets that compose the composite section for Holes 1260A and 1260B are given in Table T11. In the middle Eocene interval (38–133 mcd), the GRA bulk density data and the color reflectance data provide an excellent record that seems to reflect the cyclic variation in carbonate and silica content. These data, combined with the good RCB recovery in Holes 1260A and 1260B, allowed the construction of a nearly continuous composite section in this interval. One core gap (near 57 mcd) was not bridged. The spectral reflectance ratio (680/430 nm) data used to construct the composite section over this interval are presented in Figure F13.
In the interval from the lower Eocene to the top of the Cretaceous black shales (230–395 mcd), core recovery was excellent and definitive hole-to-hole correlations were possible throughout the section using magnetic susceptibility data. The alignment of cores in Holes 1260A and 1260B, however, resulted in numerous core gaps that could not be bridged, making it impossible to construct a continuous section through the interval (Fig. F14). The absolute depth of composite section in this interval cannot be accurately determined because some coring gaps were not spanned and the cores in this interval cannot be definitely tied to the sections above and below.
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 and NGR data sets (Fig. F15). These data sets, combined with good RCB recovery over a significant portion of the black shale interval, allowed for the construction of a nearly continuous composite section from 418 to 484 mcd. Only one core gap could not be bridged (between Cores 207-1260A-47R and 48R [436 mcd]) in this interval. Poor recovery from 394 to 418 mcd precluded the construction of a composite section in this interval of the Cretaceous black shales.
The periodic variability in the middle Eocene color reflectance and GRA bulk density data at Site 1260 will provide a good basis for postcruise cyclostratigraphic studies. Age control is excellent, with well-defined paleomagnetic datums in the section (e.g., Chrons C19n and C20n) (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 1260, three discrete splice records were assembled for the aligned cores in the intervals 38–133, 229–394, and 418–484 mcd (Table T12; Figs. F13, F14, F15). As discussed above, the middle Eocene (38–133 mcd) and black shale (418–484 mcd) splices are nearly continuous. The lower Eocene–Campanian splice (229–394 mcd) is discontinuous in nature but still provides a good record for most centimeter- to decimeter-scale studies and high-resolution centimeter-scale studies over shorter intervals. Figure F16 provides a qualitative estimate of the confidence of the core-to-core correlations and the resultant splices between cores in Holes 1260A and 1260B.
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. Because much of the distortion occurs in individual cores on depth scales of <9 m, it was not possible to align every feature in the MST and color reflectance records. 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.