Site 1257 extended to a total depth of 284.7 mbsf (Core 207-1257A-31X). A good composite section was established for the upper Paleocene (~100–150 meters composite depth [mcd]), but it was not possible to construct a complete composite depth scale for the remainder of the section. The severe biscuiting in the XCB section of Hole 1257A combined with poor recovery in parts of both the XCB- (Hole 1257A) and RCB- (Holes 1257B and 1257C) cored sections precluded rigorous correlation among holes (see "Lithostratigraphy").
At Site 1257, 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 core sections. Compressional (P)-wave velocity data were collected at 2.5-cm intervals on APC cores. Natural gamma ray (NGR) data were collected at 15-cm intervals on XCB cores from Hole 1257A and at 7.5-cm intervals on all cores from Holes 1257B and 1257C. In addition, color spectral reflectance data were collected at 2.5-cm intervals on all split cores from Holes 1257A, 1257B, and 1257C. Magnetic susceptibility data provided the primary data set that we used to correlate among holes in the interval between 100 and 150 mcd, where core recovery and quality were sufficient in each of the holes to allow core-to-core comparisons. NGR data, although collected at a lower resolution than magnetic susceptibility, provided additional verification of the hole-to-hole ties.
The biscuited and fractured nature of the XCB and RCB cores resulted in low signal-to-noise ratios for the MST sensors (e.g., GRA bulk density and spectral reflectance) that analyze small portions (i.e., ~1 cm) of the core. These sensors recorded data in individual biscuits and the slurry between the biscuits. In addition, the XCB and RCB core diameter is less consistent than APC sections, thus increasing the variability in data recorded using these high-resolution sensors. The magnetic susceptibility and NGR systems, however, sense a larger portion of the core (~5–15 cm). The data recorded by these types of sensors tend to average out some of the core-diameter variability as well as the signal variability resulting from biscuiting.
The poor recovery in much of the drilled section at Site 1257 combined with a low signal-to-noise ratio seen in much of the MST sensor data precluded placing many of the Site 1257 cores into a composite depth framework. The composite depth table (Table T11; Fig. F10) identifies the cores that could be correlated (i.e., depth adjusted) among holes and lists the offsets applied to each core. In many cases, the poor core quality (i.e., biscuited cores) resulted in only short intervals in a single core having a high-quality signal for correlation. Thus, correlated cores may only have 1- to 2-m of section with similar MST signals. The few ties, therefore, that could be made outside of the upper Paleocene interval (discussed below) are tenuous, at best.
Good core recovery and minimal drilling disturbance in the interval from ~100 to 150 mcd (Cores 207-1257B-9R through 12R and 207-1257C-3R through 7R) allowed us to construct a high-quality upper Paleocene composite section (Figs. F11, F12). The RCB cores from Holes 1257B and 1257C have excellent magnetic susceptibility and NGR records with high signal-to-noise ratios. In contrast, the XCB cores in Hole 1257A suffer from a greater degree of core disturbance (biscuiting) and are not useful for composite section construction. The composite interval from 100 to 150 mcd is nearly complete, with only one core gap (between Cores 207-1257B-10R and 11R).
The periodic variability in the magnetic susceptibility data will provide a good basis for postcruise cyclostratigraphic studies. Age control is excellent, with two well-defined paleomagnetic datums (top of Chron C26n and the top and base of Chron C25n) in the section (see "Paleomagnetism"). Preliminary investigation suggests the dominant periodicities of the magnetic susceptibility data are Milankovitch in nature.
Following construction of the composite depth section for the upper Paleocene interval at Site 1257, a single spliced record was assembled for the aligned cores over the interval from 102 to 151 mcd by patching across cores with data primarily from Holes 1257B and 1257C (Table T12; Figs. F11, F12). Only one core gap could not be bridged (the interval between Cores 207-1257B-10R and 11R).
Intervals having significant disturbance or distortion were avoided when making the splice. The Site 1257 splice can be used as a sampling guide to recover a nearly continuous upper Paleocene sedimentary sequence. When utilizing this splice as a sampling guide, it is advisable to overlap a few decimeters from different holes when 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. 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 (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.