Physical property measurements at Site 1258 were conducted on whole cores, split cores, and discrete samples. Whole-core measurements conducted with the MST included GRA bulk density, magnetic susceptibility, resistivity, and NGR. Compressional (P)-wave velocity was measured in the transverse direction on split cores at intervals of 50 cm and along both transverse and longitudinal directions on cube samples taken at a frequency of one per core in Hole 1258B. Moisture and density (MAD) were determined on discrete samples at a frequency of one per section from Hole 1258A. Intervals not recovered in Hole 1258A were sampled from Hole 1258B. A full description of the various measurement techniques can be found in "Physical Properties" in the "Explanatory Notes" chapter. Physical property sampling was minimal through the black shale sequence and across critical boundaries.
The MAD (index) properties determined at Site 1258 include bulk density, porosity, grain density, water content, and void ratio (Table T21). Bulk density was determined on whole-core sections using the MST (GRA density) and on discrete samples. The GRA method tends to underestimate the bulk density in RCB cores because the core does not completely fill the liner. At this site, the average difference between GRA and discrete sample density is 0.13 g/cm3 and is approximately constant with depth. Despite the offset, the bulk density trend with depth derived by the two methods is essentially the same (Fig. F21).
Downhole trends of index properties reflect lithologic boundaries and compositional variations in sediments in the defined lithostratigraphic units (Fig. F22). Bulk density increases with depth at a relatively constant rate in Unit I and Subunit IIA (0–280 mcd), ranging from 1.55 g/cm3 at the surface to 1.9 g/cm3 at the base of Subunit IIA. Correspondingly, porosity gradually decreases from 69% to 54%.
Between ~125 and 170 mcd, the general trend of MAD properties is interrupted by a drop in bulk density and increase in porosity, followed by an increase in bulk density and decrease in porosity between 170 and 190 mcd, just above the P/E boundary (Fig. F22).
In the interval between ~280 and ~343 mcd (Subunits IIB and IIC), bulk density and porosity are relatively constant with depth, with average values of 2 g/cm3 and 40%, respectively (Fig. F22). This interval, however, is characterized by a higher degree of variability in the data that corresponds to cyclic variations in color and hardness of the sediment (see "Lithostratigraphy"). The boundary between Subunits IIC and Unit III (343 mcd) coincides with a drop in bulk density from ~2 to 1.8 g/cm3 and in grain density from ~2.7 to 2.4 g/cm3, whereas porosity remains constant. The few data collected from the laminated black shale sequence (Unit IV) are not sufficiently representative to characterize this interval.
The grain density generally decreases between 0 and ~70 mcd, with values ranging from 2.75 to ~2.5 g/cm3. From 70 to ~343 mcd, grain density gradually increases from 2.5 to 2.7 g/cm3, with offsets corresponding to lithostratigraphic unit boundaries (Fig. F22). At the boundary between Subunit IIC and Unit III, the grain density rapidly decreases to ~2.4 g/cm3. Grain density data from Unit III are lower in magnitude but higher in variability than the overlying units.
P-wave velocity was measured on split cores using the modified Hamilton Frame apparatus. In addition, measurements of transverse (x- and y-direction) and longitudinal (z-direction) P-wave velocity were conducted on cube samples from Hole 1258B (Table T22).
Acoustic velocities increase downhole, with peaks in over-consolidated or lithified intervals (Figs. F22, F23). The depth profile correlates directly with bulk density and inversely with porosity. Velocity increases with depth in Unit I and Subunit IIA. Similar to bulk density, velocity decreases below 125 mcd and then increases rapidly at ~190 mcd. Beginning at the P/E boundary (192 mcd), variability in velocity increases significantly, with fluctuations as high as 200 m/s over short (2 m) depth intervals. This high variability continues throughout Subunit IIC and Unit III and reflects large compositional and diagenetic variations. In Unit IV, acoustic velocity varies even more than in units above because of the large density differences in the alternating black shale and limestone.
Velocity is isotropic from 0 to ~280 mcd (Unit I and Subunits IIA and IIB), with <1% difference between longitudinal and transverse directions (Fig. F23). Below 280 mcd, in Subunit IIC and Unit III, the sediment develops a small degree of P-wave velocity anisotropy with higher velocities in the transverse direction, possibly corresponding to higher clay content.
MST data from the three holes at Site 1258 have similar trends throughout the defined lithostratigraphic units once mbsf depths have been translated into mcd (see "Composite Depths"). The Miocene ooze of Unit I is characterized by moderately high NGR emissions (~10 cps) and a strong magnetic susceptibility signature (~15–20). (Magnetic susceptibility is reported as dimensionless instrument units throughout. See "Multisensor Track Measurements" in "Physical Properties" in the "Explanatory Notes" chapter for conversion factors to SI) (Fig. F24). A sharp decline in both NGR emissions (to <5 cps) and magnetic susceptibility reflects the change from the ooze of Unit I to the foraminifer nannofossil chalk of Unit II.
First-order changes in MST data in Unit II generally track trends in the carbonate content of the sediment (Fig. F24). High carbonate content is associated with low NGR emissions and low magnetic susceptibility, whereas a drop in carbonate content, potentially reflecting a more substantial contribution of sediments from terrigenous sources, is accompanied by a rise in both NGR emissions and magnetic susceptibility.
In the middle of Subunit IIA, an unconformity is present, which is expressed in Hole 1258A as a 20-m gap in the sediment record (~125–145 mcd) and in Hole 1258B as a decrease in the bulk density at 145 mcd. The P/E boundary is recorded in all the holes as a spike in both NGR emissions and magnetic susceptibility measurements at ~200 mcd and with a reduction in the carbonate content (Fig. F24). Between the P/E and the K/T boundaries, magnetic susceptibility and NGR emissions gradually increase. The K/T boundary, found in Subunit IIB, is characterized by large and variable changes in magnetic susceptibility, with values reaching a maximum of 35 in Hole 1258A and 30 in Hole 1258C. The same general trend is observed in Hole 1258B but with gaps in recovery at intervals comparable to those where peak readings were observed in other holes.
NGR emissions remain relatively constant through much of Subunit IIC and Unit III, whereas a peak in magnetic susceptibility appears at the bottom of Subunit IIC (~340 mcd) in Holes 1258A and 1258B (Fig. F24). A drop in the GRA density and magnetic susceptibility defines the transition from Subunit IIC to Unit III. At 400 mcd (Hole 1258A), a broad peak in the GRA density is also reflected in the magnetic susceptibility data; however, gaps in recovery prevent correlation with either Hole 1258B or 1258C.
The laminated shales of Unit IV show a high degree of variation in GRA density, resistivity, and NGR emissions, reflecting the alternations of organic-rich layers and highly lithified and cemented sections. Magnetic susceptibility remains low and constant through the laminated shales relative to the overlying sequence, increasing in variability and magnitude below the transition into Unit V (Fig. F24). Density generally increases with depth in all holes through Unit IV. The largest variability in the shale sequence is seen in NGR emissions, probably due to uranium enrichment in the organic-rich intervals (see "Downhole Logging") (Fig. F25).
Hole 1258C penetrated deepest into Unit V, where density and NGR emissions, although variable, tend to be relatively high (1.7 g/cm3 and 30 cps, respectively). Magnetic susceptibility gradually increases from <5 at the base of Unit IV to ~12 at the bottom of the hole.
Resistivity measurements proved difficult to correlate between holes. The NCR sensor is highly sensitive to changes in core diameter, which was exacerbated by RCB drilling during Leg 207. Where core disturbance is minimal, there is a strong correspondence between resistivity and GRA density. Marked changes in resistivity delineate major lithologic boundaries, as do other physical property data. The absolute value of the individual resistivity data points, however, is suspect because of the large variability in core diameter.