Physical property measurements at Site 1260 were conducted on whole cores, split cores, and discrete samples. Whole-core measurements, conducted with the MST include GRA bulk density, magnetic susceptibility, 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 1260B. Moisture and Density (MAD) properties were determined on discrete samples at a frequency of one per section from Hole 1260A to the depth of the Cretaceous black shale sequence. MAD measurements in the black shale sequence were performed on samples from Hole 1260B at a frequency of one per core. Sampling for MAD was minimal across critical intervals. A full description of the various measurement techniques can be found in "Physical Properties" in the "Explanatory Notes" chapter.
MAD measurements at Site 1260 include bulk density, porosity, grain density, water content, and void ratio (Table T20). Bulk density was determined on whole-core sections using the MST (GRA density) and on discrete samples. GRA tends to underestimate the bulk density in RCB cores because the core material does not completely fill the inner diameter of the liner. At this site, the average difference between GRA and discrete sample density is 0.12 g/cm3. Despite this offset, the downcore bulk density trends derived by the two methods are essentially the same (Fig. F22).
Three distinct downhole trends are seen in the MAD profile (Fig. F23). Between 0 and 37 mbsf (0–37 mcd) (lithostratigraphic Unit I and Subunit IIA), bulk density decreases slightly from 1.8 to 1.6 g/cm3 and porosity increases from 50% to 60%. Grain density values are highly variable and increase marginally with depth. This interval is a mass-failure deposit (see "Biostratigraphy" and "Lithostratigraphy").
The boundary between Subunits IIA and IIB coincides with a step decrease in bulk density from 1.6 to 1.4 g/cm3 and an increase in porosity from 60% to 68%. Between 37 and ~380 mcd, bulk density increases linearly from 1.65 to 1.8 g/cm3 and porosity decreases from 70% to 40%. Average grain density (2.7 g/cm3) remains constant throughout the interval. These downhole trends are typical of normally consolidated sediments. In Subunit IIB, from 37 to 175 mcd, MAD data (especially grain density) are characterized by high variability, reflecting frequent variations in carbonate content and radiolarian abundance. Variability is low in Subunit IIC, with nearly constant grain density and no deviation of bulk density and porosity values from their respective downhole trends. At ~276 mcd, just above the P/E boundary, the general trend is perturbed by a drop in bulk density and an increase in porosity similar to that found at the other sites over this interval, but of lower magnitude. In Subunits IIIA and IIIB from 276 to 390 mcd, variability in bulk density and porosity increases, reflecting frequent cyclic variations in color and hardness of the sediment.
The transition to Unit IV corresponds to the largest change in the MAD depth profile. Bulk density drops from 2 to 1.7 g/cm3 and grain density from ~2.6 to ~2.2 g/cm3, whereas porosity increases from 40% to 60%. Based upon the limited MAD data in this unit, variability in bulk density and porosity values is extremely high, reflecting cyclic changes in lithology between claystone and limestone. In Unit V, MAD values are nearly constant with depth, with an average bulk density of 2.1 g/cm3, an average grain density of 2.7 g/cm3, and an average porosity of 30%.
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) velocity were conducted on cube samples from Hole 1260B (Table T21).
Acoustic velocities tend to increase downhole, with peaks at overconsolidated or lithified intervals (Fig. F24). The general depth trend correlates directly with bulk density and inversely with porosity. Between 0 and 37 mcd (Unit I and Subunit IIA), in P-wave velocity increases sharply from ~1500 to ~1800 m/s. A drop in P-wave velocity magnitude to ~1500 m/s marks the transition to Subunit IIB. From 37 to 276 mcd (Subunits IIB and IIC), P-wave velocity increases linearly from ~1500 to ~1980 m/s, reflecting the normal consolidation trend seen in the bulk density and porosity data. Beginning at the depth of the P/E boundary (276 mcd) and throughout Subunit IIIA, velocity decreases slightly with depth. Cyclic variations in velocity ranging between 1800 and 2000 m/s correspond to color changes in the unit between dark and light intervals, respectively.
The K/T boundary at 329 mcd marks a major offset in P-wave velocity values. Similar offsets are not seen in bulk density or porosity profiles. Throughout Subunits IIIB and IIIC (379–390 mcd), the variations in P-wave velocity continue, reflecting cyclic lithologic alternations between dark clay-rich intervals and light-colored cemented intervals.
In Unit IV (black shale sequence), velocity values in the organic-rich shales are nearly constant with depth, averaging 1800 m/s, whereas the cemented limestone velocities vary between 2100 and >3000 m/s (Fig. F25).
Velocity is isotropic from 0 to ~150 mcd, with <1% difference between longitudinal and transverse directions. Below ~150 mcd, the sediment develops a small degree of P-wave velocity anisotropy with higher velocities in the transverse direction. The development of anisotropy correlates with an increase in clay content. The higher clay content allows the development of a preferred orientation, perpendicular to overburden pressure, in sediment grain fabric and hence to different velocities in the transverse and longitudinal direction.
MST data from the two holes at Site 1260 have similar trends throughout the defined lithostratigraphic units once mbsf has been translated into mcd (see "Composite Depths"). Unit I and Subunit IIA are characterized by moderately high NGR emissions (~10 cps) and a low magnetic susceptibility signature (~2–5; magnetic susceptibility values are reported here as raw instrument units. See "Physical Properties" in the "Explanatory Notes" chapter for conversion of this data to SI units). Both NGR emission and magnetic susceptibility decline to values close to background in Subunit IIB.
In Subunit IIC, NGR and magnetic susceptibility depth profiles are characterized by a pronounced peak at ~240 mcd. The P/E boundary is recorded as a spike in both NGR emissions and magnetic susceptibility measurements at ~276 mcd and also as a sharp decrease in the carbonate record (Figs. F26, F4).
Between 276 and 323 mcd (Subunit IIIA), magnetic susceptibility and NGR emissions gradually increase, reaching a maximum at 323 mcd before a significant drop coincident with the K/T boundary. In Subunits IIIB and IIIC, the NGR data increase linearly with depth whereas the magnetic susceptibility profile is characterized by cyclic variations.
The laminated shales of Unit IV show a high degree of variation in GRA density and NGR emissions, similar to those observed in P-wave velocity (Fig. F25). Susceptibility remains low through the laminated shale sequence relative to the overlying sediment, increasing in variability and magnitude after the transition into Unit V. Density generally increases with depth in Unit IV. Alternation of organic-rich layers and highly lithified and cemented layers is responsible for the scatter in the GRA, NGR, and P-wave velocity data throughout Unit IV. NGR emissions peak at ~440 mcd, reaching values as high as 280 cps (Fig. F25), which is probably a result of uranium enrichment in the organic-rich intervals.