Physical property measurements at Site 1257 were conducted on whole cores, split cores, and discrete samples. GRA bulk density, magnetic susceptibility, resistivity, and NGR were measured on whole cores with the MST. Split-core analysis included P-wave velocity, and discrete samples were analyzed for moisture and density (MAD) (index) properties. A full description of the various measurement techniques can be found in "Physical Properties" in the "Explanatory Notes" chapter. Three acoustic velocity measurements per section were made, whereas one MAD sample per section was taken for cores from Holes 1257A and 1257C and one per core through most of Hole 1257B. Physical property sampling was minimal through the Cretaceous shale sequence (170–230 mbsf) and across critical boundaries.
The index properties determined on discrete samples include bulk density, dry bulk density, porosity, grain density, water content, and void ratio (Table T20). Downhole trends in these parameters reflect consolidation and lithologies, with divisions between lithostratigraphic units and subunits delineated by obvious offsets in physical property curves (Fig. F18). An increase in variability of the data set at 45 mbsf coincides with a change in coring technique from APC to XCB coring. It is difficult to discern whether this increase in variability is real or an artifact from (1) drilling-induced disturbance, (2) disturbance from the splitting saw used on XCB cores, or (3) accidental inclusion of drilling slurry in discrete samples.
Good agreement exists between density and porosity determined on discrete samples (MAD) and those derived from the MST GRA measurements for all holes (Fig. F19). GRA estimates, however, are consistently lower than the MAD determinations in Holes 1257B and 1257C as a result of the smaller-diameter cores associated with RCB drilling. XCB coring in Hole 1257A induced intense biscuiting that affected GRA density data, making correlation with Holes 1257B and 1257C relatively uncertain. Additionally, instrument problems caused a systematic error in the GRA bulk density measurements throughout Hole 1257A. The error was estimated as the best-fit equation of the difference between MAD and GRA bulk density (error = 0.18849 + 0.000287 mbsf). The plotted GRA bulk density was corrected accordingly (Figs. F18, F19).
At Site 1257, bulk density generally increases downhole, with noticeable offsets at contacts between lithostratigraphic units (Fig. F19). The thin veneer of upper Neogene sediments, classified as Unit I, is represented by a single bulk density measurement of 1.59 g/cm3 and a corresponding porosity of 65.5%. Similarly, the nannofossil ooze of Subunit IIA has an average bulk density of 1.59 g/cm3 and an average porosity of 65.2%. An average bulk density of 1.61 g/cm3 and a porosity of 63.7% characterizes a gradual transition into Subunit IIB. A maximum in bulk density and a minimum in porosity toward the base of Subunit IIB (60–85 mbsf) mark a gravity flow. Subunits IIIA and IIIB have average bulk densities of 1.76 and 1.80 g/cm3 and porosities of 54.9% and 50.8%, respectively. Unit IV, representing the interval of Cretaceous shale, was poorly sampled in all holes. Six MAD samples were taken through this highly variable interval, with bulk densities ranging from 1.5 to 1.9 g/cm3 and porosities from 40% to 70%. The deepest lithologic division (Unit V) has an average bulk density of 2.14 g/cm3 and a corresponding average porosity of 33.6%. Although Unit V is classified as a single unit, the index property trends clearly change at ~255 mbsf, passing from gradually increasing density and decreasing porosity to an interval of relatively constant values.
The grain density trend is relatively flat in Unit I through Subunit IIIB; however, variability in these units is quite high (Fig. F18). The largest offset in grain density occurs across the Subunit IIB/IIIA boundary, which represents the division between the Eocene and Paleocene. Grain density decreases with depth across the Subunit IIA/IIB and IIIA/IIIB and Unit IV/V boundaries. Unit IV has characteristically low grain densities typical of organic-rich material, with three of the six samples between 2.2 and 2.4 g/cm3. Unit V has the highest grain density, with an average of 2.71 g/cm3. Grain density decreases at the top of Unit V and remains constant to the bottom of the hole.
P-wave velocity was measured on split cores using the modified Hamilton Frame apparatus. Acoustic velocities tend to increase downhole with peaks occurring in overconsolidated or lithified intervals, especially throughout Unit IV (Fig. F20A). The highly fractured and porous shale sequences tended to be drained of water, which prevented successful measurement of velocity.
Laboratory measurements of velocity (Table T21) show a high degree of alignment with downhole logging data, with residual offsets indicating some degree of rebound in the core (Fig. F20B). The largest systematic offset after correction is found in the upper part of Unit V. A distinct change in acoustic velocity occurs between the upper and lower parts of Unit V and coincides with changes in density and porosity profiles (Fig. F18). The paucity of measurements through the upper portion of Unit V results from partially saturated sediments, which prevents successful acquisition of velocity measurements.
The MST data set was successfully correlated between Holes 1257B and 1257C. Correlation of MST data between these two holes and Hole 1257A was more difficult because XCB coring in Hole 1257A caused intense biscuiting, resulting in a degraded data set (see "Composite Depths"). However, in all three holes, magnetic susceptibility, resistivity, and NGR measurements tend to reflect the larger lithologic divisions (Fig. F21).
The most pronounced changes in MST data for Holes 1257B and 1257C occur across the K/T boundary (Subunit IIIA/IIIB), where a large hiatus exists. A gradual increase in magnetic susceptibility is observed throughout Subunit IIIA, culminating in a peak at the boundary with Subunit IIIB. The increase in magnetic susceptibility is mirrored by a less pronounced rise in NGR emissions and a subsequent drop at the same boundary.
A similar trend in the MST data is observed in Hole 1257A, with the exception that the highest sustained magnetic susceptibility readings occur at 250 mbsf. This change is matched by a rise in NGR values and is coincident with a drop in carbonate content (see "Lithostratigraphy").
Low recovery in Unit IV precludes the tracking of any systematic changes throughout the shale interval. However, the low-density, high-NGR, and low-susceptibility signature of the sequence is quite pronounced. Average GRA densities are <1.5 g/cm3, with NGR readings >80 cps near the top of the unit and ~40 cps near the bottom.