At Site 1254, laboratory measurements were made to provide a profile of physical properties across a fractured interval (150-223 mbsf) and the décollement (300-367 mbsf), both previously cored in Hole 1040C, which is located 15 m to the east. All cores were initially passed through the multisensor track (MST) before being split. Gamma ray attenuation (GRA) bulk density, noncontact electrical resistivity (NCR), and volumetric magnetic susceptibility measurements were taken at 2-cm intervals, with measurements averaged from three separate 1-s data acquisitions for all cores. P-wave velocity logger measurements were not taken because of the small and variable diameter of RCB cores that generally leads to poor coupling between the core liner and recovered core. Natural gamma radiation (NGR) was counted every 10 cm for 20-s intervals. Drilling disturbance and gas expansion cracking were noted in all cores and degraded the volumetric magnetic susceptibility and GRA bulk density MST measurements. As at Site 1253, data collected with the NCR instrument exhibited significant scatter, and measured resistivity values were consistently unreasonably low compared to measurements from Leg 170, suggesting a problem with the instrument or its calibration. These data will not be discussed further. No P-wave velocity or thermal conductivity measurements were made at this site, because the cores were too fractured and disturbed to obtain P-wave arrivals using the P-wave velocity sensor system, and the cores were unsuitable for either the full-space or half-space thermal conductivity probes.
Moisture and density samples were selected from undisturbed core at regularly spaced intervals of two per section (75-cm resolution). Measurements of dry volume and wet and dry mass were uploaded to the ODP Janus database and were used to calculate water content, bulk density, grain density, porosity, void ratio, and dry bulk density. Calculated physical property data are available from the ODP Janus database for all MST and moisture and density measurements.
In the interval from 150 to 223 mbsf (Cores 205-1254A-1R through 8R), porosity determined from moisture and density measurements decreases from values of 47%-58% near its top to 39%-51% at its base, and bulk densities increase from 1.71-1.88 to 1.82-1.93 g/cm3 (Fig. F29A, F29B). Several porosity and bulk density values at ~200 mbsf deviate significantly from the overall trend in the interval from 150 to 223 mbsf (Fig. F29A, F29B). These porosity and bulk density values can be attributed to error in grain density determinations on these moisture and density samples (see discussion below).
In the interval from 300 to 362 mbsf (Cores 205-1254A-9R through 15R), porosity remains generally uniform but scattered, ranging from 42% to 55%, and increases abruptly to 57%-60% in the interval from 362 to 367 mbsf (Core 16R). Bulk density ranges from 1.87 to 2.05 g/cm3 between 300 and 362 mbsf and decreases to 1.63-1.71 g/cm3 from 362 to 367 mbsf. Both porosity and bulk density values from Site 1254 are consistent with those from Hole 1040C (Leg 170), located 15 m to the east (Fig. F29A, F29B).
When viewed in detail, porosities and bulk densities within the structurally defined décollement zone (338.5-364 mbsf) are scattered but appear to correlate with core observations (Fig. F29C, F29D). In parts of the fault zone characterized by "spiral" drilling-induced fabric, porosity is generally lower (~40%-45%, corresponding to bulk densities of 1.92-2.03 g/cm3) than in portions of the fault zone characterized by brecciation (~50%-55%, corresponding to bulk densities of 1.76-1.84 g/cm3) (see "Structural Geology"). Porosity also decreases abruptly to ~42% within a small zone at 361 mbsf, identified in the core description as both the lithologic boundary between older prism sediments and Pleistocene diatomaceous claystone (termed Unit "U1" at Sites 1039, 1043, and 1040) (Kimura, Silver, Blum, et al., 1997) and a zone of localized shear (see "Structural Geology").
GRA densities show significant scatter throughout both cored intervals, probably a result of core expansion and cracking due to gas. The highest values of GRA density are consistent with those determined from moisture and density properties and exhibit the same sharp decrease at 362 mbsf as seen in moisture and density data (Fig. F30).
Grain densities determined from dry mass and volume measurements generally range from 2.62 to 2.94 g/cm3, which are significantly greater than values of 2.40 to 2.65 g/cm3, measured at Site 1040 (Fig. F31) and also higher than that expected for most marine sediments. Grain densities determined at Site 1254 also show considerably more scatter than those from Site 1040. To evaluate the effect that possibly erroneous grain density determinations had on calculated porosity and bulk density values shown in Figure F29A and F29B, porosity and bulk density were recalculated using a constant grain density of 2.65 g/cm3, the mean value measured in samples from Site 1040 (Fig. F32A, F32B). The general shift in grain densities between Sites 1254 and 1040 and the scatter in grain densities at Site 1254 have a negligible effect on calculated porosity and bulk density values (Fig. F32A, F32B). However, several grain density determinations at ~200 mbsf, with values ranging from 1.70 to 2.25 g/cm3, deviate significantly from values above and below and do affect calculated porosity and bulk density in this interval (Figs. F29A, F29B, F32).
Dry volumes for the outliers in grain density near 200 mbsf were all measured in the same pycnometer cell, suggesting a probable miscalibration of this cell. The generally high (>2.70 g/cm3) grain densities at Site 1254 also suggest analytical error, most likely in dry volume measurement, but a specific instrumental problem has not been found.
NGR results are presented in counts per second (cps) (Fig. F33). The background, produced by Compton scattering, photoelectric absorption, and pair production (12.95 cps), was measured at the beginning and during MST runs for each core section and subtracted from the measured gamma ray values to obtain corrected counts. NGR results are relatively uniform and scattered from 9.5 to 20 cps throughout the cored intervals above 362 mbsf, with the exception of three zones characterized by several measurements of significantly higher values (21-32 cps) at 204-207, 218-223, and 300-303 mbsf. NGR values increase abruptly across the lithologic boundary at 362 mbsf, from 9-20 cps above to 18-33 cps below.
Volumetric magnetic susceptibility measured with the MST shows generally low values from 150 to 223 mbsf, with one excursion toward higher values between 180 and 195 mbsf (Fig. F34). Below 312 mbsf, magnetic susceptibility is highly variable, with two pronounced peaks at 320 and 340 mbsf. Some of the peaks in magnetic susceptibility between 312 and 367 mbsf correspond to sand layers noted in core description (see "Lithostratigraphy"), whereas others do not.
Porosities and bulk densities at Site 1254 exhibit trends similar to those seen in Hole 1040C, located 15 m to the east (Fig. F29A, F29B). Variations in porosity and density within the structurally defined décollement zone correlate with core descriptions: in general, zones of lower porosity (40%-45%) correspond to zones characterized by "spiral" deformation interpreted as drilling disturbance; zones of higher porosity (50%-55%) correspond to zones characterized by brecciation. Porosity is also low (42%-44%) between 358 and 361 mbsf, within and adjacent to a zone of localized shear (see "Structural Geology"). Porosity increases and bulk density decreases sharply below 362 mbsf across the lithologic boundary between prism sediments and Pleistocene diatomaceous claystone. A systematic error resulting in unreasonably high grain densities, with an average of 2.84 g/cm3, results in only a small error in calculated porosities and bulk densities (Fig. F32).
MST NGR counts are generally uniform throughout the cored intervals above 362 mbsf and exhibit a sharp increase from 9-20 cps above the lithologic boundary to 18-32 cps below. MST magnetic susceptibility data are generally low and uniform in the interval from 150 to 223 mbsf, with one peak at 180-195 mbsf. In the lower cored interval below 312 mbsf, magnetic susceptibility is characterized by highly variable values and several distinct peaks. Some of these peaks correlate with sand layers noted in core description, but the cause of other peaks in magnetic susceptibility in this interval is unclear.