IODP Proceedings    Volume contents     Search

doi:10.2204/iodp.proc.324.105.2010

Physical properties

Volcaniclastic rocks and sediments in Hole U1348A were characterized for physical properties as described in "Physical properties" in the "Methods" chapter. Core sections with continuous intervals longer than 8 cm were run through the Whole-Round Multisensor Logger for measurement of gamma ray attenuation (GRA) density and magnetic susceptibility. Data from whole-round measurements were filtered by a MATLAB code to remove data associated with gaps and small pieces as described in "Physical properties" in the "Methods" chapter. The filtered data were then visually double-checked against images of the core section halves. Sections longer than 50 cm were measured with the Natural Gamma Radiation Logger. Thirteen discrete oriented cubic samples were cut from the working half of the cores for measurement of moisture and density (MAD) properties as well as compressional (P-wave) velocities in three directions.

Whole-Round Multisensor Logger measurements

Magnetic susceptibility

Results for whole-round magnetic susceptibility in Hole U1348A are summarized in Figure F52. The volcaniclastic sedimentary material recovered at Site U1348 has magnetic susceptibilities of <110 x 10–5 SI. These magnetic susceptibility values are an order of magnitude less than those observed in igneous material at Sites U1347 and U1350. Magnetic susceptibility shows an overall decrease downhole roughly correlating with interpreted stratigraphic units (see "Sedimentology"). The upper interval from 210 to 242 mbsf (stratigraphic Subunits IIIa and IIIc; Cores 324-U1348A-15R through 18R) is characterized by magnetic susceptibilities of 20 x 10–5 to 50 x 10–5 SI. The lower interval, from 263 to 320 mbsf (stratigraphic Units V and VI; Cores 324-U1348A-20R through 26R) is characterized by magnetic susceptibilities of 15 x 10–5 to 30 x 10–5 SI. Moderate oscillations around these averages (e.g., highs up to 100 x 10–5 SI in intervals 324-U1348A-17R-1, 0–10 cm, and 58–60 cm; 19R-1, 106–108 cm; 20R-1, 18–118 cm; and 20R-4, 73–92 cm) and smaller variations around ±15 x 10–5 SI throughout the entire hole are likely due to sedimentary layering, possibly concentrating magnetically susceptible material in specific horizons (see "Sedimentology").

Gamma ray attenuation bulk density

The results for GRA bulk density measurements are summarized in Figure F52. At Site U1348, volcaniclastic sediment filled most of the liner diameter and GRA bulk density measurements match well with discrete sample bulk density measurements (see "Moisture and density" below).

The hole can be divided into four broad intervals based on GRA density. The transitions between these intervals are well correlated with changes in designated stratigraphic units (Fig. F52). From 190 to 203 mbsf (Sections 324-U1348A-13R-1 through 14R-3), which corresponds to stratigraphic Unit II–Subunit IIIa, described as altered granular hyaloclastite (see "Sedimentology"), the GRA density is generally >2.0 g/cm3, with values as high as 2.4 g/cm3. GRA density is markedly lower between 209 and 242 mbsf in Subunit IIIc, made up of altered granular hyaloclastite to fine hyaloclastite (Sections 15R-1 through 18R-3), with values between 1.8 and 1.9 g/cm3. From 257 to 287 mbsf (Sections 20R-1 through 23R-1), broadly corresponding to the layered granular hyaloclastite of stratigraphic Unit V (see "Sedimentology"), the density exhibits a smooth, gradual increase from 2.1 to 2.4 g/cm3. Finally, in stratigraphic Unit VI (hyaloclastite breccia; see "Sedimentology") between 295 and 315 mbsf (Sections 24R-1 through 26R-1), the density drops at the top of the unit and then again shows a gradual increase from 1.8 to 2.1 g/cm3 downhole (Fig. F52).

Natural Gamma Ray Logger

Measurements of NGR are summarized in Figure F52. Counts per second at Site U1348 are higher (5–30 cps) than seen in the relatively fresh igneous material recovered at Site U1347 (2–4 cps) but are generally not as high as the counts per second measured in altered igneous material at Site U1346 (consistently 20–30 cps). From 190 to 191.4 mbsf (Section 324-U1348A-13R-1), corresponding to stratigraphic Unit II, described as altered granular hyaloclastite (see "Sedimentology"), counts per second are between 10 and 20 cps. Subunit IIIa, composed of alternating layers of granular and fine hyaloclastite and lying between 199.5 and 203.61 mbsf (Sections 14R-1 through 14R-3), has a decreased NGR signal, generally <10 cps. Subunit IIIc is also described as layered granular and fine hyaloclastite but shows an increase to 10–15 cps compared with Subunit IIIa. For a 2 m interval between 240 and 242 mbsf (bottom of Section 18R-3 to top of 18R-4), which marks the transition from Unit III to the volcaniclastic sandstone with fossils of Unit IV, the count rate increases to >30 cps. Between 257 and 287.1 mbsf (Sections 20R-1 through 23R-1), in the layered volcaniclastics of Unit V, the signal is lower and oscillates between 4 and 28.1 cps (average = 10.9 cps). Finally, in the remainder of the hole between 285 and 320 mbsf (Sections 23R-1 through 26R-3) and representing Unit VI (hyaloclastite breccia), NGR drops to an average of 4.9 cps.

Moisture and density

A summary of results for bulk density, dry density, grain density, void ratio, water content, and porosity on discrete samples is shown in Table T7. Density and porosity data are shown in Figure F53. The samples cut from the material recovered at this site were prone to crumbling and cracking. Material could be lost during the measurement process (e.g., edges and corners crumbling away); therefore, the reported densities should be considered minima and porosities considered maxima. The high content of hydrous phases (e.g., clays and zeolites; see "Sedimentology") also resulted in the samples cracking after dehydration in the oven at 105°C. Despite cracking, the samples exhibit the expected inverse relationship between P-wave velocity and porosity and positive correlation between bulk density and P-wave velocity (Fig. F54). The discrete sample from Section 324-U1348A-23R-3 in the hyaloclastite breccia Unit VI is a clear outlier in Figure F54, displaying anomalously low P-wave velocity for its measured bulk density. However, this sample cracked during a subsequent P-wave measurement. Despite the tendency of cracking and crumbling in the samples from Site U1348, the bulk density and P-wave velocity data correspond well with downhole logging data (see "Downhole logging").

Compressional (P-wave) velocity

Downhole variation of compressional wave velocity is summarized in Figure F54 and listed in Table T8. As with MAD measurements, these should be treated with caution because of sample cracking and crumbling. The measured P-wave velocities do not show a coherent relationship with stratigraphic units, in contrast to whole-round magnetic susceptibility, GRA density, and NGR. This could be due to sample material that was prone to crumbling upon P-wave measurement and/or the heterogeneous nature of the sampled cores. However, it is clear that the recovered volcaniclastic sedimentary material is consistently slower than igneous material recovered from Sites U1346, U1347, U1349, and U1350. The measured P-wave velocities correspond closely to downhole logs (see "Downhole logging"). All measured discrete samples yielded values <3.5 km/s, ranging from 2.09 to 3.32 km/s (average = 2.78 ± 0.79 km/s; N = 13).