(Table 1) Anisotropy of magnetic susceptibility parameters of ODP Hole 134-833B

Ocean Drilling Program (ODP) Sites 832 and 833 were drilled in the intra-arc North Aoba Basin of the New Hebrides Island Arc (Vanuatu). High volcanic influxes in the intra-arc basin sediment resulting from erosion of volcanic rocks from nearby islands and from volcanic activity are associated with characteristic magnetic signals. The high magnetic susceptibility in the sediment (varying on average from 0.005 to more than 0.03 SI) is one of the most characteristic physical properties of this sedimentary depositional environment because of the high concentration of magnetites in redeposited ash flows and in coarse-grained turbidites. Susceptibility data correlate well with the high resolution electrical resistivity logs recorded by the formation microscanner (FMS) tool. Unlike the standard geophysical logs, which have low vertical resolution and therefore smooth the record of the sedimentary process, the FMS and whole-core susceptibility data provide a clearer picture of turbiditic sediment deposition. Measurements of Curie temperatures and low-temperature susceptibility behavior indicate that the principal magnetic minerals in ash beds, silt, and volcanic sandstone are Ti-poor titanomagnetite, whereas Ti-rich titanomagnetites are found in the intrusive sills at the bottom of Site 833. Apart from an increase in the concentration of magnetite in the sandstone layer, acquisition of isothermal and anhysteretic remanences does not show significant differences between sandstone and clayey silts. The determination of the anisotropy of magnetic susceptibility (AMS) in more than 400 samples show that clayey siltstone have a magnetic anisotropy up to 15%, whereas the AMS is much reduced in sandstone layers. The magnetic susceptibility fabric is dominated by the foliation plane, which is coplanar to the bedding plane. Reorientations of the samples using characteristic remanent magnetizations indicate that the bedding planes dip about 10° toward the east, in agreement with results from FMS images. Basaltic sills drilled at Site 833 have high magnetic susceptibilities (0.05 to 0.1 SI) and strong remanent magnetizations. Magnetic field anomalies up to 50 µT were measured in the sills by the general purpose inclinometer tool (GPIT). The direction of the in-situ magnetic anomaly vectors, calculated from the GPIT, is oriented toward the southeast with shallow inclinations which suggests that the sill intruded during a reversed polarity period.

Anisotropy and acoustic properties at DSDP Holes 62-463 and 62-465A

Variations of acoustic properties within the sediment column may significantly affect the propagation of acoustic energy in the upper portion of the oceanic crust. Moreover, the acoustic properties of sediments reflect their mineral compositions, fabrics, and degrees of compaction and cementation. Hence, the physical properties of indurated deep-sea sediments are of considerable geophysical and geological interest. Chalks and limestones are particularly important because substantial accumulations of biogenic carbonates are generally present at the base of the deep-sea sediment column, and high-standing features such as Hess Rise are capped by calcareous deposits. This paper constitutes a preliminary report of the compressional-wave velocities and densities of 31 indurated calcareous sediment samples recovered at DSDP Sites 463 and 465, in the Mid-Pacific Mountains and on Hess Rise, respectively. The sample set includes nine pairs of samples in which velocities were measured parallel and perpendicular to bedding to determine the velocity anisotropy of the sediment. This research is part of an ongoing study of the seismic properties of indurated deep-sea carbonates.

Consolidation properties and hydraulic conductivities of sediments from ODP Leg 207 sites

The stress history, permeability, and compressibility of sediments from Demerara Rise recovered during Ocean Drilling Program Leg 207 were determined using one-dimensional incremental load consolidation and low-gradient flow pump permeability tests. Relationships among void ratio, effective stress, and hydraulic conductivity are presented for sampled lithologic units and used to reconstruct effective stress, permeability, and in situ void ratio profiles for a transect of three sites across Demerara Rise. Results confirm that a significant erosional event occurred on the northeastern flank of the rise during the late Miocene, resulting in the removal of ~220 m of upper Oligocene-Miocene deposits. Although Neogene and Paleogene sediments tend to be overconsolidated, Cretaceous sediments are normally consolidated to underconsolidated, suggesting the presence of overpressure. A pronounced drop in permeability occurs at the transition from the Cretaceous black shales into the overlying Maastrichtian-upper Paleocene chalks and clays. The development of a hydraulic seal at this boundary may be responsible for overpressure in the Cretaceous deposits, leading to the lower overconsolidation ratios of these sediments. Coupled with large regional variations in sediment thickness (overburden stresses), the higher permeability overpressured Cretaceous sediments represent a regional lateral fluid conduit on Demerara Rise, possibly venting methane-rich fluids where it outcrops on the margin's northeastern flank.

Flow rates, hydraulic conductivities and permeabilities of sediments from ODP Holes 190-1173A and 190-1174B

Understanding the role of fluids in active accretionary prisms requires quantitative knowledge of parameters such as permeability. We report here the results of permeability tests on four samples from Ocean Drilling Program Leg 190 at the Nankai Trough accretionary prism-two from Site 1173 and two from Site 1174. Volcanic ash is present in one of the samples; otherwise, the material is hemipelagic mud. A constant-rate-of-flow technique was used at various effective pressures and rates of flow. The permeability of the four samples ranges between 10**-15 and 10**-18 m**2, with the ash-bearing sample showing the highest values.