Recovery was limited at Site 1097, which severely affected physical properties data quality and distribution. Measurements are therefore scattered and discontinuous throughout the drilled interval. Natural gamma-ray (NGR) activity, magnetic susceptibility, and gamma-ray attenuation porosity evaluator (GRAPE) density were measured on whole-round sections (see "Physical Properties" in the "Explanatory Notes" chapter). All measurements were made down to near the base of Hole 1097A to a depth of ~400 mbsf.
Whole-core magnetic susceptibility was measured at 2-cm intervals (averaged over 2 s). The raw data are provided on CD-ROM and the World Wide Web (see "Related Leg Data" in the Table of Contents), and presented in Figure F23. The data show no obvious correlation with core lithologies.
Density was measured by gamma-ray attenuation at 2-cm intervals (averaged over 2 s at each point). The raw data are on CD-ROM and the World Wide Web (see "Related Leg Data" in the Table of Contents), and appear in Figure F23. The data show no apparent correlation with lithostratigraphic variation. Plots of GRAPE density vs. magnetic susceptibility reveal no distinct populations.
Whole-core natural gamma-ray emissions (averaged over 15 s) were counted at 15-cm intervals. The raw data are provided on CD-ROM and the World Wide Web (see "Related Leg Data" in the Table of Contents), and presented in Figure F23. There is evidence for a broad decrease in the NGR signal down the cored section.
Gravimetric and volumetric determinations of index properties were made on 30 samples from Hole 1097A. One sample was taken every first and third section per core, where possible. Wet mass, dry mass, and dry volume were measured and, from these measurements, percentage water weight, porosity, dry density, bulk density, and grain density were figured (see "Physical Properties" in the "Explanatory Notes" chapter; Fig. F24; see "Related Leg Data" in the Table of Content for raw data).
A positive correlation exists between GRAPE density and density from index properties (Fig. F24A). The index properties grain density shows a weak increase with depth. Porosity decreases below 150 mbsf, which probably reflects the increasing overburden.
The low-porosity interval between 80 and 150 mbsf matches a lithostratigraphic interval interpreted as subglacial diamict containing reworked marine biogenic material (see "Lithostratigraphy" and Fig. F24C). Given the much higher porosity in the glaciomarine material between 180 and 225 mbsf, the low porosity above probably reflects subglacial shear consolidation of ~20% within the diamict (estimated on the basis of the values in Fig. F24C). Some of the intervals lower in the hole are also potentially subglacial. However, the porosity increase seen at 180 mbsf would not be expected in these lower intervals because the ambient consolidation caused by overburden would already have caused the porosity to reach similar levels to those between 80 and 150 mbsf. Similar porosity level changes in diamicts have been associated with subglacial consolidation at Sites 739 and 742 (Barron, Larsen, et al., 1989). An alternative to shear consolidation would be vertical effective stress imposed by thick ice frozen at its base. However, this would probably have compacted more of the sediment column than just the relatively impermeable diamict. It should be noted, however, that possible overconsolidation by ice overburden was not consistently expressed in the porosities of the materials at Sites 739 and 742 (compare Barron, Larsen, et al., 1989, and Solheim et al., 1991) and could be partly responsible for the porosity variation seen here.
The porosity variation of the sediments may reflect size distribution of clasts and matrix components. However, smear-slide analysis (see "Site 1097 Smear Slides") suggests the average clay, silt, and biogenic particles of the upper subglacial sequence are 27.1%, 53.3%, and 9.75%, respectively, and the lower subglacial sequence has a similar size distribution (32.0%, 46.6%, and 10.8%). The magnetic susceptibility and natural gamma emission rate of sediments depend upon grain size and mineralogy, and a crossplot of the two subglacial sequences in magnetic susceptibility-NGR space suggests that the two sets of materials belong to the same population (Fig. F25). These similarities strengthen the argument for a subglacial shear origin of porosity changes; if the porosity in the lower subglacial sequence is assumed to represent the normal consolidation porosity for the material composing both sequences, the higher subglacial sequence, which has the same porosity but less overburden, must be overconsolidated. Of course, the lower material may also be overconsolidated, but the porosity step at the base of the upper subglacial sequence cannot be completely the result of the change from poorly to well-sorted material (cf. Site 1103) and must result at least partly from subglacial shear.
Discrete P-wave velocity measurements using the Hamilton Frame sensor pair (PWS3) of the velocity-strength system were made on cores from Hole 1097A for the depth interval 83-400 mbsf (see "Related Leg Data" in the Table of Contents for raw data). Most of the samples were single pieces of matrix-supported diamictite, placed directly (without the liner) between the Hamilton Frame heads. This data set is especially important because the continuity and quality of the recovered cores did not allow any data collection from the MST P-wave logging system. The velocity and density (index properties) data correlate very well, indicating good data quality (Fig. F26). The first data point of the velocity diagram is taken from interval velocities derived from seismic reflection profiles crossing the site.
Two samples were taken from Cores 178-1097A-10R and 25R to determine the structural composition of the material by impregnated thin-section analysis. The thin section from Sample 178-1097A-10R-1, 40 cm, shows a diamict with a randomly oriented fabric of sand, silt, and clay particles, through which a mineral showing high birefringence colors was emplaced. This mineral is likely to be calcite, although there is no pervasive calcium carbonate staining in the rest of the slide. The mineral was emplaced in a discrete vein that also shows some shear-zone attributes (clear shear geometry and offset limbs). Pyrite is also clearly present within the vein. Areas of biogenic material and a few areas outside the vein have also undergone precipitation and/or replacement by the two vein minerals. The slide also shows an intraclast of reworked diamict similar to the groundmass.
The cemented nature of the material may explain the recovery improvement in this core (the material is stiffer), as well as the higher P-wave velocity found at ~90 mbsf (see "Seismic Stratigraphy"). There is no indication in the seismic profile that the shear noted in association with the mineral deposition was the result of large-scale faulting. The geometry of soft-sediment shear zones is largely pressure controlled (Arch et al., 1988). Glacial sediments of the type seen in Section 178-1097A-10R-1 are usually found to have deformed pervasively, suggesting low effective pressures that are in line with our knowledge of subglacial environments. Thus, the discrete shear geometry of the mineralized areas (in a material with such a broad and potentially disruptive grain-size range) and the absence of an orientated fabric in the surrounding material (without a pervasive calcium carbonate cement) indicate that the confining pressure at the time of shear was higher than that likely subglacially, or that the material was lithified before subglacial fracture. Both situations would have protected fabrics around the shear from reorientation. Thus, the shearing is unlikely to represent subglacial soft-bedded deformation of the original diamict per se. The ultimate origin of the remaining material may, of course, still involve subglacial deformation. Subglacial shear has been suggested for the interval between 80 and 150 mbsf on the basis on porosity (above), large-scale structure, and the quality of biogenic material (see "Lithostratigraphy"), which suggests that the highly cemented Core 178-1097A-10R is not a representative sample of the interval that it overlies, which was considerably weaker when brought to atmospheric pressure.
The thin section from Sample 178-1097A-25R-1, 91 cm (Fig. F27), is from a laminated diamict. The laminae are beds of slightly varied grain-size distribution in the fine sand to silt component, which are disrupted, discontinuous, and ~3 mm thick. A pervasive fabric is oriented in the direction of bedding. The fabric gives a strong to moderate extinction to the matrix and probably resulted from shear. Although such a fabric may be produced by deposition from a fluid, particularly after consolidation, the angle of the bedding at 20º from vertical and the pervasively disrupted and discontinuous nature of the beds suggest shear reorientation of the grains. The material also shows some burrowing, the traces of which are sites of pyrite deposition. We could not determine whether the bioturbation occurred before or after the shearing. Given the pervasive nature of the shear fabric, however, it is highly likely that the shear preceded at least the majority of the bioturbation.
The evidence above indicates the sedimentation of a material of a wide and varying size range. The beds show no internal sorting suggestive of separation through a water column, although negative evidence is not sufficient to rule out this possibility. Sedimentation was followed by deformation and bioturbation. As such, the structural and deformational evidence matches the glaciomarine origin for the material at this depth in the sediment body proposed from macroscopic evidence (see "Lithostratigraphy"). However, the evidence also encompasses the possibility of deposition into a subglacial water body that has been overridden by a glacier with only a thin or rising deformation layer. Pyrite deposition and shear alignment of the matrix grains at an oblique angle to the drilling direction may explain the relatively good recovery for this length of core.