The SEM observation shows that nannofossils and planktonic foraminifers in the sediments are well preserved throughout. Particularly, assemblage of nannofossils plays an important role in making a void space in the unlithified sediments. Although the number of spine-shaped fragments of nannofossils is small, it tends to increase with depth (Fig. F1). These occurrences suggest that weak deformation, probably due to compaction, occurred during burial. The shape and volume of the void space are controlled by the type of aggregation pattern and material features. In the case of clay minerals, such as kaolinite, this type of fabric results in an open internal floccule framework with very high porosity, (Bennett and Hulbert, 1986) possibly more than 80%. By using AMS measurements, on the other hand, we could not detect any anisotropy of the unlithified sediments. The Flinn diagram of L vs. F shows that the anisotropy is almost neutral (Fig. F3). This is consistent with the SEM observation results. The increase of the magnetic susceptibility with depth can be explained by successive compaction processes of the sediments and an increasing number of basalt clasts. The unit volume of sediment reduced during burial so that the numbers of magnetic fabrics in each unit volume increase with depth.
The results of previous experimental investigation of the behavior of clay soils during loading are relevant to the type of deformation and all rocks that contain clay materials . Although oriented fabrics can be formed in kaolinite and montmorillonite pastes, the strongest orientations occur mainly in illite-bearing pastes (e.g., Tarling and Hrouda, 1933). In addition, simple shear tends to strengthen montmorillonite clay structures but to destroy the cohesion of kaolinite clays. However, the initial fabrics are determined by gravitational and hydrodynamic forces, such as bottom current, and are controlled mainly by the size, shape, and mass of the detrital grains in a passive environment. On the other hand, if the sediments experienced a strong simple shear deformation (e.g., sediments just above the décollement zone at the Nankai accretionary prism or at the Barbados accretionary prism), the anisotropy of magnetic susceptibility of these sediments tends to be larger (e.g., Owens, 1993; Housen, 1997). The sediments in Hole 1074A contain kaolinite, montmorillonite, and illite and represent little anisotropy. This might be due to only vertical loading, although quantitative investigation will be required in the future.
The onboard gamma-ray attenuation bulk densiometer (GRA bulk density) measurement and porosity data show no significant change in either the density or porosity throughout (Fig. F2). The GRA bulk density is almost constant, varying from 1.5 to 1.7 g/cm3. Porosity of discrete samples varies from 60% to 70% throughout, except when above 10 mbsf, where porosity ranges from 40% to 70%.
The data sets in this study should be quantified for future application to active tectonic settings as a reference of the initial condition and for comparison with various tectonic settings. In fact, the behavior of unlithified sediments (i.e., consolidation processes) varies in each accretionary prism because the contents of sediments are different in each area. For example, large-sized grain (e.g., foraminifer) content might exert control on the degree of sediment consolidation. Such large-sized grain content could be quantified by counting foraminifers under the SEM or on smear slides or by weight percent through grain size separations. Compaction of sediments could be quantified through porosity measurements, although the initial porosity of a given sample will never be known. The process of compaction could be studied through consolidation tests, allowing the compaction process of various unlithified sediments to be clarified quantitatively.