Argillaceous sediments change their internal microfabrics drastically at shallow burial depths by compaction resulting from particle reorientation. This process is associated with pore volume decreases (Bennett et al., 1981). The compaction model in previous studies is summarized as follows, from shallow to deep. First, clay platelets are linked by edge-to-edge (EE), edge-to-face (EF), or stepped face-to-face (FF) contact in a random arrangement, forming a long flocculation chain. This structure is called "cardhouse" fabric. Next, during further burial, such contacts in flocculations change to FF contact, forming preferred horizontal orientation (Bennett and Hulbert, 1986). These fabric changes in argillaceous sediments have been detected by shallowing of inclination of remanent magnetization, which is mostly a result of small magnetite grains of single-domain size being reoriented in preferred horizontal orientation because of compaction (Anson and Kodama, 1987; Deamer and Kodama, 1990).
At the sediment/water interface, argillaceous sediments do not have a homogeneous cardhouse fabric but have heterogeneous fabrics because of bioturbation, bottom-current disturbance, crystallization, dissolution of minerals, and so on (Reynolds and Gorsline, 1992; Bennett et al., 1991). For example, fine-grained particles are aggregated into heterogeneous fabrics by various processes as follows. Benthic animals that eat organic materials in mud make an aggregation of fine-grained particles in their own gut (Reynolds and Gorsline, 1992). Agitation by bioturbation and bottom currents at the sediment/water interface reaggregates the fine-grained particles as a result of organic matter's adsorption and electrostatic attraction (Bennett et al., 1991; Stolzenbach et al., 1992). Then, the fine-grained particles are reoriented from EF and EE contact to FF contact by shear stress caused by the internal flow that occurs because of bioturbation and bottom-current disturbance even at the sediment/water interface. This idea was already well developed by O'Brien and Slatt (1990) and Bennett et al. (1991).
Through these complicated processes at the sediment/water interface and at shallow burial depths, fine-grained particles are eventually aggregated to what are called "peds" (Yong, 1972; Collins and McGown, 1974; Reynolds and Gorsline, 1992). Although the peds play an important role in the early compaction process of argillaceous sediments (Yong, 1972; Velde, 1996), real examples from the deep sea have not been well studied except by Collins and McGown (1974), Reynolds and Gorsline (1992), and Kawamura et al. (1999). In the present study, we will describe detailed microfabrics of the argillaceous sediments of 180-m-long cored sediments recovered from the northwest Pacific in view of peds and their interrelational development. Polarized microscope, scanning electron microscope (SEM), and magnetic fabric analyses by measuring anisotropy of magnetic susceptibility (AMS) were used for microfabric observations. Grain-size distribution and X-ray diffraction (XRD) were also conducted in this study. A new concept for the early phase of the compaction process, which gives more importance to biologic process that forms peds at various depths, is given here.