The peds in Unit I are relatively large in size (10-100 µm diameter), with porous fabric, and are composed of several kinds of particles. In contrast, those in Unit II are small-sized clay aggregations (3-30 µm diameter). The different features and scales of peds at different depths provide different formation processes, as discussed below.
How fine sediment is aggregated into a flocculated domain to form a ped plays an important role in the pore distribution and framework of the microfabric. Three characteristics, the mode of arrangement of particles, the shape and size of peds and pores, and the degree of recrystallization of minerals, systematically change downward. Together with these changes, many physical properties also change continuously downward. These relations are discussed below with the progressive compaction process.
The formation processes of microfabric in argillaceous sediments through deposition to burial are physical-chemical, bioorganic, and burial diagenesis as discussed by Bennett et al. (1991). According to them, the physical-chemical processes, including bonding mechanisms by van der Waal's attraction, electrostatic attraction, and heating effects, play a major role in microfabric development during the fluvial and eolian transport stage of particulates and on their contact with a depositional interface. The bioorganic process plays an important role in marine and coastal environments during transport and sedimentation of particulates, particularly in surficial sediments. It includes three biotic mechanisms. First is a biomechanical mechanism, which is the aggregation and disturbance of particulates by planktonic and benthic animals. For example, burrow and fecal pellets are produced by this mechanism. Second is the biophysical mechanism, in which each particulate is flocculated by organic materials as marine snow. Third are the biochemical mechanisms, in which authigenic minerals are formed by bacterial activities in sediment as a formation process of framboidal pyrite. The processes of burial diagenesis drives microfabric development when overburden or tectonic stresses dominate physical-chemical and bioorganic bonding energies (Bennett et al., 1991). In Unit I, because the original sedimentary fabrics (e.g., volcanic ash layer) are completely disturbed by bioturbation, the microfabric is affected by the bioorganic mechanism, especially biomechanical processes but not by physical-chemical mechanism.
Several studies of the microfabric of Holocene sediments have been published on micrographs of features similar to the large peds described in this study. For example, Collins and McGown (1974) described regular aggregations from several kinds of marine sediments similar to the large peds. However, they used an air-drying technique that might have changed the original texture of some artifacts; exact correlation of internal microfabrics between the regular aggregation of these studies and large peds in this study is difficult. Reynolds and Gorsline (1992) observed biofloc from hemipelagic clayey sediments (Santa Monica Basin off the California coast) whose structure is similar to the large peds described in this study. The bioflocs were observed only in the sediments and rocks with bioturbation but were not observed in nonbioturbated turbidite mud and mudstone deposited under anoxic conditions. They concluded that the origin of bioflocs is due to dissolution of fecal pellets and/or pellets of benthic animals because the bioflocs could not be observed in sediments that are not affected by biomechanical processes. Therefore, it is believed that the present peds and their aggregation are the result of biomechanical processes working in pellet formation.
The large peds in Unit I are mechanically compressed in an ovoid shape at a shallower depth than 2.13 mbsf (Sample 185-1149A-1H-2, 63-65 cm). The possible mechanism of mechanical compression in such shallow burial depth is the aggregation process by benthic animals as biofloc. The large peds are clearly distinguished from the fecal pellets in texture and color by polarized microscope observations. Fecal pellets are clearer in shape and darker than peds (Fig. F8D). This confirms our conclusion that the origin of large peds is pellets of benthic animals.
The connectors are long chains of clay platelets in EF contact. In the Mississippi Delta, sediments are dominated by domains of low- to high-angle EF contacts because of physical-chemical flocs (Bennett et al., 1981). The physical-chemical process in marine sediment is due to electrostatic attraction between clay platelets. EF-particle contacts dominate because the faces of clay minerals are negatively charged, whereas the edges are positively charged (Bennett and Hulbert, 1986). In general, the clay platelets at EF contacts form long chains at shallow burial depths as cardhouse fabric (Bennett et al., 1981). Therefore, the connectors in Subunit IIA are probably formed by electrostatic attraction in shallow burial depths and are preserved at the present depths.
The small peds of clay platelets at low-angle EF and FF contacts observed in this study are similar to floc described in some pelagic clays at 143 mbsf in the eastern equatorial Pacific (Bennett et al., 1981). The outer boundaries of the flocs are well defined, and their structure in transmission electron microscope images is not similar to physical-chemical flocs (Reynolds and Gorsline, 1992). Throughout deposition and burial, there are many possible processes that clay platelets are formed from low-angle EF and FF contact (Bennett et al., 1991). For example, FF contacts are constructed either by the burial compaction process of flocculated clay platelets of EF contacts (Bennett et al., 1981, 1991) or by dispersion depositional process that lack strong bonding (e.g., electrostatic attraction) at each contact point of the clay platelets (Moon and Hurst, 1984; Bennett et al., 1991).
Another mechanism of clay platelets at low-angle EF and FF contacts is clay aggregation processes by zooplankton, which produces a fecal pellet of ~100 µm diameter (Bennett et al., 1991). Small peds could form through fragmentation of such fecal pellets. Pellets of zooplankton play an important role in transporting clay platelets from the sea surface to the seabed under pelagic environments (Honjo, 1978). During sinking in seawater, they become porous and fragile because much of the organic matter (5.6-18 wt%) (Honjo, 1978) in the pellet is decomposed by bacterial activity (Honjo and Roman, 1978). In sediment-trap studies, fragmental pellets of several tens of micronmeters diameter have been described (Honjo, 1978, 1979). Some of the fragmental pellets probably further become porous and fragile by decomposition of calcareous and siliceous biogenic tests, whose contents are 10%-50% and <5%, respectively. Residue of pellets after such decomposition preserves biomechanical aggregation of clay platelets, and it is further dispersed by shear stress because of the internal flow by bioturbation and bottom-current disturbance. These fragmental fecal pellets are still connected by long chains of clay platelets in EF contact as a result of electrostatic attraction as small peds, which we saw in our samples in Subunit IIA.
The porosity decreases steadily down to ~60 mbsf but increases to 118 mbsf at the boundary between Units I and II and then decreases steadily again down to 180 mbsf at the boundary between Units II and III. The decrease in rate from 0 to 60 mbsf and that from 118 to 180 mbsf is different, indicating different compaction processes.
In Unit I, porosity decreases from ~70% to 60% down to 60 mbsf. Magnetic fabric reflects random orientation of magnetic minerals in Unit I (Fig. F3). The size and shape of the large peds do not change with depth (Fig. F8A, F8B). In between the large peds, however, the contact of each coarse-grained particle changes from a high-angle EF contact to low-angle EF and FF contact compaction (Fig. F8). Coarse-grained particles would slide and rotate perpendicular to the maximum effective stress direction. Even through such particle rearrangement, the total direction of the particle keeps random arrangement. The size and shape of the large ped do not change during compaction. Thus, the macropores would reduce their size during early compaction in Unit I, resulting in porosity decreases in Unit I.
In Unit II, the porosity decreases from ~60% to 50% by overburden pressure are probably due to connector breakage and small ped deformation. Subunit IIA still has distinctive connectors that interconnect the small peds, whereas they are unclear in Subunit IIB (Figs. F8, F9). The connector represents a concentration point of the effective stress in the microfabrics. As the effective stress works at the connecting edge between the peds, compaction is expected to be present by deformation and destruction of connectors, as shown by Griffiths and Joshi (1990). As a result, the macropores between the small peds would close with this connector breakage process. After the connectors are broken completely, the vertical effective stress induces the changes of shape of the small peds (Fig. F13). The contact of clay platelets in the small ped also changes from EF to FF (Fig. F9). Micropores between the clay platelets in EF contact are reduced during ped deformation in accordance with the porosity decrease. These processes would contribute to porosity decrease in Unit II.