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Diagenetic processes are essentially dewatering processes. Porosity decreases and pore water escapes from the pore space with depth in shallower portions because of mechanical compaction. Deeper, dehydration from mineral phases will occur. Therefore, diagenetic processes are strongly related to the fluid circulation system in many sedimentary basins. Diagenetic processes are also related to changes in physical properties such as porosity, permeability, and so on. Porosity and permeability decrease because of not only mechanical compaction but also chemical precipitation as a cementation process (e.g., Houseknecht, 1987; Bjørkum et al., 1998). Changes in the physical properties control fluid pressure, which directly affects the effective stress. Therefore, rock strength and frictional strength on a fault can also be related to diagenetic processes.
A subduction zone is the largest fault fluid system on Earth. Diagenetic processes in a subduction zone are important to understanding fluid circulation around the subduction zone (e.g., Bray and Karig, 1985; Cochrane et al., 1994; Saffer et al., 2000; Saito and Goldberg, 2001), which might be related to the formation of gas hydrate and earthquakes. Diagenetic processes in a subduction zone have not been well documented thus far. These processes should differ from those in an oceanic basin because of tectonically driven stress or the underthrusting of sediments themselves (e.g., Moore and Karig, 1976).
Diagenetic processes include mainly of mechanical compaction, compactive cataclasis, chemical dehydration, and cementation. Those mechanisms might occur in order from shallow to deep (Hashimoto et al., 2006). The strength of compactive cataclasis for coarser sediments is well studied in laboratory experiments (e.g., Zhang et al., 1990; Wong et al., 1997). The studies provide the relationship between effective stress, porosity, and grain size at the onset of compactive cataclasis under both hydrostatic and differential stress conditions.
Grain size distribution can be used to examine whether the sediment is formed by cataclasis. For instance, in sandstone blocks from an on-land accretionary complex, grain size distribution represents a fractal distribution, which is fit by a power function within a web structure (Hashimoto et al., 2006). A web structure is known to be formed by cataclastic deformation (Byrne, 1984). On the other hand, grain size distribution in a host part of the sandstone does not show a fractal distribution but an exponential relationship (Hashimoto et al., 2006).
The fractal distribution in grain size may relate to cataclastic deformation in some cases. Because cataclasis acts to reduce grain size and porosity (Zhang et al., 1990; Wong et al., 1997), we examine parameters related to fractal distribution in grain size as a means of estimating the degree of compactive cataclasis. The sample from the on-land accretionary complex is from a portion deep enough that after the diagenetic processes were completed all of the processes were overprinted. To understand the mechanism in total, samples from shallow to deep need to be examined. In this paper, we provide grain size distributions and variations with depth and porosity for a shallow accretionary prism, the Cascadia margin.