A long piston core taken from the Southern Nares Abyssal Plain, intersected four fractures in plastic sediments between 17 and 25 m below the sea floor Faults have been identified from seismic reflection surveys of sediments in this area The sampled fractures all occurred in oxidized brown clays Each fracture consisted of a simple plane having apparent dips ranging from 52–63" One fracture had a well developed pale brown alteration halo extending out to 15 cm along this plane Two fractures had no apparent alteration halo, but one fracture appeared to have fine scale anastomosing features surrounding the main slip plane.
Selective chemical tests for labile metal content in sediments surrounding the fractures revealed that about 70% of the reducible manganese, and 40% of the reducible iron had been leached from the sediments in the alteration halo surrounding the fracture plane These results suggest that reducing pore fluids had migrated along the fracture plane to cause the observed effects
Implications of this study are that compaction faults may act as episodic conduts for vertical advection of pore water during dewatering of unconsolidated sediments This may be a significant factor to be considered in assessing the effectiveness of deep sea sediment barriers for radioactive waste disposal.
For a number of years, acoustic reflection surveys of potential nuclear waste disposal sites in the North Atlantic have found evidence that faults occur at relatively shallow depths in the sediments (Duin et al, 1984, Buckley and Grant, 1985, Williams, 1987, Shephard et al, 1988, Perdijk, 1988) These faults have been characterized by both high and low frequency acoustic profilers Apparent displacement along the faults increases with depth in the sediment, reaching about 10 m in sediment buried 35–60 m deep Even greaterdisplacements, as much as 40 or 50 mat more than 100 m depth in the sediments, have been estimated using low frequency profiling systems (Duin et al, 1984, Williams, 1987, Perdijk, 1988) Surveys of the Madeira Abyssal Plain and the Southern Nares Abyssal Plain have identified faults at densities as great as one or two faults per line km (Shephard et al., 1988). In the Madeira Abyssal Plain the highest density of faults occur where turbidite accumulations are thickest (Williams, 1987). Clusters of faults have also been observed above or adjacent to crests of basement ridges (Buckley and Grant, 1985; Williams, 1987) Some apparent reverse displacement faults with strong diffraction hyperbolae can be resolved into normal faults by using migration techniques on the seismic data (Perdijk, 1988).
Unconsolidated deep sea sediments can be faulted as a result of differential compaction (Mayer, 1981) and these fault zones may provide sites for preferential dewatering of the sediment column Williams (1987) critically examined two alternative fault-permeability models related to the mechanism of differential compaction, and concluded that an impermeable-fault model explains several of the characteristics of faults observed in the North Atlantic abyssal plains However, both permeable and impermeable models require anisotropic distribution of permeability in the sediment column and overpressured pore fluid might still migrate along a fault at a faster rate than through unfaulted sediment.
