Multiple causes of diagenetic fabric anisotropy in weakly consolidated mud, Nankai accretionary prism, IODP Expedition 316

In the Nankai accretionary prism and its associated slope sediments early (pre-lithification) mechanical modification of mud induces preferred alignments of elongate or platy particles and the loss of intergranular porosity. Generic types of particle alignment include: 1. particles having long axes aligned in the plane of bedding, most likely as a consequence of burial compaction; 2. diverse bioturbation structures including alignments parallel to burrow walls, burrows filled with obliquely aligned phyllosilicates, and blotchy disruption of bedding; and, 3. planar deformation bands showing parallel alignments of both silt- and clay-size particles. compositional contrast between deformation bands and host rocks is consistent with loss of intergranular micropores within bands and supports the dominance of mechanical over chemical processes in their formation. Field-emission SEM imaging of Ar-ion-milled cross-sections shows that collapse of larger (>2 μm) pores, many localized at the margins of silt-size particles, reduces porosity within the bands by about 5 percent compared to the adjacent host rock. Despite the clear role of shear, evidence for particle comminution is equivocal. observations on mechanical processes in early diagenesis provide useful context for interpretation of pore types and fabric anisotropies in mudrocks across a wide range of subsurface conditions.