Bulk and Clay Mineralogy of Mud from Nankai Trough IODP Expeditions 315 and 316

Bulk powder and clay-size fraction X-ray diffraction (XRD) analyses of hemiplegic mud from IODP Expeditions 315 and 316 provide useful constraints on relative mineral abundances. The main bulk minerals include total clay minerals, quartz, plagioclase, and calcite. The calcite content shows significant variation.Values remain low (generally less than 2%) in the accretionary prism. A gradual upward increase in carbonate content within the slope apron (to 46%) is consistent with reduced carbonate dissolution and implies that the seafloor was slowly uplifted to its present water depth above CCD. At the frontal thrust sites, the calcite content is low in the sandy turbidite deposits and the underlying upper Shikoku Basin facies (average 2%). In the Kumano Basin, the basal forearc basin has greater calcite concentration (average 16%) because the depositional environment was starved of significant terrigenous influx. Dilution of biogenic carbonate by high influxes of terrigenous silt and clay began with turbidite influx at ~ 1.6 Ma. The abrupt compositional shift across the basal unconformity is noteworthy. We regard this contact as a manifestation of uplift along a system of splay faults at ~ 5 Ma. The unconformity between slope apron and accretionary prism at Site C0001 is a manifestation of uplift at ~ 3.7 Ma, with a similar shift in composition.The contents of total clay minerals increases with age in the slope and frontal thrust areas. The quartz and feldspar content show roughly a reverse correlation with the clay content. It is noteworthy that the upper Shikoku Basin facies at the frontal thrust sites has more clay (average 63%) compared to the overlying sediments (average 44%).The clay minerals smectite, illite, kaolinite and chlorite make up most of the clay-size fraction. Increasing smectite concentrations with age have been documented by ODP Legs 131, 190, and DSDP Leg 87 in the western Nankai Trough. Thus, smectite dehydration should yield more fluid in the older accretionary prism. Clays may also decrease permeability of the sediments/rocks, which increases the likelihood of overpressured pore fluids. The removal of interlayer water of smectite during drying process also causes artifacts in measurement of porosity.