Consolidation state of marine sediments west of Martinique, Lesser Antilles volcanic arc: preliminary geotechnical analyses from IODP Expedition 340

From shipboard measurements of undrained shear strength performed in hemipelagic sediments during the IODP Expedition 340 in the Lesser Antilles arc we infer the consolidation state of the upper 200 meters of marine successions in the Grenada Basin, west of Martinique. Results from consolidation tests and hydraulic conductivity measurements on sediment samples from site U1400 are used to verify observations based on undrained shear strength measurements. The selected sites (U1397, U1398, U1399 and U1400) contain stacked mass transport deposits made of volcaniclastic and hemipelagic sediments. The ratio of the undrained shear strength to the effective stress of normally consolidated hemipelagic sediments at the upper slope west of Montserrat (site U1396) is used as a reference. The drilled mass transport deposit at site U1400 comprises an upper package (∼80 metres below sea floor: mbsf) of deformed hemipelagic sediments with some layers of overconsolidated sediments. At greater depths (> 80 mbsf), layers with underconsolidated sediments suggest that pore fluid pressures exist in excess of hydrostatic. Values of excess pore pressure range from 30 and 60 % of the vertical effective stress at 84 m and 159 mbsf, respectively. Hydraulic conductivity ranges around 10-9 m/s in normally consolidated sediments and decreases to values of less than 10-10 m/s in overconsolidated sediments. At sites U1397, U1398 and U1399, hemipelagic layers interbedded within coarse volcaniclastic sediments are underconsolidated, which implies increased pore fluid pressures. We propose that shear-induced compaction during transport leads to overconsolidation and hydraulic conductivity reduction. The low hydraulic conductivity that characterises these hemipelagic sediments may reduce rates of dewatering and allow pore fluid overpressure to persist. These results suggest that excess pore fluid pressures, which reduce slope stability, may have been involved in the destabilisation and incorporation of marine sediments during past flank-collapse and submarine landslide events.