Abstract
Relatively few studies have focused on the geotechnical properties of near-seafloor (uppermost 10 m) sediments that are encountered during shallow coring or the initial phases of seafloor drilling. Such sediments are of particular interest in areas strongly affected by salt tectonics or the occurrence of shallow gas hydrates. Using sediment cores obtained at three gas hydrate and/or mud volcano sites in the northern Gulf of Mexico (Garden Banks GB425, Mississippi Canyon MC852, and Green Canyon GC185), we report on visual observations of gas hydrate, oil, and authigenic carbonates; index properties (grain size characteristics, specific surface, pH, Atterberg limits, water content/porosity); small-strain (shear wave velocity) and large-strain (undrained shear strength) mechanical properties; and electrical properties (dielectric permittivity, electrical conductivity). At all sites, sediments are dominated by clay minerals (probably illite) and the highest proportion of carbonate (up to 72%) occurs near the apparent central vent of the mud mound at MC852. Based on the synthesis of several types of data, we conclude that the strength, stiffness, and porosity of the near-seafloor sediments are governed not by overburden vertical effective stress, but rather by interparticle forces arising from the interaction of the ionic pore fluid with the high specific surface (53 to 76 m(2) g(-1)) sediment grains. In some of the shallow sediments, pore water ionic concentrations significantly exceed seawater, suggesting transport of brines from deeper salt bodies. Particle-level processes, including those associated with these high ionic concentrations, lead to a mechanistic explanation for the moussey sediment texture widely observed in cores that have experienced the dissociation of gas hydrates. Electrical conductivity measurements acquired at millimeter resolution near dissociating gas hydrate indicate that, prior to hydrate dissociation, the pore fluids are in equilibrium with those distal from the hydrate. (c) 2005 Elsevier B.V. All rights reserved.