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 m2 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.
Bibliographical noteFunding Information:
We thank the crew of the R/V Seward Johnson for their effort to acquire samples and B. Dugan and another reviewer for comments that led to improvements in the manuscript. The cruise was conducted as part of a program funded to C.R. by National Science Foundation (NSF) grant OCE0118071. Salary for T-S.Y. was supported by the Goizueta Foundation and the Petroleum Research Fund of the American Chemical Society under grant AC38418 to J.C.S, and salary for F.F. was provided by the ChevronTexaco Joint Industry Project (JIP) under DOE grant DE-FC26-01NT41330 managed by NETL. We thank G. Dickens for the core material, J. Weinberger for acquiring thermal infrared images, Joint Oceanographic Institutions for the infrared camera, and the shipboard party for the assistance in processing cores. This research was completed while C.R. was on assignment at and wholly supported by the NSF. The views expressed in this manuscript reflect those of the authors and not NSF, the JIP, or DOE.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science