Stability of Near-Surface Gassy Sediments, Amazon Continental Shelf

Abstract

Gassy sediments in continental margin environments pose constraints for subsea installations because of their inherent instability under loads imposed by earthquake activity, excessive erosion and/or deposition, near-surface faulting or diapiric activity.Evaluation of processes that control sediment stability in near-surface soils are essential for determining their viability as foundation materials for seabottom cables, pipelines, concrete footings and oil-production facilities.In modern continental shelf environments gassy sediments are encountered in rapidly-deposited deltaic environments and are concentrated beneath impermeable fine-grained deposits or trapped within thalwegs of relict (Pleistocene) channels. Highresolution shallow seismic reflection profiles depict several types of methane deposits. In each of these formations, gas may be released to the seafloor via adjacent porous sediments or through failure of adjacent or overlying strata. 1. Spatiallydiscontinuous methane deposits that are stratigraphically controlled may occur within several meters of the seabed. Continuous release of gas as bubbles through adjacent porous media results in a seabed that does not display evidence of deformational processes. 2. "Channel-bounded" deposits occur at depth within the seabed and may release a continuous supply of gas to the seafloor. Evidence of heave that can extend more than 300 m from the buried channel margins indicates that initial gas release resulted from failure in which the horizontal effective stress was greater than the vertical effective stress. 3. Subsurface "pockets" of gas frequently exceed 1-km diameter and 40-m thickness; they generally are