Stress release, joints, and instability on submarine slopes

Mass movements related to gradual stress release within a sediment section may be quantitatively important on submarine slopes, particularly when such stress release involves joint sets. The sequence of events that promotes this phenomenon has been established by numerous terrestrial studies. The process involves: mass wasting or erosion to remove vertical stress (overburden) or lateral stress (such as through canyon cutting); consequent elastic rebound of the unloaded section; and opening of existing joints and/or formation of new joint sets. The presence of joints, which constitute planes of weakness within the sediment section, controls and reduces the stability of the affected slope; that is, the stability of the slope may no longer be dependent on the inherent strength of the sediments. The results of this process have been observed on the continental slope off the Mid-Atlantic coast of the United States. There, exposed Tertiary sediments have a well-developed joint pattern that has been observed in sidescan-sonar images, from submersible operations, and in a piston core. The measured preconsolidation stress on an Eocene core sample suggests that more than 100 m (330 ft) of overburden may have been removed from parts of the area. Intact Eocene blocks, which represent apparent failure alongmore » joint planes, have fallen from canyon walls on the lower slope and moved onto the upper rise. It is suggested that this process has the potential to operate on most deeply eroded surfaces and that exhumed (overconsolidated) sediments do not necessarily represent stable conditions despite their typical high shear strengths.« less