Large river systems deliver significant quantities of fine-grained sediment to continental shelf regions. In specific areas off deltas, deposition rates are rapid and the sediment may be involved in a variety of mass movement processes on the subaqueous slopes (slumps and slides, debris flows, and mudflows) causing rapid sediment accumulation at shelf-edge depths and resulting in active progradation of the shelf edge. Seismically, the deposits appear as large-scale foresets and are commonly composed of in-situ deep-water deposits alternating with shallow-water sediments transported by mass movement. On electric logs, sands within these units are sporadic display sharp basal planes and blocky shapes. Progradation of the shelf-edge deposits is generally accompanied by overs eepening and large-scale instability of the upper self-edge slopes. Deep-seated and shallow rotational slides move large volumes of sediments and deposit them on the adjacent slopes and upper rise. Extensive contemporaneous faults commonly form at the shelf-edge. Continuous addition of sediment to the fault scarps, particularly by mass movement from nearby delta-front instability, causes large volumes of shallow-water sediment to accumulate on the downthrown sides of the faults, mostly forming large-scale rollover structures. Continued movement along the concave-upward shear planes commonly results in compressional folds and diapiric structures. Contemporaneous accumulation of shallow-water mass movement deposits may occur in association with these structures.

Massive retrogressive, arcuate-shaped landslide scars and canyons or trenches can also form at the shelf edge owing to slumping and other mass-movement processes. Such canyons and trenches can attain widths of 10 to 20 km, depths of 800 m, and lengths of 80 to 100 km. The creation of such features by shelf-edge instability results in exceptionally large volumes of shallow-water sediment yielded to the deep basins in the form of massive submarine fans. The infilling of depressions by deltaic progradation is rapid, forming large foresets near the canyon heads. The low strength of the rapidly infilled, underconsolidated sediments causes downslope creep or reactivation of failure mechanisms, resulting in multiple episodes of filling and evacuation.

End_of_Article - Last_Page 912------------