DELTAIC INFLUENCES ON SHELFEDGE INSTABILITY PROCESSES

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Abstract
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 shelfbreak depths and resulting in active progradation of the shelfedge. 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 and display sharp basal planes and blocky shapes. Progradation of the shelfedge deposits is generally accompanied by oversteepening and large-scale instability of the upper shelfbreak 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 shelfedge. 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 shelfedge owing to slumping and other mass-movement processes. Such canyons and trenches can attain widths of 10–20 kilometers, depths of 800 meters, and lengths of 80–100 kilometers. The Mississippi Canyon probably originated in this manner. The creation of such features by shelfedge instability results in the yielding of exceptionally large volumes of shallow-water sediment 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, under-consolidated sediments causes downslope creep or reactivation of failure mechanisms, resulting in multiple episodes of filling and evacuation.

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