Delivering Terrestrial Sediment to Continental Slopes: From Overview of Mechanisms

James P. Syvitski and Eric W. Hutton
U. Colorado at Boulder, Boulder, CO

The main mechanisms for moving large volumes of sediment to, and down continental slopes include 1) Sediment delivery by hypopycnal-plumes: effective on wide shelves when sea level is near the shelf-slope break. The mechanism is effective for higher stages of sea level if the shelf is narrow and discharge is large, and is affected by the magnitude of the discharge, Coriolis deflection, winds, and Ekman transport. 2) Hyperpycnal discharge: limited to small and medium-sized rivers that drain mountainous terrain capable of generating hyper-elevated sediment concentrations. 3) Sediment gravity flows generated through wave-current interactions on relatively steep continental shelves that are subjected to ocean storms. Upwelling versus downwelling conditions are important constraints. 4) Density-cascading where shelf water is made hyper-dense, flows off the shelf, converging and accelerating. Shelf water are made dense through cooling (e.g. cold winds), or through salinity enhancement (e.g. evaporation through winds, brine rejection under sea-ice). These bottom boundary currents can effectively erode the seafloor and carry sediment downslope either as a tractive current, or through conversion to a turbidity current. 5) Sediment failure may result from sediment loading and or oversteepening of the upper-slope deposits. Subsurface drainage and ground accelerations can greatly influence the size and extent of the failure surface. The failed sediment mass will transition to either a debris flows or turbidity current. 6) Theory also suggests that internal waves breaking on the upper slope may also mobilize seafloor sediment. Many other mechanisms can influence the transport of sediment down continental slopes, but are not considered volumetrically significant.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas