Dynamics of Subaqueous Gravity Depositional Processes

Clay-rich sediments, though commonly considered the least stable, possess greatest intrinsic bulk cohesion due to molecular and surface forces. Coarser materials derive stability from particle mass and physical packing; they, are essentially cohesionless. Intermediate silt and fine sand actually tend to be least stable; they are but weakly cohesive and commonly accumulate with open, mestastable packing. Hydrostatic pore pressure buoys saturated sediment masses slightly, reducing shear resistance. All fine sediments are susceptible to disturbance of their packing structures by forcible movement of fluids, thus slight shocks may deform metastable structure inducing gravity failure, even on half-degree slopes; such failures occur chiefly in areas of rapid deposition. Two fundamental dynamic boundaries distinguish elastic, plastic, and fluid behaviors. Plastic flow begins when the yield limit of cohesive sediment is exceeded; original stratification is preserved but contorted. Viscous fluid flow occurs if the liquid limit is exceeded; decreasing cohesion and churning destroy stratification and a suspension forms. This continuum of movements can be arrested at any point as is illustrated by the sedimentary record (load flows, contorted strata, pebbly mudstones). But if liquid limit is reached, cohesion normally deteriorates very rapidly by spontaneous liquefaction, almost inevitably producing a turbid suspension. This apparently accounts in part for the much greater abundance of graded bedding compared with contorted strata and pebbly mudstones. Inducement of mass failures results from disturbance by overloading, earthquakes, and hydraulic pressures (waves, tides, seepage). Turbidity currents may be initiated by mass movements, but also form by muddy rivers flowing into large bodies of water. Unfortunate misconceptions about turbidity currents have arisen, but repetitious graded bedding and displaced organisms or clastic materials are considered the most valid criteria of ancient turbidity current deposition. Volumetric importance of turbidity currents is difficult to estimate because of ambiguities among supposed criteria and destruction of graded bedding by normal currents.