FINE SEDIMENT DEPOSITS IN SHELF SEAS

From field observations it appears that the top layer of a shelf botto m in general exhibits an intricate geographical pattern of sediment fo rmations. Sediments of different composition are confined in distinct regions. This contradicts the idea that current and wave forces stir u p bottom sediment and disperse it in a random way over the shelf; the dispersal process is counteracted by sorting mechanisms. In this paper the bottom patterns of fine cohesive sediments are considered. A spec ific sorting mechanism is studied which may explain the patchy structu re of fine sediment deposits. It is shown that fine sediments can be t rapped in bottom deposits which contain a fine sediment fraction high enough to prevent pore water motion in the shelf bed. This mechanism o pposes sediment dispersal away from existing deposits. It may also exp lain the formation or the preservation of mud patches, even in regions where the bottom shear stress is relatively high.