What is a metallic sediment? and what is a "swash zone"?
I think what you are getting at is "placer beach deposits," which are often rich in metallic minerals due to their high density. The swash zone is that zone of the coastal ocean between the high and low tide marks which is called the beach face. At the shoreline the surf zone gives way to the swash zone, in which a rapid very shallow swash flow moves up the beach, carrying back sediment in partial suspension, followed almost immediately by a backwash flow down the beach. If heavy minerals are suspended in the suspended sediment, the settle rapidly to generate a thin heavy-mineral laminae.
Dr. Craig Glenn, Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822