Dish structure

A dish structure is a type of sedimentary structure formed by liquefaction and fluidization of water-charged soft sediment either during or immediately following deposition. Dish structures are most commonly found in turbidites and other types of clastic deposits that result from subaqueous sediment gravity flows.

Due to the similarity in its shape to a dish, the structure, sometimes also referred to as dish-and-pillar or dish-and-pipe , was named after the common kitchen item.

Dish structure was described scientifically for the first time by Crook in 1961 who still used the title discontinuous curved lamination. The established term was used for the first time in 1967 by Stauffer and by Wentworth. Comprehensive studies are due to Lowe and LoPiccolo in 1974 and Lowe in 1975.

The subhorizontal dish structure consists of two parts, the dish itself and the [sediment] contained within the dish plus the region stretching up to the bounding surface of the overlying dish(or dishes) above. The bounding surface of the dish can take on variable shapes, from substantially flat to bowl-like and to strongly concave up. The bounding surfaces are thin, (and usually) dark(er) laminae; they are richer in clay, silt or organic material than the surrounding sediment. The individual dishes are arranged en echelon. Their width can vary from 2 centimeters to over 50 centimeters, the vertical spacing ranges usually from less than 1 centimeter to about 8 centimeters. Their plan shape grades from circular/polygonal to oval/elliptical. Their bases are sharp, but the tops are gradational.

Commonly the dishes are separated by vertical streaks of massive sand called 'pillars'. These pillars can be small-scale structures (Type A pillars) or large and throughgoing high-discharge structures (Type B pillars).

Within an individual bed an increase in concavity combined with a simultaneous decrease in width of the dishes can often be observed towards the top.

Dish structure occurs in laterally extensive sheets. The medium in which the structure forms is usually coarse silt, but it also appears in all grades of sand. They are never found in gravels nor in clays. The containing beds are normally graded. The depositional environment of the structure is mainly deep-water marine (i.e. continental rise) comprising coarser grained turbidity currents and related high-concentration flows (grain flows, fluidized flows, liquefied flows). But dish structure can also be encountered in shallow-marine deposits and in fluviatile, lacustrine and delta environments. It is occasionally found in ash layers within marine sediments.

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