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Turbidites, and the Case of the Missing Dunes
In the deep-marine stratigraphic record classical turbidites are common and when complete comprise five vertically stacked units, which in ascending order are termed the a to e divisions. The b division consists of planar lamination, which in almost all cases is overlain by ripple cross-stratification of the c division. Conspicuously absent in this succession is dune cross-stratification, which for sediments coarser than middle fine sand in a decelerating flow should occur between the planar-stratified and ripple cross-stratified units. Here it is argued that the paucity of dune cross-stratification is the result of the deleterious effect of suspended sediment on dune inception. Specifically, high suspended-sediment concentration, and hence high density, in the bottom part of the parent turbidity current prevents the development of a sufficiently sharply defined interface between the bottom part of the current and the dense, underlying bed-load layer. This results in the absence of the necessary hydrodynamic instability whose waveform structure along the interface would otherwise mould the bed surface into the incipient bed forms from which dunes (and also ripples) grow. The almost exclusive occurrence of ripple cross-stratification above planar lamination suggests that in almost all natural turbidity currents a sufficiently well developed density interface does not become established until flow speed is in the ripple stability field.
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