Reworked pelagic detritus forms most graded beds from the upper 10-20 m of most abyssal plains. Coccoliths are by far the commonest detrital particle in many of the abyssal plains and are present with discoasters in the fine silt fraction, whereas reworked radiolaria, diatoms and planktonic Foraminifera are present in the coarse-silt and fine-sand fractions. Piston and gravity cores from the Argo and Gascoyne abyssal plains off northwest Australia contain graded beds which consist of pelagic detritus sorted into layers of either Radiolaria, diatoms or planktonic Foraminifera. These layers clearly define the basal part of many graded beds of different color shades. The source for most of the sediment must lie in the pelagic oozes of the adjacent abyssal hills and rises and not in the upper continental slope and shelf areas. There is increasing evidence that density currents commonly originate in the closer fan valleys and rises flanking the abyssal plains and uncommonly on the upper continental slope.

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