From Coastal Wiki
Nepheloid layer or nepheloid zone is a layer of water in the deep ocean basin, above the ocean floor, that contains significant amounts of suspended sediment. It is from 200 to 1000 m thick. The name comes from Greek: nephos, "cloud". The particles in the layer may come from the upper ocean layers and from stripping the sediments from the ocean floor by currents. Its thickness depends on bottom current velocity and is a result of balance between gravitational settling of particles and turbulence of the current.
In a similar way, a surface nepheloid layer may be created, due to particle flotation, while intermediate nepheloid layers may be formed at the slopes of the ocean bed due to the dynamics of internal waves.The existence of the nepheloid layer complicates bathymetry: one has to take into account the reflections of lidar or ultrasonic pulses from the upper interface of this layer, as well as their absorption within the layer. 
- Glossary of Geology, 5th Edition (American Geological Institute)