1. Bathyal or Deep-sea Deposits
These deposits are formed in the continental slope zone which is characterized by low mobility of the sea water. Accordingly there is least mechanical transport.
Besides, the absence of light and the prevalence of low temperature in this zone makes it unfavourable for benthic animals. Most of the organisms, here are planktons with carbonate or siliceous skeleton.
The terrigenous sediments of this zone are mainly derived from the shelf-zone and consist mostly of silty peiitic muds. According to the environment of deposition the muds may be blue, red and green.
The blue colour of the mud and the smell of hydrogen sulphide indicate that its formation is in a reducing environment (i .e. deficiency of oxygen and abundance of organic matter). It is found in the zone of continental slope and adjoining parts of the ocean-floor and may extend upto depths of about 5000 metres.
Red-mud consists of minute particles of silt or clay and its colour is due to the presence of oxides of iron minerals (limonite, haematite). Generally, it is widespread in the tropical regions and is formed in seas adjoining the continents where a red mantle is developed due to the weathering processes typical of humid tropical zones. These red weathering products are rich in iron oxides and are carried into the sea by rivers.
Green muds occur on the upper margin of the continental slope upto a depth of 1500-2000 metres. The colour is due to grains of a mineral called glauconite.
Glauconite is a silicate of iron and seems to form only in the presence of decaying organic matter. Green mud consists of coarser particles than the other types of mud and is, sometimes, composed of fine sand.
The biogenic sediments of the continental slope are calcareous oozes composed of the remains of planktonic organisms (the wanderers) like foraminifera, pteropoda and minute calcareous algae- cocolithophorides.
The foraminiferal oozes is composed of the minute skeletons of different foraminifera which are of calcareous nature. Pteropod ooze contain floating mulluscs called pteropoda (these are known as sea-butterflies) and accumulate generally at a depth of 3000 metres. Coccolith ooze contain a large number of coccoliths and rhabdoliths.
2. Abyssal Deposits
Beyond the foot of the continental slope, there is very little deposits of terrigenous material, and the abyssal plain is covered for most of the part by pelagic deposits which are formed to a great extent of the shells and skeletons of animals and plants which when alive float on the surface of the water.
Pelagic deposits are not entirely composed of the remain of animals and plants but also contain volcanogenic and polygenic sediments.
The pelagic deposits are classified into two groups – inorganic and organic. The inorganic group is represented by the polygenic and volcanogenic sediments whereas the organic or biogenic group comprises calcareous and siliceous oozes.