The deep ocean is the world’s largest depositional environment, encompassing all of the ocean floor below the continental shelf and slope environments.
The majority of deposition away from continental margins takes place where there is little to no flow to transport sediments in from elsewhere. Deposition consists primarily of the slow accumulation of suspended material on the sea floor.
For the most part, deep marine environments are very still and experience little to no flow. Turbidity currents may reach deep ocean, depending on the size and density of the load, but generally do not extend more than a few thousand kilometers away from continental margins. Because of this, sedimentary structures are rare and are usually restricted to the planar lamination associated with the uppermost portions of turbidite deposits.
The majority of sediments are deposited through settling of suspended material, which consists primarily of bioclasts from microscopic planktic organisms and relatively small amounts of clay- and silt-sized dust. Deposition by settling occurs very, very slowly and is usually measured in mm/ka.
Characteristics of Deposited Sediment
Sediments deposited here are generally silt-size or smaller, with the exception of Radiolarian bioclasts, which may reach up to 0.5mm in diameter. The characteristics of the deposited sediment tend to depend on the ratio of calcareous to siliceous material, as well as the ratio of bioclasts to dust.
The relative amounts of these materials are determined by the water’s ability to dissolve the bioclasts. Dissolution occurs more readily at depth, but the actual depth at which a given chemical will dissolve in seawater, known as its “compensation depth,” varies both spatially and temporally according to pressure, temperature, and the existing dissolved load. Higher pressure and lower temperature enable dissolution of chemicals like carbonates, and if a large amount of material is already dissolved, it will require higher pressures to dissolve more. This would move the compensation depth down.
The tests of planktic organisms are made of either calcium carbonate, in the case of foraminifera and coccoliths, or hydrous silica, in the case of diatoms and Radiolaria. Calcium carbonate dissolves more readily than silica, so the carbonate compensation depth (CCD) is always above the silica compensation depth (SCD).
Above the CCD, calcareous ooze accumulates the fastest of the three, so it makes up the majority of deposition there. Calcareous ooze lithifies as lime mudstone, sometimes referred to as pelagic limestone.
Below the CCD, only dust and silica can deposit, as the calcareous organisms dissolve before reaching the sea floor, and siliceous ooze accumulates more quickly than red clays. As a result, the majority of deep marine deposits are of siliceous ooze, which lithifies as chert. Chert is an amorphous hydrous silica that is characteristic of deep marine depositional environments. If Radiolaria are visible as white dots in the chert, it is known as Radiolarian chert. Small layers of carbonates are sometimes found in deep marine chert as a result of turbidites from carbonate platforms.
Below the SCD, only dust can deposit, which lithifies as mudstone. Since the majority of the dust blown across the ocean comes from aeolian environments where it has been heavily oxidized, deep sea dust deposits form red clays which lithify as red or red-brown mudstones.
In some places chemicals may precipitate directly out of the water and onto the sea floor. One relatively common precipitate is Mn-oxide, which enters the water in solution either through hydrothermal activity or continental weathering. Over time, Mn is concentrated into hard, round, black blobs known as Mn-nodules.
Planktic organisms make up a significant proportion of deposits in deep marine environments, though few are large enough to fossilize. Radiolaria up to 0.5mm may be seen in any sediments above the SCD, and macroscopic foraminifera are common above the CCD. Body fossils of larger open-water organisms such as cephalopods or whales are significantly less common, but pelagic organisms with hard shells or bones will sink to the ocean floor after death and potentially fossilize.
Trace fossils are almost exclusively of the Nereites ichnofacies, consisting of regular, surficial feeding traces. Benthic organisms in the deep ocean are rare because nutrients are rare, and the regularity of these feeding traces is thought to be a strategy to increase efficiency.
Due to the basin-like shape of the ocean floor, the mud, chert, and mud limestone facies tend to create something resembling concentric rings, with mud limestone closest to continental margins and the mud facies farthest away. Turbidite deposits become more common closer to continental margins.
Vertical facies changes are influenced by tectonic processes as well as changes the carbonate and silica compensation depths. As a deep marine basin is uplifted (or the CCD and SCD over it lowered), it will transition from mud to chert, then to mud limestone.
Because there is very little flow to transport sediment, sedimentary processes in deep marine environments are largely based around settling and chemical processes.
There are three primary facies associated with deep marine sedimentary deposition: mud limestone, which is primarily composed of calcareous ooze from calcareous planktic organisms; chert, which is primarily siliceous ooze from siliceous planktic organisms; and red mud, which is composed of dust, soot, and ash blown over the ocean. Their locations relative to one another are governed by the CCD and SCD, which determine how deep calcareous and siliceous material can go before dissolving.
Anything floating in the open ocean may make its way down to the sea floor and be incorporated into sediments, provided it does not dissolve. Because of this, pelagic fossils are relatively common in deep marine sediments.
Nichols, Gary, 2009, Sedimentology and Stratigraphy, 2nd ed.: Chichester, UK, Wiley- Blackwell.