Geography

Oceanic Deposits Classification and Distribution - Geography UPSC Notes

The loose sediments that settle on the ocean floor from various sources are known as oceanic deposits. These include materials transported by rivers, winds, ocean currents, and marine life activity. The study of ocean deposits focuses on their types, sources, distribution patterns, transport methods, and how they are layered over time.

The sediments found on the sea floor mainly come from the weathering and erosion of land-based rocks. These particles are carried into the oceans by rivers, winds, and even glaciers. Volcanic eruptions also add ash and lava to the mix. Additionally, the remains of marine organisms—such as shells and skeletons—contribute to these deposits through their natural decay and decomposition.

Classification of Ocean Deposits

Ocean deposits are classified on different bases e.g.:

• On the basis of location

• On the basis of depth

• On the basis of the origin of sediments etc

1. On the Basis of Location:

This classification is based on where specific marine sediments are typically found.

While many scientists have proposed ways to classify ocean deposits by location, the systems developed by Sir John Murray and J.T. Jenkins are the most widely accepted and recognized in oceanography.

These classifications help us understand how different types of sediments settle in different parts of the ocean, depending on factors like depth, distance from land, and biological activity.

Classification of Murray:

Sir John Murray classified ocean deposits into two broad types:

• Terrigenous deposits

• Pelagic deposits

Terrigenous deposits are mainly found along the continental shelves and slopes. These are made of coarser materials that come from the continents.

Weathering and erosion break down rocks, and rivers, wind, and glaciers carry the debris into the ocean. These deposits can appear in various colours like blue, yellow, grey, or red.

Pelagic deposits, on the other hand, dominate the deep sea floor. They are made up of finer materials, mostly formed from the skeletons and shells of marine organisms, along with some inorganic substances.

These are usually blue, grey, or red in colour and settle far away from land.

Classification of Jenkins:

J.T. Jenkins classified marine (ocean) deposits into three main groups:

• Deep sea deposits

• Shallow water deposits

• Littoral deposits (found along coastlines)

He further divided them into two categories:

• Pelagic deposits (found in deep oceans):

  • Red clay – fine reddish sediment found far from land

  • Radiolarian ooze – made of tiny plankton (radiolarians) with silica shells

  • Diatom ooze – formed from the remains of diatoms, a type of algae

  • Globigerina ooze – made of calcium carbonate shells of globigerina (marine plankton)

  • Pteropod ooze – formed from tiny floating sea snails with aragonite shells

• Terrigenous deposits (derived from land sources):

  • Blue mud – found near coasts, often rich in organic matter

  • Red mud – reddish in colour due to iron content

  • Green mud – contains glauconite, a green mineral from marine environments

  • Coral mud – formed from broken coral fragments

  • Volcanic mud – originates from volcanic ash and material

  • Gravel – coarse particles near shorelines

  • Sand – common in shallow and nearshore areas

2. On the Basis of Depth:

Deep sea deposits (Below 100 fathoms):

• Pelagic Deposits:

  • Red clay

  • Radiolarian ooze

  • Diatom ooze

  • Globigerina ooze, and 

  • Pteropod ooze.

• Terrigenous Deposits:

  • Blue mud,

  • Red mud,

  • Green mud,

  • Coral mud, and 

  • Volcanic mud.

• Shallow sea deposits (between low tide water and 100 fathoms):

  • Gravels,

  • Sands, and

  • Mud.

• Littoral deposits (Between high and low tide water):

  • Gravels,

  • Sands,

  • Mud.

General Classification of Oceanic Deposits

Oceanic deposits can be broadly classified into two major types:

Terrigenous Deposits:

These are formed from materials that originate on land and are carried to the sea by rivers, wind, glaciers, and coastal erosion. They include:

• Littoral deposits – Found along the shorelines; composed of sand, gravel, and shells.

• Shallow water deposits – Located just beyond the shore; mostly silt and coarse particles.

• Terrigenous mud – Fine sediments transported from continents, settling on the continental shelf and slope.

Neritic Deposits:

These are found in the shallow parts of the ocean (neritic zone) and transition into deeper waters. Subtypes include:

• Shallow water neritic deposits – Accumulate near continental margins; made up of sands, shells, and organic materials.

• Deep sea water neritic deposits – Found at greater depths but still influenced by continental material.

• Pelagic deposits – Formed far from land in the deep ocean basins; mostly composed of fine sediments and remains of marine organisms.

3. Classification Based on the Origin of Sediments

Oceanic deposits can also be categorized according to their source of origin—whether they come from land, a mix of sources, or marine and cosmic processes.

Littoral Deposits (Derived from land):

These deposits are primarily formed by materials eroded from land and carried into the sea by rivers, waves, and wind.

• Shore deposits – Found along coastlines; mostly sand, pebbles, and gravel.

• Shelf deposits – Found on the continental shelf; include silt, mud, and organic remains.

Hemipelagic Deposits (Mixed origin – partly land and partly marine):

These are found in intermediate depths and are a mix of terrigenous and marine materials.

• Green mud – Formed from fine sediments and decomposed marine matter.

• Volcanic mud – Derived from volcanic eruptions and ash settling in the ocean.

•  Coral mud – Formed from the breakdown of coral and marine organisms.

Eupelagic Deposits (Of marine and cosmic origin):

These are deep-sea sediments mostly made from microscopic marine organisms and some cosmic dust.

• Red clay – Fine-grained, reddish-brown clay found in deep ocean basins.

• Radiolarian ooze – Formed from the remains of radiolarians (microscopic sea organisms).

• Globigerina ooze – Made up of calcareous shells of plankton like foraminifera.

• Pteropod ooze – Composed of the shells of pteropods, small marine gastropods.

Terrigenous Deposits

The term 'Terrigenous' comes from Latin—'terra' meaning land and 'genus' meaning origin. These deposits are mainly composed of sediments that originate from land and make their way into the oceans through different transport mechanisms.

The primary sources of terrigenous deposits are:

• Rivers, which carry large amounts of sediment eroded from the land’s surface.

• Winds, which transport fine particles like dust and sand across long distances.

• Glaciers, which grind rocks into fine sediments and carry them to the sea during melting.

The composition of terrigenous sediments varies depending on their source. For instance:

• River sediments may include nutrient-rich soil, clay, and rock fragments.

• Wind-blown particles are usually very fine, such as volcanic ash or desert dust.

• Glacial deposits can carry unique minerals picked up from land during the glacier’s movement.

These deposits are commonly found on continental shelves and slopes, forming a significant part of ocean-floor sediments near coastlines.

Cosmogenous Sediments

Cosmogenous sediments have a truly extraterrestrial origin—they come from outer space! Tiny dust particles known as micrometeorites are constantly falling to Earth, and some of them eventually settle on the ocean floor.

In addition to these particles, larger meteorite fragments and tektites (glassy rocks formed during high-impact meteorite collisions) also contribute to this type of sediment.

Though cosmogenous sediments are rare, they are spread thinly across the entire ocean floor, adding a fascinating cosmic element to Earth's marine environment.

Pelagic Deposits

Pelagic Deposits are a deep-sea wonder, offering insight into life and activity in the ocean's vast, open waters. The term comes from the Greek word 'pelagos' meaning sea.

These deposits are formed primarily from microscopic marine organisms and fine suspended particles in seawater.

One of the most common forms is ooze—a soft, mud-like deposit composed of tiny shells and skeletons of marine plankton. There are two main types:

• Calcareous ooze – made from calcium carbonate shells of organisms like foraminifera and coccolithophores.

• Siliceous ooze – formed from silica-based skeletons of diatoms and radiolarians.

Another example is deep-sea clay, made of ultra-fine mineral particles. It accumulates in regions with low biological activity, where organic-rich oozes are less likely to form.

Hydrogenous Deposits

Hydrogenous deposits are formed directly from the chemical processes in seawater. The word comes from the Greek ‘hydor’ (water) and ‘gennan’ (to produce).

These sediments result from precipitation of dissolved minerals from the ocean, which then settle onto the seafloor.

A prominent example is manganese nodules—round, rock-like lumps rich in manganese, iron, nickel, copper, and cobalt. These nodules grow extremely slowly, forming layer by layer over millions of years.

Another key example is phosphorite deposits, which form in biologically active areas. When marine organisms decay, phosphorus is released into the water, where it reacts with calcium to form calcium phosphate, settling as sediment on the seafloor.

From the terrestrial echoes found in Terrigenous Deposits to the stories of marine life captured in Pelagic Deposits, and the chemical tales of Hydrogenous Deposits, each category of Ocean Deposits plays an essential part in the geological symphony that is our planet's history. 

Classification of Ocean Deposits Based on Depth

Shallow Water Deposits

• Found near coastlines, these include sand, mud, and pebbles brought by rivers, wind, and ice. Coral reefs and other biological deposits are also common here.

Continental Shelf Deposits

• Located up to 200 meters deep, these areas collect sand, silt, and clay. Sunlight supports marine life, contributing to organic sediments.

Continental Slope and Rise Deposits

• This transitional zone features turbidites—fine sediments moved by underwater landslides (turbidity currents) from the shelf above.

Abyssal Plain Deposits

• Found over 4000 meters deep, these flat plains collect pelagic sediments like siliceous and calcareous ooze, made from microscopic marine shells.

Deep-Sea Trench Deposits

• These are the ocean’s deepest parts. Sediments here include trench fill from turbidity currents and materials scraped off tectonic plates.​​​​

Final Thoughts

The classification of oceanic deposits reveals the complexity and diversity of materials found on the seafloor.

Whether derived from land (terrigenous), marine organisms (biogenous), space (cosmogenous), or chemical reactions in seawater (hydrogenous), each type reflects unique processes shaping the ocean floor.

Based on depth, these deposits vary from coarse littoral sands to fine pelagic oozes in abyssal plains and trenches. This classification helps in understanding Earth's geological history, sediment transport, and marine ecosystems, highlighting the dynamic interaction between land, sea, and even outer space in shaping our oceans.