The unconsolidated sediments, derived from various sources, deposited at the sea floors are in­cluded in ocean deposits. The study of ocean (marine) deposits includes the consideration of types of sediments, their sources, methods of their transportation, horizon­tal distribution, lithological successions or vertical variations in their distribution and composition, etc.

The sediments derived from weathering and erosion of continental rocks are transported to the oceans by rivers, winds, etc. Volcanic eruptions also provide sediments. Besides, the decay and decomposition of marine organisms (both plants and animals) also con­tribute sediments to ocean deposits.

Classification of Ocean Deposits

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

(1) On the basis of location,

(2) On the basis of depth,

(3) On the basis of the origin of sediments etc.

1. On the Basis of Location:

This classification is based on typical locations of particular marine sediment. Though several scientists have attempted to classify ocean deposits on the basis of their locations, the classifications of Sir John Murray and J.T. Jenkins are widely acclaimed.

(i) Classification of Murray:

Sir John Murray has classified the ocean deposits into two broad categories viz.:

 (a) terrigenous deposits and

(b) pelagic deposits.

Terrigenous deposits are found mainly on the continental shelves and slopes whereas pelagic deposits predominate on the deep sea floor. Terrigenous deposits are composed of coarser materials and are derived from the continents through weathering and erosional processes and are trans­ported to the oceans by various agencies.

Their color may be blue, yellow, grey, or red. Pelagic deposits consist of fine materials formed of the skeletons and shells of marine organisms and a few inorganic substances. They are generally blue, grey, or red in color.

(ii) Classification of Jenkins:

Jenkins has divided marine deposits into three groups viz.:

(a) Deep sea deposits,

(b) Shallow water deposits, and

(c) Littoral deposits.

The following is the detailed classification of Jenkins:

(A) Pelagic deposits:

(1) Red clay,

(2) Radiolarian ooze,

(3) Diatom ooze,

(4) Globigerina ooze, and 

(5) Pteropod ooze.

(B) Terrigenous deposits:

(1) Blue mud,

(2) Red mud,

(3) Green mud,

(4) Coral mud,

(5) Volcanic mud,

(6) Gravel, and 

(7) Sand.

 

 

2. On the Basis of Depth:

(A) Deep sea deposits (Below 100 fathoms):

(a) Pelagic Deposits:

(1) Red clay,

(2) Radiolarian ooze,

(3) Diatom ooze,

(4) Globigerina ooze, and 

(5) Pteropod ooze.

 

 

(b) Terrigenous Deposits:

(1) Blue mud,

(2) Red mud,

(3) Green mud,

(4) Coral mud, and 

(5) Volcanic mud.

Mud can be of four types: Red Mud: iron oxides Blue Mud: Iron + Sulfur Green Mud: (Blue mud + Sea Water), has potassium silicate Coral mud: coral reefs derivative

 

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

(1) Gravels,

(2) Sands, and

(3) Mud.

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

(1) Gravels,

(2) Sands,

(3) Mud.

2. General Classification:

(1) Terrigenous Deposits:

i. Littoral deposits,

ii. Shallow water deposits, and

iii. Terrigenous mud.

(2) Neritic Deposits:

i. Shallow water neritic deposits,

ii. Deep sea water neritic deposits, and

iii. Pelagic deposits.

3. Classification on the Basis of Origin of Sediments:

(1) Littoral deposits (derived from land)):

(i) Shore deposits.

(ii) Shelf deposits.

(2) Hemipclagic deposits (Partly from land and partly from marine origin):

(i) Green mud.

(ii) Volcanic mud.

(iii) Coral mud.

(3) Eupelagic deposits (Of marine and cosmic origin):

(i) Red clay.

(ii) Radiolarian ooze.

(iii) Globigerina ooze.

(iv) Pteropod ooze.

Terrigenous Deposits

The term 'Terrigenous' comes from the Latin words 'terra,' meaning land, and 'genus,' meaning origin. These deposits are predominantly land-derived sediments that have journeyed to the ocean depths via numerous transportation modes. 

The most common sources of Terrigenous Deposits are rivers which carry vast amounts of sediment eroded from the land's surface. Other sources include winds, which transport fine particles like dust and sand over great distances, and glaciers, which grind rocks into fine particles and carry them into the sea.

The composition of Terrigenous Deposits varies widely depending on the source. River sediments, for example, can contain everything from nutrient-rich soil to tiny fragments of rock, while glacial deposits may include unique minerals picked up during the glacier's slow movement over land.

Biogenous Sediments

This type of sediment comes from the remains of living organisms in the ocean. When tiny marine plants and animals die, their remains can sink to the bottom and build up over time, forming a kind of sediment called "ooze". If the ooze is made mostly of silica-based organisms like diatoms or radiolarians, it's called "siliceous ooze". If it's made mostly of calcium carbonate-based organisms like foraminifera or coccolithophores, it's called "calcareous ooze". These types of ooze are usually found in the open ocean, away from land.

These are the oceanic deposits derived from dead remains of living organisms. It can be classified into Siliceous Ooze Diatoms : Diatoms are single-celled micro-algae and can  do photosynthesis, i.e. autotrophs. Their cell-wall is made of silica, giving it glassy appearance. Radiolarians: Radiolarians are single celled Protozoans. They secrete silica and live on surface or upper layer of oceans. Siliceous ooze is most abundant in the Pacific Ocean. Calcareous Ooze They are formed by depositions of calcareous skeletons and carbonate secretions of planktonic organisms, viz; Pteropods: marine molluscs, abundant in Indian and Atlantic Oceans Globigerina: Protozoans, abundant in Pacific Coccolithophores: They are single-celled autotrophic phytoplanktons which have calcareous shell called coccolith.

 

Cosmogenous Sediments

Cosmogenous sediments come from outer space! Tiny dust particles, or "micrometeorites," constantly fall onto the Earth from space, and some of them end up in the ocean. Also included in this category are larger meteorite fragments and tektites, which are glassy rocks formed by the heat and pressure of meteorite impacts. Cosmogenous sediments are rare and are spread thinly across the entire ocean floor.

Pelagic Deposits

Pelagic Deposits offer an intriguing snapshot of life in the ocean's depths. Derived from the Greek word 'pelagos' for sea, these deposits are made up of the remains of marine organisms and fine particles suspended in the ocean water.

A common form of Pelagic Deposits is 'ooze,' a soft, mud-like substance made up of microscopic shells of marine organisms. There are two main types of ooze: calcareous ooze, formed from organisms with calcium carbonate shells, and siliceous ooze, from organisms with silica-based skeletons.

Deep-sea clay, also a type of Pelagic Deposit, consists of extremely fine particles that have slowly settled out of the water column. This deposit often accumulates in areas of the ocean where the supply of organic material is low, and thus oozes cannot form.

Hydrogenous Deposits

These are the chemical wizards of Ocean Deposits. 'Hydrogenous' comes from the Greek word 'hudōr' for water and 'gennan' to produce. They form through chemical reactions within the seawater, leading to precipitation of minerals that then settle onto the ocean floor.

A well-known example is manganese nodules, potato-sized lumps rich in manganese, iron, nickel, copper, and cobalt. These nodules grow very slowly over millions of years, layer by layer, from the direct precipitation of minerals from seawater.

Phosphorite deposits are another type of Hydrogenous Deposit. They form in areas of high biological productivity where the decay of organic matter releases phosphorus, which then combines with calcium in seawater to form calcium phosphate.deposits

Type of Deposit	Source of Origin	Examples	Unique Characteristics
Terrigenous Deposits	Land	Clay, silt, sand	Wide compositional variety based on the source
Biogenous Deposits	Organic accumulation of hard parts of some marine organisms	Calcareous and Siliceous oozes	They provide a record of the types of life that existed in the ocean in the past
Pelagic Deposits	Marine Life	Ooze, deep-sea clay	Provides information about past marine life
Hydrogenous Deposits	Seawater	Manganese nodules, phosphorite deposits	Forms through chemical reactions in seawater
Cosmogenous deposits	Dust from spce, meteorites debris	Meteorites debris glassy nodules	The composition of cosmogenous deposits can be quite diverse, reflecting the variety of materials found in the universe.

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

1. Shallow Water Deposits: These types of deposits are found in the relatively shallow parts of the ocean, which are typically near the coastline. They're mostly made up of sediment brought from the land by rivers, wind, and ice. This can include things like sand, mud, and pebbles. In shallow water, you also find deposits created by living creatures, such as coral reefs.
2. Continental Shelf Deposits: The continental shelf is the underwater part of each continent, stretching out from the coastline, and usually extends to a depth of about 200 meters. The type of sediment found here depends on its proximity to the shoreline and the depth of the water. It can include materials like sand, silt, and clay. Plus, due to the abundance of sunlight at these depths, a lot of marine life is present, contributing to organic deposits.
3. Continental Slope and Rise Deposits: The continental slope and rise form a transitional zone between the shallow waters near the land and the deep ocean floor. The continental slope is steep, while the rise has a gentler gradient. These areas often accumulate fine-grained sediments, called "turbidites," which come from the shallower continental shelf. These sediments are transported down the slope by underwater landslides and currents, in events called turbidity currents.
4. Abyssal Plain Deposits: The abyssal plains are the deepest, flattest parts of the ocean, found at depths of over 4000 meters. Here, you will find deposits known as "pelagic sediments". These can be made up of tiny particles that have slowly settled out of the water column over thousands of years, including dust from the land and microscopic shells from tiny marine organisms that lived near the surface and then sank when they died. The main types of pelagic sediments are "oozes", which are named after the kind of microorganism shells they contain, like siliceous ooze (from silica-secreting organisms) or calcareous ooze (from calcium carbonate-secreting organisms).
5. Deep-sea Trench Deposits: These are the deepest parts of the ocean, and the sediments here are often affected by the movement of the Earth's crust. Deep-sea trenches are typically found at subduction zones, where one tectonic plate is forced under another. The sediments here can include a mix of pelagic sediments and "trench fill" deposited by turbidity currents, as well as rocks and sediments scraped off the subducting plate.

Questions:

• Discuss the different bases for classifying ocean deposits and give a detailed account of the pelagic deposits of the oceans. ( UPSC 2007)
• Ocean Deposits ( Short Notes)(UPSC  2010)
• Describe the characteristics of different types of pelagic deposits. (UPSC  2015)
• Write a note on the marine resources of India. ( UPPSC, UP PCS, 2019, 10 Marks)
• Give a brief account of ocean deposits. ( UPPSC / UPPCS, 2019, 15 Marks)