1

MARINE SEDIMENTS AND DEPOSITS

MARINE SEDIMENTS : NATURE AND SIG NIFICANCEThe unconsolidated materials, derived from various sources and deposited at the ocean fl oors are called

marine sediments, which include weathered and eroded par cles of rocks, frag ments of dirt, dust, volcanic ashes, remains of marine organisms, fragments of meteorites etc. The se ling of marine sediments on the ocean fl oors is called ‘marine snow fall’. Besides, the broken parts of sunken ships and boats through ages have also become parts of ocean fl oor materials. Thus, the ocean fl oors are the repository of sediments of various sorts and diff erent me periods and act as the library of the earth’s geological history.’ The unconsolidated marine sediments are lithifi ed due to tectonic ac vi es and thus we fi nd layered consolidated materials on the deep ocean fl oors. Such consolidated marine sediments are called ocean deposits. In fact ocean deposits includeds both loose and unconsolidated materi als lying on the ocean fl oors, and layered consolidated sediments in the form of sedimen tary rocks. It may be men oned that most of the sedimentary rocks of the earth are of ma-rine origin i.e. these were deposited in the ocean fl oors and therea er were folded and deformed by tectonic movements (plate movements) from me to me. Thus the study of marine sediments and deposits includes the considera on of nature and signifi cance of marine sediments, their types and sources, processes of their forma on, methods of their transporta on, types of ocean deposits and their horizontal distribu on, lithologi-cal successions or ver cal varia ons in their distribu on and composi on.

The sediments derived from weathering and erosion of con nental rocks are transported to the oceans by rivers, winds, glaciers (in high la tudes) etc. The sediments derived from weath ering and erosion of coastal rocks by sea waves, tsunamis, dal and storm surges are reworked and transported by sea waves. It may be men oned that the transporta on of marine sediments by sea waves is bi-direc onal i.e. from the coasts to-wards the sea and from the sea towards the coasts.The analysis of marine sediments cores derived through deep drilling from the ocean fl oors provides valuable clues to the oceanogra phers to reconstruct the past geological and tectonic history of the earth. Thus, the study of marine deposits is geologically, biologically, culturally and climatologically very signifi cant as follows :

• The analysis of nature of marine sediments in terms of lithological succession, nature and disposi on of sedimentary beds pro vides vital proxy data for deciphering the tectonic history of the earth, mainly plate movements.

• The analysis of sediments cores provides vital clues (proxy data) to fi nd out the chronology of palaeoclimate. The nature of sediments and fossils of marine organ isms (both phytoplanktons and zooplanktons) embeded in diff erent layers of sedimentary deposits provide signifi cant proxy data which enable the geologists and climatolo- gists to fi nd out the past climate changes and sea level fl uctua ons.

• The nature and pa erns of deposits of marine sediments on the ocean fl oors give clue to trace the varia ons in the fl ow pa erns of ocean circula on mainly of ocean currents.

• The analysis of fossils of marine organisms embeded in sedimentary layers enables the biologists to trace the history of evolu on of marine life and mass ex nc on of marine organisms.

• Besides, the analysis of marine sediments and deposits provides vital clues to the following

• to assess the impacts of meteorites on the composi on of marine sediments.

• to inves gate the nature and frequency of submarine volcanic erup ons and the materials coming therefrom.

2

• to understand the nature and pa ern of movements of ocean fl oors (sea fl oor spreading) that might have taken place in the past geological history of the earth.

• to ascertain the nature of nutrients supply to marine organisms.

It may be submi ed that the marine sediments and the ocean fl oors are signifi cant archives of human culture and civiliza on, science and technology because a large number of sunken ships and boats, submarines and warships, weapons of various kinds, missiles etc. in the past centuries lying on the ocean fl oors have pre-served cultural wealth of humans. Similarly, the ‘ancient marine sediments are the informa on high ways into earth’s ancient past’ (Thurman and Trujillo, 1999).

Thus, ‘the epic stories can be read from the record that is preserved in the vast sedimentary accumula on on the sea bo om’ (P.R. Pinet, 2000).

The proxy data and clues from the ancient marine sediments about the aforesaid aspects may be sum-marized as follows :

• clues about tectonic history of the earth and plate movements,

• reconstruc on of palaeoclimate,

• understanding fl ow pa ern of ocean water, mainly ocean currents,

• evolu onary history of marine organisms,

• impacts of meteorites on the composi on of marine sediments,

• nature of undersea volcanic erup ons,

• nature and pa ern of movement of ocean fl oors i.e. sea fl oor spreading,

• reconstruc on of palaeomagne sm,

• nutrients supply to marine organisms,

• occurrence of mass ex nc on of marine organisms,

• reconstruc on of sea level and climate changes,

• cultural heritage from the sunken ships, and boats etc.

PRODUCTION, TRANSPORTATION AND DEPOSITION OF MARINE SEDIMENTS

There are 3 main mechanisms of the produc on of marine sediments as follows :

1. weathering,

2. erosion, and

3. decay of shells.

The con nental rocks are weathered through the processes of disintegra on and decomposi on into small pieces. The weathered and weakened rocks are eroded by diff erent agencies of denuda on mainly by fl uvial processes. The eroded materials (sediments) are brought to the oceans by rivers. The average annual surface runoff of 40,000 km3 from the con nents to the oceans through the rivers transports about 15,000 million to 20,000 million tonnes of sediments per year to the oceans besides 4,000 million tonnes of soluble mate-rial in suspension (Ake Sundborg, 1963). The Yellow (China, 1640 million tonnes/year), the Ganga (India and Bangladesh, 1450 million tonnes/year), the Amazon (Brazil, 850 million tonnes/year), the Brahmaputra (India

3

and Bangla desh, 703 million tonnes/year), the Yangtze (China, 480 million tonnes/year), the Indus (Pakistan, 435 million tonnes/year), the Missis sippi (USA, 300 million tonnes/year), the Irrawaddy (Mynmar, 300 million tonnes/year), the Red (Socialist Republic of Viet Nam, 130 million tonnes/year) etc. are the signifi cant contribu-tors of sediments to the oceans.

The glaciers in the high la tudes also bring glacially eroded sediments in the oceans. Wind blown sands and dusts from the coastal lands and hinterlands are deposited in the oceans.

The decay and decomposi on of skeletons of dead marine organisms provide biogenous sediments to ocean repository.

The erosion of coastal rocks by marine waves, dal and storm surges also produces substan al quan ty of sediments which are transported by the sea waves to the ocean fl oors.

The eroded materials are transported by sea waves in diff erent manner but the transporta onal work of sea waves varies signifi cantly from other agents of erosion and transporta on. For exam ple, the backwash, or undertow currents (moving from the coast and beach towards the sea) pick up the eroded materials and trans-port them seaward but the uprushing breaker waves or surf currents pick up these materials and bring them back to the coast and beaches. Thus, the transporta on of materials takes place from coastland towards sea and from sea towards the coast. When oblique waves strike the coast, longshore currents are generated. These longshore currents transport the materials parallel to the shoreline. The materials involved in the transporta on by sea waves include sands, silts, gravels, pebbles, cobbles and some me boulders. When there is equilibrium between incoming supplies of sediments by uprushing breaker waves and removal of sediments by backwash or undertow currents on the wave- cut pla orm, a profi le of equilibrium is achieved. If the wave-cut rock pla orm is characterized by steep slope towards the oceanic slope, the destruc ve waves become very ac ve and thus resultant powerful backwash removes the materi als from the landward side so that the slope of the pla orm is lessened. On the other hand, if the slope of the wave-cut pla orm is less steep, construc ve waves become more eff ec ve as they favour sedimenta on and beach deposi on on the landward side so that the slope of the pla orm becomes steeper. ‘The surface is therefore con nually modifi ed, in such away that at each point it tends to acquire just the right slope to ensure that incoming supplies of sediments can be carried away just as fast as they are received. A profi le so adjusted that this fl uctua ng state of balance is approximately achieved is called a profi le of equilibrium (A. Holmes and D.L. Holmes, 1978).

Man’s Impact on Marine Sedimenta onHuman economic ac vi es aff ect the na ture of coastal erosion, sediment produc on and their deposi on

atleast in the con nental margins and con nental shelves in a variety of ways as follows :

Dredging of ports and harbours to improve naviga on channels modifi es the pa ern and velocity of waves and currents. The materials derived from dredging are generally dupmped at many alterna ve loca ons e.g. off shore loca ons, shallow areas adjacent to the harbour, onshore shallow areas (to reclaim land), beaches (to enrich them) etc. These dumped materials are reworked and dispersed by waves in a variety of ways. The dumping of dredged materials off shore creates new mounds which modify the direc on, strength, velocity and overall pa ern of sea waves. Some mes sea fl oor is dredged to obtain materials to reclaim marshy coastal lands or to replenish eroding beaches. This ac vity deepens the sea fl oor which in turn generates long waves which erode the coastal land at rela vely faster rate than the normal waves.

Govt, of India launched a massive project of ‘Sethusamudram’ in July, 2005 for dredging the shallow por ons of sea to the south of Tamil Nadu coast in order to connect the Bay of Bengal and the Gulf of Manar through Palk Strait inorder to facilitate easy and smooth movement of commer cial ships between east and west coasts of the country. Thus circum-naviga on of Sri Lanka would be avoided. The project was launched a er proper analysis of environmental condi ons of the area such as marine, land and socio-economic environment and proper environmental impact assement. The project also ensures to protect marine ecological resources mainly coral reefs in the Gulf of Manar and Palk Bay. The work on the project has been stopped due to religious objec on.

4

Man’s ac vi es also aff ect sedimentological characteris cs of coastal environment of seas and oceans as follows :

• There is addi onal supply of waste materi als coming out of quarrying in the coastal zones. These materials are reworked and dispersed by sea waves and thus these materials are deposited in certain locali es and new beaches are formed (example- prograda on of beach ridge plain on the east coast of Jutland, Denmark, due to dumping of waste materials coming out from chalk quarry).

• Ar fi cial replenishment of eroded beaches due to altera on of sediment supply caused by construc on of break waters.

• Quarrying of beaches to obtain building materials leads to erosion of coastal land because of deple on of beach and direct exposure of coast to severe wave a ack and thus addi onal sediments are produced which are then deposited in the oceans.

• Devegeta on and extensive cul va on, in the immediate hinterlands of the catch ments of those rivers which drain the coast, result in prograda on of coastal lands, phenomenal growth in beaches and deltas because of increased supply of fl uvial materials brought by the rivers. This process has resulted into rapid rate of silta on of bays and inlets at the mouths of rivers along the Mediterranean coast due to extensive removal of vegeta on (for in creasing the cropland) and resultant accel erated rate of soil erosion and supply of enormous quan ty of sediments.

• Construc on of dams and reservoirs on major rivers (which drain into the seas) reverses the process of growth of beaches and deltas because the dams trap the sediments and force them to se le down in the reservoirs and therefore supply of fl uvial sediments through the river mouths is markedly reduced. This results in rapid rate of erosion of beaches and deltas which causes retrograda on. It has been reported that the Nile delta is suff ering from severe wave erosion which is producing more sediments. The shoreline is receding at the rate of 40m per year since the comple on of Aswan High Dam in 1970.

Man’s a empts to reduce or stop coastal erosion and therefore to check retrograda on on the one hand and to promote deposi on to encourage prograda on on the other hand have not been successful because of complex nature of mechanisms of coastal processes, both erosional and deposi onal. These direct a empts of man to manipulate and modify coastal processes for specifi c purposes (to halt erosion at harbours, to build beaches, to replenish already depleted beaches, to open inlets to encourage sea transport etc.) bring in changes in nearshore topography, mechanism of wave and current ac ons and coastal erosion, nature and pa ern of sediment movement and deposi on on the adjacent part of the coast where structural works have been ini -ated, as follows :

• Construc on of diff erent types of sea walls along the sea coasts to check cliff erosion leads to deple on of sea beaches because (1) the supply of sands and shingles from cliff erosion is stopped due to protec on provided by sea walls parallel to the coast, and (2) sea waves a er striking powerfully against the sea wals scour the beaches and remove the materials to be deposited on ocean fl oors.

• The construc on of breakwaters to shelter the harbours and the estuaries of river mouths results in accumula on of sands and singles and forma on of beaches on the updri side of breakwaters whereas beaches are eroded on the downdri side of breakwaters because of marked reduc on in the transport of sediments downdri .

It appears from the above discussion that human economic ac vi es not only aff ect but modify the pat-terns of coastal erosion by sea waves, transport of sediments and their deposi on of ocean beds.

5

Factors of Marine Sedimenta onThe processes of sedimenta on i.e. deposi on of marine sediments on ocean beds are aff ected and con-

trolled by the following 3 major factors

As stated earlier, the rivers are the major transpor ng agents of marine sediments. The con nental rocks are eroded by surface runoff and rivers and the eroded materials are brought to the oceans by these rivers. These sediments are picked up by sea waves and currents and are deposited on sea fl oor under varying condi ons. It may be men oned that terrigenous eroded sediments (of con nental origin) are reworked and dis persed by sea waves and currents before they are fi nally deposited on sea fl oor. The rate of sedimenta on depends on the rate of erosion of con nental rocks such as slow or rapid rate of erosion.

If the con nental rocks are resistant to erosion, they are eroded very slowly and hence there is very low supply of sediments by the rivers to the oceans and hence sea waves and currents have enough me to rework and disperse them. With the result the terrigenous sediments are sorted by the currents according to their size, shape and quan ty before they se le down on the sea fl oors. For example, sands are graded into coarse and fi ne categories. The terrigenous sediments grade from boulders to cobbles, peb bles, gravels, silt, sands, mud etc. On the otherhand, the weaker and less resistant con nental rocks are rapidly eroded with the result there is high rate of sediment supply and the currents do not have required me to sort out the sediments from large size to smaller size. Thus, rapid rate of supply of sediments results in the deposi on of mixed sediments. The quan ty or density of sediments also controls sor ng or non-sor ng of sediments before they are deposited. The large quan ty of sediments with large size increases the density and hence high density sediments are deposited more quickly than low density sediments. It may be noted that the rate of sedimenta on determines the degree of sor ng of par cles. Thus, high density sediments are poorly sorted while low density sediments are well sorted before they are deposited in layers on the sea fl oors.

The third important factor of marine sedi menta on is the energy condi on (energy level) of bo om cur-rents at the site of deposi on (sea fl oor). The grain size of sediments is propor onal (posi vely correlated) to the energy level of bo om currents at the me of sedimenta on on sea fl oor.high energy level

The strong bo om currents are character ized by swi ly moving turbulent water. Such swi and turbulent water carries fi ne sediments in suspension and hence does not allow them to se le down. Thus, strong bo om

• energy condi on of currents at the site of deposi on

• quan ty (density) of marine sediments

• size and shape of par clesFactors of Marine Sedimenta on

slow rate of erosion slow rate of sedimenta- well sorted sediments e.g. coarse sands, fi ne sands, silt, mud etc.

rapid rate of ero- rapid rate of sedimenta- poorly sorted sediments e.g. mixed sediments such as gravels mixed with sands or mud mixed with sands.

Strong currents high energy level deposi ons of coarser sediments

weak currents low energy level deposi on of fi ne dediments

6

currents of high energy level allow only coarser sediments to se le down. On the other hand, weak bo om currents denote low energy level and thus cannot carry coarser sediments, rather they transport only fi ne sedi-ments. Thus, weak currents of low energy level deposit only fi ne sediments. It, thus, becomes evident that the analysis of grain size of sediments, deposited on sea fl oor, may reveal the energy condi ons at the me when the sediments were deposited on the bo om, ‘fi ne grained sediments denote low-energy condi on; coarse sediments, high energy condi ons’ (P. R. Pinet, 2000).

Sources of Marine SedimentsThe marine sediments are derived and supplied from 4 major sources as follows :

• terrigenous or lithogenous source,

• biogenous source or organic source,

• hydrogenous source, and

• cosmogenous source.

The above men oned sources of marine sediments may be alterna vely grouped into the following 3 categories :

• external source (terrigenous source)

• internal source (biogenous and hydrog enous sources)

• cosmogenous source

The terrigenous or lithogenous source of marine sediments includes weathering and ero sion of con nen-tal rocks and transport of eroded materials by the rivers; coastal erosion by sea waves; and glacial erosion of con nental rocks and their transport by glaciers to the seas in high la tudes. The winds also transport dusts and sands from the hinterlands of the coasts to the oceans. The terrigenous source contributes rock fragments of varying sizes such as boulders, pebbles, cobbles, gravels etc., quartz sands, quartz silt, clay, dusts etc.

The biogenous source of marine sediments comprises the processes of decay and decomposi on of shells and skeletons of marine organismsin situ. Such sediments are grouped in two broad categories of (1) calcium carbonate (calcareous oozes) and shells of marine organisms and fragments of corals, and (2) silica (siliceous oozes). The biogenous sediments of calcium carbonate are produced in warm sea surface water while those of silica are generated in cold sea surface water.

The hydrogenous source of marine sediments includes the sediments derived from precipita on of dis-solved substances due to chemical reac ons such as phosphorites (phosphorous), oolites (cal cium carbonate), metal sulfi des (copper, silver, zinc, iron, nickel etc.), evaporites (such as gypsum and some salts).

The cosmogeneous source of marine sediments includes the sediments produced from the colli sion of meteorites in the space and thus the space dusts so produced directly fall into the oceans.

Besides, the volcanic dusts and ashes, which are ejected through con nental volcanic erup ons, are carried away by the atmospheric circula on and fi nally they fall down through precipita on into the oceans.

Mode of Marine Sedimenta onThe processes of marine sedimenta on may be grouped into the following two categories :

• bulk deposi on (bulk emplacement)

• retail deposi on

7

The process of bulk deposi on of marine sediments, geologically be er known as bulk emplacement, involves the slumping of sediments en mass including all types of terrigenous and biogenic sediments down the undersea slope under the force of gravity. The rivers unload huge amount of terrigenous sediments of vary-ing sizes (very coarse to very fi ne grained par cles) in the waters of con nental margins and inner con nen tal shelves. The con nuous build up of terrigenous materials causes slope instability due to steepen ing of slope of heaps of debris. This causes increase in gravity force which in turn causes mass movement of materials towards the outer con nental shelves and con nental slope in the forms of debris slump, debris fl ow, mudfl ow etc.

It is important to note that beddings of sedimen tary layers of terrigenous sediments are seldom disturbed, rather they are maintained while they are slumped en mass down the slope under the force of gravity. Massive undersea slides also occur in deep sea areas but such slides are not comparable to bulk slides of terrigenous sediments because the former (undersea slides) is caused by tectonic ac vi es on the sea fl oor, while the la er is caused by gravity alone.

The slumped sediments in the form of mudfl ows, known as slurries are picked up by powerful bo om currents, called as turbidity currents, and thus these turbidity currents are laden with slurries and move down the con nental slope under the force of gravity. As these slurry- laden bo om turbidity currents descend to deep sea plain, their velocity is slowed down and hence they unload coaser sediments on the sea fl oor fi rst. Further movement of these currents carries fi ne sediments in suspension which are fi nally deposited on fl at sea fl oors (fi g. 6.1). It may be men oned that the deposi on of sediments by turbidity currents shows graded beddings of sediments wherein the size of sediments becomes fi ner from the bo om upward. In other words, very coarse sediments are deposited at the sea fl oor whereas fi ne sediments are deposited in the uppermost layer of sediments. The cone-shaped deposits of graded materials at the mouths of submarine canyons are called deep sea fans. The glaciers resort to bulk deposi on of terrigenous materials in the oceans in high la -tudes by the process of ice ra ing. The ice ra ing involves the transport of terrigenous sediments embeded in the icebergs. The ice sheets associated with con nental gla ciers in polar regions carry con nental sediments. When the ice sheets are broken and dislodged from the glaciers, they fl oat as icebergs on sea surface and are carried away by ocean currents into deep sea area where they begin to melt. Thus the embeded sediments are released and se le down on sea fl oors.

8

The retail sedimenta on involves deposi on of sediments par cle by par cle in the same way as fl akes of snow fall down on the land one by one. This is the reason that fall down of par cles one by one on sea fl oor is called marine snowfall.

CLASSIFICATION OF MARINE SEDIMENTSThe marine sediments register large varia ons in terms of their origin and forma on, size and shape,

composi on, loca onal aspect etc. because they are derived from various sources such as (1) lithogenous (ter-rigenous) sources wherein sediments are produced due to weather ing and erosion of con nental rocks, and marine volcanic islands, (2) biogenous sources, which provide sediments through decay and disintegra on of marine plants and animals, (3) hydrog enous sources which include the precipitates of dissolved substances in ocean water, (4) cosmogeneous source wherein sediments are produced due to collision of meteorites in space and therea er these sediments fall in the oceans. Thus, marine sediments involve four major categories of (1) terrigenous or lithogenous sediments, (2) biogeneous sediments, (3) hydrog enous sediments, and (4) cosmogenous sediments. The sediments derived from the above men oned 4 major sources are alterna vely named as inorganic sediments (terrigenous sediments), or ganic sediments (biogenous sediments), calcare ous and siliceous sediments (hydrogenous sediments) etc.

Thus, on the basis of sources and mode of forma on marine sediments are classifi ed into the following categories :

1. Terrigenous (lithogenic) marine sediments

(1) con nental lithogenous sediments

(2) submarine volcanic lithogenous sediments

examples :

(i) gravels

(ii) sands

(iii) silt

(iv) clay

(v) mud

(a) blue mud

(b) green mud

(c) red mud

2. Biogenous marine sediments

(1) nere c sediments

(2) pelagic sediments examples :

(i) calcareous oozes

(ii) pteropod oozes

(iii) globigerina oozes

(iv) siliceous oozes

(v) radiolarian oozes

(vi) diatom oozes

9

3. Hydrogenous marine sediments (authigenic sediments)

(1) manganese nodules

(2) phosphates

(3) oolites (CaCo3, limestone par cles)

(4) metal sulfi des

(5) gypsum, halite, other salts

4. Cosmogenic marine sediments

(1) space dusts

(2) meteors par cles

(i) iron-nickel meteorites

(ii) silicate chondrites

5. Voicanogenic marine sediments

1) volcanic dusts

2) volcanic ashes

1. Terrigenous (Lithogenic) SedimentsThe lithogenic sediments are derived from the weathering and erosion of rocks (lithos = rocks, stones,

genic/genous = genera = to pro duce) whether on land (lithosphere) or in the oceans (weathering and erosion of sea volcanic islands), whereas terrigenous (terri = lands, genere = to produce) sediments include only those sediments of various sizes (ranging from boulders to clay par cles) which are produced by weather ing and erosion of only con nental rocks.

The con nental rocks are disintegrated and decomposed due to various types of weathering and thus fi ne to coarse sediments are produced. These sediments of con nental origin are called terrigenous materials which are brought to the rivers. Besides, rocks are also eroded by surface runoff and streams through the processes of surface wash, splash erosion, sheet wash, rainwash, rill and gully erosion, lateral and ver cal erosion of valleys by rivers. The weathered and eroded materials are carried by the rivers and are ul mately unloaded into the oceans and seas. ‘Some 15,000 to 20,000 million tonnes of solid materials are discharged through the rivers to the oceans annually. To this can be included a total of about 4,000 million tonnes of soluble materials. This means that for every cubic meter of water reaching the sea an average of about half a kilogram of sediments is carried away from the con nents’ (Ake Sundborg, 1983). The terrigenous sediments can be deposited in various loca ons of oceans, namely bays and lagoons near the coasts, at the mouths of rivers in the forms of deltas, in the river estuaries, parallel to the coasts form beaches. The terrigenous sediments brought by the rivers to the oceans are also carried away and reworked by sea waves and currents. The turbidity currents carry sediments to deep ocean basins.

In the high la tudes glaciers dump glacially eroded materials into the fi ords. Most of terrigenous materials are deposited in the areas of con nental margins and inner con nental shelves but high build up of terrigenous sediments on con nental shelves forms heaps (mounds) of sediments with steep slope. Thus the sediments slide down enmass under the impact of gravity along the con nental slope. The turbidity currents disperse these sediments on deep sea fl oor. The slumping of sediments is called bulk emplacement. The mode of bulk and retail sedi menta on has been explained in the preceding sec on.

The distribu on of terrigenous sediments on sea fl oor is ubiquitous i.e. widespread (omni present). In other words, terrigenous sediments have been found almost in all parts of the oceans but most concentra on is found in the con nental margins and con nental shelves. Only traces of terrigenous sediments have been found on deep sea plains. The fi ne par cles are picked up by prevailing winds from tropical and subtropical deserts and

10

are carried far away from the con nents to the deep ocean where these par cles fall down and se le down on deep sea plains par cle by par cle, this process of sedimenta on in called retail deposi on. The se ling of fi ne materials par cle a er par cle in the deep sea is called marine snowfall. It is, thus, evident that the great deserts of Asia (Arabian and Thar deserts), Africa (Sahara and Kalahari), South America (Acatama), and Australia are pools of fi ne par cles to be deposited in the oceans.

The terrigenous sediments are composed mostly of quartz mineral. The texture of terrigenous sediments is determined on the basis of grain size for which the following Wentworth scale is used.

Table 6.1: Wentworth scale of grain size for sediments

Sediments type Size range Grain size Energy conditions

(diameter inmillimeters)Gravels 1. boulder > 256 Coarse-grained High energy 2. cobble 65 - 256 3. pebble 4-64 4. granule 2-4 Sand 1. very coarse 1 -2 2. coarse 0.5-1.0 3. medium 0.25-0.5 4. fine 0.125-0.25 5. very fine. 0.0625 - 0.125 mud 1. silt 0.0039 - 0.0625 2. clay 0.0002 - 0.0039 Fine-grained Low energycolloide < 0.0002

Since there is much varia on in the size and shape of terrigenous materials, there is marked grada on of these materials when they are deposited in the ocean, i.e. coarser and larger sediments (boulders, cobbles and pebbles) are deposited near the coast and the size of sediments becomes smaller and fi ner away from the coast. Very fi ne sediments are kept in suspension in the off shore regions. On the basis of size, composi on, and chemi-cal characteris cs terrigenous sediments are divided into gravels, sands and silt, clay and muds, (table 6.1).

Gravels : The diameter of gravels ranges from 2 mm to 256 mm. There is marked grada on in the size of gravels. The following are sub-types of gravels on the basis of diameter of par cles (fi gures in the brackets indicate diameter) : boulders ( > 256 mm), cobbles (65 - 256 mm), pebbles (4 to 64 mm), granules (2 to 4) mm) etc. Since these sediments are very large in size, these are deposited near the coast on the con nental shelves by high energy currents. These sediments are further reduced in size due to further disintegra on caused by sea waves. Gravels are brought to the oceans by the rivers.

Sands : The sediments varying in diameter from 2 mm to 1/16 mm are termed sands. On the basis of size of grains sands are classifi ed into fi ve types viz. (fi gures in the brackets denote diam eter). (i) very coarse sands (1 to 2 mm), (ii) coarse sands (0.5 to 1 mm), (iii) medium sands (0.25 to 0.5 mm), (iv) fi ne sands (0.725 to 0.25 mm), and (v) very fi ne sands (0.0625 to 0.125 mm). The disintegra on and communi on of con nental rock fragments into fi ne sediments produces sands which are deposited in the oceans by rivers, surface wash and winds. There is marked grada on of sand deposits in the oceans i.e. coarser sands are deposited close to the coast while fi ne sands are deposited away from the coast.

Silt, Clay and Mud : The fi ner sediments ranging in diameter from 1/32 mm to 1/8192 mm are grouped under the category silt, clay and mud (silt = 1/32 mm to 1/256 mm, clay = 1/256 mm to 1/8192mm). Mud is s ll fi ner than clay. Some mes, silt and clay are included in the category of mud. Clay is signifi cant cemen ng element. These materials are brought from the con nents by the rivers. Clay and mud are deposited in calm seawater by low energy currents. Generally, these deposits are found at the depth of 100 to 1000 fathoms (600 to 6000 feet). Murray has divided mud into three types on the basis of colour.

(i) Blue mud includes the materials derived through the disintegra on of rocks rich in iron sulphide and

11

organic elements. These are gener ally found at greater depth of the con nental shelves. The original colour of blue mud is bluish black and it contains 35 per cent of calcium carbonate. Blue mud predominates in the Atlan c Ocean, Mediterranean Sea, Arc c Sea and en closed seas.

(ii) Red mud : The sediments derived through the communi on of rocks rich in iron oxides (FeO) form red mud. The reddish colour is mainly due to the dominance of iron content. It contains 32 per cent of calcium carbonate. The deposit of red mud is confi ned mostly to the Yellow Sea, Brazilian coast, and the fl oors of the Atlan c Ocean.

(iii) Green mud is formed due to chemical weathering wherein the colour of blue mud is changed to green mud due to reac on of seawater.

It contains green silicates of potassium and glauconite (form of iron) which cons tutes 7 - 8 per cent of total mineral composi on whereas calcium carbonate ranges from 0 to 56 per cent. The deposits of green mud are found along the Atlan c and Pacifi c coasts of N. America, off the coasts of Japan, Australia and Africa. These are generally found at the depth of 100 to 900 fathoms (600 to 5,400 feet).

2. Volcanogenic SedimentsVolcanic materials deposited in the marine environment are derived from two sources, (i) Volcanic erup ons

on the land-the volcanic materials through violent central erup ons be come very fi ne due to collision among themselves and due to further disintegra on. Fine volcanic materials nearer to the coastal lands are blown by wind and are carried to the oceans while volcanic materials of distant places are brought by the rivers via overland fl ow, rainwash, rills and small rivulets, (ii) Volcanic erup on in the oceans and the seas-in such cases volcanic materials are directly deposited. Volcanic materials resemble blue mud and are grey to black in colour.

3. Biogenic SedimentsBiogenous (bio = life, genere = to produce), also known as organic marine sediments, are the decay and

disintegra on of hard parts (skeletons) of marine organisms. Thus, the source of biogenic sediments is sea itself. The process of forma on of biogenous marine sediments includes the disintegra on of hard parts of marine animals and plants such as their bones, shells, teath etc. a er their death. Such materials fall down one a er another and are deposited on sea fl oors of varying loca ons. Primarily biogenous marine sediments are divided into the following two categories :

• macroscopic biogenic sediments, and

• microscopic biogenic sediments.

Macroscopic biogenic sediments include shells, bones, and teeth of large marine animals which are not widespread sea living organisms. Such sediments are found on con nental shelves and very rare on deep sea plains because deep sea fl oors are dominated by very small organisms. On the other hand, microscopic biogenous sediments are very small par cles of very small sea organisms. They are so small and minute that they cannot be seen without the aid of powerful microscope. There are countless ny microscopic marine organisms. The ny shells of these organisms are called tests which con nuously fall down on the sea bed a er death. In fact,

there is con nuous rain of countless tests of microscopic organisms. The accumula on of these tests on deep sea fl oors gives birth to the forma on of diff erent types of oozes. The oozes consist of 30 percent of microscopic biogenous sediments and remaining 70 percent of terrigenous clay. It may be men oned that microscopic bio-genic materials and terrigenous clay fall down together to se le down on deep sea fl oors.

The biogenic sediments are composed of two main chemical cons tuents, namely calcium carbonate and silica. Diatoms and radiolarians contribute most of silica to the microscopic biogenic sediments, while fora-minifers contrib ute most of calcium carbonate. The microscopic algae, which is called as cocolithophores also contribute calcium carbonate. The biogenous marine sediments, a er mixed with terrigenous clays and their accumula on, form diff erent types of oozes which are named a er the name of microscopic marine organisms such as diatoms, pteropods, radiolaria etc.

12

The biogenic sediments are also divided into the following two broad categories :

• nere c biogenic sediments, and

• pelagic biogenic sediments.

The nere c ma er includes skeletons of marine organisms and plant remains while pelagic ma er consists of remains of diff erent types of algae. The skeletons of animals and dead plants are subjected to decomposi- on and chemical changes. Thus, they are changed to mud and sands and are ul mately deposited on the sea fl oor.

Nere c ma er is deposited mostly on the con nental shelves and are generally covered by terrigenous materials. These include shells of molluscs and their fragments, skeletons of radiolaria and spicules of sponges, calcareous and siliceous plant remains.

Pelagic sediments consist of ma er derived from algae and are mostly in the form of liquid mud, generally known as ooze. Pelagic materials are oozes which are divided into two groups on the basis of lime and silica contents as follows.

(i) Calcareous oozes contain lime content in abundance and are seldom found at greater depth because of their high degree of solubility. They are generally found at the sea fl oor between the depths ranging from 1000 fathoms (6000 feet) to 2000 fathoms (12000 feet). On the basis of principal organisms calcareous oozes are further divided into two sub-types viz, (a) pteropod ooze, and (b) globigerina ooze.

(a) Pteropod Ooze : Most of the pteropod oozes are formed of fl oa ng pteropod molluscs having thin shells of generally conical shape with average diameter of half inch. It contains 80 per cent calcium carbonate and is mostly found in the tropical oceans and seas at the depth of 300-1000 fathoms. It decreases with greater depths and prac cally disappears beyond 2000 fathom depth. It is found mostly in the regions of corals. The main loca on of pteropod ooze includes the western and eastern parts of the Pacifi c Ocean, surroundings of Azores, Canary Island, An les, mid-Mediterranean submarine ridge and Indian Ocean.

(b) Globigerina Ooze : Though this ooze is formed from the shells of a variety of foraminifera but most of such oozes are formed of germs called globigerina. When this deposit is dried up it becomes dirty white powder. Besides milky white colour, it is also blue, grey, yellow and green in colour. The chemical composi on reveals 64.46 percent of calcium, 1.64 percent of silica and 3.33 percent of minerals. Globigerina is found mostly in the tropical and temperate zones of the Atlan c Ocean, on the eastern and western con nental shelves of the Indian Ocean and in the eastern Pacifi c Ocean. It is generally found between the depths of 2000 to 4000 fathoms and becomes absent at greater depths.

(ii) Siliceous Ooze : when silica content dominates, the ooze becomes siliceous in nature.

Silica is derived from a group of protozoa or radiolarians and benthic animals mainly sponges. This ooze does not dissolve as compared to calcareous ooze because of less calcium carbon ate and dominance of silica. Thus, such oozes are found in both warm and cold water at greater depths. This group is further divided into two subtypes on the basis of dominance of a par cular organism.

(a) Radiolarian ooze is formed by the shells of radiolaria and foraminifera. It changes to dirty grey powder when dried. Silica predominates but calcium carbonate is also present (ranging be tween 5 to 20 percent, average being 4 percent). Lime content decreases with increasing depth and it absolutely disappears at greater depth. This ooze is found upto the depth of 2000 to 5000 fathoms in the tropical oceans and seas. It covers the largest areas in the Pacifi c Ocean.

(b) Diatom ooze is formed of the shells of very microscopic plants containing silica in abundance. It also contains some clay. Calcium content varies from 3 to 30 percent. It is blue near the land and the colour changes yellow or cream away from the land. It becomes fi ne coherent white powder when dried. Diatom ooze is very frequently found at greater depth in high la tudes. Signifi cant area of this deposit includes the zone around Antarc ca and a belt from Alaska to Japan in the N. Pacifi c at the depth of600-2000 fathoms.

13

4. Hydrogenic Marine SedimentsHydrogenous sediments are also inorganic ma er and involve precipitates of dissolved substances from

water both on land and in oceans.

Majority of inorganic elements are basi cally precipitates which fall down from above. These elements fall on the land as well as in the oceans. Some of the inorganic elements are transported from the land to the oceans by various agencies. The inorganic precipitates include dolomite, amorphous silica, iron, manganese oxide, phosphate, barite etc. Besides, glauconite, phosphorite, feldspar, phillipsite and clay miner als are also found. The organic and inorganic materials are so mixed together due to chemical processes that it becomes very diffi cult to isolate them from each other.

The signifi cant hydrogenous marine depo si on includes manganese nodules, phosphates, carbonates, metal sulfi des, evaporites etc. These hydrogenous marine sediments have great eco nomic signifi cance.

Manganese nodules have round shape and consist of manganese, iron and some metals. These are formed around nuclei of coral, volcanic rock, bones of fi shes or fi sh teeths. Manganese nodules are primarily composed of manganese dioxide and iron oxide which cons tute together 50 percent by weight. The other cons tuents of manganese nodules include copper, nickel, cobalt etc. Phosphates are infact compounds of phospho rous which are precipitated as coa ngs around rocks. They are also found in the form of nodules. Phosphates are used for making fer lizers. Carbonates include two signifi cant minerals i.e. aragonite and calcite which are composed of calcium carbonate (limestone). Metal sulfi des are generally found along mid-oceanic ridges and include iron, coper, silver, nickel, zinc etc. Evaporites, as the word implies, result from excessive evapora on of seawater. They are basically salts (halite). The other evaporite minerals are gypsum and calcite.

Red clay, previously considered to be of organic origin, is the most signifi cant inorganic ma er and very important member of pelagic deposits. It covers the largest area of deep sea deposits. Silicates of alumina (85.35 percent) and oxides of iron are the chief consituents of red clay. Besides, calcium (6.7 percent), siliceous organ-isms (2.39 percent) and a few minerals are also present. It also contains decomposed volcanic material. It may be pointed out that red clay contains more radioac ve substances than any other marine deposit. It is so , plas c and greasy in character. It becomes reddish brown powder when dried. Red clay is widely distributed at the greatest depth in all the oceans. Its dominant loca ons include the zone between 40°N and 40°S in the Atlan c Ocean, eastern part of the Indian Ocean and the North Pacifi c Ocean covering 129 million km2 of area.

5. Cosmogenic SedimentsCosmogenous sediments are extraterres trial materials which are produced due to collision of meteors in

space. This is why cosmogenic sediments are called space dusts which regularly fall down on the earth’s surface (both on the lands and in the oceans). Cosmogeneous sediments comprise (1) microscopic spherules, and (2) macroscopic debris of meteors.

CLASSIFICATION OF OCEAN DEPOSITSOcean deposits are classifi ed on diff erent bases as follows :

1. On the basis of loca on

2. On the basis of depth of ocean water

3. On the basis of origin of sediments

1. On the Basis of Loca onThis classifi ca on is based ou typical loca ons of par cular marine sediment. Though several scien sts

have a empted to classify ocean deposits on the basis of their loca ons, the classifi ca ons of Sir John Murray and J.T. Jenkins are widely acclaimed.

14

Generally, ocean deposits are loca onally classifi ed into the following two categories :

• shelf deposits

• pelagic deposits

Shelf deposits include the deposi on of marine sediments of variable origin on the fl oors of con nental shelves, while pelagic deposits consist of sedimenta on of fi ne par cles on the fl oors of deep sea plains.

Classifi ca on of Murray : Sir John Murray has classifi ed the ocean deposits into two broad categories viz. (a) terrigenous deposits, and (b) pelagic deposits. Terrigenous deposits are found mainly on the con nental shelves and slopes whereas pelagic deposits predominate on the deep sea fl oor. Terrigenous deposits are composed of coarser materials and are derived from the con nents through weathering and erosional processes and are transported to the oceans by various agencies. Their colour may be blue, yellow, grey or red. Pelagic deposits consist of the materials formed of skeletons and shells of marine organisms and a few inorganic substances. They are generally blue, grey or red in colour.

Classifi ca on of Jenkins: Jenkins has divided marine deposits into three groups viz (a) deep sea deposits, (b) shallow water deposits, and (c) li oral deposits. The following is the detailed classifi ca on of Jenkins :

(A) Pelagic deposits

(1) red clay

(2) radiolarian ooze

(3) diatom ooze

(4) globigerina ooze

(5) pteropod ooze

(B) Terrigenous deposits

(1) blue mud

(2) red mud

(3) green mud

(4) coral mud

(5) volcanic mud

(6) gravel

(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

(5) pteropod ooze

15

(b) Terrigenous deposits

(1) blue mud

(2) red mud

(3) green mud

(4) coral mud

(5) volcanic mud

(B) Shallow sea deposits

(between low de water and 100 fathoms)

(1) gravels

(2) sands

(3) mud

(C) Li oral deposits

(between high and low de water)

(1) gravels

(2) sands

(3) mud

3. General Classifi ca on(1) Terrigenous deposits

(i) li oral deposits

(ii) shallow water deposits

(iii) terrigenous mud

(2) Neri c deposits

(i) shallow water neri c deposits

(ii) deep seawater neri c deposits

(iii) pelagic deposits.

4. Classifi ca on on the Basis of Origin of Sediments(1) Li oral deposits (derived from land)

(i) shore deposits

(ii) shelf deposits

(2) Heunpelagic deposits

(partly from land and partly from marine origin)

(i) green mud

(ii) volcanic mud

(iii) coral mud

16

(3) Eupelagic deposits

(of marine and cosmic origin)

(i) red clay

(ii) radiolarian ooze

(iii) globigerina ooze

(iv) pteropod ooze

DISTRIBUTION OF OCEAN DEPOSITSDistribu on of ocean deposits may be a empted in various ways as follows :

• ver cal distribu on of ocean deposits (fi g. 6.2)

• regional distribu on (ocean-wise distribu on)

• marine province-wise distribu on, such as ocean deposits on con nental shelves, and on deep sea plains.

• sediment-wise distribu on, such as terrigenous deposits, and pelagic deposits.

Distribu on of Terrigenous DepositsTerrigenous deposits include gravels, sands, muds, volcanic materials etc. which are derived through

weathering and erosion of con nental rocks by various denuda onal processes. There is marked grada on of these sediments when they are deposited in the oceans. The sequence of these materials from the coast towards the sea is gravel, sand, silt, clay and mud. The ocean currents and waves very o en disturb the grada- on and sequence of sediments. Terrigenous deposits are classifi ed into 3 categories on the basis of loca on

and depth as follows :

• li oral deposits

• shallow water deposits

• deep water deposits

(1) Li oral deposits are generally found on the con -nental shelves mainly near the coastal margins upto the depth of 100 fathoms (600 feet) but they have been also traced upto the depth of 1000 m-2000 m. Li oral deposits consist of gravels, sands, silt, clays and muds.

(2) Shallow water deposits include terrigenous sedi-ments deposited between low de water and 100-fathom depth. These deposits consist of gravels, sands, silt and clays of varying propor ons. Sea waves and dal waves help in the grada on and sor ng of sediments but undersea landslides, slumping, strong storm waves, and storms some mes disturb the ver cal stra fi ca on of sediments.

(3 ) Deep water deposits include the sediments de-posited below the depth of 100 fathoms. There is marked grada on of sediments in ver cal succession where the sequence of sediments with increasing depths is blue mud, red mud, green mud, coral mud and volcanic mud.

Distribu on of Pelagic Deposits

17

Pelagic deposits consis ng of remains of marine plants and animals in the form of diff erent types of oozes cover about 75.5 per cent of the ocean areas. Pteropod, diatom and radiolarian oozes cover 0.4, 6.4 and 3.4 per cent areas of all the oceanic deposits respec vely. Red clay cons tutes 31.1 percent of the total ocean deposits.

Table6.2: Areas covered by pelagic sediments (million km2)Sediments Atlantic Ocean Pacifi c Ocean Indian Ocean Total

Calcareous Oozes

(i) Globigerina 40.1 51.9 34.4 -

(ii) Pteropod 1.5 - - -

Total 41.6 51.9 34.4 127.9

Siliceous Oozes

(i) Diatom 4.1 14.4 12.6 -

(ii) Radiolarian - 6.6 0.3 -

Total 4.1 21.0 12.9 102.2

Red Clay 15.9 70.3 16.0 102.2

Total 61.6 143.2 63.3 268.1

Pteropod oozes are found over an area of 12,90,000 km2. Globigerina oozes cover larger areas in the

18

Pacifi c (64.5 million km2), the Atlan c (37.9 million km2) and the Indian (31.4 million km2) oceans (fi gs. 6.4, 6.5 and 6.6). Radiolarian oozes are found over an area of 5.16 million km2 in the Pacifi c and Indian oceans. Diatom oozes are spread over an area of 1,03,000 km2 in the North Pacifi c Ocean and 27.6 million km2 in the southern oceans. Red clay is distributed over an area of 129 million km2 of all the oceans.

Philippi has described a ver cal stra fi ca on of diff erent pelagic sediments wherein the sequence from top to the bo om includes pteropod ooze, globigerina ooze, radiolarian ooze, diatom ooze, and red clay. Figs. 6.4,6.5,6.6 & 6.7 depict, general pa ern of horizontal distribu on of ocean deposits. It is apparent from the

fi gures that terrigenous deposits are found along the coasts mainly on con nental shelves but they cover greater extent near the East Indies, in the North Pacifi c and along the Labrador coast. Globigerina ooze, red clay and diatom ooze dominate in the western, eastern and southern parts of the Indian Ocean whereas it contains maximum areal extent in the Pacifi c Ocean.

Ocean Deposits on Con nental ShelvesIt may be recalled that con nental rocks are the most signifi cant source of marine sediments, as con nental

rocks are weathered by diff erent weathering processes (physical, chemical and physico-bio-chemical weathering) and are eroded by surface runoff . The eroded materials are carried by the rivers and are ul mately unloaded in the oceans. Thus, terrigenous source is the major contribu ng source of sediments to be deposited on the con nental shelves. The follow ing factors determine the process of sedimental on con nental shelves :

• amount of terrigenous materials brought

• by the rivers from the lands,

• velocity of river fl ow at their mouths,

• distance from the coast,

• depth of water,

• energy condi ons of waves, and currents, etc.

19

It may be reestated that con nental shelves are broad, almost fl at and shallow pla orms of land submerged under seawater, which range in length (from coasts to the point of shelf break, fi g. 6.8) from 70 to 100 kilome-ters or even more having depths from o m (at the shoreline) to 120- 150 meters. Tidal waves, wind-generated sea waves and currents are primary energy sources. Since the energy of bo om currents decreases from the shoreline with increasing distance of con nental shelves and increasing depth of seawater, and hence there is marked grada on (sor ng) of terrigenous sediments on the fl oors of con nental shelves in the following sequence (fi g. 6.8) :

Shoreline : gravels (boulders, cobbles, peb bles granules)—>coarse to medium grained sands —> fi ne grained sands—>sand and mud —> sandy mud —>mud in off shore region —> shelf break (outer margin of con nental shelves).

Fig. 6.8: Sequence of deposi on of marine sediments on con nental shelves.

It appears from the above discussion and fi g. 6.8 that grain size of sediments on con nental shelves is propor onal to energy level of waves and currents. As the energy level decreases away from the shoreline, the grain size of shelf sediments also decreases i.e. becomes fi ner towards outer margin of shelf (shelf break). Thus shallow water is characterized by high energy condi on and coarser sediments whereas deeper water denotes low energy condi on and fi ne sediments.

It may be men oned that the aforesaid ideal sequence of marine sediments on con nental shelves is seldom found in reality because fl uctua on in sea level causes transgression and regression of seawater on coastal lands and the environment of energy levels of waves and current also changes. At the me of fall in sea level (nega ve sea level change caused either by tectonic ac vi es or glacial age) the inner parts of con nental shelves (coastward part) emerges above sea level while during rise in sea level (posi ve sea level changes caused either by tectonic ac vi es or deglacia on of ice sheets during interglacial period) seawater transgresses on to land. These events disturb the normal sequence of marine deposits on con nental shelves. The geological records reveal fall of sea level due to late Pleistocene glacia on by about 130 meters 15,000 years before pre-set from the present sea level. Therea er deglacia on during Holocene period enabled the sea level to regain its present level by rising 130m from the late Pleistocene sea level. This is why major por on of sediments on con nental shelves is relict sedi ment. Around 60-65 percent of outer con nental shelves is characterized by relict sediments of coarse texture (gravels and sands). It may be men oned that the outer con nental shelves (seaward por on) have rela vely deeper water where low energy condi on predominates. Thus coarse grained sediments cannot be deposited on quiet sea condi on of present me. On the other hand, the inner parts of con nental shelves are characterized by coarse to fi ne grained sediments which are in accordance with high energy level of bo om currents at present me. In other words, the coarse and fi ne grained sediments on the fl oors of the inner con nental shelves are modern sediments.

The worldwide distribu on of marine de posits on con nental shelves denotes la tudinal varia on. K.O. Emery (1969) has iden fi ed zonal pa ern of distribu on of marine sediments on con nental shelves at world level as follows :

(1) Tropical shelves are dominated by bio genic sediments.

20

(2) Temperate (midla tudes) shelves are char acterized by the dominance of terrigenous sediments brought by the rivers.

(3) Con nental shelves in the polar areas are dominated by glacial marine sediments ( lls and ice-ra ed debris).

Table 6.3 : Distribu on of deep-sea ocean deposits (pelagic deposits) (in percentage)

Type of sediments Composi on Atlan c Pacifi c Indian Whole Globe

Ocean Ocean Ocean

Globigerina ooze carbonate 65 36 54 47

Pteropod ooze carbonate 2 0.1 - 0.5

Ditom ooze silica 7 10 20 12

Radiolarian ooze silica - 5 0.5 3

Red clay aluminium 26 49 25 38

silicate

Oeep-Sea Ocean DepositsAs already stated pelagic deposits predomi nate in the deep sea marine deposits. The areal distribu on

of deep sea ocean deposits consis ng of calcareous oozes (globigerina and pteropod oozes), silicines oozes (diatom and radiolanian and red clay) has been shown in table 6.2 while percentages of these deposits in dif-ferent oceans (Atlan c, Pacifi c and Indian Oceans) have been shown in table 6.3. It is apparent from table 6.3 that globigerina ooze is the most widespread deposits on the fl oors of deep oceans, as it occupies 47 percent of total global deep sea deposits. Except the Pacifi c Ocean, the Atlan c and Indian Oceans are dominated by globigerina ooze as they account for 65 and 54 percent of their total deep sea deposits respec vely. It is the red clay which is most widespread in the Pacifi c Ocean (49 percent). The Atlan c and Indian Oceans account for 26 and 25 percent of their respec ve total deep sea deposits. Diatom ooze is the third signifi cant deep-sea deposit (12 percent of total global deep sea deposits). The Indian, Pacifi c and Atlan c Oceans contain 20, 10 and 7 percent of their total deep sea deposits respec vely. Radiolarian ooze is almost insignifi cant as it shares only 3 percent of total global deep sea deposits.

IMPORTANT DEFINITIONSAuthigenic deposits : The materials derived through biochemical precipita on and deposited on sea fl oors

in situ are called authigenic deposits.

Backwash : The breakers or swash or surfs a er reaching the sloping beach return towards the sea as backwash or undertow currents and rip currents.

Biogenic sediments : The sediments formed through the deposi on of skeletal remains of marine organ-isms on sea fl oors are called biogenous sediments and deposits which have at least 30 percent by volume of remains of marine organisms.

Breaker waves : The turbulent and unstable forward moving shorebound waves, which break at the shoreline, are called breaker waves or simply breakers or surf, or uprush or swash.

Breakwaters: are protec ve structures errected off shore to save the coasts from the wave erosion. They may be parallel, perpendicular or slan ng to the coasts.

Bulk emplacement : involves the enmass transport (slumping) of marine sediments down the undersea slope by gravity currents or turbidity currents under the force of gravity.

Con nental shelf : The broad, fl at, shallow and gently sloping sea fl oor extending from the coasts to the

21

point of shelf break or upper part of con nental slope is called con nental shelf.

Con nental slope : Steeply sloping sub merged sea bo om extending from the outer marging of con nental shelf or from the point of shelf break and ending into deep sea trenches is called con nental slope.

Cosmogenous sediments : The sediments of extraterrestrial origin, say from the meteorites in the space, are called cosmogenous sediments.

Deep sea fans : Cone-shaped deposits of graded materials at the mouths of submarine canyons are called deep sea fans.

Density current : The undersea gravity- driven current is called density current such as turbidity current.

Diatoms : are single celled microscopic phytoplanktons (marine plants) which are respon sible for bulk primary produc on in marine environment.

Evaporites: are deposits of dissolved subtances due to evapora on of water such as salts and gypsum.

Foraminifera: are marine protozoans having test composed of calcium carbonate, and linear or spiral or concentric shells perforated by small holes or pores.

Glacial marine sediments: are those terrigenous sediments which are transported and deposited by gla-ciers in the oceans. These also include the sediments produced through ice ra ing.

Graded bedding: denotes ver cal grading of grain size in the layered structure of sedimentary deposits where grain size becomes fi ner in ascending order.

Gravels : are coarse-grained terrigenous materials consis ng of boulders, cobbles, pebbles and gran-ules.

Gravity waves : are marine undersea waves, such as turbidity waves or currents.

Groins : are protec ve structures of either concrete or woods which are errected perpendicu lar to the coasts at regular intervals to protect harbours and beaches.

Hydrogenous sediments : The sediments derived from precipita on of dissolved sub stances due to chemi-cal reac ons such as phosphorites, oolites (calcium carbonate), metal sulfi des, gyp sum, salts etc. are called hydrogenous sediments.

Lithogenous sediments: The sediments derived from the weathering and erosion of rocks either on land or in oceans are called lithogenous sediments.

Li oral zone : The zone of benthic province between high and low de waters is called li oral zone.

Macro-biogenic sediments: are those sediments which are derived from the shells, bones and teeths of marine animals.

Micro-biogenic sediments: are small par cles of microscopic marine organisms, such as tests which con- nuously fall down on sea bo oms.

Marine snowfall: The con nuous fall of ny marine sediments on the ocean fl oors is called marine snow-fall. It resembles the fall of snow fl akes on the land.

Neri c sediments : The marine sediments deposited on the fl oors of con nental shelves are called neri c sediments.

Ocean deposits : The consolidated marine sediments in the form of sedimentary layers on sea fl oors are called ocean deposits.

Pelagic ma er : The sediments deposited on deep sea fl oors through slow sedimenta on are called pe-lagic ma er.

Phosphorites : The hydrogenic deposits having the nodules of phosphorous (P205) are called phospho-

22

rites.

Retail sedimenta on : involves deposi on of marine sediments par cle by par cle, known as con nuous rain of ny par cles.

Radiolaria : are unicellular marine animals having siliceous tests and belong to planktonic and benthos community.

Relict sediments: denote those sediments on the con nental shelves which are not of modern age as they are not in equilibrium to present environmental condi on.

Seawall : is a protec ve structure of wood, boulders or concrete which are constructed along the coasts to protect them from wave erosion.

Shelf break : is the outer edge of the con nental shelves from where starts the con nental slope.

Terrigenous sediments : are those marine sediments which are derived through the weather ing and ero-sion of con nental rocks and brought to the oceans by rivers.

Tests : The ny shells of microscopic marine organisms are called tests which con nu ously fall on sea fl oors.

Turbidity currents : are driven by the high density of sediments. They are laden with slurry of sediments and move downslope with high speed in the oceans.

Undertow currents : The breaker waves (surfs) a er reaching the sloping beach returns to the sea as a backwash or undertow current.