The document discusses marine sediments, outlining their formation, types, and significance in geological oceanography. It explains the classification of sediments based on their origin and texture, emphasizing the role of various processes in transporting and depositing sediments in the oceans. Additionally, it highlights the economic importance of marine sediments, which are crucial for understanding Earth's geological history and for resource exploitation.

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MARINE SEDIMENTS
by
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science
University of Mysore
India

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Introduction:
Oceanography is an interesting subject to
learn. It is a part of earth sciences. Geological
oceanography is the study of Earth beneath
the oceans.
A geological oceanographer does three major
works.
a)The geological oceanographers study the
topography, structure, and geological
processes of the ocean floor.

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b)They discover how the Earth and oceans
were formed.
c) They also analyse how the ongoing
processes may change the oceans in the
future.
The subject of geological oceanography
includes many sub-disciplines, like
geophysics and plate tectonics,
petrology and sedimentation processes, and
micropaleontology and stratigraphy.

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Geological oceanographers also explore
many features of the oceans such as
continental rises and ocean ridges, ocean
trenches, seamounts, abyssal hills, the
oceanic crust, marine sediments, erosional
processes, volcanism, and seismicity.
The growth of Science and Technology made
it possible to explore the deep ocean basins
in several parts of the world.

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The ocean morphology and relief were
extensively studied using sophisticated
instruments and detailed mapping
techniques.
Ocean water circulation, distribution of
temperature, density and salinity, are very
dynamic features in the seas and oceans.
Oceans act as reservoirs of energy. They are
the home for many marine life.

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Oceans also contain a huge amount of
mineral resources. Deep ocean basins are
the zones of continuous sedimentation. The
oceans are the final destination for many of
all the sediments to be deposited. About 200
million years of earth history are available in
the fossiliferous sediments deposited within
the ocean basins.

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Understanding of these marine sediments
become an inevitable aspect of
oceanographic studies. Hence, in this
module, the following marine sediments are
highlighted:
1. Types of sedimentary deposits
2. Sediment texture and transport
3. Composition of marine sediments
4. Distribution of oceanic sediments
5. Ocean sediments as resources

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1. TYPES OF SEDIMENTARY DEPOSITS
Sediments are the earth’s materials like
gravel, sand, mud and lime that are
transported and deposited by wind, water,
ice and gravity.
They get accumulated and cemented to form
sedimentary rocks.

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Based on the mechanism of accumulation,
sediments are classified into
a) Residual sediments
b) Mechanical sediments
c) Chemical sediments and
d) Organic sediments.
Residual sediments are formed due to
weathering. Eg. Laterite and bauxite.

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Mechanical sediments are formed due to
deposition after transportation in water. Eg.
Sandstone and conglomerates.
Chemical sediments are formed due to
precipitation of soluble constituents by
water. Eg. Limestone.
Organic sediments are formed due to the
accumulation and preservation of organic
matter.

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Continuous sedimentation of organic and
inorganic substances takes place in all
aquatic ecosystems.
Benthic fauna thrives using these resources.
Bioturbation is the stirring of a sediment by
benthic fauna. This often destroys the
structure of sediments.
Recently laid sediments of lakes or seas may
contain some detritus food and micro
organisms.

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Benthic fauna, feed through burrowing of
these recently laid sediments. The bedding
and lamination structures already created in
these sediments are disturbed.
Marine sediments are formed by several
processes. Marine sediments are brought in
by the rivers, glaciers and wind alongwith
shells and skeletons of organisms.

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Deep sea rocks that formed from these
sediments are good source of information
about the past sedimentary environments.
Sediments are much useful in deciphering
the source rock contribution and the
locations of their origin. Sediments vary in
their sizes, shapes, chemistry and density.
Some are larger in size and some are very
fine particles.

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Marine sediments are also classified based
on their origin and particle size.
The notable categories of marine sediments
are;
a) Lithogenous Sediments or terrigenous
sediments
b) Biogenous sediments
c) Hydrogenous sediments or authigenic
sediments and d) Cosmogenous sediments.

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The term lithogenous means “derived from
rocks”.
They originate from erosion of land, volcanic
eruptions, and blown dusts.
These are mainly existing as continental
masses, and as volcanic islands inside the
oceans. These are important sources of
lithogenous sediments.

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Weathering of minerals, on continental
lithospheric rock bodies, provide the input
for lithogenous sediments in the oceans.
Quartz sands, clays, muds are the examples.
Clays are the major products of mineral
weathering reaction on land.

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The major products of such processes are
a) Clays – are derived from decomposition of
feldspars
b) Chlorite – are derived from decomposition
of ferromagnesian minerals
c) Montmorillonite –is a notable clay
mineral
d) Illite – is another clay mineral
e) Kaolinite –is a product of Potash Felspar
and mica.

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f) Zeolite – is a hydrated aluminum silicate
resulting the weathering of feldspars.
Wind plays a significant role in the transport
of lithogenous sediments towards the
oceans.
Biogenous sediments are derived from
organisms.

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The materials include carbonate minerals
created by organisms, such as corals,
molluscs, and foraminifera.
These are deposited at ocean floors with
layers of calcite forming limestone.
The other biogenic rocks include
stromatolites, the flint nodules found in chalk
and coal.

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Bones, teeth and insoluble remains of
organism are continuously deposited inside
the oceans.
The most common chemical compounds
found as biogenous sediments are silica and
calcium carbonate.
Microscopic diatoms, animals like
radiolarians and organisms like foraminifera
are responsible for producing Calcium
Carbonate component.

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Growing Algae and coral reef are the
outcome of such biogenic sediment
formation.
Hydrogenous sediments are mainly derived
from water. They are formed by the
chemical reaction and precipitation that
occur within the seawater.

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The notable minerals formed under this
category are
Manganese deposits , Phosphorites and
Glauconites.
The rate of accumulation of these
hydrogenous sediments is very slow. One of
the prominent marine sediments having
great economic value, is the occurrence of
Manganese nodules. These are polymetallic
nodules. They are found in deep ocean floors.

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The major components of manganese
nodules are Mno and Fe2O3. In addition, they
contain copper, cobalt and nickel. The
genetic cause of these nodules may be
related to the volcanic products, runoff from
continents and other mineral-weathering
reactions happening on rocks.
Bacteria is an active player in the formation
of manganese nodules.

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The other marine sediments include a good
amount of Phosphorites, Glauconites and
Carbonates.
2. SEDIMENT TEXTURE AND
TRANSPORT
They are mostly particles of varying sizes
and shapes.

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Sediments are characterized by various
properties like sphericity, roundness, sorting
and textures.
Sediment textures are determined by their
grain size. They range from large sized
boulders to minute colloids. Wentworth has
proposed a size range for various particles
for sedimentological analysis.

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The scale of particles are classified according
to their size ranges. This is a popular
approach for understanding the sediment
textures.
Let us see the Size Range (mm) in diameter
of various particles.
256mm particles are called as Boulders
64mm particles are called as Cobbles
4mm particles are called as Pebbles
2mm particles are called as Granules

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1mm particles are called as Very Coarse
sands
0.5 particles are called as Coarse sands
0.25 particles are called as Medium sands
0.12 particles are called as Very fine sands
0.06 particles are called as Silts
0.002 particles are called as Clays
And the rest are called as Colloids.

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The cobbles, pebbles, granules, sand, silt and
clay sized particles are the major categories
of sediments deposited inside the seas.
Larger particles like cobbles and boulders
are deposited by strong waves, in areas of
high energy. When the energy level is low,
clay-sized particles are deposited by the
waves.

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Sorting of sediments are normally done by
the water masses very effectively. Well-
sorted sediments are seen in beach sands.
Poorly sorted sediments are found in
continental shelf regions.
Sediments are also characterized by their
angularity.

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During the course of a long journey by a
river, the angular fragments are rolled up by
the stream water, and become rounded in
their shapes and also smaller in their sizes.
Roundness and sphericity increase over a
period of time and ultimately at the mature
stage they settle in some parts of the ocean.

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Maturity of sediments are indicated by
decreasing clay content
increasing degree of sorting and
increasing roundness of grains.
A beach sand is an ideal example for a
mature sedimentary deposit. The sediments
derived from the continents and from the
margins of the oceans gradually settle out to
fill the bays and become the deltas.

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The great volume of sediments, when
transported by rivers and longshore
currents, get deposited beyond the shelf
region and reach the deep-ocean basin.
Erosion, Transportation and deposition of
sediments depend on two major factors like
velocity and particle size.

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Very small sized particles, less than 20
millimicron in diameter, are carried far
inside the open ocean by even the surface
winds.
Marine deposition is an everlasting
phenomena of all the oceans in the world.
Most of the sediments are transported by
current systems in streams, along shorelines
and in shallow seas before they are
deposited.

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Due to these, sedimentary rocks show
layering and other structural features. The
notable structures are
a) Stratification
b) Cross-bedding
c) Graded bedding
d) Ripple marks and mud cracks.
These features, are called as primary
sedimentary structures.

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3. COMPOSITION OF MARINE
SEDIMENTS
Composition of Marine sediments
Marine sediments are of two major
categories as
a) Neritic sediments and
b) Oceanic sediments

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Neritic sediments are those that are
deposited near the continents and oceanic
sediments are the characteristic of deep-
ocean basins.
The major outputs of these are
a) Turbidites
b) Glacial deposits and
c) Carbonate deposits.

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The sediments of deep ocean basins include
a) Abyssal clay and
b) Calcareous and Siliceous oozes
Neritic sediments refer to mainly lithogenous
sediments. Oceanic sediments refer to
organic and hydrogenic categories of
sediments. Marine sediments are called as
Pelagic sediments when they accumulate in

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the abyssal plain of the deep ocean, far away
from terrestrial sources.
Pelagic sediments are primarily limited to
the continental shelf.
Pelagic sediments that are mixed with
terrigenous sediments are known as
hemipelagic.

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There are three main types of pelagic
sediments:
1. Siliceous oozes
2. Calcareous oozes
3. Red clays.
Ooze does not refer to a sediment's
consistency, but to its origin.

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Oozes are primarily biogenic that is,
composed of planktonic debris while red
clays are non-biogenic, with little organic
material.
Any sediment that is composed of more than
30% of microscopic skeletal debris is
classified as an ooze. The type of sediment
that accumulates at a given locale is
determined by the location's distance from
land, water depth, and overall fertility.

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Oozes are not present on the continental
margin.
The Calcareous ooze is composed primarily
of the shells--also known as tests--of
foraminifera, coccolithophores, and
pteropods. This is the most common pelagic
sediment by area, covering 48% of the world
ocean's floor.

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This type of ooze is limited to depths above
the Carbonate Compensation Depth at time
of burial.
It accumulates more rapidly than any other
pelagic sediment type, with a rate that varies
from 0.3 - 5 cm / 1000 yr.
The Siliceous ooze is composed of the
debris of plankton with silica shells, such as
diatoms and radiolaria.

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This ooze is limited to areas with high
biological productivity, such as the polar
oceans, and upwelling zones near the
equator.
The least common type of sediment, it covers
only 15% of the ocean floor. It accumulates
at a slower rate than calcareous ooze: 0.2-1
cm / 1000 yr.

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The Red clays are also known as pelagic
clays. They accumulate in the deepest and
most remote areas of the ocean. They
Contain less than 30% biogenic material,
their composition is a varied. There will be a
mix of very fine quartz and clay minerals.
The authigenic deposits precipitated directly
from seawater, and also from
micrometeorites.

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The red clays take the color of oxidized iron
minerals. Sometimes, it is brownish in color.
Its ultimate origin is uncertain. Red clay
seems to be mostly derived from distant
rivers, and windblown dust.
These deposits cover about 38% of the ocean
floor. They accumulate more slowly than any
other sediment type, at only 0.1-0.5 cm /
1000 yr.

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There are several kinds of oozes recognized
based on their content and composition.
The notable ones are:
a) Diatom ooze
b) Radiolarian ooze
c) Foraminifera ooze
d) Pteropd ooze and
e) Globigerina ooze occur in deep oceans.

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Destruction of skeletal remain occurs due to
solution.
Dilution is a major process of oceanic waters.
The origin of Marine sediments are easily
indentified from their mineral composition.
Occurrence of rock-farming minerals like
quartz and feldspar are indicative of their
origin from rocks of the land.

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Occurrence of silica (sio2) and calcite (CaCo3)
may be attributed to the compound derived
from organisms.
Numerous minerals also get precipitated
from ocean waters.
Phosphorites are found at depths above
1000m and Glauconites are found at a depth
of 2500m.

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Aragonites and Calcites are the minerals of
Carbonate group.
Precipitation of these minerals happen in the
form of Oolites. Due to the occurrence of
much carbonate masses, oceans are
considered to be the reservoirs of carbonates
and a store house of carbon-di-oxide.
The cosmogenous sediments are of cosmic in
origin.

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These are the microscopic spherules rich in
iron and rocky chondrules.
Meteoric impacts are the major reasons for
the occurrence of such sediments in oceans.
Metallic element like iridium is found in
marine sediments, which is indicative of
cosmogenic origin.

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4. DISTRIBUTION OF OCEANIC
SEDIMENTS
There are two aspects related to marine
sedimentation.
One is the settling rate and the other one is
the sedimentation rate.

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Let us see the Settling of sediments:
Water can carry the sediments as suspended
load and bed load.
These sediments are settled when there is a
change in the velocity of flow or due to any
obstruction for movement.
The settling velocity is the minimum velocity
a water flow must have in order to transport
the sediments rather than depositing them.

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This is given by the Stoke's Law:
SV= 2( Dp-Df) gr^2/ 9 m
where SV is the settling velocity,
D is density
p indicates particle
f indicates the fluid
g is the acceleration due to gravity,
r is the radius of the particle and
m is the dynamic viscosity of the fluid.

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If the flow velocity is greater than the settling
velocity, sediment will be transported.
Otherwise, they are settled.
The rate of sedimentation depends on
various factors like daily input, seasonal
inputs and annual inputs from various
sources.

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Oceanic sediments cover about 75% of the
ocean floors. In this, the calcareous oozes
alone cover about 48% of the deep sea floors.
Abyssal clay covers about 38% and siliceous
oozes cover about 14% of the total area.
Calcareous oozes are the dominant oceanic
sediments of the Indian and Atlantic oceans.
Abyssal clays are the principal marine
deposits of the Pacific ocean.

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One of the major marine sediments confusing
the oceanographic studies is due to the
occurrence of Fecal Pellets. Theses are
derived from the surface directly above the
seas and oceans. Fecal Pellets produced from
tiny animals living in water are also the
major contributors.

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The following are the statistical
observations:
a) The rate of accumulation of lithogenous
particles in 1.27 cu.km / year.
Biogenous particles 0.38 cu.km / year.
Both of them put together contributes to
1.65 cu.km / year.
b) The rate of volcanic sediment
accumulation is 0.05 cu.km / year.

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c) Seamounts add 0.2 cu.km / year of mass
to the ocean crust and volcanic islands add
1.1 cu.km / year.
All of these put together contribute 3 cu.km
/ year of sediments into the ocean floors.

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5. OCEAN SEDIMENTS AS RESOURCES
Potential mineral resources and organic
resources are contained within the marine
deposits. The only problem is the high cost
involved in exploitation of these resources.
Some are exploited and a few are being
explored.

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One of the primary resources explored from
oceanic bottoms is the oil resources.
Nearly one-third of the world's oil comes
from offshore fields in our oceans.
Areas most popular for oil drilling are the
Arabian Gulf, the North Sea, and the Gulf of
Mexico.
Sand and Gravel are plenty in oceans. They
also contain rich and valuable minerals.

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Precious stones including Diamond are found
from ocean bottoms. Sulphur, Phosphorites,
Polymetallic crusts and nodules are the
major marine deposits exploited from
oceans. Metallic minerals containing cobalt,
copper and nickel.
Ocean sediments are not uniformly
distributed. They are very rare along mid
oceanic ridges.

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The rate of sedimentation in the global
oceans are as follows:
a) in the deep ocean floors, it is 0.5 to 1.0
cm/ 1000 years.
b) In continental margins, it is 10-50 cm/
1000 years
c) In deltas, bays and estuaries, it is
500cm/1000 years.
The economic minerals are also related to
the depths of their orgin.

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Polymetallic nodules occur at a depth from
4,000 to 6,000 m containign Nickel,
copper, cobalt, and manganese.
The manganese Crusts occur at a depth
ranging from 800 to 2,400 m containing
mainly cobalt, some vanadium, molybdenum
and platinum.
The sulfide deposits occur at a depth
ranging from 1,400 to 3,700 m containing

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Copper, lead and zinc and some gold and
silver.
Diamonds are also mined from the seabeds.
Ocean mining sites are usually around large
areas of polymetallic nodules or active and
extinct hydrothermal vents at about 1,400 -
3,700 meters below the ocean’s surface.

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The vents create sulfide deposits, which
contain precious metals such as silver, gold,
copper, manganese, cobalt, and zinc.
The deposits are mined using either
hydraulic pumps or bucket systems that take
ore to the surface to be processed.

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The rising demand for precious metals in
Japan, China, Korea and India has pushed
these countries in search of new sources.
Now, the interest has recently shifted toward
the hydrothermal vents as the source of
metals instead of scattered nodules.
Marine waters are also under serious threats
by other human activities.

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During the last five decades, high level
nuclear wastes have been found to be
accumulating inside the seas and oceans.
These are mainly due to the production of
nuclear weapons and power generation.
Global mining in the deep seas, raises a lot of
questions about the environmental damages
they create to the surrounding areas.

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Experts are also apprehensive about these
removal of parts of the sea floor.
Removing parts of the sea floor disturbs the
habitat of benthic organisms, causing
permanent disturbances.
There are also leakage, spills and corrosion
effects. It is necessary to properly handle the
marine sediments for human consumption.