Sedimentary Rocks

Syllabus
Sedimentary Rocks
• Introduction, Classification of Sedimentary
• Rocks, Common Structures and Textures of
Sedimentary Rocks, Descriptive Study of
Common Sedimentary Rocks Descriptive
Study of Common Sedimentary Rocks
(Sandstone,
Limestone,
Shale,
Conglomerate, and Breccias)

Introduction
• Sedimentary rocks are those which have formed out of
sediments.
• Sediments are rock fragments which are product of weathering.
Weathering has already been defined as natural processes of
disintegration (i.e.. mechanical breakdown) and decomposition
(i.e.. chemical decay) of rocks.
• Sediments which have formed out of disintegration are loose
materials of various sizes like clay, sand and pebbles. Since
sediments represent secondary or derived materials from the
pre-existing rocks, rocks formed out of them are also called
secondary rocks.

Sedimentary rocks on the earth’s crust
• The earth is nearly 8000 miles in diameter. The
top portion of the earth is called the crust. The
crust being less than 40 miles is too thin
compared to the size of the earth as a whole.

• This crust, is mainly made up of sima and sial. Sima is
the underlying portion of the ocean floor and Sial is the
overlying granitic portion forming the continental part.
• Above these, lies a thin veneer of sedimentary rocks
covering large tracts of the surface. This sedimentary
layer is discontinuous and of varying thickness.
• Quantitatively, sedimentary and related rocks together
constitutes only 5 % of the earth’s crust, the remaining
95 % being igneous rocks and their metamorphic
equivalents.

• Among different sedimentary rocks, shale is the
most abundant; sandstone and limestone are of
next order.
• The three represents approximately 4 %, 0.75 %
and 0.25 % of the earth’s crust. Since most of the
sedimentary rocks in nature occur as beds, these
are also called stratified rocks. Essentially,
sedimentary deposits are aqueous, i.e.. formed in
the bodies of water such as river, lakes and seas.

Classification of Sedimentary Rock
• The classification of sedimentary rock is closely related
to their origin. Broadly, they are classified into detrital
and non-detrital rocks. Detrital rocks, which are
popularly called clastic rocks, are formed out of
physically broken and transported rock fragments.
Coarser rock fragments are cemented by a finer matrix
and give rise to reduaceous and arenaceous rocks.
Conglomerates, breccias, tiilites and sandstones
represent these kinds of rocks. Still finer sediments
like silt and clay mainly undergo compaction and
become hard mudstones and shales.

• Non-detrimental rocks or non—clastic rocks are
formed either by precipitation, evaporation or
by accumulation of hard parts of plants or
animals. Coals, various types of limestone,
spring deposits, flint, salt beds, etc, are formed
this way.

• Any rock, when subjected to alternating dry and wet
climates over a considerable length of time, undergoes
through decay and decomposition. This ultimately
leaves behind chemically inert and insoluble residual
matter. This makes up a group of sedimentary rock
known as residual rocks. Usually these rocks are dull
looking, porous, spongy and represent a mixture of
oxides of ferric iron, manganese, silicon, titanium and
hydroxides of aluminum.
• Laterite, bauxite and terra rossa and soils are of
residual origin.

• Due to intense and prolonged decomposition of rocks,
soluble matter is leached and transported in solution form by
surface water or ground water. Due to prolonged
disintegration, rocks are broken down into smaller and
smaller particles. When large bodies of rocks are reduced to
a convenient size in this manner, they are transported
mechanically.
• Thus, rivers and streams transport rock matter both
physically and chemically, of these, mechanically transported
sediments undergo deposition whenever favorable condition
occur. These are subsequently cemented or compacted to
give rise to another group of secondary rocks called detrital
rocks. These comprises the most important varieties such as
conglomerates, some breccias, sandstones and shales.

• The rest of the river-transported matter, carries as solution, is
also deposited sometimes, under favorable condition due to
chemical processes like precipitation and evaporation. A large
varieties of limestone, dolomites, salt deposits, are typical
example of this kind are called chemical deposits.
• Yet another group of sedimentary deposits are formed due to
the accumulation of hard parts of organisms. Under favorable
conditions, a part of the dissolved matter is extracted by growing
plants and trees and utilized as their food materials. These
plants and trees may subsequently form coal deposits. The
remaining dissolved material ultimately reach the sea or ocean
and get extracted by deposits.

• Such a group of sedimentary deposits which are
formed out of active involvement of plants and
other organisms are called “ organic deposits”.
• Sedimentary types of deposits are the most
abundant, and account for nearly 95 % of all
the types put together.

Residual Deposits
• The residual deposits are the insoluble products of rock
weathering which have escaped erosion by geological
agents like wind and rivers and which still cover the
rocks from which they have been derived, i.e.. they are
in situ.
• Compositionally, they are made up of unaltered
minerals and insoluble products of decomposition of
other minerals of the original rock. The insoluble
products are usually quartz and muscovite.
• Terra Rossa
• Laterite
• Soils are the chief varieties of residual deposits.

Residual Deposits
Terra Rossa
• This is reddish clayey soil covering limestone in dry
regions. It is formed when argillaceous, limestones are
dissolved, insoluble clay content along with other
mineral matter is left behind as residual (terra rossa),
while calcium carbonate content is carried away in
solution form.

Residual Deposits
Laterite
• The term laterite is derived from the greek word later
meaning brick laterite was used as a substitute for brick
in many places.
• When freshly quarried laterite is very soft, wet and can
be cut with a knife, but it hardens on drying.
• Laterite is formed from a variety of rocks under
tropical weathering.
• Usually, it shows patches of different color like yellow,
brown, red, black and white. It is often porous,
concretionary and earthy in appearance.

Residual Deposits
• As building material, laterite is used as a substitute
for brick, in places where it is available, because it
can be cut to the required size easily.
• As it naturally occurs on the surface, it is very
cheap. But as it is not strong, it can be utilized only
for small or poor quality works and where heavy
loads are not involved.
• As it crumbles easily, it cannot makes a good road
metal also. Thus laterite is not an important
building material, generally.

Residual Deposits
Soil
• Soil is the loose porous material made up of
sediments (i.e.. rock debris which is inorganic
mineral matter) and organic (humus) matter.
The associated voids are filled by air or
moisture. In soils, generally, sediments are
more in proportion than organic matter.

Residual Deposits
Types of soils
• Soils are geologically classified into insitu soils
and drift soils.
• Insitu soils are genetically related to underlying
rocks and they are derived from them. Insitu
soils are infertile as they have undergone
leaching. These soils are also called residual
soils or sedimentary soils.

Residual Deposits
• The drift soils, on the other hand, had formed
at some place but occur elsewhere after
undergoing considerable transport.
• As they occur stratified, they have uniform
thickness and by virtue of their mode of
formation, these soils are fertile. Such soils are
also called as transported soils.

Residual Deposits
• A soil is also classified on the basis of its
dominating constituent i.e.. sand, silt, clay, lime or
humus.
• Accordingly there are sandy soils, loamy soils (sand
+ silt or clay), marl (clay or silt + lime), silty soils,
clayey soils, calcareous soils, peaty soils, etc. Of
these , sandy and loamy soils are light, dry, friable,
porous and are devoid of soluble matter.
• On the other hard clayey soils are dense,
impermeable, wet and possess a considerable
amount of soluble matter.

Residual Deposits
Detrital Rocks
• By virtue of their frequency and abundance of
occurrence, detrital rocks are treated synonymous to
sedimentary rocks and hence these are very important.
As already stated, these rocks are the clastic
sedimentary rocks and represent those which have
been formed from rock fragments, due to geological
agencies like wind, river and glaciers. Conglomerates,
breccias, sandstones and shales belong to this group.

Residual Deposits
Chemically formed Rocks (Chemical Deposits)
• During weathering of rocks, some of the soluble
constituents are leached and carried away in the form
of solution. Such dissolved matter comes out as solid
material subsequently, either due to precipitation or
evaporation. Which are physicochemical processes.
The resulting rocks or deposits are called chemical
deposits.
• Limestone is a common example.

Chemically formed Rocks
(Limestone)

Residual Deposits
• Precipitation produces very fine material. So, rock
formed by this process, like some limestone, are
very fine grained. On the other hand, products of
evaporation are large crystal or coarsed grained.
Salt, calcite and gypsum are typical example are
typical example of this kind.
• Concretions, secretions, colloids, some siliceous
deposits and limestone, dolomites, bog iron ore
and different types of salt deposits are
representative of this category.

Organically Formed Rocks
( Organic Deposits)
• Such rocks are sedimentary deposits which have formed with
the active involvement of plants and animals.
• If these are mainly due to plants, they are called phytogenic and
if they are mainly due to animals they are called zoogenic.
• Compositionally, the following five types of organic deposits
occur in nature:
• Calcareous type
• Phosphatic type
• Ferruginous type
• Siliceous type
• Carbonaceous type.

Organically Formed Rocks
( Organic Deposits)

Common Structures and Textures of
Sedimentary Rocks
Stratification
• Sedimentary rocks have the general character of
occurring in the form of series of beds. This
phenomenon is known as “ Stratification” or “Bedding”.
When the thickness of individual beds is very small it is
called “lamination”.
• This is an inherent, primary character of sedimentary
rocks is not a continuous process. But occur in stages
i.e.. deposition of sediments ceases temporarily and
then, after some time, the deposition is resumed.
• This short break in sedimentation is represented by the
bedding plane. Bedding plane is the plane of contract
between any two adjacent beds of strata.

Sedimentary Rocks
Cementation
• Conglomerates, breccias and sandstones are the
important sedimentary rocks which are formed mainly
due to cementation, In these, finer material act as the
binding medium in between the coarser constituents.
• This cementing material is usually secondary silica,
calcium carbonate, clay or ferruginous material.
Gypsum and barytes also occur rarely as cementing
materials. In case of conglomerates, often the
cementing material is heterogeneous, though silica,
fragments of quartz, occur often in the matrix.

Sedimentary Rocks
Fossil Occurrence
• Fossils have been defined as “ relicts and remnants of
ancient plants and animals preserved inside the rock by
natural processes”.
• Remains of plants and animals when they get buried under
sediments, become fossilized. Of course, all buried
creatures or plant materials do not become fossils. Only a
few with hard parts become fossils under favorable
conditions. In most of the cases, the soft part of buried
organisms or plants get decayed and disappear without
becoming fossils.
• Thus fossils can occur only in sedimentary rocks and
remain in them as their integral part.

Sedimentary Rocks

Sedimentary Rocks
Ripple Marks
• In Stagnant and shallow water bodies, the waves and
current on the surface of water produce sympathetic
impressions in the form of minor undulations on the
loose and soft sediments which lie at the bottom. These
are known as ripple marks and develop under shallow water
condition. Some sandstones, limestones, shales, etc. Show
this character. As can be seen from its mode of formation
ripple marks can be expected to occur only in sedimentary
rocks. The igneous rocks and metamorphic rocks cannot
have this features.
• Ripple marks are of two types symmetrical and
asymmetrical . Symmetrical ripple marks are formed
due to wave action and asymmetrical ripple marks are
formed due to current action.

Sedimentary Rocks
Mud Cracks or Sun Cracks
• Water of a sea or lake, depending on different conditions
(like tide effect and season effect) covers the gentle
sloping sides on and off. When such a wet surface is not
covered, it dries up and develops vertical polygonal
cracks which are wedge shaped (tapering downwards).
Subsequently, when the surface is covered by water, loose
sediments may be deposited on it, filling up the underlying
cracks. Under favorable conditions, this features of mud
cracks is retained in these rocks. Such rocks when
broken along the relevant plane, exposes the earlier mud
crack like features. Thus mud crack originates and
occurs in some sedimentary rocks. Like ripple marks,
mud cracks too, when found in unknown rocks, confirm that
the concerned rocks are sedimentary rocks.

Sedimentary Rocks
Rain Print or Rain Marks
• Rain prints or rain marks develop under the
same conditions as those of mud cracks and are
preserved on surface of some rocks. A rain print
is a slight shallow depression encircled by a low
ridge which is raised by the impacts of rain drop.
If the rain falls obliquely on loose soil the ridge
on the leeward side is higher than that on the
windward side. All this indicates that the rain
print can occur only in sedimentary rocks and
not in other group of rocks.

Sedimentary Rocks
Tracks and Trails
• Tracks and trails are the marking indicating
the paths of some animals or worms, over a
soft sediment, which is able to take and retain
the impression.

Sedimentary Rocks
Peculiar Form
• These forms occur only in sedimentary rocks.
Hence their presence in unknown rocks
indicates that they are sedimentary rocks.

Sedimentary Rocks
Conchoidal Fractures
• Some massive and compact limestones are
extremely fine grained, as they are formed out of
participated materials. This type of rocks, when
broken, develop conchoidal fractures on the
surface. This is because these rocks are very
fine grained as well as non-crystalline. Shales,
too, sometimes show vague conchoidal fractures
by virtue of fine grained character.

Descriptive Study of Common
Sedimentary Rocks
Conglomerate
• A conglomerate is a clastic, rudaceous
sedimentary rock, which is made up of
rounded or surrounded pebbles and gravel.
Occasionally, cobbles and boulders also
encounter in some conglomerates. Miner
logically, the constituent pebble are usually
jasper, flint, quartz or rock pieces.

Sedimentary Rocks
Properties and Uses
• (i) The compositional heterogeneity of
pebble and cementing material result in
difference in their physical character- this
contribution to the weakness of rocks.
• (ii) The incomplete cementation contributes
to the usual porosity and permeability
associated with conglomerates this makes
them good aquifer but incompetent rocks.

Sedimentary Rocks

Sedimentary Rocks
• (iii) The rounded shape of pebble of
conglomerates does not allow firm grip for
cementing material; hence less cohesion in
the rock. For all these reasons conglomerates
are undesirable at the site of foundation of
major civil engineering structures.
• One rare but economically very important
aspect is that the conglomerates of upper
Vindhyans are famous as diamondiferous.

Sedimentary Rocks
Breccias
• Like conglomerate, breccias also made up of
pebbles, gravels, etc. and the fine grained
cementing material provide the binding
medium. But unlike conglomerates, these have
pebbles and gravels which have sharp and angular
edges. This angular character indicates that
they had undergone negligible transport and
their parent rocks are likely not to be far from
the place of their occurrence.

Sedimentary Rocks
• The mechanically transported pebbles and
gravel, when deposited, give rise to the most
common type of breccias. In addition to these
there are a few other types of breccias, namely,
volcanic breccias, fault breccias etc.

Sedimentary Rocks
Sandstones
• Among sedimentary rocks, sandstones are
very abundant and are next only to shales in
this respect, they represent nearly 15 % of
the sedimentary rocks of the earth’s crust.
These are very common clastic rocks made up
of sand and are described as arenaceous rocks.
Unlike conglomerates, sandstones are
stratified and sometimes fossiliferous too.

Sedimentary Rocks
• Sand grains in sandstone are mostly quartz, In
addition to these the other minerals are mica, magnetite
and feldspar.
• Nature of Sand Grains and Cementing Materials
• Based on the size of sand grains the sandstones may be
coarse grained, medium grained or fine grained. But
generally the sand grain in any sandstones will be
nearly of the same size.
• The sand grain present may be rounded or surrounded
or angular in shape.
• Sandstones occur in different colors such as white, red,
yellow grey and green. The color of sand grain is due
to cementing material.

Sedimentary Rocks
Porosity Permeability and Uses
• By virtue of their mode of formation, like
conglomerates, sandstones also generally
porous and permeable. The porosity of
sandstones is dependent on the size and shape
of the sand, Further the degree of cementing
also contribute to the porosity. Therefore
sandstones vary considerable in their porosity
and permeability.

Sedimentary Rocks
• From the civil engineering point of view,
however, careful study is necessary before
considering a rock as competent or incompetent,
useful or useless. When sandstone is
considerably porous and permeable i.e.. when
the degree of cementation is less, it will not
make up a good rock. & if the sand stone are well
cemented and if the grains are angular and quartz
in composition, the siliceous sandstone are the
best and highly satisfactory for all civil
engineering purpose.

Sedimentary Rocks
Shales
• Shales are the most abundant sedimentary rocks.
Representing nearly 80 % of them. In other words shales
are more abundant than all other sedimentary rocks put
together.
• Like conglomerates and sandstone shales are also typical
elastic rocks i.e. formed out of a mechanically
transported and deposited sediments. These are made up
of solid particles of extreme fine size i.e. silt and clay.
Stratification is best in shales because the individual
layers are very thin and differently colored.

Sedimentary Rocks
• Shales often contain fossil fuels i.e.. flora
and fauna. Shales are uniform in
composition. They are mostly hydrous
aluminum silicates in composition because
their constituent clay minerals are product
of weathering of feldspar and other primary
silicate minerals.

Sedimentary Rocks
Porosity and Permeability
• Being extremely fine grained clastic rocks,
shales are highly porous. Since permeability is
related to porosity, shales are normally expected
to be highly permeable too. But due to the
surface tension phenomenon of water and extreme
fine intergranular space, shales are impermeable.
Rocks of this kind which are porous but not
permeable are called aquiclude.

Sedimentary Rocks
Civil Engineering and Other Uses.
• Shales when saturated with water, under
pressure, are likely to produce lubricating
material making a slippery base for
overburden; therefore they are unsuitable at
the site of foundation of important civil
engineering structures.

Sedimentary Rocks
• Shales are the most incompetent rocks in nature.
They may yield plastically and also may undergo
subsidence.
• Shales are unsuitable for civil engineering purposes
• But are highly useful in following respect,
• On disintegration, they produce soils which are
indispensible for agriculture.
• They form one of the chief raw material for cement
manufacture
• They are invaluable as cap rocks in the occurrence
of oil and gas deposits.

Sedimentary Rocks
Limestone
• Among the sedimentary rocks, in the order of
abundance, limestone rank third, next to shales
and sandstones. These comprises 5 % of all
sedimentary rocks occurring on the earth’s
surface.
• Limestone are typically non-detrital rocks. They
are formed either chemically, due to precipitation
of calcium carbonate from surface water. Or
organically, due to accumulation of hard part
of organisms

Sedimentary Rocks
• In hand specimens, limestones show different
colors like white, gray, buff, yellow and black.
• If the limestone have formed out of chemical
precipitation, they are extremely fine grained.
In case of organic, limestone have formed out
of chemical precipitation, they are extremely
fine grained. In case of organic, limestone, the
grain size may vary depending on the type of
organisms responsible for its formation.

Sedimentary Rocks
Types of limestones
• Massive compact fine grained limestones with
grey or any other light color are common in
nature the porous types are yellowish, brownish
etc.
• The other important types are
• Chalk, stalactites, stalagmites and drip stones,
travertine, kankar, fossiliferous or shell
limestone, flaggy limestone, lithographic
limestone, magnesium limestone, argillaceous
limestone, siliceous limestone.

Sedimentary Rocks
Porosity and permeability
• Limestones are the most deceptive and most
unpredictable rocks in terms of porosity and
permeability. This is because of the inherent character
of limestone, that is getting corroded by carbon
dioxide, bearing water at anytime. But inside they
may have solution cavities and solution channels of
varying magnitudes. Highly irregular and uneven
topography formed this way in limestone region is
known as Karst topography.
• Thus the range of porosity and permeability is
unreasonable large in limestone. Therefore they are
either one of the best or worst aquifer in nature.

Sedimentary Rocks
Civil Engineering and Other Uses
• Massive and compact limestones are
reasonable competent to support civil
engineering structures. They are suitable as
road metal, railway ballast and as construction
material but may not be very durable.
Further if hollow, their load-bearing
strength also will get reduced.

Sedimentary Rocks

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (6)

Similar to Sedimentary Rocks

Similar to Sedimentary Rocks (20)

More from GAURAV. H .TANDON

More from GAURAV. H .TANDON (20)

Recently uploaded

Recently uploaded (20)

Sedimentary Rocks