Sedimentary rocks you still got itttttttttttttttttt
March 10, 2010 by blessan
Sedimentary rocks are formed due to the aggradations and compaction of the sediments.
The word sedimentary has been derived from the Latin word which means settling down.
The sedimentary rocks are also Called as Stratified or layered rocks and hence Consists
of different types of layers and strata of various sediments. Some sedimentary rocks do
not consist of layers, ex: loess.
Characteristics of Sedimentary rocks
Sedimentary rocks are formed of sediments derived from older rocks,
Plants and animal remains thus these rocks contains fossils of animals
The sedimentary rocks are widespread about 75 % of the globe consists of
sedimentary rocks and the remaining 25% by igneous and metamorphic
rocks. Nevertheless the sedimentary constitute only 5 % of the composition
of earth’s crust.
The deposition of sediments takes place in a certain sequence. The size of
the Sediments decreases from the littoral margins to the centre of the
The sedimentary rocks are consolidated and compacted by cementing
elements Such as Silica, iron compounds, Calcite, Clay Etc
The consolidation of layers of sedimentary rocks depends upon the nature
of cementing materials and the rocks are Seldom found in their original
manner because of the lateral compressive and tensile forces
Sedimentary rocks are seldom crystalline but are Characterized by Joints
perpendicular to the bedding Planes.
The connecting Plane between two consecutive beds or layers is called as
Types of formation in Sedimentary rocks
When two beds are uniform along the bedding Plane then this type of formation is
When two beds are not uniform then the structure is called as Unconformity.
Types of Unconformity
Non-conformity (where sedimentary rocks succeed igneous &
Angular unconformity (where horizontal sedimentary beds are deposited
over the previously folded strata)
Disconformity (where two conformable beds are separated by Changes in
Paraconfirmity ( where two sets of conformable beds are Separated by
same set of sediments)
Sedimentation units in the rocks which are more than One centimeter is
The upper and lower surface of the beds are called BEDDING PLANE or
Sometimes the lower surface of the bed is called SOLE. There are further sedimentary
units within a bed. The units having more than one centimeter is called layer or Strata
Where as the units below one centimeter are Called laminae. Thus several strata and
laminae make up a bed. When beds are deposited at an angle to the depositional surface
they are called cross beds and the general phenomena of inclined layers are called cross
lamination or cross bedding.
Soft mud or alluvial deposits by rivers develop cracks when baked in sun and these
cracks are generally of Polygonal Shape, such cracks are Called MUD CRACKS OR
Most of the sedimentary rocks are Permeable and Porous but few of them are
nonporous and Impermeable
Classification of sedimentary rocks
ON THE BASIS OF NATURE OF SEDIMENTS
o Mechanically formed or clastic rocks
o Chemically formed sedimentary rocks
o Organically formed sedimentary rocks
ON THE BASIS OF TRANSPORTING AGENTS
o Argillaceous or aqueous rocks
o Aeolian sedimentary rocks
o Glacial sedimentary rocks
Mechanically formed sedimentary Rocks
Previously formed rocks are subjected to mechanical or physical disintegration
these are called fragmental rock materials or Clastic materials which become the
source materials for the formation of mechanical or clastic sedimentary rocks.
These materials are further broken down in to fragments during their
transportation by the exogenous process such as wind, running water glacier etc,
some of the important members of this group are Sandstones, conglomerates, Silt,
shale, Clay etc.
Sandstones are formed mostly due to the deposition, Cementation,
compaction of Sand grains. The Sand grains are divided into five categories on
the basis of their size
Classification of sands by grain size
Sand types Grain size (in mm)
1. Very Coarse sand
1. Coarse Sand
0.5 to 1.0
1. Medium sand
0.25 to 0.5
1. Fine sand
0.125 to 0.25
1. Very Fine sand
0.0625 to 0.125
These grains are deposited in the water bodies, and are aggravated and consolidated by
Cementing elements such as Silica, Calcium, iron oxide Clay etc. The Colour of the
sandstone Varies according to the nature of the cementing elements. Sandstones become
red and gray when cemented by iron oxide and in to white when the cementing element is
replaced by calcium Carbonate. They become hard and resistant when cemented by
Silica. On an average the sandstones are generally porous rocks.
On the basis of mineralogical and textural Characteristics sandstones are
classified in to the following types
Quartz Arenites (composed entirely of quartz grains)
Arkose Sandstones (feldspar being the dominant Mineral)
Lithic Arenites (composed of fine grained rock fragments derived from
Shales, schist, slates and volcanic rocks).
Greywackle Sandstones (Composed of feldspar, rock fragments, quartz).
Note: The English word Arsenite is derived from Latin word Arena-meaning thereby
They are formed due to the consolidation and cementing of pebbles of various Sizes
with Sands. Polished round fragments of size with 4 mm of diameter are called Pebbles
and those with a diameter 256mm are called boulders. Angular rock fragments are Called
Clay rock and shale
They are formed due to the deposition and cementation of fine sediments. Sediment
having grain size of 0.3mm to O.4mm are called silts where as Clays are formed When
the grain size is between 0.4mm to O. 00012 mm are Cemented or consolidated. Silt and
Clay are soft weak rocks but are defiantly impervious. Clay rocks are formed exclusive
Kaolin minerals and pure White in colour but they Change to different colours because of
being mixed with impurities.
Chemically formed sedimentary Rocks
When Running Water containing chemicals come in touch with the rocks containing
Soluble materials are washed away and come in contact with the Chemicals. The
Chemical materials are then settled down, compacted and cemented to form Chemical
sedimentary rocks such as gypsum and salt rocks.
Organically formed sedimentary rocks
The sediments derived from the disintegration and decomposition of Plants and
animals deposit and consolidate to form organic sedimentary rocks. The rocks are divided
in to three categories on the basis of its carbon and lime content. They are
Calcareous rocks are formed due to the deposition and consolidation of sediments
derived from the remnants of animals and plants which contain lime. Limestone is one of
the examples of the calcareous rocks. Limestone is formed in the following Manner
Calcium oxide (Cao) reacts with water to form Calcium hydroxide Ca (0H)
2. This is given by the following equation,
Then Calcium hydroxide reacts with Carbon dioxide (CO2) to give Calcium
carbonate CaCo3 this is given by the following equation,
The calcareous rocks are collectively called as carbonate rocks or carbonates. Calcium
carbonate (Caco3), Magnesium carbonate (Mgco3), dolomite (CaMg (co3)2 are
important calcareous rocks. Limestone is both thinly beaded and thickly bedded. The
most dominant minerals are calcite (Hexagonal), Aragonite (orthorhombic). Though
Limestone are weak rocks in humid regions they become more resistant when exposed to
dry climate, because of its uniform or homogenous structure they are not easily affected
by differential expansion and contraction due to temperature. The Carbonate rocks after
Chemical weathering give birth too karst topography. Chalks another form of Carbonate
rocks are formed due to the Precipitation of Carbonate minerals desired from
microorganisms like foraminifera.
ARGONITE ROCKS (ORTHOTHROMBIC) CALCITE ROCKS
These rocks contain Carbonic minerals which are mainly remains of vegetation. These
rocks are formed when these remains are buried in the ground and undergo Changes
because of pressure from overlying deposits and earth movements. The Initial form of
carbonaceous rock is Peat which is dark grey in colour, and then subsequently it
transforms into rocks with greater Proportion of carbon like lignite, bituminous,
anthracite Coals. The coal layers one called coal seams. They are more important
economically than geomorphically.
They are formed due to the dominance of silica Content. These rocks are formed due to
the aggradations and compaction of sediments derived from radiolarian organisms
(marine organisms that absorb silicon compounds during their lifecycle), sponge, diatom
plants. Geyserites are also deposits of Silica around geysers.
ON THE BASIS OF TRANSPORTING AGENTS
Based on major transporting agents the sedimentary rocks are classified in to three
These are also known as aqueous rocks because they formed in the water areas and are
also transported by waters. They are called argillaceous rocks because of the presence of
Clay and these rocks are characteristically soft. They are generally impervious rocks and
are further sub-divided in to three types
Marine argillaceous sedimentary rocks
Lacustrine argillaceous sedimentary rocks
Riverine argillaceous sedimentary rocks
Marine argillaceous sedimentary rocks
These rocks are mainly formed due to the deposition and compaction of
sediments on the littoral margins of the oceans and seas. The process of sedimentation
is well ordered and sequential in Character. The size of the particles decreases
progressively as we go from the coastal areas towards the Centre of the sea or oceans
some of the examples of these types of rocks are Limestone, sandstone, dolomite and
Lacustrine argillaceous sedimentary rocks
These rocks are formed generally when sediments are deposited near water bodies
such as Lakes. Generally the rocks are found in the floor of the Lakes. Unlike the
argillaceous rocks there is no ordering of size or deposition in-Lacustrine rocks.
Riverine argillaceous Sedimentary rocks
These rocks are formed due to the deposition of sediments in the Riverine
environment. The sediments are deposited on the beds of the rivers and in the flood
Plains. These deposits include alluvia which are dominated by Clay. The alluvia are
deposited by rivers during floods and are renewed almost every year .These alluvial
deposits develop Polygonal cracks due to their exposure to isolation.
They are formed due to the deposition of Sands brought out by the wind. Pre existing
rocks are greatly disintegrated due to mechanical weathering and this result in the
formation of large quantity of Sands of different sizes. These Particles are further
comminuted in to finer Particles due to the attrition while they are being transported from
one place to another. Continuous deposition results in the formation different layers but
these layers are not well consolidated and hence are very porous in nature. Sometimes
there is complete absence of layers in rocks such as Loess. In India loess are found on the
banks of the Paleochannel and the Valley of Narmada River.
The materials deposited by the Glaciers are called glacial drifts which are deposited in
four conditions and hence they are four types of morainic deposits
Lateral moraines- When Glacier Materials are deposited on the either Side
of the Glacier.
Medial Moraines-When glaciers Materials are deposited along the joining
of the Glaciers.
Ground moraines- When Glacier materials are deposited on the bed of
Terminal Moraines- These formed when glaciers are ablated and materials
are deposited there.