Sedimentary rocks (2)

Sedimentary Rocks


Transportation of sediments and formation of sedimentary
rocks by mode of river water- deposition on the continent and
on the ocean floor.
Fluvial deposits
Marine deposits



SEDIMENTARY ROCKS: are the secondary rocks which are
formed from the loose fragments or
detrital or clastic sediments produced
by weathering of older rocks.
• Almost 90% of earth crust is made up of igneous rocks
• 75% of land surface on the earth is covered by thin
veneer of sediments or sedimentary rocks.
• These sediments are transported and deposited by
river water, wind or by movement of glacial ice.
Transportation is either in suspension or in solution.
• When settle down on the beds of ocean, river and lakes
undergo compaction/cementation for millions of years
to form SEDIMENTARY ROCKS


IMPORTANCE OF SEDIMENTARY ROCK
“Present is the key to the past”
• Helps in knowing depositional environment viz.
marine (ocean deposits), fluvial (river deposits),
aeolian (wind deposits), glacial, estuarine,

Lacustrine (lake deposits) etc.
• Helps in knowing the provenance (i.e. source
area of the sediments); change in climatic
conditions i.e. in knowing and understanding
old climate=paleoclimate.


TYPES OF SEDIMENTARY ROCKS
Clastic rocks
•
•
•
•

Chemical & Organic rocks

Sandstones
Conglomerates
Breccia
Shale/mudstones

Evaporitic rocks
These rocks are formed
due to evaporation of saline
water (sea water)
eg. Gypsum, Halit
(rock salt)

Carbonate rocks

Organic rocks

Form basically from
CaCO3 – both by
chemical leaching and
by organic source
(biochemical)
eg.
Limestone; dolomite

Form due to
decomposition of
organic remains
under temperature
and pressure eg.
Coal/Lignite etc.


CLASTIC ROCKS
• formed from broken rock fragments weathered
and eroded by river, glacier, wind and sea
waves. These clastic sediments are found
deposited on floodplains, beaches, in desert and
on the sea floors.
solidify

Clastic rocks

• Clastic rocks are classified on the basis of
the grain size: conglomerate, sandstone,

shale etc.


GRAIN SIZE
Gravel
>256-2 mm
Boulder: >256mm
Cobble: 64-256 mm
Pebble: 4-64 mm
Granule: 2-4mm
Fine gravel

Sand
2- 0.062 mm

Silt
0.062-0.004 mm

Clay
<0.004 mm


• Clastic rocks mainly
comprise
broken
fragment of older
rock – they are also
know as Terrigenous
rocks

Matrix:
is the finer grains
or material that
surrounds the
larger clasts. It
consist of either
clay, silt and sand.

Any rock fragment
(size is > 4 mm=Pebble)

Cement:
dissolved
substance
that bounds
the
sediments.
1. Calcareous
2. siliceous

Fine-gravel/
Granule
(size <4mm)


Degree of roundness helps in knowing the distance of transportation
•Angular clasts- short distance transport from the source
•Rounded clasts- long distance transport

When clastic fragments are
cemented or undergo consolidation
they are called
CONGLOMERATES

Similarly when angular clasts
undergo consolidation they are
called BRECCIA


Sorting of the sediments also suggest the mode of deposition and transportation.
Long distance transport= well-rounded and well-sorted sediments,
Short distance transport = poorly sorted angular grains.
Also helps in knowing the energy conditions of the river.



DIFFERENT CATERGORIES OF CLASTIC ROCKS
•

RUDACEOUS ROCKS: made up of rounded or subrounded Pebbles and cobbles eg. conglomerate

•

ARENACEOUS ROCKS: made up of mainly sand
eg. Sandstone. These rocks are either accumulated by
wind action or deposited under water action or marine
or lake environment

•

ARGILLACEOUS ROCKS: made up of clay size
sediments eg. Shale, mudstones, siltstones.


IMPORTANCE
• CONGLOMERATE comprise clastic sediments like pebbles
and cobbles (heterogeneous)
• If the cementation is good (voids between the clasts) = then the
conglomerate will be hard and competent hence act as strong
foundation, but not good rock for ground water source.
• However, if the cementation is poor = it makes the rock more
porous with high porosity = act as good reserve for ground water
(aquifer), but is undesirable at the site for foundation of major CE
structures.
• Due to heavy seepage along the conglomerate may result in failure
by sliding. Eg. Failure of St. Francis dam, US.


Cementation:
• Cementing material is usually secondary Silica
(Siliceous cement), Calcium carbonate
(Carbonate cement), Iron rich (ferruginous
cement)
• Cement itself to some extent is the source of
weakness in the sedimentary rocks
• Because cementing material and the clastic
sediments
are

usually
of
different
composition, leading to heterogeneity in their
physical characteristics.


• Hence such rock will not behave homogeneously under stress,
resulting into development of cracks or fissures which develops
in cementing material.
• If the cement is Calcium Carbonate- it is undesirable, because
it is susceptible to dissolve in Carbon-dioxide in water
• However, if cementation process continuous for longer span of
time= cementation will become more complete, which reduce
the porosity and permeability in the rock mass and increase
competence.
• Shape of grains: i.e. if coarser grains are rounded or
subrounded=cement material will not have firm grip=such
rocks behave as incompetent rocks


• SANDSTONE:
• Made up of sand grains dominantly of Quartz and
Feldspars, where quartz is highly resistive to
weathering
• Cementation plays similar role in this rock as seen in
conglomerate
• However, Siliceous cement are best and highly
desirable for CE purposes, also the ferruginous sst.

Different type of Sst. (based on their composition)

Quartz Sst.

Arkose (Feldspar)

Graywacke/lithic arenite


SHALE

Shale-fissile
•

• Shales are clastic rocks, made up
mainly fine silt/clay
• They
are
most
abundant
sedimentary rocks, accounts for
about 80% of them
• Often contain fossils
• Mostly hydrous aluminum silicate
in composition = from weathered
feldspars
• Deposition takes place under low
fluvial regime or under weak water
current. Eg. Offshore or in Lagoon

Shales are made of fine well sorted silt and clayey sediments, where
normally one can expect high porosity and permeability.

•But due to surface tension phenomenon of water and extremely fine
intergranular space shales are impermeable, hence does not yield water
when tapped


CE IMPORTANCE
•
•
•

•

When shales are saturated with water – under pressure
they are likely to produce slippery foundation for any
structure- therefore not suitable for CE structures
eg. Lafayette dam of US constructed on argillaceous
rocks sunk by 20 feet.
Srisailam Dam in Andhra Pradesh (One of the 12th
largest Hydroelectric Projects in the India) faced similar
problem, however, precautions were take by grouting to
stop the seepage along the weak zones.
Because of its impermeable and porous nature it acts as
cap rocks in the occurrence of Oil and Gas.


EVAPORITIC ROCKS

These rocks are formed within the a depositional basin
from chemical substances dissolved in the seawater or
lake water.

Gypsum

CaSO4.2H20

Halite

(NaCl)


Economic importance of Evaporites

• SALT: other then daily use of salt for cooking,
•
•
•
•
•

it is used
For production of Paper,
Soap
Detergents
Antiseptics
As chemical for dyeing etc.

• GYPSUM:

is used for plaster and in
manufacturing construction materials.


CARBONATE ROCKS
• Limestone: It is a non-clastic rock formed either
chemically or due to precipitation of calcite (CaCO3)
from organisms usually (shell). These remains will
result in formation of a limestone.
• Limestones formed by chemical precipitation are
usually fine grained, whereas, in case of organic
limestone the grain size vary depending upon the type
of organism responsible for the formation
– Chalk: which is made up of foraminefera is very fine grained
– Fossiliferous Limestone: which medium to coarse grained, as it
is formed out of cementation of Shells.