Sedimentary rocks (2)

Transportation of sediments and formation of sedimentary rocks by mode of river water- deposition on the continent and on the ocean floor.. • When settle down on the beds of ocean, river

Sedimentary Rocks

Transportation of sediments and formation of sedimentary rocks by mode of river water- deposition on the continent and

on the ocean floor.

Marine deposits

Fluvial deposits

• 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

SEDIMENTARY ROCKS: are the secondary rocks which are

formed from the loose fragments or detrital or clastic sediments produced

by weathering of older 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.

• Sandstones

• Conglomerates

• Breccia

• Shale/mudstones

TYPES OF SEDIMENTARY ROCKS

Clastic rocks Chemical & Organic rocks

Evaporitic rocks

These rocks are formed

due to evaporation of saline

water (sea water)

eg Gypsum, Halit

Carbonate rocks

Form basically from CaCO3 – both by chemical leaching and

by organic source (biochemical) eg

Limestone; dolomite

Organic rocks

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

• 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.

CLASTIC ROCKS

Clastic rocks solidify

• Clastic rocks are classified on the basis of

the grain size: conglomerate, sandstone, shale etc.

• Clastic rocks mainly

fragment of older rock – they are also

rocks

Any rock fragment

sediments.

1 Calcareous

2 siliceous

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

sub-rounded 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.

• 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.

• 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

• 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

Shale-fissile

• 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 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.

CE IMPORTANCE

EVAPORITIC ROCKS

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

(NaCl)CaSO4.2H20

Economic importance of Evaporites

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

• As chemical for dyeing etc.

• GYPSUM: is used for plaster and in

manufacturing construction materials.

• Limestone: It is a non-clastic rock formed either chemically or due to precipitation of calcite (CaCO 3 ) 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.

CARBONATE ROCKS

• used as a building stone and in the manufacture of lime, carbon dioxide, and cement.

• Massive and compact lst Are competent to support CE-structure

• However, if it occur in huge thickness then it is not advisable,

Chalk

Sedimentary structures

Feature of a sed Rock

GRADED BEDDING

Fine gravelly lithounitMedium-coarse sandy lithounit (cross stratified)

Laminated layers of fine silt and clay

Cross-stratified sst Paleo-flow from right to left

Typical cross stratification in

Sand Dunes

Ripple marks

Foot prints

Biogenic structures

Snake Trails

Mud cracks