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 formeddue to evaporation of saline water (sea water)eg. Gypsum, Halit (rock salt)

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 rockssolidify

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

GRAIN SIZE

Gravel>256-2 mm

Sand2- 0.062 mm

Clay<0.004 mm

Silt0.062-0.004 mm

Boulder: >256mm

Cobble: 64-256 mm

Pebble: 4-64 mm

Granule: 2-4mmFine gravel

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

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

Fine-gravel/Granule(size <4mm)

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

Cement: dissolved substance that bounds the sediments.

1. Calcareous2. 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.

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• 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 tapped

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

Gypsum Halite

(NaCl)CaSO4.2H20

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.

• 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.

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, because of its typical CAVING character.

Chalk

Sedimentary structures

• Bedding is most imp. Feature of a sed. Rock

• Beds are usually > 1 cm• Laminae < 1 cm• Orientation of bedding

helps in knowing the paleo-current direction of the old rivers

GRADED BEDDING

Fine gravelly lithounit

Medium-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