Almost 90% of earth crust is made up of igneous rocks


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Almost 90% of earth crust is made up of igneous 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.



Sandstones

  • Sandstones

  • Conglomerates

  • Breccia

  • Shale/mudstones



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.

  • 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.



GRAIN SIZE



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

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















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.







SANDSTONE:

  • 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)

  • Different type of Sst. (based on their composition)



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



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.





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.



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.




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









Typical cross stratification in



Ripple marks



Foot prints



Mud cracks



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