Sedimentary Rocks

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Sedimentary Rocks
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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
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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

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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
  climatepaleoclimate.

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TYPES OF SEDIMENTARY ROCKS
Clastic rocks
Chemical Organic rocks

• Sandstones
• Conglomerates
• Breccia
• Shale/mudstones

Carbonate rocks
Organic rocks
Form due to decomposition of organic remains
under temperature and pressure eg. Coal/Lignite
etc.
Form basically from CaCO3 both by chemical
leaching and by organic source (biochemical) eg.
Limestone dolomite
Evaporitic rocks
These rocks are formed due to evaporation of
saline water (sea water) eg. Gypsum, Halit (rock
salt)
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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.

Clastic rocks
solidify

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

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GRAIN SIZE
Sand 2- 0.062 mm
Clay lt0.004 mm
Silt 0.062-0.004 mm
Gravel gt256-2 mm
Boulder gt256mm Cobble 64-256 mm Pebble 4-64
mm Granule 2-4mm Fine gravel
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• Clastic rocks mainly comprise broken fragment of
  older rock they are also know as Terrigenous
  rocks

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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.
• Calcareous
• siliceous

Fine-gravel/ Granule (size lt4mm)
Any rock fragment (size is gt 4 mmPebble)
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Degree of roundness helps in knowing the distance
of transportation

• Angular clasts- short distance transport from the
  source

• Rounded clasts- long distance transport

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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.
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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.

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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.

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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.

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• 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 subroundedcement material will not
  have firm gripsuch rocks behave as incompetent
  rocks

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• 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.

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

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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

• 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

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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.

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EVAPORITIC ROCKS
These rocks are formed within the a depositional
basin from chemical substances dissolved in the
seawater or lake water.
Halite
CaSO4.2H20
(NaCl)
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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.

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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.

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• 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.

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Sedimentary structures

• Bedding is most imp. Feature of a sed. Rock
• Beds are usually gt 1 cm
• Laminae lt 1 cm
• Orientation of bedding helps in knowing the
  paleo-current direction of the old rivers

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GRADED BEDDING
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Medium-coarse sandy lithounit (cross stratified)
Fine gravelly lithounit
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Laminated layers of fine silt and clay
Cross-stratified sst. Paleo-flow from right to
left
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Typical cross stratification in
Sand Dunes
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Ripple marks
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Foot prints
Biogenic structures
Snake Trails
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Mud cracks