Sedimentary Rocks

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Sedimentary Rocks

• Earth Science

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Sedimentary Rocks

• Sediments pieces of solid material that have
  been deposited on Earths surface by wind, water,
  ice, gravity, or chemical precipitation.
• When sediments become cemented together, they
  form sedimentary rocks
• The formation of sedimentary rocks begins when
  weathering and erosion produce sediments

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Sedimentary Rocks

• Weathering
• Chemical weathering occurs when the minerals in a
  rock are dissolved or otherwise chemically
  changed.
• Physical weathering occurs when fragments simply
  break off the solid rock along fractures or grain
  boundaries. During physical weathering, minerals
  remain chemically unchanged

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Sedimentary Rocks

• Weathering
• Weathering produces rock and mineral fragments
  known as clastic sediments.
• Clastic sediments range in size from huge
  boulders to microscopic particles.
• Clastic sediments usually have worn surfaces and
  rounded corners caused by physical abrasion
  during erosion and transport.

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Sedimentary Rocks
Classification of Clastic Sediments
Particle Size Sediment Rock
gt256 mm 256-64 mm 64-2 mm Gravel Conglomerate
2-0.062 mm Sand Sandstone
0.062-0.0039 mm Silt Siltstone
lt0.0039 mm Clay Mudstone or shale
Boulder Cobble Pebble
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Sedimentary Rocks

• Erosion and Transport
• Erosion - the removal and movement of surface
  materials from one location to another.
• Four main agents of erosion wind, moving water,
  gravity, and glaciers.
• Eroded materials are almost always carried
  downhill

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Sedimentary Rocks

• Erosion and Transport
• Deposition when sediments are laid down on the
  ground or sink to the bottoms of bodies of water
• Sediments in wind and water tend to form layers
  when deposited largest particles at the bottom
• Glaciers and landslides, however, move all
  materials with equal ease

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Sedimentary Rocks

• Burial
• Most sediments are ultimately deposited on Earth
  in depressions called sedimentary basins
• Some sedimentary basins can contain layers of
  sediment over 8 km thick
• As more sediment is deposited in that area, the
  bottom layers are subject to increasing pressure
  and temperature

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Sedimentary Rocks

• Lithification
• The increasing temperature and pressure cause
  lithification.
• Lithification is the physical and chemical
  processes that form sediments into sedimentary
  rocks.

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Sedimentary Rocks

• Lithification
• Begins with compaction. The weight of overlying
  sediments forces the sediment grains closer
  together.
• For example water is squeezed out of layers of
  compacted mud.
• Some sediments, such as sand, resist
  lithification due to it being well-compacted
  during deposition

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Sedimentary Rocks

• Cementation
• Sediments that are buried 3-4 km deep experience
  temperatures that are high enough to start the
  chemical changes that cause cementation.
• Cementation occurs when mineral growth cements
  sediments together into solid rock.

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Sedimentary Rocks

• Cementation
• Two common types of cementation
• First type occurs when a new mineral grows
  between sediment grains as dissolved minerals
  precipitate out of groundwater. (Calcite growing
  between quartz grains)
• Second type occurs when existing mineral grains
  grow larger as more of the same mineral
  precipitates from groundwater and crystallizes
  around them. (quartz crystallizing around quartz
  grains)

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Sedimentary Rocks

• Features
• Bedding the horizontal layering of sedimentary
  rocks
• Bedding can range from a millimeter-thick layer
  of shale to a sandstone deposit several meters
  thick
• The type of bedding depends on the method of
  transport.

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Sedimentary Rocks

• Features
• Graded Bedding
• Bedding in which the particle sizes become
  progressively heavier and coarser towards the
  bottom layers
• Often observed in marine sedimentary rocks that
  were deposited by underwater landslides

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Sedimentary Rocks

• Features
• Cross-bedding
• Formed as inclined layers of sediment move
  forward across a horizontal surface.
• Small-scale cross-bedding can be observed at
  sandy beaches and along sandbars in streams and
  rivers.
• Most large-scale cross-bedding is formed by
  migrating sand dunes

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Sedimentary Rocks

• Evidence of past life
• Fossils are the preserved remains, impressions,
  or any other evidence of once-living organisms.
• When an organism dies, it may be buried before it
  decomposes
• If its remains are further buried without being
  disturbed, it might be preserved as a fossil.

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Types of Sedimentary Rocks

• Classification of sedimentary rocks depends on
  how they were formed.
• Three main types clastic, organic, and chemical

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Types of Sedimentary Rocks

• Clastic sedimentary rocks
• The most common type of sedimentary rock
• Formed from the abundant deposits of loose
  sediments found on Earths surface
• Clastic sedimentary rocks are further classified
  according to the sizes of their particles.

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Types of Sedimentary Rocks

• Coarse-grained clastics
• Sedimentary rocks consisting of gravel-sized rock
  and mineral fragments
• Conglomerates
• coarse-grained sedimentary rocks that have
  rounded particles
• Gravel is transported by high-energy flows of
  water.
• During transport, gravel becomes abraded and
  rounded as the particles scrape against one
  another

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Types of Sedimentary Rocks

• Breccias
• Coarse-grained sedimentary rocks that have
  angular particles
• The sediments from which they formed did not have
  time to become rounded.
• This suggests that the particles were transported
  only a short distance and deposited close to
  their source.

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Types of Sedimentary Rocks

• Medium-grained clastics
• Sedimentary rocks that contain sand-sized rock
  and mineral fragments
• When medium-sized sediments are buried and
  lithified, sandstone is formed.
• Sandstone has relatively high porosity the
  percentage of open spaces between grains in a
  rock
• Sandstone often holds underground oil, natural
  gas, or groundwater deposits due to its high
  porosity.

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Types of Sedimentary Rocks

• Fine-grained clastics
• Sedimentary rocks consisting of silt and mud
• Siltstone, mudstone, and shale
• Siltstone is mostly composed of silt-sized grains
• Shale is mostly composed of silt and clay-sized
  particles
• Shale has very low porosity it usually hinders
  the movement of underwater liquids.

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Types of Sedimentary Rocks

• Chemical sedimentary rocks
• During chemical weathering, minerals can be
  dissolved and carried into lakes and oceans.
• As water evaporates, the minerals are left behind.

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Types of Sedimentary Rocks

• Also, when the concentration of dissolved
  minerals in a body of water reaches saturation,
  crystal grains precipitate out of solution and
  settle to the bottom
• The layers of chemical sedimentary rocks that
  form as a result of this process are called
  evaporites
• The three most common evaporites are calcite,
  halite, and gypsum

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Types of Sedimentary Rocks

• Organic sedimentary rocks
• Formed from the remains of once-living things
• The most abundant organic sedimentary rock is
  limestone.
• Formed from the shells of ancient shell fish
• Coal is also an organic sedimentary rock
• Formed from the remains of ancient plants