Sedimentary Rocks

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Sedimentary Rocks

• Form as sediment (rock fragments) that have been
  eroded (moved) from one location to another are
  deposited (placed).
• Most sediments are deposited in large bodies of
  water, and tend to be rounded by their erosional
  agent.
• The formation of sedimentary rocks gives us clues
  about the environment at the time of rock
  formation.

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Sedimentary Rock Formation

• Most sedimentary rocks form as a result of
  compression and cementation of broken rock
  material, into one solid rock.
• Compression
• The pressure created by water and other sediments
  on top of a layer of sediment, squeezes the water
  out of the sediment.
• The sediment then hardens into solid rock.
• Most shale forms this way as clay particles made
  of mica are compressed.

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Shale

• Shales are made of compacted clay particles.
• Do you think you can see individual grains in a
  shale?
• No grains are visible.
• Shale is the most common sedimentary rock type on
  earth.

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Sedimentary Rock Formation Cont.

• Cementation
• When broken rock fragments are glued together
  by a cementing agent.
• Usually silica or calcite acts as a cementing
  agent and glue broken rock fragments together.
• What would be a good way to determine if the
  glue is silica or calcite?
• Think back to your Identifying Minerals Lab.
• Calcite reacts w/ acid.
• Sandstones and Conglomerates form in this manner.

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Sandstone
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Conglomerates

• A rock containing gt50 clasts larger than .2 cm
  (the limit for coarse sand),
• Though conglomerates usually have pebble size
  grains or larger.
• Clasts may be any rock type, and are commonly a
  mixture in any one outcrop.

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Sedimentary Rock Formation Cont.

• Chemical Process
• Some sedimentary rocks form as the result of
  chemical processes, namely evaporation and
  precipitation.
• Water can dissolve some minerals (halite for
  example) and carry them in solution.
• When the water begins to evaporate the amount of
  the minerals it can hold decreases.
• The mineral then begins to form crystals again.
• Rocks that form in this manner are called
  Evaporites.
• Examples See ESRT pg. 7

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Gypsum

• Rock gypsum this rather featureless rock is made
  almost entirely of gypsum, colored with a trace
  of hematite dust.
• Gypsum beds can be extensive, and represent
  evaporites left from the evaporation of saline
  water.

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Sedimentary Rock Formation Cont.

• Biological Formations
• Are these rocks made out of minerals?
• No. They are ORGANIC in origin so are not made of
  minerals!
• Bituminous Coal
• Forms when the remains of plants are deposited in
  shallow, still water, where they are compressed
  into rock.
• Limestone
• Forms when corals use minerals like calcite to
  form hard shells, when the corals die they leave
  the shell behind to be compacted into rock.

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LimestoneRemember it Reacts with Acid!
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Bitmuminous Coal

• Forms in areas that were once swampy, and rich in
  plant life
• The plant matter was then compacted into solid
  rock.
• Coal is one of the major sources of energy for
  Electrical Power plants.

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Where Sedimentary Rocks are Found.

• Sedimentary rocks are found as a thin layer over
  75 of the Earth.
• Sedimentary rocks are not found deep underground
  because the processes that form them occur at the
  surface only.
• Look at Pg. 3 of the ESRT.
• Note the Distribution of Sedimentary Rocks.

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Features of Sedimentary Rocks

• Kinds of Sedimentary Rocks
• There are 3 main kinds of sedimentary rocks,
  Clastic, Chemical or Organic (Bioclastic).
• Clastic
• Form from fragments of other rocks by compaction
  and cementation
• Chemical
• Form when mineral crystals form as precipitates
  or other chemical reactions.
• Organic (Bioclastic)
• Sedimentary Rocks forming from the remains of
  plants and animals.

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Properties/Characteristics of Sedimentary Rocks

• Discrete Layers
• Layers form as one sediment settles on top of
  another, and then are compressed into rock.
• Fragmental Particles
• Called Clastic sedimentary rocks.
• These rocks are made of pieces of rock cemented
  together.
• These rocks are classified based on the size of
  the grains of sediment.
• Look at your ESRT pg. 7

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Grain Sizes

• Grain Sizes are recorded on your ESRT pg 6 also.
• Your ESRT lists the sizes of each particle in
  centimeters.

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Properties/Characteristics of Sedimentary Rocks

• Organic Composition
• Some rocks are composed partially or entirely out
  of the remains of once living things.
• Such remains are called Fossils.
• These rocks are referred to as Bioclastic.
• These rocks are identified by composition and
  texture.
• Predominance of One Particle Size
• Gives us clues that the environment the rock
  formed in was steady over a long period of time.

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What sedimentary rocks tell us

• The environment in which a sedimentary rock forms
  is often determined by its characteristics.
• Composition
• The presence of marine fossils indicates that the
  rock formed at the Earths Surface in a large
  body of water (ocean).
• If salt is present we know it was probably formed
  by the evaporation of water, meaning it was
  shallow water.
• If limestone is present we know it was a warm and
  shallow environment, like those that corals form
  at today.

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What sedimentary rocks tell us (2)

• Structure
• The layers in rocks offer clues as to the
  environment of formation.
• Layers with ripple marks, cross bedding, and
  fossil shells indicate these rocks formed in
  shallow marine environments.
• The size of the clasts in a rock tell us where it
  formed, if large particles are present only, then
  the water above was moving enough to carry only
  small particles of sediment.

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What sedimentary rocks tell us (3)

• Texture
• The shape of the rock fragments tell us a lot
  about where the rock formed in relation to the
  source material (original rock).
• Rounded sediments tell us the sediment has been
  moved a great distance by water.
• Angular sediments tell us it hasnt been moved
  far, or was not transported by water.

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Texture

• On left, angular on the right rounded sediments