Sediments and Sedimentary Rocks

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Sediments and Sedimentary Rocks
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Sedimentary rocks

• Volume of upper crust 5
• Area of continents 75
• Record of geological events e.g., Himalayas
  will someday be sediment

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Sediment stages
1. Weathering
2. Erosion
3. Transportation via water, glaciers and wind
4. Deposition
5. Burial and compaction
6. Diagenesis
Fig. 8.4
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Diagenesis

• Diagenesis The physical and chemical changes
• Heat
• Pressure
• Chemical reactions
• by which buried sediments are lithified and
  become sedimentary rocks.

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Diagenetic Processes
Sediments are buried compacted and lithified at
shallow depths in the Earths crust.

or subducted where they are subjected to high
pressure and temperature
Fig 8.11
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Types of sedimentary rock

• Clastic Rock resulting from the consolidation
  of loose sediment that has been derived from
  previously existing rocks and accumulated in
  layers.
• Clastic particles Physically transported rock
  fragments produced by the weathering of
  pre-existing rocks.
• Clastic sediments Accumulations of clastic
  particles.
• (b) Chemical Rock formed by the precipitation
  of minerals from solution by either organic or
  inorganic processes.

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Diagenetic Processes
Compaction squeezes out the water
Precipitation addition of new minerals cements
the sediment particles
Compaction squeezes out the water
Fig. 8.11
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Types of sediments
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Types of sediments
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Clastic rocks
Fig. 8.14
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Relative Abundance of Sedimentary Rocks
Fig. 8.13
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Sedimentary Environments
Fig. 8.4
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Clastic sedimentary environment
Fig. 8.4
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Chemical and biochemicalsedimentary environments
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Transport affects the type of sediment

• Movement of sediment by
• wind (e.g., dunes)
• ice (e.g., glaciers)
• or water (e.g., rivers, streams).
• Mode of transport produces distinctive
  deposits.
• Most sediments are carried by rivers (25 billion
  tons/year)

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Transport affects the type of sediment

• Strong currents
• Faster than 50cm/s
• Carry gravel finer material
• Moderately strong currents
• 20-50cm/s
• Carry sand
• Weak currents
• Slower than 20cm/s
• Carry mud and silt
• Faster flow Carry larger particles

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Sorting

• Sorting Measure of the variation in grain sizes
  in a clastic rock or sediment.
• Well-sorted sediments indicate that they have
  been subjected to prolonged water or wind
  action.
• Poorly-sorted sediments are either not
  far-removed from their source or are deposited
  by glaciers.

Fig. 8.2
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Roundness

• Roundness Measure of how rounded the corners
  are.
• Angular grains close to source
• Rounded grains transported for a great distance

Fig. 8.3
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Sedimentary structures

• Bedding or stratification Parallel layers of
  grains of different size or compositions.
  Bedding ranges from mm to m thick. Generally
  horizontal at the time of deposition (not
  cross-bedding).
• Whats up
• Cross bedding Sets of bedded material
  deposited at angles as large as 35o.
• Graded bedding Coarse grains at base and
  progressively finer grains towards the top.
• Ripples Small dunes of sand or silt whose
  long dimension is at right angles with the
  current.
• Bioturbation Remnants of burrows and tunnels
  by clams, worms, etc.

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Cross bedding
Grains are deposited on the lee side of the dune
Fig. 8.6
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Cross bedding
Fig. 8.5
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Ripples

• Very small dunes.
• Long axis is at right angle to the prevailing
  current.
• Can be symmetric or asymmetric.

Fig. 8.8
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Ripples
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Modern rippled sand
Fig. 8.7
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Ripple Marks (Ancient Rippled Sand)
Fig. 8.7
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Making ripples
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Bioturbation
Fig. 8.9
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Continental slope
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Turbidity currents

• Suspension of water, sand, and mud that moves
  downslope (often very rapidly) due to its greater
  density than that of the surrounding water.
• Often triggered by earthquakes.
• Speed of turbidity currents first appreciated in
  1920 (breaking of phone lines on the Atlantic
  seafloor). Also gave indication of distance
  traveled by a single current.

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Graded Bedding

• Found in continental slope environments.
• Beds can be cm to m.
• Each layer has coarse grains at bottom, fine
  grains at top (due to decrease in current that
  deposited the grains).

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River sediment
Fig. 8.10
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Classification of (bio)chemical sediments
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Chemical and biochemical sedimentary rocks
Fig. 8.17
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Chemical and biochemical sedimentary rocks

• Evaporites Precipitations due to evaporating
  seawater.
• arid climate
• freshwater supply from rivers is low
• connections to open sea are restricted

• Silica sediment chert
• source of silica diatoms or other silicate
  microorganisms
• - diagenetic alteration

• Iron oxide
• banded iron formation iron ore
• older formed when the atmosphere had less oxygen
• ocean contained oxygen producing microorganism
  oxidizing Fe

• Phosphorites
• cold deep ocean water rising along the
  continental margin
• deep ocean water rich in phosphorous

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Carbonate platforms
Fig. 8.16
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Within the reef lagoon, growth of
carbonate-secreting organisms, including
forminifera, coral, algae and mollusks, is rapid,
and carbonate sediments form quickly.
Eventually a carbonate platform grows with steep
sides towards the sea.
If sea level rises, the reef continues to grow
towards the light and lagoon sedimentation
outpaces sedimentation in the open ocean.
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Coral Reefs and Atolls
Bora Bora atoll, South Pacifc
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Coral Reefs and Atolls
Process first described by Charles Darwin
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Evaporites
1. During Miocene, the Mediterranean became a
shallow basin
2. Reduced exchange with open sea
3. Evaporation removed water
4. Fresh water inflow was limited
5. Gypsum and halite crystallize, forming
evaportites
Fig. 8.19