Sediments and Sedimentary Rocks

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Sediments and Sedimentary Rocks
Tom Bean/DRK
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Sedimentary Rocks

• Make up 5 by volume of the upper crust, but
• Form 75 of the rocks exposed on the continents
  areas
• Source of most fossil fuels
• Often the only record of geologic events

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Processes of the Rock Cycle Related to
Sedimentary Rocks

• Weathering
• Erosion
• Transportation
• Deposition (sedimentation)
• Burial
• Diagenesis

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Sedimentary Stages in the Rock Cycle
Fig. 7.1
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Major Categories of Sedimentary Rocks

• Detrital or Clastic - Rock resulting from the
  consolidation of loose sediment that has been
  derived from previously existing rocks and
  accumulated in layers
• Chemical - Rock formed by the precipitation of
  minerals from solution by either organic or
  inorganic processes

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Transport and Deposition of of Clastic Sediments

• Movement of sediment by wind, ice or water.
• Mode of transport produces distinctive deposits.

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Transport affects the sediment in several ways

• Sorting measure of the variation in the range
  of 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 deposited by
  glaciers.

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Sorting
Well-sorted
Poorly-sorted
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Transport affects the sediment in several ways

• Roundness measure of how rounded the corners
  are
• Sphericity measure of how much it is like a
  sphere
• Sorting, roundness, and sphericity all increase
  with amount of transport.

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Well-sorted Sand
Fig. 7.2
Rex Elliott
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Poorly-sorted Sand
Fig. 7.2
Rex Elliott
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Roundness and Sphericity
Fig. 7.3
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Types of Detrital Rocks

• Based on the size of the particles
• Conglomerate (rounded) or Breccia (angular)
• Sandstone (quartzite, arkose, greywacke)
• Siltstone
• Shale (laminated) or Claystone or Mudstone
  (massive)

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Conglomerate
Fig. 7.15a
Breck Kent
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Sandstone
Fig. 7.15b
Fig. 7.15a
Breck Kent
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Classification of Sandstones

• Sandstones are sometimes given special names
  depending upon he composition of the detrital
  grains

Fig. 7.16
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Shale
Fig. 7.15c
D. Cavagnaro/Visuals Unlimited
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Lithifaction

• The process by which unconsolidated sediments are
  turned into sedimentary rocks.
• Compaction decrease in the pore space between
  clastic grains. Compaction may be up to 40
  reduction in volume in shales
• Cementation the precipitation of mineral
  within pore spaces, this tends to bind grains
  together and further reduce pore space

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Common Cement Minerals

• Calcite
• Silica (SiO2)
• Iron oxide

Calcite Cement
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Common Cement Minerals

• Calcite
• Silica (SiO2)
• Iron oxide

Silica Cement
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Chemical Sedimentary Rocks

• May form due to direct chemical precipitation or
• Form due to biochemical activity in which animals
  and plants leave skeletons composed of calcite,
  silica, or phosphate
• Major types of chemical sedimentary rocks
• Limestone
• Dolomite
• Chert
• Evaporites

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Limestone

• Composed of calcite (CaCO3)
• Most common type of chemical sedimentary rock and
  the most common type of rock in Florida

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Varieties of Limestone

• Coquina composed of coarse shell fragments
• Chalk composed of nearly 100 microscopic
  marine skeletal remains
• Oolitic limestone composed of small round
  grains (ooids), which form by calcite
  precipitation around a seed grain
• Travertine formed by inorganic precipitation of
  calcite in caves and near hot springs

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Coral Reef Surrounding Volcanic Island
Fig. 7.18
Jean-Marc Truchet/Tony Stone Worldwide
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Fossiliferous Limestone
Peter Kresan
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Chemical Sedimentary Rocks (2)

• Dolomite composed of the mineral dolomite,
  Ca,Mg(CO3) usually forms by the alteration
  (replacement) of limestone
• Chert composed of silica (SiO2) formed by
  organic or inorganic mechanisms and by the
  replacement of calcite in limestone by silica

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Chert
Fig. 7.19d
Breck Kent
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Chemical Sedimentary Rocks (3)

• Evaporites formed by precipitation of minerals
  in seas and lakes in regions of rapid evaporation
  
• Examples
• Rock salt (halite)
• Gypsum

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Halite
Fig. 7.19c
Breck Kent
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Gypsum
Fig. 7.19b
Breck Kent
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One Model for the Formation of Evaporites
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Playa Lake
Fig. 14.23
David Muench
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Sedimentary Structures

• Provide information about the environment in
  which the sedimentary rock formed
• Most common structure is bedding or
  stratification (each bed is separated by a
  bedding plane)
• Structures also help determine if a bed is
  right-side-up. - this is important in deformed
  rocks

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Fine layering in Sandstone
Fig. 7.15b
Fig. 7.15a
Breck Kent
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Tom Bean/DRK
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Modeling Sedimentary Environment

• Sedimentary structures
• Sorting, roundness, sphericity
• Sequence of beds

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Graded Bedding

• Turbidity currents carry gravel, sand, and mud in
  an underwater avalanche
• Once motion stops, larger particles settle first

Scale mm to meters
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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 in the Atlantic
  also gave indication of distance traveled by a
  single deposit

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Other Stratigraphic Up Indicators

• Cross-bedding
• Ripple marks
• Mudcracks
• Raindrop impressions
• Fossils (some may have been preserved in growth
  position)

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Cross-bedded Sandstone
Fig. 7.6
Peter Kresan
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Formation of Cross-beds
Fig. 7.7
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Modern Rippled Sand
Fig. 7.8
Raymond Seiver
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Ancient Ripple-marked Sandstone
Fig. 7.8
Reg Morrison/Auscape
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Fig. 7.9
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Mudcracks
MODERN
ANCIENT
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Bioturbation Tracks and Tunnels
Chip Clark
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Relative Abundance of Sedimentary Rock Types
Fig. 7.14
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Common Sedimentary Environments
Fig. 7.5