Chapter 6 Sediments and Sedimentary Rocks

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Chapter 6 Sediments and Sedimentary Rocks
GEOL 101 Introductory Geology
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Sediments

• Collective name for loose, solid particles
  originated from weathering and erosion, thus
  Unconsolidated.
• Classification of sediments based on sizes
• Gravel gt 2mm
• Sand between 1/16 and 2 mm
• Silt between 1/256 mm and 1/16 mm
• Clay lt 1/256 mm

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Change from Sediments to Sed Rxs

• Transportation
• Roundness round distance from source
• Sorting well sorted - distance from source
• Deposition settle of particles
• Preservation

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Change from Sediments to Sed Rxs

• Lithification
• Compaction reduction in pore space
• Cementation seal out the pores
• Crystallization growth of crystals from
  solution.

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Sedimentation Process
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What is Sedimentary Rock?

• Sedimentary rocks are products of mechanical and
  chemical weathering
• Account for about 5 (by volume) of Earths
  upper crust
• Contain evidence of past environments
• Provide information about sediment transport
• Often contain fossils
• Economic importance
• Coal
• Petroleum and natural gas
• Sources of iron, aluminum, and manganese

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Sedimentary Rocks

• Rock types
• Classification
• Sedimentary Environments
• Fossils

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Sedimentary Rock Classification

• Classified according to material type
• Clastic
• Chemical
• Two major textures are used in the classification
  of sedimentary rocks
• Clastic
• Discrete fragments and particles
• All detrital rocks have a clastic texture
• Nonclastic
• Pattern of interlocking crystals
• May resemble an igneous rock

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Sedimentary Rocks Types

• Sediment originates from mechanical and/or
  chemical weathering
• Rock types are based on the source of the
  material
• Clastic rocks transported sediment as solid
  particles
• Chemical rocks sediment that was once in
  solution

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Detrital Sedimentary Rocks

• The chief constituents of detrital rocks include
• Clay minerals
• Quartz
• Feldspars
• Micas
• Particle size is used to distinguish among the
  various types of detrital rocks
• Particle shape and sorting give indication of
  transport distance (maturity)

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Particle Size Classification
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Sorting, Shape, Setting
Sorting
Sphericity Roundness
Depositional Setting
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Clastic Sedimentary Rocks

• Common Clastic sedimentary rocks (in order of
  increasing particle size)
• Shale
• Mud-sized particles in thin layers that are
  commonly referred to as laminea
• Most common sedimentary rock
• Sandstone
• Composed of sand-sized particles
• Forms in a variety of environments
• Sorting, shape, and composition of the grains can
  be used to interpret the rocks history
• Quartz is the predominant mineral

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Detrital Sedimentary Rocks

• Conglomerate and breccia
• Both are composed of particles greater than 2mm
  in diameter
• Conglomerate consists largely of rounded gravels
• Breccia is composed mainly of large angular
  particles

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Shale containing plant remains
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Sandstone
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Conglomerate
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Breccia
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Chemical Sedimentary Rocks

• Consist of precipitated material that was once in
  solution
• Precipitation of material occurs in two ways
• Inorganic processes
• Organic processes (biochemical origin)
• Common chemical sedimentary rocks

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Chemical Sedimentary RocksCommon Types

• Limestone
• Dolostone
• Chert
• Evaporites
• Coal

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Limestone

• Most abundant chemical rock
• Composed of calcite carbonate CaCO3
• Marine biochemical limestones form as coral
  reefs, coquina (broken shells), and chalk
  (microscopic organisms)
• Inorganic limestones include travertine and
  oolitic limestone

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Coquina
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Fossiliferous Limestone
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Chemical Sedimentary Rocks

• Dolostone
• Composed of calcite, magnesium carbonate CaMgCO3
• Typically formed secondarily from limestone
• Chert
• Made of microcrystalline quartz
• Varieties include flint , jasper and agate
  (banded form)
• Evaporites
• Evaporation triggers deposition of chemical
  precipitates
• Examples include rock salt and rock gypsum

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Chert
Agate Banded Chert
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Coal

• Common chemical sedimentary rocks
• Different from other rocks because it is composed
  of organic material
• Stages in coal formation (in order)
• Plant material
• Peat
• Lignite
• Bituminous

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Coal
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Coal Formation Stages
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Sedimentary Environments

• A geographic setting where sediment is
  accumulating
• Determines the nature of the sediments that
  accumulate (grain size, grain shape, etc.)
• Identified by sedimentary facies, fossils, and
  structures

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Sedimentary Environments

• Continental
• Dominated by stream erosion and deposition
• Glacial
• Wind (eolian)
• Marine
• Shallow (lt 200 meters)
• Deep (seaward of continental shelves)
• Transitional (shoreline)
• Tidal flats
• Lagoons
• Deltas

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Depositional Environments
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Depositional Environments
Marine
Continental
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Turbidity Currents
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Turbidity Currents
Graded Bedding
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Evaporite Formation
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Sedimentary Facies

• Different sediments often accumulate adjacent to
  one another at the same time
• Facies possesses a distinctive set of
  characteristics reflecting the conditions in a
  particular environment
• Merging of adjacent facies tends to be a gradual
  transition

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Sedimentary Facies
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Sedimentary Structures

• Useful for interpreting sedimentary environment
• Strata or beds, layers of similar sediment
  (most characteristic of sedimentary rocks)
• Lamina thin layers (lt 1cm)
• Bedding planes separate strata
• Cross-bedding
• Graded beds
• Ripple marks
• Mud cracks

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Bedding
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Bedding Structures
Graded bedding
Lamina
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Cross Beds
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Ripple Marks
unconsolidated sediment
lithified rock
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Mud Cracks
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Grand Canyon
o
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Fossils Evidence of past life

• By definition, fossils are the traces or remains
  of prehistoric life now preserved in rock
• Fossils are generally found in sediment or
  sedimentary rock (rarely in metamorphic and never
  in igneous rock)

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Dinosaur footprint in limestone
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Fossils Evidence of past life

• Geologically fossils are important for several
  reasons
• Aid in interpretation of the geologic past
• Serve as important time indicators
• Allow for correlation of rocks from different
  places
• Studied in detail in Historical Geology (GEOL
  102), offered in Spring 2004

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Chalk Cliffs of Dover, England
Tiny planckon shells
Deposited on sea floor Later uplifted