Fine Grained Clastic Rocks

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Fine Grained Clastic Rocks

Fine grained rocks are difficult to characterise, but are most abundant (75% of all sedimentary rocks). They may contain the fossils and not have much cement between the grains.

The following rocks are very common in low energy environments,mostly marine but also on the flood plain of rivers or in lakes. 

Reminder: clastic rocks are made up of pre-existing minerals and rocks. 

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  • Dark grey
  • Very fine
  • Contains clay minerals, mica and quartz
  • Indivudal minerals cannot be identified with a hand lens
  • Not layered
  • No plasiticity (the ability of a material to permanently change shape without fracuring)
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  • Very fine grained – more compacted than mudstone 
  • 40% water
  • Mostly clay
  • Variety of colours – dark brown/red/green (depends on carbon or iron content)
  • The higher the organic content, the darker the clay.
  • Plastic (can be moulded)
  • Forms layers with distinct bedding planes. 
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  • Dark coloured 
  • Fine grained
  • Distinctive layers due to mineral alignment 
  • Clay minerals are flat and parallel to the beds at 90 degrees to the pressure from overlying rocks.
  • Splits easily along laminations = fissile 
  • Composed of clay, mica and quartz
  • Not plastic, but hard, brittle and impermeable. 
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Limestones - group 1

There are 2 groups of limestones:

  • Chemical limestone from the precipitation of CaCO3 from sea water.
    • Oolitic limestone
    • Chemically formed
    • Sub spherical can sized grains (1mm) = ooliths
    • Oolitsh have concentric layers of CaCO3 surrounding a nucleus of a shell fragment or sand grain.
    • Ooliths have a ine calcite mud matrix (micrite) or a crystalline cement (sparite)
    • Form in sub-tropical seas in shallow water, agitated by high energy waves
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Limestones - group 2

There are 2 groups of limestones:

  • Biological limestone form organic remains e.g. shell fragments. 
    • Chalk
    • Biologically formed limestone
    • Composed of coccoliths – calcareous discs from the skeletons of algae
    • White = pure calcium
    • Commonly formed in depth water shelf environments. 
    • Fossiliferous Limestones
    • Bioclastically formed from fossil fragments
    • Bioclastic, but not clastic
    • Micrite or spartite cement
    • High energy environments if fossils are broken e.g. sea bed
    • Low energy environments if fossils are whole e.g. lagoons
    • E.g.
      • Shelly limestone – made form shells e.g. bivalves
      • Reef limestone – made form sections of coral
      • Crinoidal limestone – made form sections of crinoids. 
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  • Most limestones are 90% CaCO3 – minerals composition is a good indicator
  • Modern limestone is mostly composed of aragonite (a form of CaCO3) that is unstable in rocks and changes to calcite.
  • Calcite reacts with HCL – easy to identify. 
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