Revising Geology #10

SEDIMENTARY ROCKS

Sedimentary rocks are important because

  • they make up much of the surface of the crust
  • they contain information about the history of the earth – the conditions of deposition
  • they provide raw materials for industry, building etc

What does the classification of sedimentary rocks tell you about their mode of formation?

Write a 12-word description of clastic sedimentary rocks which says how they formed and how they were formed.

Write a 12-word description of calcareous sedimentary rocks which says how they formed and how they were formed.

Write a 12-word description of evaporite deposits which says how they formed and how they were formed.

Compile a chart which compares the typical features of sedimentary rocks compared with igneous and metamorphic rocks.

How would you recognise a sedimentary rock

  • in the field?
  • on a geology map?


Clastic sedimentary rocks

Clastic sedimentary rocks are classified by grain size. Why?

Rudaceous rocks are made of grains larger than 2mm.

  • Why is 2mm chosen? Is there any connection with coarse-grained igneouis rocks?
  • What are the features of a clast larger than 2mm compared with one which is smaller than 2mm?
  • Where do clasts larger than 2mm come from. How are they formed?

Argillaceous rocks are made of grains less than 0.004mm.

  • Why is this figure chosen>
  • What are the features of ‘grains’ less than 0.004mm.
  • Where do they come from? How are they formed?

Arenaceous rocks are between 0.004mm and 2mm in diameter.

  • What are the features of grains of this size?
  • Why is quartz commonly found in arenaceous  clastic sedimentary rocks?

Textures of sedimentary rocks

List the factors which control the texture of clastic sediments.

Write 10-word definitions of

  • texture
  • sorting
  • rounding

Explain each of the following textures which involve the degree of rounding:

  • angular clasts in a rudaceous rock;
  • angular clasts in silt-size  arenaceous rock;
  • rounded clasts in rudaceous rock;
  • moderately well rounded clasts in arenaceous rock;
  • very well-rounded clasts in arenaceous rock.

Explain each of the following textures which involve the degree of sorting:

  • a rudaceous rock which is not sorted;
  • a rudaceous rock which is poorly sorted;
  • an arenaceous rock which is moderately sorted;
  • an arenaceous rock which is very well sorted;
  • an arenaceous rock which has mica flakes larger than 2mm.

Geopix Study Guide Sedimentation will help you revise.