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
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.