Types of Sedimentary Rocks

• Clastic – Made from weathered rock fragments
• Chemical – Minerals that crystallize directly from water.
• Biochemical – Cemented shells of organisms.
• Organic – The carbon-rich remains of plants.

Clastic Sedimentary Rocks

Clasts are broken bits of rock, and range in size from very large to very small. Like Igneous Rocks, Clastic Sedimentary Rocks are classified on the basis of texture and composition. The following are analysed to determine the type of clastic sedimentary rock:

• Clast (grain) size
  
  

  Clast	Size
  Boulder	> 200 mm
  Cobble	64 – 200 mm
  Pebble	4 – 64 mm
  Gravel	2 – 4 mm
  Sand	1/16 – 2 mm
  Silt	1/256 – 1/16 mm
  Clay	< 1/256 mm

  
  
• Clast composition - What are the clasts composed of?
  • Clay minerals
  • Quartz
  • Feldspars
  • Micas
  • Other minerals
  • Rock fragments
    
    
• Angularity and sphericity - Indicate degree of transport. Fresh detritus is usually angular and non-spherical. Grain roundness and sphericity increases with transport.
  
  
• Sorting.
  • Well-sorted - Uniform grain size indicates a uniform amount of energy, such as waves on a beach.
  • Poorly sorted - Wide variety of grain sizes suggests a depositional environment with a varying amount of energy, such as an alluvial fan.
    
    
• Character of cement - Minerals that fill sediment pores. There are two processes at work here. The first is mineral-enriched water. These waters flow through the sediment during burial, coating the clasts (grains) with that mineral, 'glueing' them together. The second is the pressure of the overlying sediments as they get burried. The grains get compacted into a tighter configuration, reducing pore spaces and increasing the cement/clast ratio, making the clasts 'stick' together more.
  • Weakly cemented (friable) – Grains easily pulled from rock.
  • Strongly cemented (indurated) – Grains hard to dislodge.
  • Common cements: Quartz, Calcite, Hematite, & Clay minerals
    
    
• Maturity – No, not how old the rock is, but rather a measure of the degree of “processing.”
  • Textural maturity – Degree of roundness and sorting.
  • Mineral maturity – Degree of unstable mineral removal.
  • In general, the more round the grains and the more stable minerals there are in a sediment, the mature the sediment is said to be. Time and transport causes sediment evolution. Maturity is used to reconstruct depositional conditions.